r/RTLSDR • u/Bulky_Salad4056 • 2d ago

ACARS and the problem with MLAT

I use ADSB.im on RI 4 to feed ADS-B and AIS, everything works fine. I bought a third SDR dongle and antenna to play with ACARS / VDL2 / HFDL.

ACARS works and receives messages, but I noticed that the MLAT light was red and the web stats pages that I feed showed errors that it was not counting the MLAT.

I thought there was a bottleneck somewhere on the RP4, so I took a second RP4 just for ACARS only. I noticed that when it's set to ACARS only, I still have the on map blue ADS-B and green ACARS labels.

I think the ADS-B Feeder sees the planes twice and the MLAT twice, and it interferes with the MLAT calculation and sending data.

CPU usage -30% , RAM -1GB, and temps are fine, no undervolting. I'm using active USB hub for dongle. I don't see anything in the options that I could change.

I tried it on 2 RP 4 on official and beta ADS-B Feeder Image. How to set what to change to have 3 SDR for ADS-B, AIS and ACARS on 1 RP4 without problems with MLAT? I can use 2 RP4s but I prefer to have everything on 1.

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3

u/kc2syk 2d ago

MLAT is typically for ADSB and not ACARS.

For MLAT to work, you need your exact position and a good timing source (GPS, etc.).

2

u/Bulky_Salad4056 2d ago

Without ACARS enabled, everything works fine. GPS is set. ACARS somehow messing with MLAT or ADS-B when they are together. I don't think there will be a problem with RP 4 performance because the stats and resource usage are low.

1

u/kc2syk 2d ago

Hmm.. I would ask the adsb.im people, since you are using their image. That may not be a supported configuration. They have a discord and zulip chat channel, or you could open a github issue. GL

1

u/elmarkodotorg 1d ago

Sounds like a port/data path clash? There's no reason one would impede on the other usually.

1

u/thebaldgeek 2d ago

The ACARS implementation of ACARS on the .im build is mostly broken. (It 'works' for very very very few people)
The dev is not interested in working with anyone to fix it, so it's as you find it. It just causes major breakage when you enable it.

If you are interested in ACARS, you will need to build a system just for it, not a different system running .im and ACARS, a different software stack.
I will be talking about a new 'fork' of dumpVDL2' that is really amazing in a few days over on r/ACARS but again, to be clear, you will be running it on stock Pi OS.
If you cant wait, there are a few different ways to install dumpVDL. If you want to run PoA or HFDL, you will need the same Pi OS base install, just different packages, SDR's and antennas. (Well, for HFDL, PoA and VDL2 can use the same SDR and antenna - but cant do both at once).

1

u/Bulky_Salad4056 1d ago

That explains a lot. ADS-B Feeder from adsb.im looks great and works fine for ADS-B and AIS. I'll have to do some more research and read up about ACARS. I have a spare PI 4 and will install dumpVDL2 and test other programs. Thanks for the explanation.

2

u/jreykdal 2d ago

Doesn't MLAT need to a considerable distance between units to work?