FlightAware Discussions

Broke MLAT

Okay not sure why, but yesterday I finally combined my UAT receiver that was running on a seperate Pi onto the 1090 pi. Everything appears to be working fine, MLAT is green and shows it’s in sync yet I’ve lost sync with stations near me. Anyone have a clue what could be the problem here? I’m using an AirSpy for 1090 and I’m not showing any lost samples.


You can’t use the airspy with another dongle on an RPI. The airspy uses too much USB bandwidth. I tried it and it failed every time.

It should work on an Odroid, or something with a decent USB setup.

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I don’t have any lost samples, so it shouldn’t be a USB issue.

Unplug the UAT dongle and see if the error goes away.

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@jonhawkes2030 is absolutely correct - MLAT flat out won’t work if another USB device is plugged in on a Raspberry (any iteration) alongside the Airspy. You may not be showing dropped packets, but there is not enough bandwidth on the shared USB hub on a Raspberry. It will work on a RockPi 4b, but have not tested Odroid or any others - most SBC’s (currently) don’t have seperate USB hubs built-in to handle the excessive bandwidth if another device is plugged in.


You can loose samples in a way that’s not visible to the software.

Switch the Airspy from 20 MHz to 12 MHz if you want to run another SDR on the same system.

Enabling the airpsy bit packing could also work, but changing to 12 MHz is the safer bet.


Oaky well po, I just don’t understand how you can lose samples and not having a timing error. Back to two Pi’s.

I had/have an Odroid and I was never able to get it to work on it before, but it’s been several months since I’ve tried.

Oh ok, i read that wrong.

Well … if your timing is really bad, it can’t even get a sync to nearby receivers to generate a sync error.
This is hard to differentiate from working MLAT and no stations / aircraft to sync to / with.

Anyhow if you change to 12 MHz … and it improves then you know what the issue is.

Well reducing it to 12MSPS did get it to sync again. Interesting that the timing could be so bad that it can’t generate a sync error.