MLAT on the Skydex feeder network
Several feeders asked for MLAT, most recently in our Discord this week. It is live now: Skydex runs an MLAT server next to the ADS-B ingest, and any station that already feeds us can join it with one extra line.
What MLAT does
Most aircraft send their own position in ADS-B. Some don't: older Mode S transponders, a lot of military traffic, part of general aviation. Their transponders still answer, though, and when four or more receivers hear the same reply, the server can work out where it came from by comparing the arrival times. That is multilateration. It only works if the receivers' clocks can be lined up, which the server does by watching ordinary ADS-B aircraft whose positions it already knows.
How to switch it on
On your station's page at skydex.online/feed, the Setup tab now has a second line under the first one. It looks like this, with your own station id:
mlat,feed.skydex.online,31090,uuid=<your uuid>;
Put it in ULTRAFEEDER_CONFIG next to the ADS-B line and recreate the container. On adsb.im, paste it in the same place as the first line: Setup, Expert, "Add additional Ultrafeeder arguments". The uuid has to be in the line. Without it ultrafeeder sends the id it uses for other networks, and we can't match the connection to your station.
The same tab then tells you what the server sees: not connected, connected but with no stations in sync yet, in sync with N stations, or connected with its timing ignored for now. That last one nearly always means the antenna position or altitude in your settings is off.
Where it works today
MLAT needs neighbours. We counted on 27 October: of 51 stations online, two areas already have enough of them, the US Southeast with five sites and the English Midlands with four. The Netherlands has three. Everywhere else the connection works and simply has nothing to compute until more stations join nearby, so don't read an empty result as a fault.
What happens to the data
Positions go back to the stations that helped compute them and show up on your own tar1090 map. They don't go into the Skydex map or the game yet; that comes after we have watched the server under real load for a while. Your exact antenna position and altitude go to the MLAT server only. Public pages keep showing the same rounded location as before.
The server is wiedehopf's mlat-server, the one adsb.lol and adsb.fi run, unmodified. It is AGPL software, and it tells every client where its source lives when it connects.
If you feed ships too, the docker-shipfeeder post has the one-variable setup for AIS. Questions go to the feeders channel in our Discord.