Apparatus and methods for differentiating between synchronous and asynchronous response signals in secondary radar devices

Claim 1

A secondary radar device in which the interrogation pulse repetition rate is derived from the pulse repetition rate of a primary radar device comprising a variable controllable frequency divider a pulse generator supplying pulse repetition rate pulses to said primary radar and to said variable frequency divider, a random generator also connected to said variable frequency divider, a trigger preparation device connected to the output of said variable frequency divider so that the interrogation pulse repetition frequency of the secondary radar for the interrogation signal changes continuously with the divider ratio, a memory control circuit, said trigger preparation circuit connected to key said secondary radar transmitter and emitting an indicator impulse to said memory control when an interrogation signal is transmitted, a storer connected to said memory control and storing information, an adder connected to said storer, a decoder connected to the receiver of said secondary radar and supplying to said adder the decoded answer signals, said storer under the control of said memory control and storing decoded answer signals from said adder in synchronism with the timing of the interrogation signals triggered by the trigger preparation circuit so that nonsynchronous answer signals are suppressed and that synchronous ones are admitted, and a threshold circuit connected to said storer, a presentation means connected to said threshold circuit and receiving an output signal from said storer only when a threshold value is exceeded by the stored answer signals.