Feedback: PAW pressure and Mode C/S

[Easy Cruising]

I am posting some feedback from yesterday's flying experiences around Stuttgart area, with PAW in a non-pressurized light aircraft. This is a new posting but relates to my previous post "Barometric Pressure Way-off ?".



Barometric Pressure on Ground
-----------------------------
Yesterday on the ground at my airfield (in AIP the altitude is quoted as 2227ft) I set the Dynon Skyview and Winter TSO analogue altimeter to the AIP airfield altitude, then checked the pressure on PAW :

PAW : 1037mb (picture 1)
Winter: 1038/1039mb (picture 2)
Dynon: 1038mb (picture 3)
Tower : 1038mb 

.. see the first 3 pictures attached. so it looks like all were in agreement quite closely. Also note that PAW had 10 satellites/dgps/3d even on the ground, so very good, and GNSS=2253ft which is close to the airfield altitude of 2227ft.

Negative Altitude of other aircraft
-----------------------------------
On the ground a twin then taxied right past my nose. It appeared with a tailnumber and 'circle' on SkyDemon, and a red, square notification box on paw radar, but both showed a relative altitude to me of -1.6, which confuses me somewhat. 

PAW pressure during flight
--------------------------
Climbing to 5500ft on the Dynon 1038mb and approx 40nm from the home airfield, regional Info also quoted regional QNH as 1038, same as home airfield. Dynon gps altitude 5391ft. Looking at PAW it showed GNSS=5396ft ,very close to the Skyview GPS altitude, with 'GPS fix SPS',  11 satellites and 3d. However, paw now showed QNH 1032mb. (Picture 4)

Phantom Airliner
----------------
Proceeding south-west of Stuttgart at 3600ft msl, I got a 'circle' on SkyDemon (and box on paw radar, no adsb) showing a relative altitude of -3 . That would put it below ground level. The associated tail-number shown was TC-NBB, which according to flightradar (https://www.flightradar24.com/data/aircraft/tc-nbb) is an Airbus A320. There's no way it could have been below me or anywhere near. After a minute or so, the circle disappeared.

Proceeding southwest another 30nm, I got another 'below ground' signal (circle on SD, box on paw radar, no adsb) , different tail number .. cant remember but wasnt a . Signal disappeared again soon. This time I was really far away from any controlled airspace or any major airfield.

Generally, this situation with 'phantom' signals has happened several times on other flights, always when flying within about 2000ft of the ground.

Flying Config
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Generally, PAW picks-up adsb aircraft really well, also small aircraft with adsb, of which there are many in my area. The in-flight Mode S/C signals are also very accurate , and I can usually spot the target, which makes the 'phantoms' even harder to explain. I have never  spotted any aircraft when the 'phantom' signal appeared.


As a consequence of the above, I think the barometric pressure is not way-off at all as I had originally thought. However, I seem to get spurious 'below ground level' Mode S/C signals which I cannot explain. If anyone on paw team can suggest what might be the issue, I'd appreciate it.

[grahambaker]

I would prefer it if the Status screen didn't use the term 'QNH' for the GPS derived value, as clearly seen here is the difference at altitude between the real QNH and the GPS derived value. It just confuses people when they see a value that differs from what they think it should be.

In this case the difference between indicated altitude and the GPS altitude and a bit of temperature compensation explains the difference between the two, and the higher you go and the further the conditions are from ISA, the greater that difference will be.

Perhaps the label should be 'GPS calc QNH' to at least remind people that it is a synthetic value.

[Admin]

Graham, good point, dont mind re labeling this.
Its really used for testing to lookup the qnh and compare against 1013 reference

EC, please take a look at this thread
http://forum.pilotaware.com/index.php/topic,693.msg8982.html#msg8982
I dont believe in ghosts or phantoms!
If you want me to investigate your tracks, very happy to do so.
In paw RADAR the altitude difference is relative, not absolute. For SkyDemon shoul also be relative assuming you are upto date - there was a bug in an earlier version of SkyDemon
Thx
Lee

[Easy Cruising]

My SkyDemon and PAW software are on latest versions. I read the other thread. I realize that the 'number' shown by PAW for Mode S traffic is a relative altitude in thousands of feet. But then why did the aircraft on the ground in front show -1.6 when all qnh readings were in agreement? Is it because the other aircraft is transmitting it that way ?
 
I'm only really interested here in knowing how far I can trust the relative altitudes for non adsb traffic. Usually it seems quite good from visual observation.

Concerning looking at logs: good idea,thanks, I'll get them next time I go to the airfield.

Concerning the airliner I mentioned, it was definitely in the area, going to Stuttgart, so could well have been 3.6 above me (although I'm fairly sure it read -3.6 on paw), but I'm unsure why I got a 'circle' alert and not an adsb alert, because that aircraft shows full adsb tracks on flightradar.

[The Westmorland Flyer]

Perhaps the label should be 'GPS calc QNH' to at least remind people that it is a synthetic value.

Yes, I think this makes a lot of sense. Even with DGPS the GPS derived altitude wanders around too much to give a reliable QNH accurate to the nearest mb.

[exfirepro]

Morning EC,

Just had a brief read here as I am desperately trying to get out flying, but during all my PAW flight testing I can't recall having any serious concerns about the reported vs visible relative altitude for bearingless targets - at least at 'Microlight' altitudes (up to 10,000 ft fairly regularly in my case).

I realise this is 'subjective' and will re-read and digest all the data later, but just thought you would like to know my experiences.

Regards

Peter

p.s. I would definitely suggest taking Lee up on his offer to have a look at your PAW Track Logs. If the logs are large I generally put them in a dropbox and PM him a link. Sorry if that sounds like trying to teach Granny to suck eggs.

[Easy Cruising]

Will do. Thx.

[Paul_Sengupta]

I am desperately trying to get out flying

Grumble...

http://www.bbc.co.uk/news/uk-38356112

Anyway, I can't really comment on the aircraft which taxied past, I would guess that it was transmitting incorrectly - perhaps it didn't have Mode C selected before it got to the runway.

As for airliners, something I've noticed is that if the signal strength is way down, just at the limits of reception, you may get an altitude of "0" MSL (or FL 0!) for it - I guess this is just that it hasn't been received properly. What do you have your Mode C/S sensitivity set to? You may be displaying things at the edge of reception. I have mine set to short range. At the edges of reception it may also not get an ADS-B position for the airliner, though not all airliners have ADS-B...in the UK notably FlyBe don't have it.

[exfirepro]

Paul,

Good points, I hadn't thought of that.

Peter

p.s. I did get flying BTW, though just a quick 45 mins before dark. Really need to get lights in my instruments if I want to fly any later (only joking). Strobes look good at that time of day though.

[Easy Cruising]

My Mode C/S is set to medium sensitivity. Will get back with logs when I can get them.