 The rapid introduction of new types of radars working on different frequencies meant the IFF system had to respond to an ever @-@ increasing list of signals , and the direct response of the Mk . II required an ever @-@ increasing number of sub @-@ models , each turned to different frequencies . By 1941 it was clear that this was going to grow without bound , and a new solution was needed . The result was a new series of IFF units which used the indirect interrogation technique . These operated on a fixed frequency , different from the radar . The interrogation signal was sent from the aircraft by pressing a button on the radar , which caused the signal to be sent out in pulses synchronized to the radar 's main signal . The received signal was amplified and mixed into the same video signal as the radar , causing the same extended blip to appear . 
