MLAT coverage zones and EU NATO flights


#1

Hi all,

My (very general, handwavey) understanding of the way FlightAware handles MLAT is that the system groups various feeders in an geographical area into a single group and uses data from those feeders to compute MLAT positions. However, data from feeders in different groups – even if they’re physically relatively close and have overlapping coverage areas – don’t get used. Is this correct?

My location, #11261, gets good coverage over nearly all of Switzerland, eastern France, southern and south-western Germany, Luxembourg, and a little bit of western Austria. Maximum range is around 300mi. However, it looks like certain flights (usually NATO military flights in Germany or France) are in different MLAT groups from my feeder and although I can see the Mode-S data (call sign, ICAO hex ID, and altitude) those planes show up in the dump1090 web interface with only a white background, not the blue indicating my feeder contributed to an MLAT location.

I occasionally see other NATO and Swiss Air Force flights show up with MLAT positions, so it’s not due to military flights being blacklisted from MLAT processing.

Is there a map of MLAT zones in Europe? Is it possible to have my feeder be a part of several zones to help contribute to more locations?


#2

This is a tricky problem.

The size of a zone is limited by what a single server can handle. There is overlap between zones so yes you can be in more than one zone. The overlap is on the order of around 60 miles. The zones are based on receiver location, not aircraft position, which makes the actual coverage area a bit unclear - it is the coverage overlap of receivers that are located in the zone, which might be quite a lot larger than the actual zone (and probably quite oddly shaped). It’s not possible to manually place receivers in particular zones, it’s entirely determined by receiver location vs. zone polygon.

So yes it’s possible that, say, you can see an aircraft over Germany, and several other receivers (maybe in Germany) can also see it, but you are not in the same zones so they can do mlat on the aircraft but you can’t. The global system fairly reliably sees the aircraft in this case, as usually at least one zone will have enough receivers for mlat, but that doesn’t necessarily mean you’ll see mlat results yourself. This is just a limitation of how the system currently works, unfortunately.

In the case where two zones can see an aircraft and generate mlat timings, both (all) sets of timings are used to provide solutions and update the estimated track, so you will benefit from data from nearby zones even if you’re not in them so long as you have at least 4 receivers in your own zone that can see it.

I have ongoing work to try to select the zones better, and also to improve the per-server capacity so zones and their overlaps can be larger. But it might be a while.


#3

Interesting, thanks for the details.

I’m not sure how easy it’d be to generate, but I’d find it really interesting to see a map of MLAT zones and the locations of feeders within each zone. It’d be really cool to see how that changes over time as new sites are added.

As for the “unfortunate limitation”, it’s not really a problem and makes sense.

Though it does seem a bit odd that FlightAware’s “no military aircraft” blacklist seems to only apply to mainly US flights: FlightAware shows French, German, and Swiss Air Force flights but doesn’t show US Air Force-operated flights operating in Europe as part of NATO operations (at least not so far as my limited testing has shown). Thus, even though the “global system”, as you put it, can compute the location of US Air Force flights using MLAT, they’re not displayed or listed on the website – that’s why I’m interested in seeing them in my local web interface rather than relying on the FlightAware website to see such flights.

Other flight-tracking sites that do MLAT, such as FlightRadar24, show those flights on their public website without any issues. Go figure.

Anyway, thanks again for all your work with MLAT and other things.

Edit: I was mistaken: the NATO E-3 Sentry flights in Germany are not done using US-registered aircraft, but rather Luxembourg-registered, NATO-owned aircraft. Now I’m even more confused as to why this particular plane wasn’t showing up on the FlightAware website.