Communication system

Abstract

1. In combination: a signal source for producing a signal having characteristic variations representative of intelligence, a first code source for producing a first code signal having apparently random characteristic variations but actually having characteristic variations conforming to a first particular code sequence which repeats after a relatively short time interval, a second code source for producing a second code signal likewise having apparently random characteristic variations but actually having characteristic variations conforming to a second particular code sequence which may repeat after a relatively long time interval, modifying circuit means responsive to the intelligence signal from the signal source and to a coded signal for modifying the characteristics of the coded signal in accordance with the characteristics of the intelligence signal, and means responsive to the signals from the first code source and to the signals from the second code source for initially introducing the first code signal to the modifying means for modification of such signal in accordance with the characteristics of the intelligence signal and of the first code signal and for subsequently introducing the second code signal to the modifying means for modification of the second code signal in accordance with the characteristics of the intelligence signal.

Claims

We claim:

1. In combination: a signal source for producing a signal having characteristic variations representative of intelligence, a first code source for producing a first code signal having apparently random characteristic variations but actually having characteristic variations conforming to a first particular code sequence which repeats after a relatively short time interval, a second code source for producing a second code signal likewise having apparently random characteristic variations but actually having characteristic variations conforming to a second particular code sequence which may repeat after a relatively long time interval, modifying circuit means responsive to the intelligence signal from the signal source and to a coded signal for modifying the characteristics of the coded signal in accordance with the characteristics of the intelligence signal, and means responsive to the signals from the first code source and to the signals from the second code source for initially introducing the first code signal to the modifying means for modification of such signal in accordance with the characteristics of the intelligence signal and of the first code signal and for subsequently introducing the second code signal to the modifying means for modification of the second code signal in accordance with the characteristics of the intelligence signal.

2. In combination: a signal source for producing a signal representative of intelligence to be transmitted from one point to another point, a source for generating a carrier signal, means coupled to said signal source and to said generating source for modulating the carrier signal in accordance with the intelligence signal to produce a modulated intelligence signal, a first code source for producing a first code signal having pseudo-random characteristic variations which are repetitive after a particular relatively short time interval, a second code source for producing a second code signal having pseudo-random characteristic variations which are repetitive after a particular relatively long time interval, modifying circuit means responsive to the modulated intelligence signal and to a coded signal for modulating the coded signal in accordance with the characteristics of the modulated intelligence signal, and selecting means responsive to the signals from the first code source and the second code source for initially introducing the first code signal to the modifying means for modification of the first code signal in accordance with the characteristics of the modulated intelligence signal to produce a short-code coded intelligence signal and for subsequently introducing the second code signal to the modifying means for modification of the second code signal in accordance with the characteristics of the modulated intelligence signal to produce a long-code coded intelligence signal.

3. The combination defined in claim 2, including, an indicating signal source for producing an indicating signal, and means responsive to the short-code coded intelligence signal for causing the indicating signal to be transmitted from said one point to the other during the production of the short-code coded intelligence signal.

4. The combination defined in claim 2, including, an indicating signal source for producing an indicating signal, and means responsive to the short-code coded intelligence signal for causing such indicating signal to replace the intelligence signal and be transmitted from said one point to the other just prior to the production of the long-code coded intelligence signal.

5. In combination, a signal source for producing a signal representative of intelligence to be transmitted from one point to another, a source for producing a binary carrier signal having a first voltage state and a second voltage state, modulating means coupled to said sources and responsive to the intelligence signal and to the binary carrier signal for frequency modulating the carrier signal in accordance with the intelligence signal to produce a modulated intelligence signal having first and second voltage states and exhibiting transitions between such voltage states, a first code source for producing a first binary code signal having first and second voltage states and exhibiting transitions between such voltage states in conformance with an apparently random particular code sequence which is repetitive after a relatively short time interval, a second code source for producing a second binary code signal having first and second voltage states and exhibiting transitions between such voltage states in conformance with an apparently random particular code sequence but which may be repetitive after a relatively long time interval, inverting circuit means responsive to the modulated intelligence signal and to the first and second code signals for passing the code signals without any change in amplitude during the occurrence of the first voltage state in the modulated intelligence signal and for changing the amplitude of the code signals from the first voltage state to the second voltage state and from the second voltage state to the first voltage state during the occurrence of the second voltage state in the modulated intelligence signal, and means responsive to the first code signal and the second code signal for initially introducing the first code signal to the inverting circuit means for modification of the first code signal in accordance with the voltage characteristics of the modulated intelligence signal and for subsequently introducing the second code signal to the inverting circuit means for modification of the second code signal in accordance with the voltage characteristics of the modulated intelligence signal.

6. The combination defined in claim 5 and in which said intelligence signal has a particular identifying frequency during the introduction of the first code signal to the inverting circuit means.

7. The combination defined in clam 5 and in which said intelligence signal has a particular identifying frequency just prior to the introduction of the second code signal to the inverting circuit means.

8. In a coded communication system for operating upon a coded intelligence signal which is initially coded in accordance with a first code signal having apparently random characteristic variations but actually having characteristic variations conforming to a first particular code sequence having a repetitive pattern after a particular relatively short time interval and which is subsequently coded in accordance with a second code signal also having apparently random characteristic variations but actually having characteristic variations conforming to a second particular code sequence having a repetitive pattern after a particular relatively long time interval, a receiving station including: means for receiving the coded intelligence signal, a code signal producing means for selectively producing signals corresponding to the first code signal and to the second code signal, decoding circuit means responsive to the signals from the receiving means and from the code signal producing means for detecting the coded intelligence signal to obtain the intelligence signal, and means coupled to the code signal producing means for initially obtaining the introduction of the first code signal from the code signal producing means to the decoding circuit means during the coding of the intelligence signal with the first code signal to obtain the detection of the intelligence signal by the decoding circuit means and for subsequently obtaining the introduction of the second code signal from the code signal producing means to the decoding circuit means during the coding of the intelligence signal with the second code signal to obtain the detection of the intelligence signal by the decoding circuit means.

9. The combination defined in claim 8, including, clock signal generating means coupled to the code signal producing means for controlling the clock frequency of the code signals produced by the code signal producing means.

10. In a coded communication system for operating upon a coded intelligence signal coded in accordance with a code signal having apparently random characteristic variations but actually having characteristic variations conforming to a particular code sequence having a particular clock frequency, a receiving station including: means for receiving the coded intelligence signal, a decoding signal producing means for producing at least one decoding signal corresponding to the above mentioned code signal, decoding circuit means coupled to the receiving means for detecting the coded intelligence signal to obtain the intelligence signal, means coupled to the decoding signal producing means for obtaining the introduction of the decoding signal from the decoding signal producing means to the decoding circuit means to obtain the detection of the intelligence signal, clock signal generating means coupled to the decoding signal producing means for producing clock signals at an adjustable frequency and for introducing the clock signals to the decoding signal producing means to control the frequency at which the decoding signal is produced, control means coupled to the clock signal generating means for obtaining a particular change in the clock frequency from the particular value during a search interval to achieve correlation between the decoding signal produced by the decoding signal producing means and the coding of the received coded intelligence signal, and means coupled to the clock signal generating means and responsive to the decoding signals and to the coded intelligence signal for locking the clock signal generating means for the production of a particular frequency upon the achievement of correlation between the decoding signal produced by the decoding signal producing means and the coding of the received coded intelligence signal.

11. The combination defined in claim 10, including, signal correlation circuit means coupled to said receiving circuit means and to said decoding signal producing means for producing a control signal indicative of variations at the receiving means between the received coded intelligence signal and the decoding signal produced by said decoding signal producing means, and means coupled to the correlation circuit means and to the clock signal generating means for introducing said control signal to the clock signal generating means to control the frequency of the clock signals and to maintain correlation between the decoding signal produced by the decoding signal producing means and the coding of the received coded intelligence signal.

12. In a coded communication system for operating upon a coded intelligence signal which is initially coded in accordance with a first code signal having apparently random characteristic variations but actually having characteristic variations conforming to a first particular code sequence having a repetitive pattern after a particular relatively short time interval and which is subsequently coded in accordance with a second code signal also having apparently random characteristic variations but actually having characteristic variations conforming to a second particular coded sequence having a repetitive pattern after a particular relatively long time interval, a receiving station including: means for receiving the coded intelligence signal, a decoding signal producing means for selectively producing first and second decoding signals respectively corresponding to the first code signal and to the second code signal, decoding circuit means responsive to the signals from the receiving means and from the coding signal producing means for detecting the coded intelligence signal to obtain the intelligence signal, control means coupled to the decoding signal producing means for initially obtaining the introduction of the first decoding signal from the decoding signal producing means to the decoding circuit means during the coding of the coded intelligence signal by the first coding signal to obtain the detection of the intelligence signal and for subsequently obtaining the introduction of the second decoding signal from the decoding signal producing means to the decoding circuit means during the coding of the intelligence signal by the second coding signal to obtain the detection of the intelligence signal, clock signal generating means coupled to the coding signal producting means for producing clock signals at an adjustable frequency and for introducing the clock signals to the decoding signal producing means to control the frequency of the decoding signals produced by the decoding signal producing means, and means coupled to the clock signal generating means for causing the clock frequency to shift from one value to another for a particular time interval until correlation is achieved with the received coded intelligence signal when such coded intelligence signal is coded in accordance with the first code signal and for causing the clock frequency to shift periodically between two values for a subsequent time interval until correlation is achieved with the received coded intelligence signal when said coded intelligence signal is coded in accordance with the second code signal.

13. In a coded communication system for operating upon a coded intelligence signal which is initially coded in accordance with a first code signal having apparently random characteristic variations but actually having characteristic variations conforming to a first particular code sequence having a repetitive pattern after a particular relatively short time interval and which is subsequently coded in accordance with a second code signal also having apparently random characteristic variations but actually having characteristic variations conforming to a second particular code sequence having a repetitive pattern after a relatively long time interval and which includes an indicating signal for indicating when the changeover from the initial coding to the subsequent coding is to occur, a receiving station including: means for receiving the coded intelligence signal, a decoding signal producing means for selectively producing first and second decoding signals respectively corresponding to the first code signal and to the second code signal, decoding circuit means coupled to the receiving means and to the decoding signal producing means for detecting the coded intelligence signal to obtain the intelligence signal, control means coupled to the decoding signal producing means for initially obtaining the introduction of the first decoding signal from the decoding signal producing means to the decoding circuit means during the coding of the coded intelligence signal by the first code signal to obtain the intelligence signal and for subsequently obtaining the introduction of the decoding signal from the decoding signal producing means to the decoding circuit means during the coding of the coded intelligence signal by the second coding signal to obtain the intelligence signal, and means responsive to the indicating signal included in the coded intelligence signal for causing the control means to terminate the introduction of the first decoding signal to the decoding circuit means and to initiate the introduction of the second decoding signal to the decoding circuit means.

14. The combination defined in claim 13, including, clock signal generating means coupled to the decoding signal producing means for producing clock signals at an adjustable frequency and for introducing the clock signals to the decoding signal producing means to control the frequency of the first and second decoding signals produced by the decoding signal producing means, and means responsive to the indicating signal for causing the clock frequency to shift periodically between two values for a particular time interval after the introduction of the second code signal to the coded intelligence signal and until correlation is achieved between the second decoding signal and the coded intelligence signal.

