Years ago I built a "moving target simulator" for a K-band CW (homodyne) radar. I simply took the Gunnplexer module out of a K-band automatic door opener (a 24.125 GHz resonant cavity complete with feed horn, Gunn diode local oscillator and RX mixer diode) and fed my simulated doppler AF signal into it's RX mixer diode (I did not energise the Gunn diode - so the microwave oscillator was NOT operating).

When I illuminated it with 24 GHz RF from the radar, it produced good reflections at RF + AF and RF - AF. I did not have access to good K-band microwave test equipment so I was unable to quantify the strength of the reflections or mixing conversion loss, etc.

So I would say that yes, subject to sufficient link budget, if you had an antenna in orbit, coupled to a mixer diode, and you drove that mixer diode with AF modulation, then that antenna will re-radiate the mixing products of the RF illumination and AF modulation. Would this be an interesting experiment for an ulta-low-power bird that does not have the power budget to run an RF transmitter?

73 ZL2WRW Ross Whenmouth