Mode C and S detection - SkyDemon oddity...or not

[exfirepro]

Well Rob, didn't you open a can of worms here!

Thank you for checking your PilotAware QNH against your aircraft / local tower as I requested. As Lee has already said - your test (within 1mB - i.e. within 30ft of the reported 'Tower' QNH) is well within tolerance for PilotAware - or any other pressure based setup. I would therefore certainly not be concerned about your Baro sensor. I had already considered the 'known' issue whereby the current version of SkyDemon can misrepresent relative altitude due to a conversion issue between height reported in metres and feet (which by the way has been corrected in the latest Beta version of SkyDemon, which I tested this morning and which should be released later this week) and had ruled this out as a likely cause as the reported relative altitude difference is usually far greater then the 400ft you originally reported.

If you can as suggested provide Lee with your track log and both aircraft's registrations, he will be able to check this for you. Otherwise I suggest updating SkyDemon as soon as the new release comes out (your device will notify you) and let us know if the problem persists after that. If so I would suspect a defect with the other aircraft's pressure reporting or as Lee and I have both suggested, that the aircraft displaying on your screen was NOT the one you were observing.

Regards

Peter

[RobertPBham]

Hi Lee,

Sorry - I thought when you mentioned there should be no discrepancy that it was faulty - I didn't think there was a big issue with 1mb. :-)

I'm happy to supply the track file - the aircraft I was following was G-WARU - it was a mode C or S target as it was bearing less.

Who should I send the track file to?

The version of SkyDemon I have is the latest - from the Apple App Store - not sure if this one has the bug or not.

I'm not too worried as you mentioned it could be an issue with the other aircraft reporting height but just thought I'd mention it in case I was interpreting things incorrectly.

Thanks
Rob

[RobertPBham]

And sorry for opening a can of worms! :-)

This was my first real chance to test and see the bearing less target alerts in action - following another club aircraft at a safe distance and seeing the information appear. I've checked the track file using the Java app that you can download from the site - I cannot see G-WARU registration in the track file - I've been through virtually every listed plane using the app and none are closer than 7KM (I may have missed a plane).

Thanks
Rob

[Admin]

Hi Robert
Dont forget bearingless have no coordinates, hence no range, so nothing to show ini the post processor.
If you give me access to the trk file and an approx time I may be able to work it out

Thx
Lee

[RobertPBham]

That explains it - can I email it to you? It's 7MB - compressed using 7Zip is 603KB.

I can approximate the time - it will be 1 hour 15 minutes in and I'm sure we were following G-WARU for a good 30 to 45 minutes. The plane I was flying was G-GYTO.

I've just opened the track file in Notepad++ and still cannot find WARU call sign listed - odd as when I saw the alert on SkyDemon I know that's the call sign list - I mentioned it to people in the plane - one of them was taking photos of it while we followed and said it's definitely WARU with the zoom lense in action.

Thanks
Rob

[exfirepro]

Hi again Rob,

I was only joking about the can of worms.  I thought you told me your flight was uneventful!! Not so the post-flight analysis it would seem. All part of the learning curve. Lee will no doubt be able to use his 'magic' to get to the bottom of things and let us know what he finds.

Best regards

Peter

[RobertPBham]

Hi Peter,

I know you were joking! :-)

I've had a bit more of a nosey at the track file in Notepad++ and am guessing that you are listing the Mode C and S targets via Hex ID rather than call sign? I converted the clubs aircraft call signs to Hex ID's and can find many occurrences within the file.

I have no idea what the other numbers mean though on each line but interesting all the same - I like to investigate things. The hex ID occurrences appear towards the middle to end of the file so would tally roughly with the times I mentioned and the detections.

Thanks
Rob

[Admin]

Hi Robert,

Can you tell me the settings you have selected for
Mode-C/S
- Separation
- Detect
- Select

Initial findings, PAW started running at 10:13:08 (Zulu Time)
so 1:15:00 into flight would be about 11:28:00

at 11:32:01 I see the following
$PFLAA,3,1500,,741,1,,,,,,0*72
this indicates an A/C 741 metres above - 2431 ft, in SD this should have been shown as 2.4

then at 11:32:49
$PFLAA,3,1500,,-1,1,,,,,,0*5C
this indicates an A/C 1 metre below !

This repeats for 10 seconds, very close, or at your height
$PFLAA,3,1500,,-1,1,,,,,,0*5C
$PFLAA,3,1500,,-2,1,,,,,,0*5F
$PFLAA,3,1500,,-3,1,,,,,,0*5E
$PFLAA,3,1500,,-4,1,,,,,,0*59
$PFLAA,3,1500,,-3,1,,,,,,0*5E
$PFLAA,3,1500,,-2,1,,,,,,0*5F
$PFLAA,3,1500,,0,1,,,,,,0*70
$PFLAA,3,1500,,1,1,,,,,,0*71
$PFLAA,3,1500,,2,1,,,,,,0*72
$PFLAA,3,1500,,5,1,,,,,,0*75

this should have shown in SD as 0.0

more importantly, it sounds like you are receiving your own transponder, which is why I wanted the settings above, this repeats a few times whilst I presume your transponder is interrogated.

