A Synchronized Pulse Communication System with Pseudorandom Interpulse Period

The security of a pulse type communication system can be improved by the incorporation of an element of randomness in the time of transmission. A system is simulated in which a remote receiver is caused to be gated on at apparently random intervals which are in synchronism with the arrival of the signal from the controlling local transmitter. The time of transmission is not random, but is determined by an extremely complex program. By duplicating the program in the remote equipment, the receiver can anticipate the time of arrival of the local transmissions. The time of transmission is varied ± 10 per cent about a nominal period which corresponds to a message repetition frequency of 100 cps. The pseudorandom intermessage variations are governed by the output of an eighteen stage magnetic core shift register. The period of repetition of the pseudorandom transmission is in excess of six minutes and much longer periods are readily obtainable. Special scrambling techniques are used to increase the burden of a would-be code breaker. The system can be operated in either the periodic or pseudorandom mode, and a fail-safe feature is provided to return the system to the periodic mode if the local and remote units become unsynchronized while in the pseudorandom mode. Receiver synchronization is maintained by a voltage controlled clock frequency oscillator which is phase locked to the local transmission. The phase lock compensates for Doppler shift in the received frequency and for any drift inherent in the remote clock source.