15. In a coded communication system for operating upon a coded intelligence signal which is coded in accordance with a code signal having apparently random characteristic variations but actually having characteristic variations conforming to a particular code sequence having a particular clock frequency, a receiving station including: means for receiving the coded intelligence signal, a decoding signal producing means for producing at least one decoding signal corresponding to the above mentioned code signal, decoding circuit means responsive to the signals from the receiving means and from the decoding signal producing means for detecting the coded intelligence signal to obtain the intelligence signal, means coupled to the decoding signal producing means for obtaining the introduction of the decoding signal from the decoding signal producing means to the decoding circuit means to obtain the intelligence signal, clock signal generating means coupled to the decoding signal producing means for producing clock signals at a frequency adjustable from a particular frequency and for introducing the clock signals to the decoding signal producing means to control the frequency of the decoding signal produced by the decoding signal producing means, first control means coupled to the clock signal generating means for obtaining particular variations in the clock frequency from the particular value during a search interval to achieve correlation between the decoding signal produced by the decoding signal producing means and the coding of the received coded intelligence signal, means operatively coupled to the clock signal generating means for locking the clock signal generating means at a particular frequency to maintain the correlation between the decoding signal and the coding of the received coded intelligence signal, and second control means coupled to the first control means to activate the first control means during periods of reception of the coded intelligence signal below an established threshold value.

16. The combination defined in claim 15, in which the second control means includes memory means for delaying the activation of the first control means by the second control means for a particular interval and for establishing the frequency of the clock signal generating means at a fixed value during each such particular interval.

17. In combination for use in each of a first station and a second station displaced from one another by a distance to be measured, a transmitting section including: a first source for producing a signal to be transmitted, a second source for producing a code signal having a controllable clock frequency, transmitter control means coupled to the second source for controlling the clock frequency of the code signal produced thereby, and coding circuit means coupled to the first source and to the second source for producing a signal coded in accordance with a particular code sequence and which code sequence has a particular clock frequency established by the transmitter control means; a receiving section for receiving the coded signal from the transmitting section of the other station and including means for receiving the coded signal, a decoding signal producing means for producing a signal corresponding to the code signal produced by the transmitting section of the other station, decoding circuit means coupled to the receiving means and to the decoding signal producing means and responsive to the decoding signal for decoding the received signal, receiver correlation means coupled to the receiving means and to the decoding signal producing means for producing a control signal indicative of the displacement of the received signal and the decoding signal, and receiver control means coupled to the correlation means and to the decoding signal producing means for controlling the clock frequency of the decoding signal in response to the correlation control signal from the correlation means to obtain a correlation between the received signal and the decoding signal; first switching means coupled to said transmitting and receiving signals for causing the transmitting section of the first station to transmit the coded signal to the receiving section of the second station, second switching means coupled to said transmitting and receiving sections for subsequently causing the transmitted section of the second station to transmit the coded signal to the receiving section of the first station and to set the receiver control means at the first station to a condition such that said clock frequency is established at a particular displaced value; and range indicating counter means coupled to the receiver correlation means of the first station and to the receiver control means of the first station to indicate the time required for correlation to be established between the signal received from the second station and the signal produced by the code signal producing means of the first station.

18. A system for measuring the range between a first station and a second station including: means at the first station for transmitting a signal and for producing a particular code signal to code the transmitted signal, first means at the second station for receiving the coded signal from the first station and for producing a decoding signal corresponding to the particular code signal and displaced in phase with respect thereto by an amount corresponding to the range between the first and second station so as to decode the received coded signal, second means at the second station and coupled to the first means at the second station for transmitting a signal coded in accordance with the decoding signal produced at the second station, second means at the first station for receiving the coded signal from the second station and for producing the particular decoding signal to decode the received coded signal, control means at the first station and coupled to the second means at the first station for varying the particular decoding signal produced by the second means at the first station until correlation with the coded signal received from the second station is achieved, and range indicating means coupled to the last named control means for providing an indication as to the range between the first and second stations in accordance with the time required to achieve correlation between the decoding signal and the coded signal received from the second station.

19. A receiving station for use in a coded communication system for receiving a coded intelligence signal coded in accordance with a code signal, said receiving station including: means for receiving the coded intelligence signal, decoding signal producing means for producing a first decoding signal having characteristics corresponding to the code signal and for producing a second decoding signal similar to the first decoding signal and time delayed with respect thereto, decoding circuit means responsive to the decoding signals produced by the decoding signal producing means for decoding the coded intelligence signal, detector means coupled to the decoding circuit means for respectively producing first and second control signals in response to the first and second decoding signals, and control means responsive to the first and second control signals produced by the detector means to control the producing means and maintain a synchronized condition between the decoding signals and the received coded intelligence signal.

20. In a coded communication system for operating upon a coded intelligence signal coded in accordance with a code signal having apparently random characteristic variations but actually having characteristic variations conforming to a particular code sequence having a particular clock frequency, a receiving station including: means for receiving the coded intelligence signal, a decoding signal producing means for producing a first decoding signal and a signal decoding signal individually corresponding to the above mentioned code signal and displaced in time with respect to one another, decoding circuit means responsive to the first and second decoding signals produced by the decoding signal producing means for decoding the coded intelligence signal and for producing a first signal having a first particular frequency in response to the first decoding signal and a second signal having a second particular frequency in response to the second decoding signal, clock signal generating means coupled to the decoding signal producing means for controlling the clock frequency of the first and second decoding signals produced thereby, control means coupled to the decoding circuit means for selecting and comparing the first and second signals produced thereby to produce a control signal, and means coupled to the control means and to the clock signal producing means and responsive to the control signal from the control means for controlling the frequency of the clock signal generating means so as to maintain the first and second decoding signals synchronized with the received coded intelligence signal.

21. A system for producing a code signal having apparently random characteristic variations but actually having characteristic variations conforming to a particular code sequence which repeats after a relatively long time interval, said system including: first code signal generating means for producing a first code signal having characteristic variations conforming to a first code sequence which repeats after a first particular time interval, second code generating means for producing a second code signal having characteristic variations conforming to a second code sequence which repeats after a second particular time interval different from the first particular time interval, and means coupled to the first code generating means and to the second code generating means for producing a third code signal having characteristic variations conforming to a code sequence which is repetitive only after a long interval as compared with the repetition intervals of the first and second code sequences and which is patterned in accordance with the repetition intervals of the first and second code sequences.

22. A system for producing a code signal having apparently random characteristic variations but actually having characteristic variations conforming to a particular code sequence which repeats after a relatively long time interval, said system including: first shift register means for producing a first plurality of successive pulses individually occurring at apparently random times but conforming to a first code sequence which repeats after a first particular time interval, second circulating register means for producing a second plurality of successive pulses individually occurring at apparently random times but conforming to a second code sequence which repeats after a second particular time interval different from the first particular time interval, and mixing means coupled to the first circulating register means and to the second circulating register means for producing a code signal conforming to a code sequence which is repetitive only after a long interval as compared with the repetition intervals of the first and second code sequences and which is patterned in accordance with the repetition intervals of the first and second code sequences.

23. A system for producing a code signal having apparently random characteristic variations but actually having characteristic variations conforming to a particular code sequence which repeats after a relatively long time interval, said system including: a first code selector for establishing a first particular code sequence, a first circulating register, means coupled to the first circulating register for introducing a pulse to the circulating register, means coupled to the first circulating register for sampling at successive intervals the introducing pulse during the circulation of the pulse through the first circulating register to produce a plurality of successive pulses in response thereto and for introducing the successive pulses to the first code selector to obtain a sereis of output pulses corresponding to said first particular code sequence, means coupled to the first code selector for introducing the serial output pulses therefrom to the first circulating register to be circulated therein and thereby forming a first code signal, a second code selector for establishing a second particular code sequence, a second circulating register, means coupled to the second circulating register for introducing a second pulse to the second circulating register, means coupled to the second circulating register for sampling the second pulse at successive intervals during the circulation of the second pulse through the second register and for introducing the successive pulses to the second code selector to obtain a series of output pulses corresponding to said second particular code sequence, and means responsive to the first and second code signals for producing a third code signal patterned in accordance with the sequences of the first and second code signals and having a code sequence which repeats only after a long interval as compared with the repetition intervals of the code sequences of the first and second code signals.

24. In combination at a first station for operating upon signals coded on a continuous basis in a particular asynchronous pseudo-random relationship with respect to time and continuously modulated by intelligence signals to obtain modulated intelligence signals and transmitted in a continuously modulated form from a second station, means at the first station for receiving the modulated intelligence signals from the second station, means at the first station for producing decoding signals continuously coded in the same particular asynchronous pseudo-random relationship with respect to time as the coded signals, synchronizing means at the first station and responsive to the decoding signals and to the modulated intelligence signals received at the first station for varying the times of occurrence of the decoding signals relative to the coding of the modulated intelligence signals during an asynchronous relationship between the decoding signals and the coding of the modulated intelligence signals and until the production of such a synchronous relationship, means at the first station and operatively coupled to the synchronizing means and to the decoding-signal-producing means for locking the operation of the decoding-signal-producing means to maintain the times of occurrence of the decoding signals upon the production of a synchronous relationship between the decoding signals and the modulated intelligence signals, means at the first station and responsive to the decoding signals and to the modulated intelligence signals for combining the signals to obtain the intelligence signals, and means operatively coupled to the synchronizing means and responsive to the decoding signals and to the modulated intelligence signals for obtaining an operation of the synchronizing means only upon each subsequent deviation of the decoding signals and the coding of the modulated intelligence signals from a sychronous relationship.

25. In combination at a first station for operating upon signals coded in a particular asychronous pseudo-random relationship with respect to time and modulated by intelligence signals to obtain modulated intelligence signals and transmitted in modulated form from a second station as the modulated intelligence signals, means at the first station for receiving the modulated intelligence signals from the second station, signal means at the first station for producing decoding signals in the particular asynchronous pseudo-random relationship with respect to time, means responsive to the decoding signals and to the modulated intelligence signals for varying the rate of occurrence of the decoding signals until the production of a particular relationship between the decoding signals and the occurrence of the coding in the modulated intelligence signal received at the first station, means operatively coupled to the synchronous means and the signal means for locking the signal means for the production of the decoding signals at a particular rate upon the occurrence of synchronization between the rate of occurrence of the decoding signals and the occurrence of the coding in the modulated intelligence signals, and means operatively coupled to the last mentioned means and responsive to the decoding signals and to the modulated intelligence signal received at the first station for combining these signals, only upon the occurrence of the particular relationship between the decoding signals and the coding of the modulated intelligence signals, to produce the intelligence signals.

