Minor issue with US ADS-B

The "to-do" list for Virtual Radar Server is as long as my arm, but if you want to pile more work onto it then this is for you.
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Minor issue with US ADS-B

Post by dsfh2992 » Mon Mar 21, 2016 4:56 pm

As you may know, 978 Mhz ADS-B out is an option in the US. Some smaller planes are equipped with it, but may also have a Mode S transponder on 1090.

I have had reports of targets appearing as MLATs, while also broadcasting ADS-B via 978 (which is also being fed from another feed). In this case, it was reported that the MLAT track took precedence over the 978 data since it was visible first. The fix for this, I believe, would be to prefer "MLAT = no" positions over "MLAT = yes" positions when coming from two separate feeds. Is this already the case?


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Re: Minor issue with US ADS-B

Post by agw » Sun Mar 27, 2016 2:53 pm

Yes, kind of. If the MLAT receiver is the first receiver to send messages for an aircraft then it'll be the only receiver that the merged feed will listen to until 3 seconds go by without any transmission for the aircraft from the MLAT receiver. You can configure the 3 second timeout in the merged feed settings.

VRS will ignore MLAT positions for an aircraft if the nominated receiver is already sending positions for the aircraft, so if the UAT receiver is the first one to send messages for the aircraft, or it takes over from the MLAT receiver, then after that any MLAT positions will be ignored.

Maybe the fix is to have some way for users to say "this feed should have a low priority in the merge - if it's a choice between this feed and another as a source of messages for aircraft X then go with the other one". At the moment all feeds have equal priority, MLAT feeds should ideally defer to other feeds and just add positions.

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