# SSR Frequency Dif Reasons RNAV

After the SSRSSR —Secondary Surveillance Radar transmits its interrogation pulses, it then listens out for the response to its pulses. If the SSRSSR —Secondary Surveillance Radar listened out on the same frequency, there is every chance that it could mistake reflections from the ground as the response from the aircraft - and thus display the distance to the ground and not to the aircraft.

This problem is solved entirely by having the aircraft respond on a frequency that is 60 MHz removed from the SSRSSR —Secondary Surveillance Radar transmission. (This 60 MHz difference is also an exam fact.) If the SSRSSR —Secondary Surveillance Radar hears the response on the same frequency that it transmitted on, it must be a reflection and can be ignored.