How does MLAT function without ADS-B aircraft

[SweetPea11]

There has been much discussion over the years about MLAT and how it works.

My understanding was that there was a requirement for at least one ADS-B out aircraft to be visible to at least 4 ground stations to provide timing for the MLAT calculations performed by Flightaware with the results returned to those who had provided the sightings.

Last night was very quiet and there were no ADS-B planes but two police helicopters tracked with MLAT.

In these circumstances how are the MLAT calculations performed and why are the results showing on my Skyview?

Thanks

S.

[obj]

You can’t get mlat sync without either ADS-B aircraft or a GPS-synchronized receiver (radarcape, some flightfeeders, etc).

There is a grace period where you’ll continue to receive mlat even after losing sync, so a single screenshot doesn’t really help - what did it look like a couple of minutes earlier?

Also, in this particular case, note how SkyAware is telling you “with positions: 0”. That means that there are no aircraft with recent positions (last 60 seconds). But if you click on an aircraft, as you’ve done here, you will still see the last position heard - even if it’s very old.

PiAware (fa-mlat-client) will periodically log how many ADS-B aircraft it hears and how many are being used for sync.

(also, that’s a pretty old dump1090-fa you have there)

[SweetPea11]

@obj

You can’t get mlat sync without either ADS-B aircraft or a GPS-synchronized receiver (radarcape, some flightfeeders, etc

Thanks for confirming that.

@obj

There is a grace period where you’ll continue to receive mlat even after losing sync, so a single screenshot doesn’t really help - what did it look like a couple of minutes earlier?

I wasn’t paying much attention until I noticed that the were two MLAT and no ADS-B but that screen shot was probably three minutes after I noticed the situation.

It has been quite odd here the last few weeks in lockdown. Rarely is there more than two or three aircraft with flight numbers and if it is good flying weather there may be more than 30 GA aircraft mainly doing circuit work.

Interestingly, yesterday I noticed 15 aircraft with 3 ADS-B and 12 MLAT.

@obj

But if you click on an aircraft, as you’ve done here, you will still see the last position heard - even if it’s very old.

Yes but the last position was only 4 seconds ago.

Unfortunately, I was looking on a tablet so I cannot see if I was contributing to these positions in the Tracklog and I didn’t think to check if I was synced with other receivers.

@obj

(also, that’s a pretty old dump1090-fa you have there)

Yes it is but I don’t like the new user interface. In this release I get the popup with all the information for the aircraft I click on.

In the new UI, most of the information is hidden off screen and I have to scroll down to try and find the information of interest and often I need to do two screen dumps because not all the information is displayed at once.

Would the outcome have been different had I been running the current release?

As usual, thankyou very much for taking the time to explain the inner workings,

S.

[obj]

@SweetPea11

Yes but the last position was only 4 seconds ago.

No, the last message (direct from the aircraft) was 4 seconds ago. The last position was more than 60 seconds ago; the “aircraft with position” number counts aircraft where the last position was <60 seconds ago, and that number is zero in your screenshot. i.e, at the time of the screenshot, you were not receiving mlat positions.

There used to be a thing that told you the time since last position, but that got lost during the SkyAware overhaul and only made it back in recently, IIRC.

[SweetPea11]

@obj

No, the last message (direct from the aircraft) was 4 seconds ago. The last position was more than 60 seconds ago; the “aircraft with position” number counts aircraft where the last position was <60 seconds ago, and that number is zero in your screenshot. i.e, at the time of the screenshot, you were not receiving mlat positions.

Thanks.

I’m not meaning to nit-pick but we are in an unique position with the lockdown and flight restrictions to see things we would normally not see.

If the trailing grace period I get MLAT positions after my last ‘contribution’ to the MLAT process is 3 minutes does that imply I must have seen an ADS-B aircraft in the minute before that?

I would normally look at the track log to see if I had contributed to an MLAT position but this Police helicopter is not reported on Flightaware.com. Is there any other way to identify my contribution?

Also, if there was a suitable ADS-B plane in range of three other receivers would that be enough to calculate the position? (if the MLAT fairy was having a good day AND the stars aligned)

@obj

There used to be a thing that told you the time since last position, but that got lost during the SkyAware overhaul and only made it back in recently, IIRC

You recall correctly.

I can see only a last seen field in 5.7.1 but I can also see a last position in 3.8.1

S.