CAP1391 shows as bearingless in SkyDemon via PAW

[a.alexeev.p]

Hi all,

Recently was trying to watch some aircraft through PAW while on the ground and noticed a strange thing - couple aircraft that I know for sure had an ADS-B out CAP1391 devices on (and no Mode S transponder as one was actually parked) were showing up as bearingless. What could be the reason for it? I believe they were showing normal after take off…

[steveu]

Hi all,

Recently was trying to watch some aircraft through PAW while on the ground and noticed a strange thing - couple aircraft that I know for sure had an ADS-B out CAP1391 devices on (and no Mode S transponder as one was actually parked) were showing up as bearingless. What could be the reason for it? I believe they were showing normal after take off…

Do you mean no mode S as not fitted, or no mode as not flying/in standby?

Does the CAP1391 have the setting for flying/not flying enabled? I think the default may be 10 knots to switch over? Been a while since I looked...

[a.alexeev.p]

So aircraft electrics completely off as somebody just forgot their CAP1391 device on in a parked aircraft. Not sure what their transmit settings are, but it was visible on a flight tracking app and clearly shown as ADSB as I would expect and it is just PAW that for some reason was treating it as bearingless

[exfirepro]

Hi Alex,

What PAW software version was in use?

I seem to recall there was an issue where some DF18 devices (SkyEchos) were showing as DF17 (Transponders), but I can't recall whether this was due to a PAW or ATOM software fault. I'm pretty sure it was fixed at the time though.

Regards

Peter

[a.alexeev.p]

I don’t recall the exact version, but it was the latest one that was released in the past 1-2 months. Same behaviour on my old classic paw and my friend’s brand new rosetta. It is just the classic picked up one aircraft and rosetta another (both with SE2 left on).

[Admin]

Do these aircraft also have mode S transponders?
Are you sure that the transponder is giving a bearingless, and cap1391 is not detected?

[a.alexeev.p]

Just to reconfirm:

Aircraft does have a mode s transponder only (not ADSB), but it is not very relevant because it was parked and completely off, just the pilot left their 1391 device on. I am sure of that also because on a flight tracking app I could see the exact position and clear marking that the signal was ADSB.

Two different PAW devices were nevertheless showing it as bearingless. When the the aircraft took off later - it started to show normally (although I can’t be 100% sure if when showing normally it was direct from the 1391 device or Mode S MLAT function of PAW). It was still showing as ADSB on the flight tracking app.

[Admin]

A skyecho has a vso setting
Anything less than this and it will not emit a position report
Can you check its setting

[a.alexeev.p]

I can definitely try and ask, however, as a flight tracking app was definitely showing it’s position on the ground (definitely not MLAT) and clearly marking it as ADSB - then it is fair to assume it was transmitting?

[Admin]

I can definitely try and ask, however, as a flight tracking app was definitely showing it’s position on the ground (definitely not MLAT) and clearly marking it as ADSB - then it is fair to assume it was transmitting?

Yes if it was not moving
Do you have the icao of the se2 devices, and date/time of flight?
Thx
Lee

[a.alexeev.p]

Thanks Lee! Yes sure:)

40632D around 14:30-15:00 UTC on 17th April 2023

[exfirepro]

Edited

Alex,

That aircraft reports as a Cessna 172 out of Goodwood.

I don't of course have the full picture, but looking at PilotAware Replay and the PilotAware Database for the date in question, the aircraft was reporting what appears to be a very weak Mode S signal (via MLAT) around the south coast in the Goodwood / Selsey area from about 9:15am - initially operating what appear to be circuits out of Goodwood until about 09:30, but with pretty erratic position reporting. The weak MLAT reports on our network were to be fair probably partly attributable to the aircraft operating at very low level (less than 1000ft). It then appears again from about 10:30 to 11:30 - again as an extremely poor Mode-S (MLAT) signal, but with very occasional bursts of ADSB 18 - probably from a poorly sited CAP1391 device. It next appears from about 12:15 to 12:30 - again as a very irregular CAP1391 ADSB 18 Only signal between Goodwood, Selsey and Hayling Island, before suddenly switching from CAP1391 ADSB Only back to Mode-S again (as if the transponder was suddenly switched on) and thereafter showing a poor mix of ADSB / Mode-S MLAT back to Goodwood - again with extremely erratic position reporting (at low level) and lots of gaps. There is then a single 'ping' at 14:06 at 375ft (at Goodwood - presumably in the circuit or on the ground), followed by a re-appearance as a mixture of CAP1391 ADSB and MLAT in the Goodwood /Selsey/Bognor Regis area from 15:40 to 16:15, after which it seems to be operating as purely Mode-S at a fairly consistent altitude of between 750 and 950ft until it returns to Goodwood at around 16:50.

The erratic nature of the Mode-S reports would mean that PilotAware would keep changing between known-position (when the MLAT is reliable) and Bearingless (when it is not) and the aircraft would be reported as such on SkyDemon (provided you had 'Show Bearingless Targets' enabled). This would certainly explain (at least as far as I can tell from what you have sent us) what you were experiencing - i.e. intermittent known position reports from the poor MLAT/CAP1391 interspersed with random intermittent Bearingless target reports from the Mode-S - NOT the CAP1391.