26. In combination for use to provide for a transmission of intelligence from a first station to a second station removed from the first station, a signal source at the first station for producing a sequence of coding signals occurring in a particular asynchronous pseudo-random pattern, modulating means responsive to the sequence of coding signals from the signal source for modulating the coding signals in the sequence in accordance with the intelligence to produce modulated intelligence signals, means responsive to the modulated intelligence signals for obtaining a transmission of such signals to the second station, means at the second station for receiving the modulated intelligence signals transmitted from the first station, signal means at the second station for producing a sequence of decoding signals in a pattern corresponding to the pattern of the coding signals, synchronizing means at the second station and responsive to the received signals and to the decoding signals for varying the rate of occurrence of the decoding signals during a lack of a particular relationship between the decoding signals and the coding of the received signals until the production of the particular relationship between the decoding signals and the coding of the received signals, means operatively coupled to the synchronizing means and to the signal means for locking the signal means at a particular rate of occurrence corresponding to the production of the particular relationship between the decoding signals and the coding of the received signals, and means responsive to the received signals and to the decoding signals for detecting the received signals only upon the occurrence of the particular relationship between the decoding signals and the received signals, to produce the intelligence.

27. In combination for use at a first station to provide for a transmission of intelligence to a second station removed from the first station, a signal source at the first station for producing a sequence of coding signals occurring in a particular asynchronous pseudo-random pattern, modulating means responsive to the sequence of coding signals from the signal source for modulating the coding signals in the sequence in accordance with the intelligence to produce modulated intelligence signals, means responsive to the modulated intelligence signals for obtaining a transmission of such signals to the second station, means at the second station for receiving the modulated intelligence signals transmitted from the first station, signal means at the second station for producing a sequence of decoding signals in a pattern corresponding to the pattern of the coding signals, synchronizing means at the second station and responsive to the decoding signals and the received signals for initially obtaining a particular relationship between the occurrence of the decoding signals and the occurrence of the corresponding coding signals in the modulated intelligence signals received at the second station, means at the second station and responsive to the decoding signals and the received signals and operatively coupled to the synchronizing means for detecting the received signals in accordance with the decoding signals to obtain the intelligence only upon the occurrence of the particular relationship between the occurrence of the decoding signals and the occurrence of the corresponding coding signals in the modulated intelligence signals, means at the second station and operatively coupled to the decoding means and to the synchronizing means for locking the operation of the decoding means at a particular rate of occurrence of the decoding signals upon the occurrence of the particular relationship between the occurrence of the decoding signals and the occurrence of the corresponding coding signals in the modulated intelligence signals received at the second station, and means at the second station and responsive to the decoding signals and the received signals and operatively coupled to the synchronizing means for initiating an operation of the synchronizing means only upon each subsequent lack of the particular relationship between the occurrence of the decoding signals and the occurrence of the corresponding coding signals in the modulated intelligence signals to obtain such a synchronization.

28. In combination for use at a first station to provide for a transmission of intelligence to a second station removed from the first station, a first signal source at the first station for producing in a pseudo-random pattern a first sequence of signals having characteristics to promote a synchronization between the operation of the first and second stations, a second signal source at the first station for producing in a pseudo-random pattern a second sequence of signals different from the first sequence of signals and having characteristics for modulation by the intelligence means at the first station and responsive to the second sequence of signals and the intelligence for modulating the second sequence of signals by the intelligence to obtain the production of modulated intelligence signals, means at the first station and responsive to the signals in the first and second sequences for initially obtaining a transmission of the signals in the first sequence and then a transmission of the modulated intelligence signals in the second sequence, means at the second station for receiving the signals transmitted from the first station, means at the second station for producing decoding signals in the first and second sequences, means at the second station and responsive to the signals received at the second station in the second sequence and to the decoding signals in the second sequence to obtain a decoding of the intelligence transmitted from the first station in the modulated intelligence signals in accordance with a modulation of the modulated intelligence signals and the decoding signals and upon the production of a particular relationship between the received signals in the second sequence and the decoding signals in the second sequence; and means at the second station and responsive to the signals received at the second station and operative during the reception of the signals in the first sequence to obtain a particular relationship between the signals produced by the decoding means in the first sequence and the reception of the signals in the first sequence from the first station and means at the second station and responsive to the production of the particular relationship between the signals received at the second station in the first sequence and the decoding signals in the first sequence for obtaining the production of the particular relationship between the signals received in the second sequence and the decoding signals in the second sequence.

29. In combination at a first station for operating upon signals coded in a particular asynchronous relationship with respect to time and modulated by intelligence signals to produce modulated intelligence signals and transmitted in a modulated form from a second station as the modulated intelligence signals, means at the first station for receiving the modulated intelligence signals from the second station, signal means at the first station for producing decoding signals having the same asynchronous pattern with respect to time as the coding signals, means at the first station and responsive to the modulated intelligence signals and to the decoding signals for varying in a particular time relationship the pattern of occurrence of the decoding signals with respect to the pattern of the coding of the modulated intelligence signals until a synchronous relationship between the occurrence of the coding signals and the pattern of the coding of the modulated intelligence signals, means operatively coupled to the synchronizing means and to the signal means for locking the signal means for the production of a particular asynchronous pattern upon the occurrence of the synchronous relationship between the occurrence of the coding signals and the pattern of the coding of the modulated intelligence signals received at the first station, means operatively coupled to the locking means and responsive to the decoding signals and to the modulated intelligence signals received at the first station for combining these signals to detect the intelligence signals upon the occurrence of the synchronous relationship between the occurrence of the coding signals and the pattern of the coding of the modulated intelligence signals received at the first station, and means responsive to a failure of reception of the modulated intelligence signals for maintaining the production of the decoding signals in the particular asynchronous pattern with respect to time and in a substantially constant timed relationship.

30. In combination at a first station for operating upon signals coded in a particular asynchronous relationship with respect to time and modulated by intelligence signals to provide modulated intelligence signals and transmitted in a modulated form from a second station as the modulated intelligence signals, means at the first station for receiving the modulated intelligence signals from the second station, signal means at the first station for producing decoding signals in the same particular asynchronous relationship with respect to time as the coded signals, synchronizing means at the first station and responsive to the decoding signals and the modulated intelligence signals received at the first station for varying the pattern of occurrence of the decoding signals with respect to the pattern of the coding of the modulated intelligence signals to obtain a particular relationship between the decoding signals and the coding of the modulated intelligence signals, means operatively coupled to the signal means and the synchronizing means for locking the operation of the signal means in a particular pattern upon the occurrence of the particular relationship between the decoding signals and the coding of the modulated intelligence signals, means operatively coupled to the locking means and responsive to the decoding signals and the modulated intelligence signals received at the first station for combining these signals to produce the intelligence signals upon the occurrrence of the particular relationship between the decoding signals and the coding of the modulated intelligence signals, and means responsive to the decoding signals and to the modulated intelligence signals and operatively coupled to the synchronizing means for placing the first station on a stand-by basis upon a failure to achieve the particular relationship between the occurrence of the decoding signals and the coding of the modulated intelligence signals within a particular period of time.

31. In combination at a first station for operating upon signals coded in a particular asynchronous relationship with respect to time and modulated by intelligence signals to provide modulated intelligence signals and transmitted in a modulated form from a second station as the modulated intelligence signals, means at the first station for receiving the modulated intelligence signals from the second station, signal means at the first station for producing decoding signals in the same particular asynchronous relationship with respect to time as the coded signals, synchronizing means at the first station and responsive to the decoding signals and to the modulated intelligence signals for varying the frequency of occurrence of the decoding signals until the production of a particular relationship between the decoding signals and the coding of the modulated intelligence signals received at the first station, means operatively coupled to the signal means and the synchronizing means for locking the operation of the signal means in a particular pattern upon the occurrence of the particular relationship between the decoding signals and the coding of the modulated intelligence signals, means operatively coupled to the locking means and responsive to the decoding signals and to the modulated intelligence signals for combining these signals to produce the intelligence signals upon the occurrence of the particular relationship between the decoding signals and the coding of the modulated intelligence signals received at the first station, means responsive to the modulated intelligence signals and operatively coupled to the locking means for maintaining the frequency of occurrence of the decoding signals for a particular period of time in the absence of reception of the modulated intelligence signals, and means operatively coupled to the locking means and responsive to the modulated intelligence signals for activating the synchronizing means upon a failure to receive the modulated intelligence signals for the particular period of time.

32. The combination set forth in claim 31 in which means are included for imposing a stand-by operation upon the receiving means upon a failure of the control means to obtain the particular relationship between the decoding signals and the coding of the modulated intelligence signals within a particular period of time after the activation of the synchronizing means.

33. In combination at a first station for operating upon signals coded in a particular asynchronous relationship with respect to time and received from a second station wherein the second station produces the coded signals in a relationship displaced from corresponding coding signals at the first station by a period of time corresponding to the distance between the stations, means at the first station for receiving the coded signals from the second station, means responsive to the coded signals received at the first station and responsive to the coded signals produced at the first station for varying in a particular pattern the frequency at which the coded signals are produced at the first station until the occurrence of a particular relationship between the coded signals received at the first station and the coded signals produced at the first station, and means responsive to the coded signals received at the first station and the coded signals produced at the first station to provide an indication as to the range between the first and second stations in accordance with the period of time required to produce the particular relationship between such signals.

34. In combination for operating upon signals coded in a first particular asynchronous relationship with respect to time and modulated by intelligence signals to provide modulated intelligence signals and transmitted in a modulated form from a first station to a second station as the modulated intelligence signals, means at the first station for providing signals coded in a second particular asynchronous relationship different from the first particular asynchronous relationship and coded to facilitate a synchronous relationship with the operation of the second station, means at the first station for providing a modifying signal, means at the first station and responsive to the signals coded in the second asynchronous relationship for obtaining a transmission only of such signals to the second station and for subsequently obtaining a transmission of the modifying signal to indicate that the modulated intelligence signals will follow and for thereafter obtaining a transmission of the coded signals in the first particular asynchronous relationship, means at the second station for receiving the coded signals, means at the second station for producing decoding signals initially coded in a pattern corresponding to the coded signals in the first and second particular asynchronous relationships, synchronizing means at the second station and responsive to the decoding signals and to the signals received at the second station for varying the frequency of occurrence of the decoding signals to obtain a particular relationship between the decoding signals and the coded signals received at the second station in the second particular asynchronous relationship and subsequently in the second particular relationship, means at the second station and operatively coupled to the synchronizing means and responsive to the modulated intelligence signals for detecting such signal upon the occurrence of the particular relationship between the decoding signals and the coded signals received at the second station in the second particular asynchronous relationship and subsequently in the first particular asynchronous relationship, and means at the second station and responsive to the modifying signal received from the first station for activating the last mentioned detecting means to obtain a detection of the modulated intelligence signals upon the occurrence of the particular relationship between the decoding signals and the coded signals received at the particular station in the first particular asynchronous relationship.

