I live in the Rochester NY area and recently noticed the ATS-B tracking for the Goodyear blimp (N1A) was very erratic on my PiAware when it came here for the PGA Championship recently. I checked the FlightAware tracking and it was smooth, but their track log showed only UAT plus a lot of MLAT positions. I assume that the MLATs were the reason for the jagged tracks that I saw. There are a couple of things that puzzle me:
1 I don’t have UAT support on my system, why was I getting any ADS-B reports from N1A at all?
Why would they choose to use UAT instead of Mode-S?
I set up my feeder almost a year ago. I considered adding UAT support but, when I checked the 25 nearest feeders to me, only one was reporting UAT, and that one was only getting about 20 aircraft per month. I had assumed that, since we are so close to the Canadian border, local plane owners choose Mode-S in case they want to fly in Canadian airspace.
Their older 1090 transponder is mode-s, so it does not broadcast position like a full ads-b transponder. It also supports mode-a/c.
When they decided to upgrade, it looks like they added a 978 MHz UAT transponder. This broadcasts position all the time, but they still need something that supports 1090, so they kept the 1090 mode-s.
978 MHz UAT is GPS based and very accurate, but has limited range, and FA has limited receivers. Whenever possible, FA most likely uses the 978 UAT as top priority, since it is the most accurate. You were not seeing this locally, since you do not receive 978.
When out of 978 receiver range, then it looks like FA switches to 1090 mode-s mlat. That is what you were receiving, since you were likely receiving 1090 locally and contributing to the mlat calculations.
A third option for FA would be to use the public FAA track data for a known flight. That can be good, but will never match GPS. FA mlat is often better than the FAA public tracks.
For LawrenceHill. Yes, ADS-R is definitely another option. I am within 3 nm of an FAA site that does rebroadcast on both 1090 and 978, so I see that traffic on both frequencies. My best guess here is that he was seeing the 1090 mode-s mlat on his system.
