I just bought a 260B that has been sitting for the last 10 years. The old narco transponder won't even power up anymore. Since the ADS-B mandate is around the corner, I am considering to have a Garmin GTX 345 transponder installed. It comes with its own optional WAAS GPS installed, or without WAAS GPS. I understand that to have ADS-B out capability one needs to have some sort of WAAS GPS linked to the transponder. I may want to upgrade the avionics by installing a GTN 650 in the next few years (before 2020). The 650 could then provide WAAS to the transponder.

My questions is: Will the GTX 345 transponder (without its own optional GPS) work as a "non ADS-B out" transponder? In other words, will it function as a mode C or mode S transponder, or is it completely useless without a WAAS GPS source?

It will need WAAS from the start. It will not function as a made S or C transponder.

Thank you for your info.

I have the 335 installed without GPS or WAAS input and it functions as Mode S. Not sure why the 345 wouldn't operate in the same way?

I asked an expert about the Trig TT-22 and was told that it will work fine as a transponder until I install the GPS.