I then see an A/C at 11:42:09
$PFLAA,3,1500,,-355,1,,,,,,0*5E
1100ft below

I then see an A/C at 12:03:02
$PFLAA,1,4500,,283,1,,,,,,0*7E
928ft above

There are lots more contacts, but this looks like the period you were referring to.


Thx
Lee

[Keithvinning]

Wellesbourne QNH was 1008Mb as reported by the tower.
PAW QNH was 1009mb, QNE was 265ft and GNSS was 154ft.

Hi Robert et Al

Sorry for getting to this thread a bit late but my internet has been out for 3 weeks thanks to a thunderstorm and a VDSL modem that took a lighting strike.

So I was down at Wellesbourn today and have loaned them a PilotAware unit so that Jon in the tower can set up a virtual radar for you good PilotAware users when you are in cct. This will also allow you to take photos of your transmissions if you want to convert Mode S to ADS-B and present them to CAA.

However, the control tower is made up of two shipping containers stacked on top of each other with a pod on the top is at a rough estimate 20-25ft high. The difference between 1008 and 1009 Mb is 27.4ft so I think that we can discount the inaccuracy of the barograph.

Ken Richardson and Alan Brame? have both got PAW so we should get a little cluster going at Wellesbourne.

Robert I guess that when you are in circuit you should let Jon Mike, or Frankie know that you are Downwind PilotAware so that they get used to using the facility.

That would be great if the standard at all provincial and private airfields and airstrips became G-ABCD Downwind - PilotAware. Then you would know that you are being seen.

At the moment Wellesbourn, Tatenhill and Damyns Hall have set up or are in the process of setting up ground stations for this facility. Do you think thesis a good idea?

Cheers

Keith
 

[RobertPBham]

Hi Lee,

Thanks for looking at the track file - interesting findings!

I've checked the settings for Mode-C/S and they are as follows:
Display - Req-ID
Separation - +/- 4000ft
Detect - Medium Range
Select - Mode-CS(Beta)

I did originally think that the alert could be my plane as I thought I may have forgotten to change the Hex-ID within PilotAware - however, I checked this and it was set correctly for G-GYTO (40438D).

It is definitely possible that G-WARO was at a similar height to me before I spotted them when they were above - we were all following a similar course! I'm guessing that the Mode C and S targets aren't saving there call signs in the track file. I'm certain I saw G-WARU listed and a -0.2 on SkyDemon.

Looking at all the details you have found it appears that it could be just the SkyDemon bug with translating heights from metres to feet.

I'm happy to monitor once the new version is released. This has been the only opportunity I've really had to follow a plane and 'see' what happens with PilotAware etc.

Hi Keith,

Great news that Wellesbourne now have PilotAware - lets hope the airfield stays open!! Very interesting on the QNH - I always assumed it was taken from ground level but that would definitely explain the difference!

More than happy to let Jon, Mike and Frankie know I'm about with PilotAware - great to get them used to it - not sure on changing the phraseology for downwind though - completely understand the logic but it may add some confusion to anyone who doesn't know what it is - they will be thinking the Pilot is Aware of what?? Just my thoughts anyway!

Thanks
Rob

[Admin]

Hi Rob

Aha!
OK, if you have a transponder, you should select the option with FILTER, this will attempt to remove your mode C return

Thx
Lee

[RobertPBham]

Hi Lee,

Now set to Mode-CS+Filter(Beta). I might be lucky as all the planes I fly have a transponder!

That may explain a few things with what's being reported in the logs! I'm hopefully flying on Friday, so will report back any further details although waiting for the new version of SkyDemon may be more sensible! I bet a won't be lucky enough to be following a plane! :-)

Thanks
Rob

[Sean McDonald]

I think Derby are also setting up a ground station?

[grahambaker]

I hope all these people are going to set them to 'Rx' only, the last thing we need are spurious traffic warnings springing up.

I wonder what software they will be using to display traffic, as it seems a bit clunky to use a full blown nav package in flight mode.

[exfirepro]

I hope all these people are going to set them to 'Rx' only, the last thing we need are spurious traffic warnings springing up.

I was thinking that myself, Graham. The last thing you need on short final is a sudden unexpected danger warning from the tower  (though in reality you would have 'seen' their PAW from several miles out).

Regards

Peter