35. In combination at a first station for operating upon signals coded in a particular asynchronous relationship with respect to time and modulated by intelligence signals and transmitted in a modulated form from a second station, means at the first station for receiving the modulated intelligence signals from the second station, and means at the first station and operatively coupled to the synchronizing means and responsive to the production of the particular relationship between the decoding signals and the coding of the modulated intelligence signals for locking the operation of the clock means to maintain the production of the clock signals at a frequency to maintain the production of such particular relationship, means at the first station for producing decoding signals in the same particular asynchronous relationship as the coded signals, clock means at the first station for producing clock signals on a periodically recurrent basis, means at the first station and responsive to the clock signals for synchronizing the occurrence of the decoding signals with the frequency of the clock signals, means at the first station and responsive to the decoding signals and to the modulated intelligence signals received at the first station for detecting the modulated intelligence signals to produce the intelligence signals, synchronizing means at the first station and responsive to the decoding signals and the modulated intelligence signals for varying the frequency of the clock signals to maintain a particular relationship between the decoding signals and the coding of the modulated intelligence signals.

36. In combination at a first station for operating upon signals coded in a particular asynchronous relationship with respect to time and modulated by intelligence signals to provide modulated intelligence signals and transmitted in a modulated form from a second station as the modulated intelligence signals, means at the first station for receiving the modulated intelligence signals from the second station, means at the first station for producing decoding signals in the same particular asynchronous relationship as the coded signals, clock means for producing clock signals on a recurrent basis, means at the first station and responsive to the clock signals for synchronizing the production of the decoding signals with the frequency of occurrence of the clock signals, synchronizing means at the first station and responsive to the decoding signals and the modulated intelligence signals and operatively coupled to the clock means for varying the frequency of the clock signals in a particular pattern until the production of a particular relationship between the decoding signals and the coding of the modulated intelligence signals, means at the first station and operatively coupled to the clock means for locking the operation of the clock means at a particular frequency for maintaining the particular relationship between the decoding signals and the coding of the modulated intelligence signals upon the production of such particular relationship and means at the first station and responsive to the decoding signals and the modulated intelligence signals and operatively coupled to the last mentioned means for detecting the modulated intelligence signals only upon the occurrence of the particular relationship between the decoding signals to obtain the intelligence, and the coding of the modulated intelligence signals.

37. In combination at a first station for operating upon signals coded in a particular asynchronous relationship with respect to time and modulated by intelligence signals to provide modulated intelligence signals and transmitted in modulated form from a second station as the modulated intelligence signals, means at the first station for receiving the modulated intelligence signals from the second station, means at the first station for producing first decoding signals in the same particular asynchronous relationship as the coded signals, means at the first station and responsive to the first decoding signals for producing second decoding signals in the same particular asynchronous relationship as the coded signals and on a delayed basis with respect to the first decoding signals, means at the first station and responsive to the first and second decoding signals and to the modulated intelligence signals for producing control signals representing phase displacements from a particular phase relationship between the modulated intelligence signals and the first and second decoding signals, means at the first station and responsive to the control signals produced by the last mentioned means to vary the times of occurrence of the first and second decoding signals in accordance with the characteristics of the control signal for the production of a particular relationship between the first decoding signals and the coding of the modulated intelligence signals, and means at the first station and responsive to the modulated intelligence signals and the first decoding signals and operatively coupled to the last mentioned means for detecting the modulated intelligence signals, upon the production of the particular relationship between the first decoding signals and the coding of the modulated intelligence signals, to produce the intelligence signals.

38. In combination at a first station for operating upon signals coded in a particular asynchronous relationship with respect to time and modulated by intelligence signals to provide modulated intelligence signals and transmitted in modulated form from a second station as the modulated intelligence signals, means at the first station for receiving the modulated intelligence signals from the second station, means at the first station for producing first decoding signals in the same particular asynchronous relationship as the coded signals, means at the first station and responsive to the first decoding signals for producing second decoding signals having the same particular asynchronous relationship as the coded signals and having a particular phase displacement from the first decoding signals, means at the first station for producing clock signals on a recurrent basis, means at the first station and responsive to the clock signals for obtaining the production of the first and second decoding signals in accordance with the frequency of the clock signals, means at the first station and responsive to the first and second decoding signals and to the modulated intelligence signals for producing control signals in accordance with the relative phase between the first decoding signals and the coding of the modulated intelligence signals and between the second decoding signals and the coding of the modulated intelligence signals, means at the first station and responsive to the control signals produced by the last mentioned means for varying the frequency of the clock signals to obtain a particular relationship between the first decoding signals and the coding of the modulated intelligence signals, and means at the first station and responsive to the first decoding signals and the modulated intelligence signals for combining these signals to obtain the intelligence signals.

39. In combination for use to provide for a transmission of intelligence from a first station to a second station removed from the first station, a signal source at the first station for producing a sequence of coding signals occurring in a particular asynchronous pseudo-random pattern, modulating means at the first station and responsive to the sequence of coding signals from the signal source for modulating the signals in the sequence in accordance with the intelligence to produce modulated intelligence signals, means at the first station and responsive to the modulated intelligence signals for obtaining a transmission of such signals to the second station, means at the second station for receiving the modulated intelligence signals transmitted from the first station, signal means at the second station for producing decoding signals in the same asynchronous pattern as the coding of the modulated intelligence signals at the first station, synchronizing means at the second station and responsive to the decoding signals and to the modulated intelligence signals received at the second station for varying the times of occurrence of the decoding signals to provide a particular relationship between the decoding signals and the coding of the modulated intelligence signals, locking means operatively coupled to the signal means and responsive to the production of the particular relationship between the decoding signals and the coding of the modulated intelligence signals for locking the operation of the signal means to maintain such particular relationship, and means at the second station and responsive to the decoding signals and to the modulated intelligence signals for combining these signals, upon the production of the particular relationship between the decoding signals and the modulated intelligence signals, to produce the intelligence signals.

40. In combination for use to provide for a transmission of intelligence from a first station to a second station removed from the first station, a signal source at the first station for producing a sequence of coding signals occurring in a particular asynchronous pseudo-random pattern, means at the first station and responsive to the coding signals and to the intelligence for modulating the coding signals in accordance with the intelligence to produce modulated intelligence signals, means at the first station and responsive to the modulated intelligence signals for transmitting the signals to the second station, means at the second station for receiving the modulated intelligence signals, clock means at the second station for producing clock signals on a recurrent basis, means at the second station and responsive to the clock signals for producing decoding signals at a rate related to the frequency of the clock signals and in a pattern corresponding to the asynchronous pattern of the coding signals, synchronizing means at the second station and responsive to the decoding signals and to the coding of the modulated intelligence signals for varying the frequency of the clock signals to maintain a particular relationship between the decoding signals and the coding of the modulated intelligence signals, means at the second station and responsive to the production of the particular relationship between the decoding signals and the coding of the modulated intelligence signals for locking the operation of the clock means to maintain the production of the clock signals as a rate for maintaining such particular relationship, and means at the second station and responsive to the decoding signals and to the modulated intelligence signals and operative upon the production of the particular relationship between the decoding signals and the coding of the modulated intelligence signals for detecting the intelligence.

41. In combination to provide for a transmission of intelligence from a first station to a second station removed from the first station, a signal source at the first station for producing a sequence of coding signals occurring in a particular asynchronous pattern, means at the first station and responsive to the coding signals and to the intelligence for modulating the coding signals in accordance with the intelligence to produce modulated intelligence signals, means at the first station and responsive to the modulated intelligence signals for transmitting the signals to the second station, means at the second station for receiving the modulated intelligence signals transmitted from the first station, means at the second station for producing first decoding signals in the particular asynchronous pattern, means at the second station and responsive to the first decoding signals for producing second decoding signals having a particular phase relationship to the first decoding signals, means responsive to the first and second decoding signals and to the modulated intelligence signals for varying the times of occurrence of the first and second decoding signals to maintain particular phase relationships between the first decoding signals and the coding of the modulated intelligence signals and between the second decoding signals and the coding of the modulated intelligence signals, and means responsive to the first decoding signals and the modulated intelligence signals for detecting the modulated intelligence signals to produce the intelligence.

42. In combination for use at a first station to provide for a transmission of intelligence from the first station to a second station removed from the first station;

first means for providing a relatively short sequence of first signals in a pseudo-random code for a particular period of time to obtain a synchronization in the operation of the first and second stations,

second means operatively coupled to the first means for providing for the transmission of a second signal during the transmission of the first signal to provide an indication to the second station that the first signals do not represent intelligence,

third means operatively coupled to the first means for providing a relatively long sequence of third signals in a pseudo-random code after the production of the first signals,

fourth means operatively coupled to the third means for modulating the third signals in accordance with the intelligence to provide modulated intelligence signals, and

fifth means operatively coupled to the fourth means for obtaining a transmission of the modulated intelligence signals to the second station.

43. In combination for use at a first station to provide for a transmission of intelligence from the first station to a second station removed from the first station;

a signal source for providing a sequence of coding signals,

first control means operatively coupled to the signal source for obtaining the production of first signals by the source in a first asynchronous sequence for a first particular period of time to provide for a synchronization in the operation of the first and second stations;

second control means operatively coupled to the first control means for obtaining a transmission of a second signal during the transmission of the first signals to indicate that the first signals do not represent intelligence,

third control means operatively coupled to the first control means and the signal source for obtaining the production of third signals by the source in a second asynchronous sequence different from the first asynchronous sequence and for obtaining the production of the third signals after the production of the first signals, and

means operatively coupled to the third control means for modulating the third signals with the intelligence to obtain modulated intelligence signals.

44. In combination for use at a first station to provide for a transmission of intelligence from the first station to a second station removed from the first station;

first means for providing clock signals at a particular frequency,

second means operatively coupled to the first means for providing a relatively short sequence of first encoding signals in a pseudo-random code for a particular period of time in synchronism with the clock signals to obtain a synchronization in the operation of the first and second stations,

third means operatively coupled to the second means for providing for the transmission of a second signal with particular characteristics during the transmission of the first signal to provide an indication to the second station that the first signals do not represent intelligence,

fourth means operatively coupled to the first and second means for providing a relatively long sequence of second encoding pulses in a pseudo-random code in synchronism with the clock signals and after the production of the first encoding signals, and

fifth means operatively coupled to the first and fourth means for modulating the second encoding signals in synchronism with the clock signals to provide modulated intelligence signals.

45. The combination set forth in claim 44, including,

sixth means operatively coupled to the fifth means for obtaining a transmission of the modulated intelligence signals.

Description

The present invention relates to communication systems in which communcation is provided in coded form between two or more stations.

For private communication between two or more stations, the signals transmitted between the stations to represent intelligence must be coded or otherwise rendered unintelligible to unauthorized receivers. The coded intelligence signals are transmitted to an authorized receiving station, however, in a form such that the signals can be detected and decoded at such a receiving station.

Certain essentials are required to provide a satisfactory coded communication system. One requirement is that the intelligence signals be coded at the transmitter in such a manner that they may be easily decoded at the authorized receiving stations. Furthermore, the coding at the transmitting station and the decoding at the receiving station should be accomplished without any loss of intelligence. A further concurrent requirement, however, is that the coding of the intelligence signals be of such a nature that the code cannot easily be broken by unauthorized stations which may receive the coded intelligence signals. Many attempts have been made to provide a system meeting these requirements, but these attempts have not been entirely satisfactory.

However, a suitable system of the type referred to in the preceding paragraph is disclosed and claimed in copending application Ser. No. 735,089 which was filed May 9, 1958, in the name of Joseph P. Gleason et al. In the system disclosed in the copending application, the criteria outlined in the preceding paragraphs are met by coding the intelligence signal with a coding signal having a "pseudo-random" characteristic.

