I wonder if APRS transmitter would be acceptable as transponder. It should be possible to make a tiny, cheap ADS-B transponder. They are similar to APRS, with

I wonder if APRS transmitter would be acceptable as transponder. It should be possible to make a tiny, cheap ADS-B transponder. They are similar to APRS, with GPS and VHF radio. But there really isn't a market now for small ones, and there are regulations that make expensive.