This is the xpond_red-pitaya.grc Github PiRadar example. As a first test of the transponder alone, one could simply receive the AM broadcast band (530 kHz - 1710 kHz) and transpond it up to a frequency where you have a receiver, whether in the 40 MHz or 27 MHz or wherever you have a receiver for. That would show that the idea of transponding works with this hardware.

The essential functions are:

1. amplify incoming radar signal
2. frequency translate incoming radar signal to frequency range radar receiver is listening at

When using a transponder, the radar needs to transmit and receive at different frequencies to avoid a feedback loop of having transmit and receive of the transponder on the same frequency.

To operate the radar in an ionospheric sensing mode, and with the ability for the “target” to add impairments including:

1. fading
2. frequency selective fading
3. random phase shift

and other errors requires an active transponder. There are blocks in GNU Radio that simulate these channel impairments.