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I know that the Secondary surveillance radar (SSR) interrogates at 1030MHz, and the reply is at 1090MHz.

What happens if the radio signal sent to the plane is less than/more than 1030MHz?
Also what happens if the radio signal with which the plane responds is less than/ more than 1090MHz?

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    $\begingroup$ The signal received at both ends of the radio channel is anyway affected by Doppler shift. While I don't have a good reference at hand, SSR is a pulse modulation, hence tolerant to small carrier frequency sweep as long as it remains within the passband filter bandwidth of the IF stage. If nobody answers, I'll search for the exact information. $\endgroup$ – mins Feb 20 '18 at 18:33
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With respect to the radar frequency drift, I am afraid you are asking an impossible question, the interrogation frequency is continuously auto monitored, any drift tendance is electronically prevented. However should this occurs, the aircraft transponder has a sharp filter limited just to light Doppler effect, beyond that the transponder will not answer to the received signal.

I do understand a drift in the aircraft transponder, that is a frequency drift in the aircraft transponder answer, that’s not a problem; even though it is quartz generated, that is nearly impossible to drift, the ground radar accepts a frequency drift of + or - .5MHz, beyond that the received signal by the radar could be interpreted as a response to a lobesides signal received by the aircraft transponder, and may get mutually eliminated.

However the radar memorizes all present mode A received information, if the received signal concerns an aircraft identification not received previously, the radar will accept the signal and not reject it as being a response to a lobesides signal.

If the frequency drift is important, this is equivalent to having lost the transponder. The total loss of transponder information may sometimes be mitigated by the availability of primary radar data. However, if false primary tracks appear too often, a “real” track may be considered to be fake. This is especially true when no other data is available (e.g. information from the previous sector).

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