Finally managed to take my software radio outside and watch airplanes overhead. Lots of fun stuff there. Also I built a crappy 1090MHz dipole antenna. I have no idea what I’m doing, but it seems to work reasonably well. I wonder if I can measure how well?
Thanks to @lemonodor I learned about http://www.airframes.org/ which lets me look up those 6 digit ICAO codes. For instance ae2923 flying at 16,300 is a US Air Force registered B350 King Air. I’m wondering if it’s a chase plane for drones. COUGR11 (ae2f5f) at 14,325 and SPARTA53 (053977) at 15,300. Neither of those show up in airframes, although ae2f5f may be another King Air and 053977 is a bogus code, 1234567 in octal.