It's certainly one of the most erratic reports I have ever looked at. If it was my aircraft I would be extremely concerned about the poor quality signals which appear to be being transmitted from it - with ADSB CAP1391 (perhaps understandably) only reporting out to less than 10Km, but my greater worry is the extremely poor quality of the MLAT reports which may well indicate a problem with the aircraft's Mode-S transponder.

Best Regards

Peter
p.s. you can check this for yourself at https://playback.pilotaware.com/playback/groundstations/ by selecting the Transmission Type, putting in the ICAO code and Start Time (3 hour segments max, though shorter periods give a more detailed expanded report) and clicking on 'Search'.

[Admin]

Thanks Lee! Yes sure:)

40632D around 14:30-15:00 UTC on 17th April 2023

Take a look at this icao code using vector
https://www.pilotaware.com/analysis/vector
Be sure to disable ALL days except 17/4

You will notice the performance is not great, and this is base upon a 12db antenna on the ground stations. A 2db / 3db gain antenna airborne will be considerably worse

Thx
Lee

[a.alexeev.p]

Edited

Alex,

That aircraft reports as a Cessna 172 out of Goodwood.

I don't of course have the full picture, but looking at PilotAware Replay and the PilotAware Database for the date in question, the aircraft was reporting what appears to be a very weak Mode S signal (via MLAT) around the south coast in the Goodwood / Selsey area from about 9:15am - initially operating what appear to be circuits out of Goodwood until about 09:30, but with pretty erratic position reporting. The weak MLAT reports on our network were to be fair probably partly attributable to the aircraft operating at very low level (less than 1000ft). It then appears again from about 10:30 to 11:30 - again as an extremely poor Mode-S (MLAT) signal, but with very occasional bursts of ADSB 18 - probably from a poorly sited CAP1391 device. It next appears from about 12:15 to 12:30 - again as a very irregular CAP1391 ADSB 18 Only signal between Goodwood, Selsey and Hayling Island, before suddenly switching from CAP1391 ADSB Only back to Mode-S again (as if the transponder was suddenly switched on) and thereafter showing a poor mix of ADSB / Mode-S MLAT back to Goodwood - again with extremely erratic position reporting (at low level) and lots of gaps. There is then a single 'ping' at 14:06 at 375ft (at Goodwood - presumably in the circuit or on the ground), followed by a re-appearance as a mixture of CAP1391 ADSB and MLAT in the Goodwood /Selsey/Bognor Regis area from 15:40 to 16:15, after which it seems to be operating as purely Mode-S at a fairly consistent altitude of between 750 and 950ft until it returns to Goodwood at around 16:50.

The erratic nature of the Mode-S reports would mean that PilotAware would keep changing between known-position (when the MLAT is reliable) and Bearingless (when it is not) and the aircraft would be reported as such on SkyDemon (provided you had 'Show Bearingless Targets' enabled). This would certainly explain (at least as far as I can tell from what you have sent us) what you were experiencing - i.e. intermittent known position reports from the poor MLAT/CAP1391 interspersed with random intermittent Bearingless target reports from the Mode-S - NOT the CAP1391.

It's certainly one of the most erratic reports I have ever looked at. If it was my aircraft I would be extremely concerned about the poor quality signals which appear to be being transmitted from it - with ADSB CAP1391 (perhaps understandably) only reporting out to less than 10Km, but my greater worry is the extremely poor quality of the MLAT reports which may well indicate a problem with the aircraft's Mode-S transponder.

Best Regards

Peter
p.s. you can check this for yourself at https://playback.pilotaware.com/playback/groundstations/ by selecting the Transmission Type, putting in the ICAO code and Start Time (3 hour segments max, though shorter periods give a more detailed expanded report) and clicking on 'Search'.

Thank you so so much! All super interesting and very details and triggered lots of thoughts, so might not be a short answer from myself either as some points are carry over from the original post idea and some completely new

1) The erratic behaviour you shared – does that mean that if I have mode-s 3D enabled – I am fairly likely to be displayed with similar output in my PAW? If so – I must admit I would probably go for disabling the 3D MLAT as got me thinking that I might be wrongly looking for a plane in a certain direction, so rather look all around due to a bearing less notification.

2) I in fact flew today and experienced a situation when an aircraft was first showing as bearlingless target and then next minute as a specific position. My Mode S 3D was disabled then, so in such circumstances it would totally make sense that possibly that other aircraft had some portable device that was visible sometimes to my PAW but not always. See first screenshot attached.

3) I have to point out that original question is unfortunately still remains a bit of a mystery because it was not really about MLAT or anything. I know for sure that they left a CAP1391 on by mistake in a parked and completely shut down aircraft and I was only 50m away from it, but still saw it as bearing less through PAW (second screenshot).

[a.alexeev.p]

Also found the following log line:

$PALOG,20230417,152910,PAWRT,40632D,1,A,1,C,1,S,0,ES,0,P,0,F,0,U,0,M,00,valid,00,vers,00,nacp,00,nica,00,sil,00,sils,*07

which sounds like a signal was decoded as Mode AC only? As it was on the ground with engine off for a long time – there is no doubt on my side that that all standard electrics including normal onboard transponder were completely off at the time...