ADSB Exchange Blue dongle captures UAT 978

FlightAware ADS-B Flight Tracking
1

The other night I started running the optimize gain program and went to bed, and at some point it hard failed and apparently left the Pi in a state where dump1090-fa was down. When I got a chance to look at it at lunchtime the next day I noticed the UAT graphs had some blips, which I’d never seen before and didn’t expect because I’m running the ADSB exchange Blue SDR (supposedly filtered to 1090) with their 2ft 1090/978 antenna. Today I ran a test by stopping dump1090-fa and running “sudo piaware-config uat-receiver-type sdr”, and started picking up a few UAT planes per hour despite the blue SDR saying it’s filtered to 1090. I just thought it was interesting, I figured I’d share. And now I’m wondering if an SDR filtered to 978 would give me more than the few-planes-per-hour the blue SDR is giving me…

The SDR in question:

2

A couple of things –

  1. Filters aren’t an all-or-nothing thing, they have a response curve where frequencies outside the passband get attenuated more – but the passband can be quite wide, and the curve might let quite a bit of energy through outside the passband. For example one of the FlightAware external filters deliberately has a passband that’s wide enough to cover both 978 and 1090MHz.

  2. It may just be garbage data, noise that’s being misinterpreted as a real message. If you throw enough noise against the wall, occasionally some of it sticks. Does the data you see look realistic (has position / callsign that makes sense, you receive >1 message per aircraft, etc)?

3

Yes, I was thinking it might be the situation you described in your part 1. The data seems good, I saw a couple planes with color/position on the map when I had a chance to look, and yesterday afternoon/evening I got 13 planes and 1100+ positions reported.