The term "psuedo-random" is intended to mean signals satisfying most of the requirements of randomness so as to preclude the reception of the intelligence by unauthorized stations. However, the characteristic changes in the pseudo-random coding signal, in reality, proceed in accordance with a fixed pattern or program, and this pattern repeats after a certain finite period. These characteristics of the pseudo-random coding signal enable the coding signal to be easily duplicated in an authorized receiver and used for decoding purposes.

In the system described in the copending application Ser. No. 735,089, a coding signal generator is provided at the transmitting station to produce signals having first and second amplitude levels. When no intelligence is being transmitted, the coding signal generator produces signals which alternate between the first and second amplitude levels at a constant frequency. However, upon the production of intelligence such as audio signals, the signals produced by the coding signal generator become modulated in frequency in accordance with the characteristics of the intelligence. The resulting modulated intelligence signals are combined with the coding signal in a suitable coding unit, such as a balanced modulator, to produce the intelligence signals which are used for transmission to the distant receiving station. At the transmitting station of the copending application, the psuedo-random coding signal produced by the coding signal generator is inverted every time the amplitude of the modulated intelligence signals changes. For example, the pseudo-random coding signal may be considered to have high and low amplitudes at successive periods which do not have any apparent periodicity. During the time that the modulated intelligence signal has a high amplitude, the coding signal continues in its original form. However, when the modulated intelligence signal exhibits its low amplitude level, the coding signal becomes inverted in polarity. This causes the high amplitude portions of the original coding signal to become low amplitude portions and vice versa.

The receiving station of the copending application has a decoder which produces signals in a pseudo-random pattern corresponding to the signals produced by the coder at the transmitting station. The pseudo-random decoding signal is mixed at the receiving station with the coded signal received from the transmitting station so that the modulated intelligence signal may be recovered. The recovered modulated intelligence signal is then introduced to an appropriate detector which enables the intelligence to be recovered.

The present invention is related to a coded communication system similar to the system disclosed in the copending application Ser. No. 735,089 discussed above. The present invention, however, in one of its aspects is more particularly concerned with the control of the transmitting and receiving stations in such a system so that correlation may be achieved, and maintained, between the pseudo-random coding signals developed at the transmitting station and the coding signals developed at the receiving station. In the system of the present invention, such correlation is achieved and maintained in an automatic, rapid and efficient manner.

The coded communication system to be described in predicated upon a principle known as "noise correlation modulation". In accordance with this principle, narrow-band signal information is modulated by the pseudo-random coding signal described above and which exhibits wide-band characteristics. This coding causes a low amplitude transmission to be obtained, the presence of which is extremely difficult to detect by an unauthorized receiver.

This technique of noise correlation modulation causes the energy of the transmitted signal to be spread over a wide frequency range so that the radio frequency energy per unit band width is greatly diminished as compared with narrow-band transmission. Because the transmitted energy is so spread over a wide band and appears as noise, the transmitted signal has a composition which renders it most difficult for an unauthorized receiver to detect and demodulate it or even to be aware of its actual existence. The system of the invention, in the embodiment to be described, includes one or more receiving stations which each include a pseudo-random decoding signal generator, which generator serves to produce a pseudo-random decoding signal which is identical to the coding signal used at the transmitter.

The pseudo-random decoding signal source at each receiving station in the coded communication system of the invention is synchronized with the psuedo-random coding signal source at the transmitting station in a manner to be described. In accordance with the embodiment of the present invention, which will be described, such correlation or synchronization is achieved by first causing the transmitting station to transmit a coded signal which is coded in accordance with a "short code". This "short code" repeats itself after a relatively short time interval so that it is a relatively simple matter for the decoding signal source at an authorized receiving station to achieve correlation with the short-code coding signal. After such correlation is achieved, the transmitting station shifts over to long-code operation. During long-code operation, the transmitted coded intelligence signal is coded in accordance with a pseudo-random coding signal which does not repeat itself until an extremely long period of time has elapsed. However, since the receiver is brought into step with the transmitter during the short-code operation, and since it falls out of step by a very slight amount during the changeover to long-code operation, re-correlation for the long-code may be achieved by a relatively simple searching sequence. This will be described in detail in the subsequent specification.

Another feature of the invention is the provision of a coded communcation system which exhibits a high degree of message privacy. The pseudo-random code generator at the transmitting station of the coded communcation system of the invention, and the identical pseudo-random decoding generators at the different receiving stations in the system, have been constructed in accordance with the concepts of the invention, as will be described, to provide such a degree of message security that unauthorized reception is rendered virtually impossible even with automatic equipment. The coding and modulation of the system of the present invention permits a receiving station which is properly correlated with the transmitting station to receive the transmitted intelligence, while an uncorrelated receiver at one tenth the range would find it most impossible to detect even the presence of signal energy.

The system of the invention is also advantageous in that it exhibits a high degree of immunity to interfering or jamming signals. The system also exhibits high quality and fidelity characteristics for the transmission of speech. Another feature of the invention is that it may be conceived to permit the use of reliable and relatively simple circuitry in both the transmitting and receiving sections of each station. The system to be described also incorporates an ancillary capability, and that is the ability to measure range between any two stations constructed to include apparatus embodying the concepts of the invention.

Other features and advantages of the invention will become evident from a consideration of the following drawings in which:

FIG. 1 is a schematic representation of a coded communication system of the type disclosed in the copending application Ser. No. 735,089, referred to above, the illustrated representation showing in block form the various components which make up the transmitting and receiving stations of a typical station in the system described in that application and this illustration serving as a convenient means for describing the principles upon which the present communication system is predicated:

FIG. 2 shows a plurality of curves illustrating the various wave forms developed in the transmitting section of the system illustrated in FIG. 1, these curves being useful in explaining the operation of the system of FIG. 1 and in explaining the purpose and function of the different components making up that system;

FIG. 3 is a block diagram illustrative of the components which are incorporated into a typical transmitting-receiving station of the system of the present invention and which is capable of carrying out the different control forms and other objectives of the present invention;

FIGS. 4a and 4b constitute a composite logical diagram of a controller which is included in the system of the invention and which constitutes one of the components illustrated in FIG. 3, the controller serving to initiate the different controls which are incorporated into the system;

FIG. 5 is a block diagram of the actual receiver which is incorporated into the receiving section of the station to be described, the illustrated receiver serving to receive, decode and demodulate the coded intelligence signal received from the transmitter;

FIGS. 6, 7 and 8 are circuit diagrams of the various components which are included in the receiver of FIG. 5;

FIG. 9 is a fragmentary circuit diagram illustrating the manner in which a received signal is input into the first station of the receiver circuit;

FIG. 10 is a logical block diagram of a clock generator assembly which is included in the station of 3, the illustrated generator assembly serving to generate clock pulses whose frequency are subject to different controls exerted on the clock generator;

FIG. 11 is a logical block diagram of a coding signal generator, which illustrates the basic means whereby a pseudo-random coding signal, or similar decoding signal, suitable for use in the system of the present invention may be generated;

FIG. 12 is a logical block diagram of the actual coding signal generator utilized in the embodiment of the invention to be discussed;

FIG. 13 is a logical diagram illustrating the different components which make up the order used in the embodiment of the invention to be described and also illustrating the appropriate connections between these components;

FIG. 14 is a circuit diagram of a code mixer circuit which is used in the coder of FIG. 13;

FIG. 15 is a diagram of a receiver decoding stage which includes two balanced modulators in parallel and which is useful in explaining the actual decoding stage used in the embodiment of the invention to be described;

FIG. 16 is a circuit diagram of a hybrid balanced modulator which is used in the receiver of the embodiment of the invention to be described to constitute the decoding stage of the receiver;

FIG. 17 is a block diagram of the modulator unit which is incorporated into the embodiment of the invention to be described;

FIG. 18 is a circuit diagram of the modulator unit shown in block form in FIG. 17;

FIG. 19 is a block diagram of a transmitter power amplifier which may be used as a final amplifier for the transmitting section of a station constructed in accordance with the teachings of the invention;

FIG. 20 is a circuit diagram of a radio frequency amplifier which may be used in the receiver section of the station;

FIG. 21 is a block diagram illustrating a suitable frequency generator for use as a carrier wave generator for the transmitting section of the station and as a local oscillator for the receiving section;

FIG. 22 is a circuit diagram of a frequency multiplier for use in the generator of FIG. 21; and

FIGS. 23a and 23b illustrate the logic circuitry and mechanical indicators and counters which may be incorporated into the system for the performance of the range measuring function.

The system illustrated in FIG. 1 is similar to the system disclosed and claimed in copending application Ser. No. 735,089, as indicated above. The system includes a source of intelligence such as an audio source 11. This source may be a microphone, or it may be any other source of voice signal or of other types of intelligence signals to be transmitted by the system. The source 11 may be connected to an amplifier such as an audio amplifier 13 which serves a well-known function of amplifying the audio or intelligence signals. The amplifier 13 is, in turn, connected to a signal generator and frequency modulator 15 so that the amplified intelligence signals from the amplifier may be introduced to the unit 15 to modulate a carrier signal generated by that unit.

The signal generator portion of the unit 15 may, for example, be a frequency modulated since wave oscillator connected to drive a bistable flip-flop circuit. The sine wave generator portion of the unit may, for example, have a center frequency of 25 kilocycles so that the bistable flip-flop circuit will generate a signal having a rectangular wave shape and having a center frequency of 25 kilocycles.

Alternately, the signal generator portion of the unit 15 may be a multivibrator having a natural frequency equal of, for example, 25 kilocycles. The frequency of the multivibrator may be directly controlled by the intelligence signal from the amplifier 13 so that a frequency modulated signal may be directly developed and at any instant be dependent upon the characteristics of the intelligence signal from the amplifier 13.

The signal from the audio amplifier, as noted above, is introduced to the frequency modulator portion of the unit 15 to frequency modulate the rectangular wave carrier signal from the signal generator portion of the unit. The audio amplifier may, for example, change the frequency of the carrier signal from the signal generator portion of the unit by a frequency of .+-.5 kilocycles during the frequency modulation process. As previously described, the frequency of the carrier signal at any instant is dependent upon the characteristics of the intelligence signal at that instant, such as the amplitude of the audio signal from the source 11.

The signal generator portion of the unit 15, therefore, generates a rectangular wave carrier signal. This signal exhibits amplitude transitions between two fixed amplitude values and the timing of these transitions changes in accordance with the characteristics of the intelligence signals from the audio amplifier.

When a sine way oscillator and flip-flop circuit are used to constitute the signal generator portion of the unit 15 as suggested above, the signal from the audio amplifier 13 can be used to frequency modulate the output signal of the signal generator by any usual frequency modulator network associated with the sine wave oscillator.

The system shown in FIG. 1 also incudes a coding signal generator 17. This generator produces a coding signal having a rectangular wave shape. This coding signal has amplitude transitions between two fixed amplitude values which are controlled to occur in accordance with a pseudo-random predetermined coding sequence. As indicated above, the pseudo-random nature of this coding sequence is actually in accordance with a definite pattern and is repetitive so as to facilitate the production of similar decoding signals at the receiving station.

