I use one of these (in a way) whenever I fly. I have an RPi running Stratux with a WAAS GPS reciever and two modified RTL-SDR dongles, one for each ADS-B band. It then rebroadcasts all this info over a wifi network that my ipad picks up and displays with foreflight. The whole thing sans ipad was ~$100, is super reliable, and fits cleanly in a small case. The commercial version (Stratus 2s) is $900 and has reliability issues.
On a side note, these only pick up AC that are broadcasting ADS-B which is most commercial aircraft and a minority of commuter/personal planes. This will change at the beginning of 2020 when the FAA will mandate all aircraft will need it (or at least anyone planning to ever operate in a mode C ring/near a major city)
Not permanently mounted flight hardware, no TSO required. Only certification required for that would be basic FCC ISM transmission stuff, the same as your router. If they used a precertified module (like most internet of shit devices) you only need the harmful emission testing. It's also very unlikely that they voluntarily went through any of it because the devices regularly overheat and fail.
Pilot. I rent 1970s aircraft so it's nice to have a backup set of avionics.
I should mention, this is also a fun thing to have on airline flights as you can see the route you're flying, other planes around, speed, altitude, where you are in line for the runway, etc. There are even free alternatives to foreflight for android (avare) so you only have to make the Stratux hardware.
On a side note, these only pick up AC that are broadcasting ADS-B which is most commercial aircraft and a minority of commuter/personal planes. This will change at the beginning of 2020 when the FAA will mandate all aircraft will need it (or at least anyone planning to ever operate in a mode C ring/near a major city)