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

[RobertPBham]

Hi all,

Just got back from a fly out this weekend to Jersey with the club I am with - first trip across water and overseas and it was amazing.

I'm sure your not interested in my trip ramblings but spotted the following when I was flying over the water to Jersey - five aircraft made the trip and as I got over the water a bearing less alert appeared on SkyDemon - it was picking up one of the club planes. I spotted the reported aircraft - approximately 1/2 KM ahead and about 200 feet above. SkyDemon was reporting the aircraft as -0.2 - now I thought this meant they were below us and originally that's where my scan started. Does the -0.2 mean distance from my aircraft or is it height - it it's height is there any reason it was reported below (in my understanding) or am I understanding the -0.2 incorrectly and it means that my aircraft is 200 feet below the reported target.

Thanks
Rob

[exfirepro]

Hi Rob,

Well done... I bet you enjoyed your trip. I well remember my first channel crossing. I left Headcorn with a colleague, but he took off like a scalded cat heading for Dover, so I just dawdled along on my own at 5,500ft casually taking photos of ships and the view. Had I realised that one of my mags had accidentally got switched off (long story), I might have been more concerned, but flying from East Fortune we get used to flying over water so wasn't concerned 'til my engine eventually gave out on final into Abbeville - very embarrassing when I realised why!

The figure attached to the bearingless target ring denotes the relative altitude of the other aircraft to your own, so should as you say have been showing as +0.2 if the other aircraft was definitely 200ft above you. It is possible for the reported relative altitude to vary slightly within the tolerances of the technology, but 400ft out seems excessive. Is it possible that the alert you saw was coming from one of the other aircraft in your group - perhaps below you and behind rather than the one you saw in front and above you?

Regards

Peter

[RobertPBham]

Hi exfirepro,

Your trip sounds very interesting - mine was no where near as exciting! It was a great experience though - stopped off at Alderney on the way back - I was a passenger on the way back so managed to get some great photos.

I'm sure it was the club aircraft ahead and nothing behind - we were the last two across the water (us being the last plane). I tracked G-RU for a good 20 to 30 minutes and the alerts displayed the call sign and then the height (as I remember it). The alerts varied from -0.2 to -0.8 - there attitude seemed very stable to ours so not varying very much in that time. I've checked the SkyDemon log and my altitude was between 2200 and 2400 for the time we were behind G-RU - they were always above and ahead of us - at approximately 1 o'clock. We were in contact with various control zones and had no other reports of traffic behind and below - especially that close.

There's a possibility the PilotAware antenna was at an angle in my bag - I don't think this should effect the reading but just thought I'd mention it.

Thanks
Rob

[exfirepro]

Hi again Rob,

No, the angle of your PilotAware antenna(s) might have had an effect on signal strength, but shouldn't have any effect on the reported altitude.

Judging the altitude of another aircraft relative to your own in flight is always difficult, however if as you say the bearingless target ring was displaying -0.2 to -0.8 with the aircraft always ~200ft above you as you saw it, this would mean a reporting error of between 400 and 1,000 feet - which I would certainly not consider to be within tolerances. It's unlikely, but I do know of one case of a faulty barometric sensor on a PAW bridge, which could certainly lead to incorrect altitude reporting. Next time you are flying, check the barometric pressure reported on your PilotAware Home Screen against the actual QNH setting from your own altimeter before you take off. If there is a significant variation please let us know. As I say, not likely, but possible and worth checking.

Regards

Peter

[RobertPBham]

Hi Peter,

I'll definitely check that and report back.

One uncertainty - the day before I had a flight and had set the incorrect QNH - SkyDemon was reporting the correct altitude - I know this as it alerted me to airspace above at the same time a friendly controller did! Does SkyDemon use the barometric sensor from PAW? If so, I'm sure the sensor is fine.

Also the SkyDemon log from from flight to Jersey matches the altitudes I was flying - all assuming it's using the PAW sensor for this though.

Thanks
Rob

[Paul_Sengupta]

I believe Sky Demon shows the GPS altitude.