The coding signal generator 17 may be similar to the generator described in a report by Neal Zierler entitled "Several Binary-Sequence Generators", Massachusetts Institute of Technology, Lincoln Laboratories Technical Report No. 95. The coding signal generator 16 may also be constructed in a manner similar to that disclosed in copending application Ser. No. 714,459 filed Feb. 6, 1958, in the name of Joseph G. Gleason, or in copending application Ser. No. 750,591 filed Feb. 2, 1959 in the name of Robert J. Grady.

The signals from the coding signal generator 17 are introduced to a balanced modulator 19 and the frequency modulated carrier signal from the signal generator and frequency modulator 15 is also introduced to the balanced modulator 19. The balanced modulator functions to invert the coding signal from the generator 17 in response to the frequency modulator signal from the unit 15. Thus, the coded intelligence signal produced at the output terminal of the balanced modulator 19 is a signal having a rectangular wave shape as illustrated in the curve D of FIG. 2 and having certain amplitude transitions from one of its fixed amplitudes to the other. These amplitude transitions result in part from inversions of the coding signal from the generator 17, the coding signal being shown in the curve B of FIG. 2. The amplitude transitions in the coded intelligence signal also results in part from the amplitude transitions in the modulated intelligence signal from the frequency modulator portion of the unit 50, one transition of which is shown in curve C of FIG. 2. The coded intelligence signal from the balanced modulator 19 may be transmitted to the receiving section of a distance station by any known means. For example, the coded intelligence signal from the balanced modulator 19 may be introduced to a suitable radio transmitter for radiation to the distant station.

The system also preferably includes a clock pulse generator 21. This generator, as will be described, produces clock timing pulses which recur at a stabilized and constant repetition frequency, but whose frequency may be controlled for searching purposes. The clock pulses from the generator 21 are introduced to the coding signal generator 17 so that each amplitude transition in the rectangular wave coding signal from that generator will occur in timed coincidence with a corresponding one of the clock pulses. Also, the clock pulses are introduced to the signal generator portion of the unit 15 so that each amplitude transition in the modulated intelligence signal from the unit 15 will also occur in timed coincidence with one of the clock pulses. The frequency of the clock pulse generator is relatively high with respect to the frequency of the amplitude transitions of the modulated intelligence signal from the unit 15 so that the control exerted by the clock pulses on that signal will not shift the transitions sufficiently to cause notable distortion in the modulated intelligence signal produced by the unit 15 or in the coded intelligence signal produced by the balanced modulator 19.

The sequence of clock timing pulses from the generator is shown in the curve A of FIG. 2. The coding signal from the generator 17 is shown in the curve B of FIG. 2, and that signal may be provided with a rectangular wave shape as shown. The amplitude transition of the coding signal shown in the curve B of FIG. 2 is in accordance with the pseudo-random coding sequence described above. The amplitude transition of the coding signal shown in the curve B of FIG. 2 is also controlled by the clock pulses from the generator 21, so that each amplitude transition occurs in timed coincidence with the leading edge of a clock pulse.

The modulated intelligence signal produced by the unit 15 is shown in the curve C of FIG. 2. This modulated signal also has a binary rectangular wave shape in the illustrated embodiment. The times of occurrence of the amplitude transitions in the modulated intelligence signal shown in curve C of FIG. 2 are controlled by the intelligence signal from the audio amplifier 13 in the manner described.

The intelligence signal from the audio amplifier 13 is therefore caused to frequency modulate a carrier signal from the signal generator portion of the unit 15. The resultant modulated intelligence signal at the output of the unit 15 may be suitably shaped by any known network to have negligible amplitude modulation. As shown in the curve D of FIG. 2, the coding signal from the generator 17 is passed without inversion by the balanced modulator 19, when the amplitude of the modulated intelligence signal from the unit 15 is in one of its two binary states. Alternately, the balanced modulator 19 operates to invert the amplitude of the coding signal from the generator 17 when the amplitude of the signal from the unit 15 is in its second binary state. The resulting coded intelligence signal produced at the output of the balanced modulator 19 is shown in the curve D in FIG. 2.

The use of the clock pulses of the curve A to control the operation of both the coding signal generator 17 and the signal generator portion of the unit 15 assures that all amplitude transitions involved in the production of the coded intelligence signal (shown in curve D of FIG. 2) will occur with the same basic timing. Therefore, it is virtually impossible for the coded intelligence signal to be analyzed at an unauthorized station and a distinction to be made between the amplitude transitions in the modulated intelligence signal and in the coding signal itself.

The receiver section of the distant station includes a balanced modulator 23 which may be similar to the balanced modulator 19 at the transmitter. The receiver section also includes a decoding signal generator 25. As noted previously, this decoding signal generator generates a pseudo-random decoding signal which has a rectangular wave-shape in which amplitude transitions occur in the same coding sequence as those of the coding signal from the coding signal generator 17 at the transmitter section. The balanced modulator 23 at the receiver, which serves as the decoder, may be connected to a band-pass filter 35 which, in turn, is connected to a frequency modulation discriminator-detector 27. The discriminator-detector 27, in turn, is connected to an amplifier 29 from which the intelligence may be recovered. The decoding signal introduced by the generator 25 to the balanced modulator 23 causes the balanced modulator to re-invert the coded intelligence signal from the transmitter at the precise points at which it was inverted at the transmitter by the coding signal generator 17. Therefore, the balanced modulator 23 develops an output signal similar to the signal shown in the curve C of FIG. 2.

The output signal from the balanced modulator 23 has a rectangular wave-shape and exhibits amplitude transitions at the same frequency as the amplitude transitions of the frequency modulated signal from the signal generator and frequency modulator 15. The frequency modulates signal from the balanced modulator 23 is passed by the band-pass filter 35 to the frequency discriminator-detector 27. The frequency discriminator-detector 27 produces an intelligence signal which duplicates the signal originating at the audio source 11 in the transmitter. This intelligence signal is amplified in the amplifier 29, and the intelligence represented by it is recovered by any suitable transducer (not shown) such as a loud speaker.

The filter 35 may be a narrow band, pass-band filter of known inductance-capacity type. This filter serves to pass the modulated side bands of this signal from the balanced modulator 23 to the discriminator-detector 27. However, the filter 35 discriminates against the background noise signals having a relatively wide frequency band outside the frequency range of the modulated intelligence signal.

The use of the balanced modulator 23 and the band-pass filter 35 provide certain advantages. This is because the production of the modulated intelligence signal and the combination of this signal with the coding signal to produce the coded intelligence signal increases the bandwidth of the transmitted signal. This results from the fact that the coding signal has a wide band width of the order, for example, of 1 megacycle. Since the energy level represented by the coded transmitted intelligence is now distributed over a band width of at least one megacycle, the energy level of the intelligence in the coded intelligence signal is considerably decreased in comparison to the energy level of the intelligence itself. Actually, the energy level of the intelligence in the coded intelligence signal may be below the noise level in the atmosphere. This means that an unauthorized receiver has difficulty in detecting even the actual presence of the coded intelligence signal.

The balanced modulator 23 at the receiver reduces the band width of the received signal from one megacycle to a band width of, for example, 10 kilocycles at a center frequency of, for example, 25 kilocycles. The filter 35 then serves to filter signals having frequencies below 20 kilocycles over and above 30 kilocycles. The signal passed by the filter 35 now has sufficient amplitude over its spectrum, with respect to the residual noise signals translated by the filter, so that the intelligence signal may be detected in the discriminator 27 and its intelligence recovered at the output of the amplifier 29.

In the system of the present invention to be described, and as noted above, two separate pseudo-random coding signals are used. One of these is termed the "short code" and, as described, that code sequence repeats itself after a relatively short time interval. At the beginning of each transmission, the short code is first transmitted from a transmitting station to a receiving station. This permits the decoding signal generator at the receiving station to be brought into correlation with the coding signal generator at the transmitter in a relatively short time interval, as will be described.

When correlation has been achieved in the short code mode, the transmitting station then changes to the long code mode of operation. The long code, as described above, repeats itself only after an extremely long time interval. This long code is, therefore, advantageous from a security standpoint, but it is difficult for a receiver to achieve correlation with such a long code without the initial short code correlation step. The changeover from short code to long code by the transmitter is indicated to the receiver by a particular signal which is transmitted from the transmitting station to the receiving station just before the changeover is made. At the moment that the changeover is made at the transmitting station, the decoding signal generator at the receiver is first speeded up and then slowed down until correlation with the coding signal generator at the transmitter is again achieved. When such correlation is achieved, the decoding signal generator at the receiving station is held in a locked-on condition with the received coded intelligence signal, this being obtained in a manner to be described.

The correlation sequence described above permits correlation in the long code mode to be achieved in a relatively short time interval. This is because such a sequence enables the receiver to start producing the pseudo-random long code decoding signal at approximately the same time that the transmitter starts producing the pseudo-random long code coding signal. Therefore, the displacement between the long-code coding signal generated at the transmitter and the long-code decoding signal generated at the receiver is relatively small. It is, therefore, possible to achieve full correlation at the receiver between the received coded intelligence signal and the locally generated decoding signal in a relatively short time interval, and by a relatively simple searching sequence as will be described.

A transmitting-receiving station constructed in accordance with the present invention is illustrated in FIG. 3. This station includes a transceiver unit 10, a radio frequency unit 12, a range control 14 and a control box 16. An intercommunication set 18 and a direction finder 20 are coupled to the control box 16.

The transceiver unit 10 includes a controller 22, a clock generator 24, a coder 26, a modulator 28 and a receiver 30. The controller 22 controls the clock generator 24 to obtain correlation or synchronization in the system in a manner to be described. The controller also starts an initial code injection into the coder 26 for the short code and also for the long code, as will be described. The controller also determines whether the coder 26 is to produce the short code or the long code. As will be explained, the controller 22 also causes the coder 26 to generate a decoding signal for the receiver 30 for side tone reception when the station is in its transmitting mode and for normal reception when the station is in its receiving mode. The side tone reception enables the station to monitor the coded intelligence being transmitted to the distance receiver. In addition, the controller 22 controls certain components of the radio frequency unit 12 and certain components of the transceiver 10 to control the transmit and receive modes of operation of the system.

The clock generator 24 provides clock pulses for the coder 26 and for the range counter 14. The coder 26 introduces the pseudo-random coding signal to the modulator 28 when the station is in its transmit mode; and it provides the pseudo-random decoding signal in a delayed and in an undelayed state to the receiver 30, for reasons to be described, when the station is in its receive mode. The modulator 28 introduces audio signals to the coder 26 to produce the coded intelligence signal for wide band transmission. The modulator 28 produces a 60 megacycle uncoded amplitude modulated signal, and this signal is applied to a radio frequency transmitter power amplifier 32 in the radio frequency unit 12 for narrow band transmission; and the modulator 28 also produces a 60 megacycle balanced modulated coded intelligence signal, and the latter signal is applied to the power amplifier 32 for wide band transmission.

