If you decide to set up an SDR for ADS-B, you might want to consider setting up a WebSDR with something like OpenWebRX. This would let people listen to all the signals in the bandwidth that you set.
If you’re interested, receiverbook.de is a list of most WebSDRs.
They do, but compounding errors are always a problem with inertial navigation.
Instead of GPS, they can use fixed radio beacons like VOR and TACAN (which I think are both just US systems, but there are similar systems around the world and at major airports). This is basically the system that was in use before GPS.
EDIT: grammar