Barometric Pressure Way-off ?

[Easy Cruising]

Maybe I don't understand correctly, but my baro pressure on the PAW was 1012mb when the area info was giving me 1021mb. Seems like a big difference. Does anyone know if this is expected to happen, or should it be about the same ?

[efrenken]

Area info is QNH, while PAW is giving you QFE.

Regards

Eric

[Easy Cruising]

Thanks for the reply but on the PAW status page it says QNH 1012, whilst area gives QNH 1021. I'm wondering if my PAW pressure sensor is broken. What am I missing ?

[efrenken]

Maybe I'm wrong. It just came to my mind that PAW might be able to calculate QNH from barometric pressure and GPS altitude.

Can one of the pros throw in his thoughts, please?

Thank you and regards

Eric

[exfirepro]

Hi EC / Eric,

Sorry for the delay. I did start answering this thread this morning but had to leave before completing my reply. Not sure I would count myself as a 'pro' by the way, but here goes anyway.

PAW reports barometric QNH on the home screen, which it then uses to compare the altitude of your aircraft with other aircraft using 'barometrically derived QNH', namely ADSB, Mode 'C' or Mode 'S' aircraft. The altitude for the P3i side of PAW is GPS derived. This dates to before a barometric sensor was incorporated into the PilotAware system, and although GPS derived altitude is technically less accurate than barometric, this decision ensured that all P3i aircraft are compared to each other using the same reference, which is the important point.

EC, when you say 'my baro pressure on the PAW was 1012mb when the area info was giving me 1021mb' I presume you are talking about the Regional Pressure Setting? This can often be considerably different to your local QNH as it is the lowest forecast QNH across a fairly wide region, whereas the PAW barometrically derived QNH will always be local. If making a comparison, you need to compare your PAW QNH against a reliable local source, such as your airfield tower weather system, or an aircraft on the ground which has recently had its altimeter checked against such a reference source.

As separation distances at the levels we operate are rarely critical, it is acceptable for there to be a slight variation in barometric QNH. From memory, although we have had one or two baro units which were faulty, I can recall very few. Please feel free to recheck yours against a reliable QNH source and let us know if there is still a significant variation.

Regards

Peter

[Easy Cruising]

Thanks exfirepro for your answer. You are correct that I was referencing the area QNH. When I next fly I'll check the airfield actual QNH with what PAW gives me. The difference I noticed still seems a bit too big but will see. Thx.

[Admin]

Hi All
Sorry for late response on this, let me try to shed some light on this.
If my memory serves correctly it goes like this

The sensor provides an altitude based upon standard pressure 1013.25mb
I compare this altitude to the GPS altitude, and calculate the QNH, based on the difference

The real test would be to set your altimeter to 1013.25, and read off the altitude, assuming your altimeter is calibrated, this should be within 100ft of the reported sensor altitude

Now I must say the sensors we have tested have all been pretty accurate, excluding 3 known failures which had wild results. The sensor itself has a calibration trim for fine tuning and drift over time. We are not using that currently, but it is a software fix to add that feature.

Firstly please compare to the altimiter to see if it is out of calibration

Thx
Lee

[Paul_Sengupta]

GPS altitude can vary wildly...it does here. The calculated QNH will vary with it. For best results make sure you have a good clear view to the south so that you receive the EGNOS satellites. You can tell it has, when on the web page the GPS fix is listed as "DGPS fix".

excluding 3 known failures which had wild results

I still have my "wild" one, I'm currently using it as my ground station. Did you ever get any traffic from be over the internet by the way?

[Admin]

Hi Graham,

I think what is in question here, is the accuracy of the value in the QNH field.
As I mentioned this is derived by comparing barometric vs gps altitude

Thx
Lee

[grahambaker]

If you are repying to my post, I deleted it! (not least because I went back into the PAW config screen a few secons later and the field had disappeared, or at least I couldn't find it again ).

[exfirepro]

Graham,

Didn't see your post, but if you were talking about the QNH box on the 'Configure' page, it disappears once PAW gets GPS lock and sets the QNH automatically (on the 'Home' screen).

Peter

[grahambaker]

Graham,

Didn't see your post, but if you were talking about the QNH box on the 'Configure' page, it disappears once PAW gets GPS lock and sets the QNH automatically (on the 'Home' screen).

Peter

That explains it. Thanks.

[Easy Cruising]

Thanks to all. From this I get that I should check the PAW QNH against my alt instruments set to 1013mb (I have an analog guague and a SkyView). That should tell whether the paw sensor is about ok. Then, GPS altitude can be the issue. My paw is on the dash with no major hinderances, so I expect the GPS to be getting the DGPS fix all the time. When I noticed the issue I was 6000ft on a nice day (in Germany), so I expect the gps reception was good. Can't remember whether PAW shows gps alt (is that the gnss reading?), but if it does I'll compare to the SkyView gps alt (antenna on dash also). I never normally get gps drop-outs. Will report back.

[Admin]

On the home page, paw reports
Barometric altitude
Gps altitude

You need to set your pressure setting to 1013
Read the altitude
Compare to pressure altitude

Thx
Lee

[The Westmorland Flyer]

I'm just getting back into PAW after a rather lengthy hiatus. I have the new bridge and it's working as far as I can tell (after six days not a single P3i transmission received, despite an external antenna out in the clear, but then I am in the far north!).

I am lucky to know my exact altitude (705ft) and I also have a calibrated barometer that is corrected to QNH on site. The GPS reported GNSS altitude does, of course, vary but it is typically within 20ft of actual (less than 1mb error). Correcting for that as necessary, PAW is consistently reporting my calculated QNH as around 2.5mb higher than actual, i.e. around 70ft out. This seems to be a barometer reporting error.

I realise that 70ft is nothing to worry about in practice but I just wonder what sort of nominal accuracy I should be expecting from the bridge barometer? The barometers on the old Harkwood ARF boards, which I still have, were both accurate to significantly better than 1mb.