The receiver 30 demodulates the 60 megacycle coded intelligence signal received from a distant station and introduced to a radio frequency receiver amplifier 34 in the radio frequency unit 12. The receiver 30 provides an audio signal for the intercommunication set 18 by way of the control box 16, and it also provides a correlation signal to the controller 22 and a phase correction signal to the clock generator 24.

The radio frequency unit 12 includes the radio frequency transmitter power amplifier 32 and the radio frequency amplifier 34, referred to above. The radio frequency unit 12 also includes a frequency generator 36. An antenna 38 is connected to the armature of a relay 40. When the relay 40 is de-energized, the antenna 38 is connected to the receiver amplifier 34. However, when the relay 40 is energized by a "transmit" signal (Ck) from the controller 22, it causes the antenna 38 to be connected to the transmitter power amplifier 32. One of the fixed contacts of the relay 40 is connected to the transmitter power amplifier 32, and another of the fixed contacts of that ralay is connected to the radio frequency amplifier 34.

The transmitter power amplifier 32 mixes the modulated signal from the modulator 28 with a radio frequency signal from the frequency generator 36 and amplifies the resulting signal for transmission purposes from the antenna 38. The frequency and the power level of the transmitted signal are selected at the control box 16.

The radio frequency receiver amplifier 34 amplifies the coded intelligence signal from a distant station which is intercepted by the antenna 38, and this amplifier converts that signal to a frequency centered around 60 megacycles. The frequency generator 36 serves as a local oscillator for both the transmitter power amplifier 32 and the radio frequency receiver amplifier 34.

The range counter 14 counts the clock pulses necessary for the system to be re-correlated after having once transmitted and then received a signal. In this manner and as will be described in detail, the range counter generates a display of the range between the two particular stations between which such communication is carried out.

The control box 16 provides means for selecting wide band or narrow band transmission and reception, for selecting tone transmission, for selecting automatic direction finding operation, and for selecting range measurement interrogation and range measurement response. The control box also provides a means for selecting the frequency of the radio frequency channel (as noted above), a "code-of-the day" and the radio level of the receiver. The control box also provides a means for destroying a "code-of-the-day" in an emergency.

When the station is in the wide band transmit mode, the apparatus transmits a coded intelligence signal which comprises a radio frequency carrier balanced-modulated with a pseudo-random modulated coding signal and with audio information contained within the modulation of the coding signal in the form of an ultrasonic subcarrier which is frequency-modulated with audio information.

When coded wide band transmission is initiated, the control box 16 conditions the transmitting components of the system, and a control signal is introduced to the controller 22 to initiate the "transmit" sequence within the controller.

The controller first causes the clock oscillator in the clock generator 24 to be established at a predetermined fixed and stabilized frequency of, for example, 4.853 megacycles. This may be achieved by switching the control of the clock oscillator to a suitable crystal which is resonant at the particular frequency. This particular frequency is advantageous in simplifying range measurements, as will become apparent subsequently.

At the same time, the controller 22 causes a short code initial condition pulse to be injected into the coder. This initial condition pulse causes, in a manner to be described, a selected short code to be generated by the coder. After the initial condition selection has been completed, the coder 26 begins to generate the repetitive pseudo-random short code. The short code generation continues for a period of, for example, 0.25 seconds. At the end of that period, the controller 22 introduces a 5 kilocycle audio signal to the modulator 28 for a short interval of, for example, one millisecond. This 5 kilocycle audio signal is transmitted to the receiving station as an indication to the receiving station that the transmitter is about to switch to long code operation.

The controller 22 now causes a long code initial condition pulse to be injected into the coder 26. This latter initial condition pulse causes the selected long code to be generated by the coder, as will be described. After this latter initial condition operation is completed, the coder 26 generates the pseudo-random long code. During the entire interval the transmitter is operating in the short code mode, a 1 kilocycle audio tone is transmitted by the transmitting system. This tone serves as a warning signal to the operator that communication may not be established at that time.

When one of the stations is conditioned to a transmitting mode, in the manner described above, at least one other similar distant station is in a stand-by mode ready to receive the coded intelligence signal from the transmitting station. When the distant station is in the stand-by mode, its coder 26 is generating a selected short code decoding signal which is identical to the short code coding signal generated at the transmitting station. At this time, the clock generator at the distant stand-by station is operating at a frequency of, for example, 4.848 megacycles. This is 5 kilocycles below the clock frequency at the transmitting station during the short code transmission.

There will, therefore, be a displacement in code phase between a coded intelligence signal received from the transmitting station and the locally generated decoding signal at the receiving station. This enables the locally generated decoding signal at the receiving station to assume all possible time relationships with the received coded intelligence signal in a period of 0.2 seconds, and it assures that when a coded intelligence signal is received which is coded by a short-code coding signal, correlation will occur within that time. Should a signal, coded in accordance with a selected short code, be received from the transmitting station while the receiving station is in its stand-by condition, correlation in the short code mode will occur within 0.2 seconds. The receiver 30 at the receiving station then causes its controller 22 to establish its clock generator 24 at the fixed frequency of, for example, 4.853 megacycles, and this is carried out at the instant the short code correlation occurs. This causes the locally generated short code to be synchronized with the received short-code coded intelligence signal and such synchronization is maintained in a manner to be described by appropriate correlation locking circuits in the receiver.

After an interval of, for example, 0.25 seconds and as discussed above, the transmitting station transmits a 5 kilocycle signal to the receiving station to indicate that the transmitting station is about to change from short code to long code transmission. This 5 kilocycle signal is used at the receiving station to cause the controller to insert the long code initial condition into the coder 26. At the same time, the controller at the receiving station causes the clock generator frequency to change so as to initiate a search for synchronism with the long code which is now received from the transmitting station. The control of the clock generator 24 at the receiving station is such that the generator is set first at a frequency of 5 kilocycles above the normal frequency of 4.853 megacycles and then at a lower frequency.

Because of delays inherent in the receiving channels, the phase of the totally generated long code will tend to lag behind the received long code coded intelligence signal. It is, therefore, necessary initially to increase the frequency of the clock generator at the receiving station when the changeover to long-code transmission is made at the transmitting station in order to achieve correlation. A search period of 70 milliseconds at the 5 kilocycle search rate is sufficient to bring the locally generated long code at the receiving station into correlation with the long-code coded intelligence signal received from the transmitting station. Should correlation fail to occur due to fading conditions or other difficulties, the system at the receiving station will make repeated attempts to achieve correlation. This is carried out by following a search sequence of performing a search at a frequency of 5 kilocycles lower than normal clock frequency after a 100 millisecond period of searching the frequency 5 kilocycles above the normal clock frequency.

When long code correlation is achieved between the received coded intelligence signal and the locally generated long code, the clock frequency is returned to its normal value of, for example, 4.853 megacycles, and the clock signal at the receiving station is locked in phase with the received long code coded intelligence signal. By correlation control circuitry to be described, the receiving station is now provided with a locally generated long code which is synchronized with the lone-code coded intelligence signal received from the transmitting station. The receiving station is now able to derive the audio intelligence contained in the coded intelligence signal received from the transmitting station.

Should a break in the transmission occur, either due to fading or a termination of transmission, a keyed memory is energized in the clock generator 24 at the receiver. This memory holds the clock frequency at the value it last had when the break occurred. The reappearance of the signal from the transmitter again causes the clock frequency at the receiver to lock with the received signal, should the former signal reappear within the interval established by the keyed memory.

However, should the signal from the transmitting station fail to reappear after an interval of, for example, 1 second after such a transmission break, the controller 22 causes the clock generator 24 to initiate a search sequence. This does not cause any change in the operation of the coder 26. This controller search will be referred to as "auto search". If the signal from the transmitter should again be received during the search and should long code correlation again be achieved, the controller 22 at the receiving station causes the clock generator 24 again to lock in phase with the coded intelligence signal received from the transmitting station. However, if after a search of 300 milliseconds, correlation has not occurred, the receiving station is automatically set by the controller to its stand-by condition. This latter operation is termed "auto reset".

Range measurements are performed by allowing one station, designated as the "Interrogator" to transmit to a second station designated as the "Responder". When synchronization in long code has been achieved, the responder is placed in the transmit mode without resetting its coder; and the interrogator is placed in receive search mode without resetting its coder. At this instance, the code signals produced at each station are displaced in phase by an amount proportional to the distance between the two stations. It is, therefore, merely necessary for the interrogator to measure the number of cycles of clock displacement necessary to achieve correlation with the responder in order to measure the range between the two stations.

The interrogator will be placed in stand-by condition before the interrogation procedure is begun. Interrogation is initiated by the operator who causes the control box 16 of the interrogator to send a control signal to the controller 22. Upon receipt of this control signal, the controller causes the clock generator 24 to produce the normal clock frequency of, for example, 4.853 megacycles. At the same time, the controller 22 at the interrogator causes the short code initial condition to be inserted into the coder 26 and the short code to be generated. The transmitting section of the interrogator is placed in its transmit mode and the receiving section of the interrogator is conditioned for side tone operation.

In the manner described above, the interrogator transmits the short code for an interval of, for example, 0.25 seconds. At the end of the short code transmission, and in a manner similar to that described previously, the interrogator transmits the 5 kilocycle tone signal to the responder. The interrogator then changes to long code transmission in the described manner.

When the interrogator begins its long code transmission, the 5 kilocycle tone signal is again sent to the responder to produce a side tone signal at the responder. After a period of 0.5 seconds, the controller 22 of the interrogator removes the 5 kilocycle tone signal from the modulator 28 of the interrogator, and it also conditions the interrogator for reception. The clock generator at the interrogator is now placed in its search mode by the controller which sets its frequency 5 kilocycles below normal.

The procedure described above results in a 0.5 second period of wide band transmission in which the audio generator is modulated with a 0.5 kilocycle control tone after the system is in long code operation. The interrogator is now returned to its received condition but the coder 26 continues to produce the long code.

During the operation described above, the controller 22 introduces signals to the range control 14 to reset the range control and to permit a count to be performed. The clock generator 24 compares the search clock frequency with a highly accurate crystal control oscillator running at the normal clock frequency. This provides the range control 14 with a direct indication of clock displacement.

The responder may be in any condition when the respond operation is begun. Respond operation is initiated by a control signal which is sent from the control box 16 to the controller 22. No change in output from the controller occurs at this time, and this permits the responder to be used in the normal communication mode when range measurement is not being made. However, if the responder receives a signal from the interrogator, the short code/long code sequence described above occurs.

When the responder is in long code condition, the 5 kilocycle tone from the interrogator is sent from the receiver 30 of the responder to the controller 22. This causes the controller to set the responder to the transmit mode. Then after a period of, for example, 3 seconds, the controller sets the responder to stand-by condition. The procedure outlined in the preceding paragraph permits the responder as noted to be used in normal communication mode when a range measurement is not being made. When a range measurement is being made, the controller 22 causes the responder to synchronize in long code mode with the interrogator, and then causes the responder to transmit for a 3 second period with no change in the code being generated. This permits the interrogator to synchronize with the transmitting responder and thus obtain a range measurement on the counter 14.

