FlightAware Discussions

Sometimes MLAT is a bit confusing (or confused)


#1

It is hard to work out what this helicopter is doing.

The airport to the left (YMMB) is a busy with GA and training.

Note the heading is about 90° to the track

Fortunately, MLAT is only used on less and less GA aircraft and all commercial aircraft in Australia are tracked with ADS-B

Only 5 of 50 aircraft in view are MLAT today.

S.


#2

It’s a Bell 206. MLAT with the local receivers is not accurate enough to work out what it is doing.
Better receivers may provide more accuracy but they are a lot more expensive.

I did an endorsement in one (EPQ and NBP) when I did my PPL(H) in 2002. I love the startup of a turbine.


#3

Hence the title says MLAT is a bit confusing (or confused)

MLAT around here is only used by some light planes, some helicopters and the Police Air Wing (mainly helicopters). Interestingly, Media helicopters don’t broadcast at all and are untracked. Most of the time the MLAT tracks are a reasonable representation of the aircraft with some aberrations as to be expected.

We often have a Police helicopter doing circles around here and the MLAT track is reasonable.

The track from VH-VBX (VBX) by local MLAT standards is very poor even though each point in the track log shows 5 to 10 MLAT stations contributed.

It is an imperfect system.

S.

If you look at the


#4

MLAT for helos is worse because they tend to fly low so their receiver range is not good.
I rarely fly helos above 2000ft AGL.
It makes it hard to get a good MLAT fix on them,

There will be more ADS-B in the future. I think it is already required for IFR in Aus(I have flown IFR in Aus for years). They may add more requirements for congested airspace too.


#5

That problem occurs only since the extended squitter decoding was introduced.

It interprets some messages as a BDS(5,0) message:
(https://mode-s.org/decode/ehs/bds50-track-n-turn.html)

The problem with these extended squitter messages is that the type of message or what the data means is only know to the interrogating radar. Therefore the decoder needs to guess which kind of message it is.
It seems the decoder is regularly guessing wrong in regards to this BDS(5,0) message and thus MLAT planes get a wrong heading.

obj would need to change the code for this problem to change. Or maybe the planes are just broadcasting garbage data who knows.

I’ve learned to just ignore it :wink:

(Before the change the heading would only be the track from the last to the current MLAT position i believe)


#6

Thanks for the education.

I watch MLAT planes around here including the low flying Police and Ambulance helicopters who are often down to 500’ and also flying circles.

The track I captured was particularly messy even for the low flying stuff around here.

I wasn’t complaining or suggesting that something needs to be fixed but merely pointing out a very rare example where it hasn’t quite worked as we have come to expect and as a result the above explanation has improved my knowledge.

Another few years and MLAT will be almost all gone and then it will just disappear into the archives and the next generation will have no idea what we are talking about.

S.