PilotAware
British Forum => General Discussion => Topic started by: Easy Cruising on December 10, 2016, 04:22:05 pm
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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 ?
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Area info is QNH, while PAW is giving you QFE.
Regards
Eric
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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 ?
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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
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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
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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.
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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
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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?
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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
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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 :-\).
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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
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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.
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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.
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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
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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.
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The barometers on the old Harkwood ARF boards, which I still have, were both accurate to significantly better than 1mb.
I used MPL3115A2 modules from NXP, I presume the Bridge uses the same, they are very accurate. Unless you vapour phase solder them, then some of the fluid can condensate inside the sensor and may cause issues down the line.
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Superficially it does look like the same barometric sensor and I note that the specified accuracy is circa 1ft, so this one being around 70ft out is something of a surprise.
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fyi .. I created a new post called "Feedback: PAW pressure and Mode C/S" , to give feedback after testing the pressure as suggested in this post.
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My PAW has now been on for ten days just sitting at home watching the world fly by. Interestingly, the pressure sensor is slowly drifting into spec - it's now just about 1mb out, which is insignificant for the intended application.
Still yet to receive a single P3i transmission but PAW is thin on the ground as yet, up here in the frozen north.
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John,
You should try a flight up my way (East Fortune) we now have a small but solid core (and growing) of aircraft 'PAW operational' here and also a few out of Fife (Glenrothes) and Perth. Let me know if/when you can visit and I'll arrange to meet up.
Regards
Peter
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Hi Peter,
I've flown past East Fortune numerous times, en-route Fife/Perth/Dundee/other places north, but never actually landed there. I should do that some time!
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Ahha, broken my duck! Something with PAW flew past a while ago and I picked up 79 P3I transmissions. Wonder who it was?
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The log will tell you
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Yes, I thought about that. I need to RTFM to find out how all that works. I guess I needed to enable some of the Log Interface Messages options in config but I hadn't done that, as most are too cryptic for me to immediately grasp what they do. The settings don't seem to save either. I'd better go and look out that manual... there's only so much one can discover by doing the blokey thing of not reading it.
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John,
It's not critical to preset anything. Just plug a USB stick into your PAW while it is running. If all your USBs are being used you can unplug anything except the WiFi (PAW doesn't like it if you unplug the WiFi when it's running). Also don't use any port you have configured for ADSB out or FLARM in - or PAW can do things you might not like with any data on your stick.
Log in to 192.168.1.1. Go to the tracks page, Sync tracks you want (or all) onto your stick (PAW automatically sets up a PilotAware folder with a Tracks folder inside it). This only takes seconds - indicated by the progress bar moving across the screen. Remove your stick and transfer the track files to a PC or laptop. You can then open and read them with Notepad or Wordpad.
Look for entries at or near the time you had the contact. IIRC P3i entries will have the callsign enclosed in # 'brackets' unless the Group ID has been changed from the standard 'PAWGRP'.
Regards
Peter
Edit: John - just noticed over on another thread that you have already worked most of this out, but it might help someone else, so I will leave it on.
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Thanks Peter. Having finally got the track data onto my memory stick I've been playing around with the data and have discovered that it was one G-CCEJ, presumably of this parish, who buzzed the place at around 14:30 today, getting within a few hundred metres, so I think he must have seen the PAW alert and gone looking for the source! He would have found a white cottage on top of a hill with some VERY big aerials, none of which are anything to do with PAW. I think the RAF has it programmed in as a VRP.
I'm cracking this slowly...!
EDIT: I see that G-CCEJ is indeed in the PAW map and I have just added myself back there after my hiatus.
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Hi John
Have you undertaken a permanent installation on the SC. Quite a few SC's have. It makes a tremendous difference on range
I have done it on G-MOOV
Keith
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Not yet, Keith. It's in my to-do list over the next few weeks, together with changing out my transponder for one that squitters appropriately. Why do I always seem to be doing maintenance work on my aircraft in the middle of winter?!
I'd be interested to know where you put the various antennas. I was thinking that the front of the engine bay might be a good place for the PAW antenna. I'd like to avoid taping it to the hatch if possible. The GPS and ADS-B antennas are rather less critical of course. I was thinking of running all the cables back to behind the seats, via the tubes under the central console and then putting the hardware there, out of the way. What did you do? If you'd prefer to take this to PM then that's fine.
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Why do I always seem to be doing maintenance work on my aircraft in the middle of winter?!
So it won't eat into summer flying time ;) But it is a pain if the hangar/shed is cold and dark
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Why do I always seem to be doing maintenance work on my aircraft in the middle of winter?!
So it won't eat into summer flying time ;) But it is a pain if the hangar/shed is cold and dark
Got it in one!
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I now have log files concerning the 'ghost' airliners mentioned earlier in this thread. Admin, can I send one to you to look at ? How do I do that ?
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I now have log files concerning the 'ghost' airliners mentioned earlier in this thread. Admin, can I send one to you to look at ? How do I do that ?
Hi EC,
I have sent an email regarding supplying the data.
Can you post a link to the original posting, I cannot find anything earlier in this thread related to 'ghost' airliners
Also which PilotAware version are you running ?
Thx
Lee
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Have sent you an email. Was running the version of PAW that was current 17.12.016, since I just updated it before that.