A Secure Digital Command Link

The command link described was developed for use with a radar tracked, surface-to-surface guided missile. However, it has features which are applicable to a variety of other uses, wherever communications must be disguised and coded to prevent undesired interference, and to insure secrecy. The command link is of the time division variety, utilizing a pseudorandom command code, which code pulses are interspersed among a completely random set of pulses. A large variation is possible in the time between successive command pulses, in the time between complete commands, in the time of a complete command, in the time between random pulses (maximum and minimum values), and in the nature of the pseudorandomness. Automatic and continuous synchronism between transmitter and receiver is maintained by transmitting at intervals (not necessarily periodic) a single synchronizing pulse which is a part of the pseudorandom code. The synchronizing action can also be obtained from any or all of the other commands without the need for a separate synchronizing pulse, if desired. Note that the synchronizing pulses are nonperiodic, and are fully encoded. The synchronizing process in the receiver is unique in that a single pulse received does the job. The circuit utilizes 3-stage counter, operated continually in a counting mode from an internal oscillator, with the received synchronizing pulse resetting the counter to a fixed count. This reset serves to introduce a fixed time delay between the received pulse and the output of the third stage.