identification -- friend or foe systems which utilize computer control continuously changing interrogation signals and responding signals have been developed . the signals are varied in a time dependent manner , however , it is possible that an intruder might intercept a proper responding signal and repeat the code of this responding signal , and thus confuse the interrogating station . the present system allows a responding signal to be used only once during a particular time interval so that an interrogating station will know that if an otherwise proper responding signal is received twice during a time interval , that the second signal is spurious .