Air 2 Air

[son of brommers]

Have been watching on ADS-B numerous tankers flying racetracks north of Norfolk over the last week, I couldn't help notice that as the receiver arrives in what appears to be close proximity to the tanker before connecting, the receiver stops broadcatsing on ADS-B and then pops back up once fueling is completed whilst the tanker keeps broadcasting.
Any ideas, is this normal practice or just an issue with 2 aircraft being so close together that ADS-B cannot recognise the 2 seperately?

[sycamore]

Normal practice...

[Hot 'n' High]

Originally Posted by son of brommers

.......... Any ideas, is this normal practice or just an issue with 2 aircraft being so close together that ADS-B cannot recognise the 2 seperately?

Certainly older equipment I looked after suffered from "garbling" when transponders/IFFs were in close proximity to one another and this IEEE paper seems to cover that aspect in ADS-B too. Thus it was (and probably still is) as per the post from sycamore to prevent such issues.

[6foottanker]

Stops the tanker TCAS from whinging at the crew. Normal ops.

[son of brommers]

Thanks all, makes perfect sense.

[MechEngr]

Originally Posted by Hot 'n' High

Certainly older equipment I looked after suffered from "garbling" when transponders/IFFs were in close proximity to one another and this IEEE paper seems to cover that aspect in ADS-B too. Thus it was (and probably still is) as per the post from sycamore to prevent such issues.

Garbling is when two messages are transmitted too closely in time, not due to spatial proximity. The density in that paper refers to the number of aircraft that can be heard, not the number of planes per cubic meter. This is similar to two people in the same room talking at the same time to a third person.

The mention of TCAS makes the most sense.

[Hot 'n' High]

Originally Posted by MechEngr

Garbling is when two messages are transmitted too closely in time, not due to spatial proximity. ................................. The mention of TCAS makes the most sense.

You may be correct (twas a long while ago I'm going back now) but I understood it was to do with overlapping (in space) of replies when they are displayed back on ATC Radar screens as well as the fact that the ground processor would be dealing with co-incident (in time) replies from a single interrogation when the a/c are very close - not that the ADS-B is triggered that way.

Have I mistakenly combined "garbling" with "superimposition" - with the first being a processing issue and the other being a display issue? TCAS issues does make sense as well!

[beardy]

I seem to recall that tanker and receivers were part of one formation with the tanker being the leader. One formation, one squawk that of the leader, the tanker.