There's a difference between civilian transponders which generally squawk all the time and military IFF, which typically only responds to a coded challenge (cal

There's a difference between civilian transponders which generally squawk all the time and military IFF, which typically only responds to a coded challenge (called an IFF interrogation). If a military IFF doesn't receive a valid interrogation, it remains silent. It is Very Bad if an enemy can either spoof a valid "friend" IFF response or cause IFFs to respond and give up one's position. The latter was done with great effect in both WW2 and the Vietnam War.