For narrow band uncoded transmission, the radio frequency unit 12 transmits an amplitude modulated carrier over the antenna 38. An amplitude modulated signal for this purpose is obtained from the modulator 28 of the transceiver unit 10. The radio frequency transmitter power amplifier 32 mixes the amplitude modulated signal with the radio frequency signal from the frequency generator 36, and the amplifier 32 linearly amplifies the signal for transmission by the antenna 38.

For narrow band reception, the antenna 38 is connected to the receiver amplifier 34 by the relay 40, and the amplifier 34 supplies the received amplitude modulated signal in amplified form and heterodyned to 60 megacycles by the frequency generator 36 to the receiver 30. The received signal is converted to 6 megacycles in the receiver, then amplified and converted to 55 kilocycles and then it is detected in the usual manner to recover the amplitude modulations.

The logical components which make up the controller 22 are shown in block form in FIGS. 4A and 4B. The controller includes logical circuitry which operates to control the receiver 30, the modulator 28, the coder 26, the clock generator 24 and the range counter 14. The controller provides signals for the receiver to control the side tone circuits. It also introduces signals to the coder to inject the initial conditions into the coder and to determine whether operation is to be in long code or short code. The controller also introduces control signals to the clock generator 24. In addition, the controller sends signals to the range counter 14 to control the coding or range pulses and to reset the range counter.

The controller 22 as shown in FIG. 4A includes an "and" gate 50. This type of gate is well known to the electronic digital computer art, and appropriate circuitry for the gate is also well known. For example, the "and" gate 50 may be constructed in a manner similar to that described and shown on page 32 of "Arithmetic Operations in Digital Computers" by R. K. Richards (published by D. Van Nostrand Company of Princeton, N.J., in 1955). The "and" gate includes a plurality of input terminals at which a corresponding plurality of different terms are introduced. The output terminal from the "and" gate is true only when all the input terminals are true.

The "and" gate 50 is connected to a "one-shot" multivibrator (Of). "One-shot" or "monostable" multivibrators are also well-known to the electronic art. These multivibrators have a stable state and an unstable state. The introduction of an input signal to the multivibrator causes it to be triggered from its stable state to its unstable state. The multivibrator then returns to its stable state after a time interval determined by its internal parameters. One-shot multivibrators such as the multivibrator Of may be constructed in a manner similar to that shown and described on pages 5-52 and 5-53 of "Control Engineers Handbook" published by McGraw-Hill Book Company, Inc., in 1958.

The "one-shot" multivibrator Of is connected to an "or" gate 52 and to an "or" gate 54. This latter type of logic gate is also well known to the electronic digital computer art. The "or" gate has a plurality of input terminals to which a corresponding plurality of input terms are introduced. The "or" gate produces an output term which is true if any of the input terms are true. The "or" gates 52 and 54 may also be constructed in a manner similar to that described and shown on page 32 of "Arithmetic Operations in Digital Computers" by R. K. Richards.

A plurality of terms E, C and Rs are introduced to the "and" gate 50. These terms, like others to be described, are derived from multivibrators which are respectively designated by the same letters. The unbarred terms are derived from the "true" output terminals of their corresponding multivibrators, and the barred terms are derived from the "false" output terminals. The "true" output terminal of each multivibrator appears near the upper left corner of the box designating the multivibrator, and the "false" output terminal of each multivibrator appears near the upper right corner of the box designating the multivibrator. In like manner, the "true" and "false" input terminals to each multivibrator respectively appear near the lower left and right corners of the box designating the multivibrator.

The terms C and I are introduced to an "and" gate 56, and this "and" gate is connected to the "or" gate 52. The "or" gate 52 is connected to an inverter 58 of any suitable construction. The output term of the inverter is true when its input term is false, and vice versa. The input term (C.multidot.I+Of) is introduced to the inverter 58, and it produces the term (C.multidot.I+Of) in response to the input term.

A "transmit" term (Pt) and a "receive" term (Pr) are introduced to an "or" gate 60, and the "or" gate 60 is connected to an inverter 62. The transmit term (Pt) is derived from a network including the grounded armature and a normally-closed fixed contact of a transmit-receive relay K1. The fixed contact is connected to a resistor 64, to the cathode of a clamping diode 66 and to the "or" gate 60. The resistor 64 is connected to the negative terminal of a source of direct voltage having a value, for example, of 18 volts. The anode of the diode 66 is connected to the negative terminal of a source of direct voltage having a value, for example, of 6 volts.

A group of signals (Rj), (Rh) and (Ra) from the receiver 30 of FIG. 1 are introduced to a corresponding plurality of trigger circuits (J), (H) and (R3) respectively. A filter 150 receives the signal (Ra), this filter being connected to the cathode of a diode 152. The anode of the diode 152 is connected to the trigger circuit (Rs) and to a grounded resistor 154. The resistor 154 is shunted by a capacitor 156. The trigger circuit (J) introduces the term (J) to an "or" gate 158 and to the "false" input terminal of the flip-flop (D). The trigger circuit (H) supplies the term (H) to the clock generator 24 of FIG. 3, and it supplies the term (H) to the "and" gates 82, 86 and 140. The trigger circuit (Rs) supplies the term (Rs) to the "and" gates 50 and 86.

The "or" gate 158 also receives the term (Q). This "and" gate is connected to the "false" input terminal of the flip-flop (I). The secondary winding of a transformer 160 is also connected to the "or" gate 158 and to ground. The primary of that transformer receives the reset switch term (So) from the control box 16 of FIG. 3. The primary is also connected to a resistor 162 and to a grounded capacitor 164. The resistor 162 is connected to the negative terminal of a 6 volt direct voltage source.

The secondary of a transformer 166 is connected to ground and to the "false" input terminal of the flip-flop (I). The primary of the transformer receives the term (Si) from the interrogate switch in the control box 16 of FIG. 1. The primary is also connected to a resistor 168 and to a grounded capacitor 171. The resistor 168 is connected to the negative terminal of the 6 volt direct voltage source. The flip-flop (I) introduces the term (I) to the "and" gates 56 and 72 and to the diode 124 and to the range counter 14 of FIG. 1. The flip-flop (I) introduces the term (I) to the "and" gate 100 and to the diode 128.

The secondary of the transformer 160 is also connected to an "or" gate 170. The terms (of) and (Q) are also introduced to the "or" gate 170. The "or" gate 170 is connected to the "false" input terminal of the flip-flop (E). The secondary of a transformer 172 is connected to ground and to the "true" input terminal of the flip-flop (E). The term (Sr) from the "respond" switch in the control box 16 of FIG. 1 is introduced to the primary of the transformer 172. The primary is also connected to a resistor 174 and to a grounded capacitor 176. The resistor 174 is connected to the negative terminal of the 6 volt direct voltage source. The flip-flop (E) supplies the term (E) to the "and" gate 50.

The receive term (Pr) is derived from a network including the grounded armature and a normally open fixed contact of the transmit-receive relay K1. This latter fixed contact is connected to a resistor 68, to the cathode of a clamping diode 70, and to the "or" gate 60. The resistor 68 is connected to the negative terminal of a source of direct voltage having a value, for example, of 18 volts. The anode of the diode 70 is connected to the negative terminal of a source of direct voltage having a value, for example, of 6 volts.

The transmit term (Pt) is also introduced to an "or" gate 72, as is the term (I). The "or" gate 72 is connected to a one-shot multivibrator (Oe). The "true" output terminal of the multivibrator (Oe) is connected to the respective anodes of a pair of diodes 74 and 76, and this output terminal is also connected to the "or" gate 54.

The inverter 58 is connected to each of a plurality of "and" gates 80, 82 and 84. The inverter 62 is also connected to the "and" gate 84, and it supplies the term (Pt+Pr) to that "and" gate. A term (Od.sup.2) is also introduced to the "and" gate 82, as is a term (H). A term (Op.sup.2) is applied to the "and" gate 80.

The circuitry of FIG. 2 also includes a plurality of "and" gates 86, 88 and 90. A group of terms (C, H and Rs) are introduced to the "and" gate 86; a pair of terms (H) and (Od.sup.2) are applied to the "and" gate 88; and a pair of terms (H) and (Oc.sup.2) are applied to the "and" gate 90.

The "and" gate 80 is connected to an "or" gate 92 and to an "or" gate 94. The "and" gate 82 is connected to an "or" gate 96 and to the "or" gate 92. The "and" gate 84 is connected to the "or" gates 92 and 96. The "and" gate 86 introduces the term (C.multidot.H.multidot.Rs) to the "or" gate 92 and to an "or" gate 98. The "and" gate 88 is also connected to the "or" gate 98, as is the "and" gate 90. The term (Q) is applied to the "or" gates 92, 96 and 98, and the term (Pr) is applied to the "or" gate 98.

The "and" gate 90 is further connected to a "one-shot" multivibrator (Od) which, in turn, is connected to a blocking oscillator (Od.sup.2). Blocking oscillators are believed to be sufficiently well known to the electronic art so as to preclude the need for a detailed description in the present specification. The blocking oscillator introduces the term (Od.sup.2) to the "and" gates 82 and 88.

The "or" gate 92 is connected to the true input terminal of a flip-flop (A) and to the true terminal of a flip-flop (B). Flip-flop multivibrators are well known to the electronic digital computer art. These units are bi-stable networks, and are triggerable between a "false" state and a "true" state by input terms respectively introduced to their "false" input terminals and their "true" input terminals. When in the "true" state the flip-flop develops a true term at its "true" output terminals. When in a "false" state, the flip-flop develops a true term at its "false" output terminals.

The flip-flop (A) develops the term (A) at its "true" output terminals, and it develops the term (A) at its "false" output terminals. The flip-flop (B) develops the term (B) at its "true" output terminals, and it develops the term (B) at its "false" output terminals.

The "or" gate 96 is connected to the "true" input terminal of the flip-flop (C), and the "or" gate 94 is connected to the "false" input terminal of that flip-flop. The flip-flop (C) develops the term (C) at its "true" output terminals, and it develops the term (C) at its "false" output terminals.

A pair of terms (Og) and (Lf.sub.2) derived from the coder 26 are introduced to the false input terminals of the flip-flops (A) and (B) respectively. The terms A, A, B, B, C and C are all introduced to the coder 26 of FIG. 1, as described above.

The "or" gate 98 is connected to a "one-shot" multivibrator (Ob) and to the "true" input terminal of the flip-flop (D). A term (J) is applied to the "false" input terminal of the flip-flop (D). The multivibrator (Ob) supplies the term (Ob) to an "and" gate. A term (I) is also introduced to that "and" gate. The "and" gate 100 develops an output term (V=I.multidot.Ob). The terms (V) and (Ob) are introduced to the clock generator 24 of FIG. 1, as described above. The flip-flop (D) develops the term (D) which also is applied to the clock generator 24 of FIG. 1, as described above.

A 5-kilocycle oscillator 102 develops an output signal (A5), and a 1 kilocycle oscillator 104 develops an output signal (A1). The output signal A5 is introduced to the anode of a diode 106 and to the anode of a diode 108. The output signal (A1), on the other hand, is connected to a pair of normally open contacts of a tone relay (K2), and to the anode of a diode 110. The normally open contacts also connect to the cathode of a diode 112.

The anode of the diode 112, and the