It was my baro sensor which is faulty, but it shows about 5-6000ft out! However, with suction around the cabin, the pressure inside the cabin can be lower than static, and this will cause the baro sensor to think you're higher than you actually are. A couple of people have reported this sort of thing now. I suggested to Lee that a "calibration" setting be put in the menu to adjust accordingly. It could even suggest a value based on the GPS altitude (best to do while receiving EGNOS).

[grahambaker]

I would advise against using GPS altitude to modify barometric.

The further away from ISA temperatures, and the higher you go, the further they will be apart. A manual method of syncing the baro chip to the QFE on the ground would be better.

[Admin]

Just to clear up some possible confusion here:-

Altitude used in SkyDemon is the GPS altitude, whether you are using Location Services or PilotAware.
This is all derived from the GPS/NMEA data.

Traffic data that is passed to SkyDemon (and others) contains relative height differences.
In the case of ADS-B, Mode-C and Mode-S traffic data, the relative height difference is calculated using Barometric Pressure.
In the case of Mode-C this is only accurate to the closest 100ft, I recall in Mode-S/ADS-B it can be a higher resolution of 25ft

The original discrepency (if correct) was a 400ft discrepancy - which is pretty large.
First thing to check is the reported QNH on the PilotAware homepage, this will report both the Pressure and the calculated
altitude, please ensure these reported values are correct, if they are, then the issue is either the other aircraft pressure sensor, or this was not the aircraft reported.

One final mention, is that there was a fix in SkyDemon (last 2 weeks) which resulted in miscalculations for Mode-C/S traffic due to a metres to ft conversion

Final word on this, PilotAware to PilotAware traffic uses GPS altitude comparison, which seems to be a more accurate and finer resolution than Barometric.

Thx
Lee

[Paul_Sengupta]

A manual method of syncing the baro chip to the QFE on the ground would be better.

This wouldn't help as the problem is the dynamic pressure within the cabin when the aircraft is at cruise speed.

[RobertPBham]

I've just had chance to pop in to Wellesbourne and jump in to a plane - no flying so all of this was tested on the ground.

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

On the ground there doesn't appear to be a major discrepancy - not sure how this translates to in the air though. I would guess the pressure could be quite different inside the cockpit to outside and hence the possible discrepancy?

If I get some aircraft time I'll try and see what the reported details are in the air.

Thanks
Rob

[Admin]

On the ground there doesn't appear to be a major discrepancy - not sure how this translates to in the air though. I would guess the pressure could be quite different inside the cockpit to outside and hence the possible discrepancy?

Hi Rob,
is it a pressurized cabin ?
If not there should be no discrepency.
Thx
Lee

[Paul_Sengupta]

If not there should be no discrepency.

Many enclosed aeroplanes have a slightly lower cabin pressure due to the suction effect of the airflow around the fuselage.

[RobertPBham]

Hi Lee,

It's not a pressurised cabin - the test was done on the ground with the door open on a stationary PA28. Only using the plane to get the QNH from the tower.

Sounds like I may have a faulty sensor if that is the case!

Thanks
Rob

[Admin]

Sounds like I may have a faulty sensor if that is the case!

Hi Robert,
Hmm, this looks pretty accurate to me

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

What makes you think it is faulty ?

my comment

If not there should be no discrepency.

Was referring to the fact that either the A/C you saw was not the A/C reported by PAW
or the plane you saw was not returning the correct ALT
or the version of SkyDemon has the bug I mentioned earlier

As Peter mentioned earlier, the Track files will tell us diffinitively, we are just postulating at the moment.
(Edit: that was another thread, doh, are you able to supply the track file from your flight ?)

Finally, was the discrepency you observed an ADS-B, ModeC or ModeS return ?
and do you possibly know the reg of the A/C you had a visual, we can check this against the track file

Thx
Lee

[grahambaker]

A manual method of syncing the baro chip to the QFE on the ground would be better.

This wouldn't help as the problem is the dynamic pressure within the cabin when the aircraft is at cruise speed.

Accepting that every individual aircraft could be slightly different, but every aircraft I've flown I flip to alt-static just to see the effect on altitude and airspeed, so I know what to expect if I have to do it for real. I don't think I've ever seen anything greater than 1 or 2 mb, say 50' in altitude, i.e well inside the tolerance for an altimeter anyway.