"Do It Yourself" airborne proximity warning device

On Thursday, October 11, 2018 at 8:41:00 AM UTC-7, wrote:
Alas the 915 MHz band and other ISM bands are already congested with many other uses, and (from the wikipedia page) "communications equipment operating in these bands must tolerate any interference generated by ISM applications, and users have no regulatory protection from ISM device operation." These bands were not intended for reliable communications use. It is a shame that we are forced into those marginal wastelands in the spectrum to do these things, while there are many hundreds of, e.g., aviation band comm frequencies available, now that they've been divided into 25 KHz channels (and 8.33 KHz in Europe and perhaps someday in the USA too). Of which gliders are officially allowed to use, what, 2 channels (dating to the 100 KHz spacing days), for air-to-air coordination?

The ISM band is congested in an urban setting but certainly not congested where gliders do their flying. The range of influence from all those many ISM devices out there is limited because their power output is limited by regulation. So the band becomes a pretty good choice actually to take advantage of the chips, antennas etc that are available at low cost.