Pressure altitude

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This topic contains 20 replies, has 8 voices, and was last updated by  rafalopez 9 years, 10 months ago.

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    <p>I assume that FS uses/displays GPS altitude, not pressure altitude, as only combined GPPS/Vario flight computers record pressure altitude in their IGC file. Is this true?</p>
    <p>There seem to have been a number of problems with pilots viewing their instruments showing pressure altitude, but their GPS track logs showing GPS altitude and they are getting penalized.</p>



    IGC files have pressure and GPS altitude. I can see both of them in SeeYou. Here they give the same results, but that doesn’t mean that the altitude shown on the 6030 display is the same as the one displayed later in SeeYou (or FS, for that matter). I’ve seen huge differences there, and it doesn’t matter which altitude SeeYou is displaying.

    I believe that SeeYou adjusts the altitude displayes to the terrain. It moves your launch altitude to match the take off spots altitude as it knows it and then adjusts everything to that altitude. I don’t know if FS does that also.

    I just wonder if FS or SeeYou can be used to provide the altitude that the pilot actually saw displayed on their 6030. I very seriously doubt it.

    Which one is right?



    Hi Davis,
    You are probably aware this only affects the Brauniger & Flytec Instruments.
    They are currently rewriting their firmawares to have the user field to display Baro or GPS altitude, the newer 6030,6020 are done with th Compeo & Competino not far off.





    The scoring systems should use pressure altitude if it is available, not GPS altitude.


    Oyvind Ellefsen

    It’s GPSDump that decide what to save in the tracklog. Historically it’s been the GPS altitude that’s been used, but since Garmin started releasing sensor models this is strictly not the case anymore.

    I think the scoring system should be able to handle both altitude systems, and the scorer should be aware of all the implications of both systems. There is no easy fix, everyone (Pilots, scorers, CIVL, etc.) needs to learn what the issues are.

    – Oyvind



    There are actually three altitude systems – Garmins with a pressure sensor record a height based on user calibration, GPS height and pressure readings.

    If I use GPSDump to save a tracklog, which one of these systems is recorded, and is it possible to use this tracklog, without conversion, to check for altitude violations based on GPS height or barometric height?

    If this cannot be done then I need to use additional software and convert tracklogs – FS / GPSDump does not provide the complete solution that I need.




    A garmin gps only log one altitude as far as I know and gpsdump reads this altitude. You can’t use any other software to get any other information.

    Other instruments like the compeo log both barometric and gps altitude and gpsdump only use the gps altitude. Other software can log both.

    I can set my garmin 76s to either base its altitude an a barometer, an auto calibrated barometer altitude that shouldn’t be of more than 30 m from gps altitude and according to the manual only gps altitude but this doesn’t work on my unit.

    I am not sure which altitude that “should” be used. Most relevant airspace here in Norway is based on amsl altitude and I have always though that this is the real altitude and a gps tries to show real altitude and is thus the best option. One advantage to use gps altitude is that we don’t need to calibrate anything.

    The other airspace is flight level and thus depends on the pressure.




    If you set your 76s to base its altitude as a barometer (auto-calibrate OFF?) and then initialise the height to be 200m lower than the actual height, what values are recorded? ie. will all recorded values be 200m lower than the actual height?







    I know only of the Flytec/Brauniger models and the Flymaster to have pressure altitude available. For these models I know what is what.

    In the Garmin protocol there is only one altitude parameter. It just says “altitude in meters”. No indication of what type of altitude, and I can’t see how I can make the GPS to select anything.

    The same for all other instruments.

    If a barometric altitude is available GpsDump can download it. There isn’t much programming to do to retrieve it. Currently the data bytes are just ignored.

    When a track log is saved as an .igc file the pressure altitude is set to zero. The exception is when saving the raw data downloaded from the Brauniger/Flytec models. The only altitude in the .kml file is the GPS altitude.

    It also shouldn’t be dificult to add the barometric altitude to the GpsDump screen and the .kml/.igc files.

    Stein S



    I am doing a little digging into this too.

    As far as I am aware Flytec and Brauniger give the Pressure Altitude (altitude with respect to 1013.2hPa), so any calibration of the instrument is ignored. The gps altitude is also available.

    I think Aircotecs give QNH (altitude with respect to the pressure at mean sea level), so it depends on the instrument calibration. Gps altitude is not available. I think the instrument calibrates on power up assuming there is gps coverage. I wish to confirm this with Aircotec.

    MLRs and Garmins without pressure sensors give gps altitude.

    Garmins with pressure sensors give QNH auto calibrated to the gps height or manually over-ridden. I believe they record what is displayed on the screen but will confirm this with garmin as some have said this is not the case. I’d also like to know when they auto-calibrate. Manually changing the height while flying may or may not be detectable in the track log. If it is then it may look remarkably like an autocalibration.

    I think it is a good idea for gpsdump to extract and save both altitudes if available. It is extra work for SteinS tho’. Some instruments only give barometric and some only give gps. Which to display by default in FS I don’t know.

    I believe in most cases we would be using Pressure Altitude (flight levels) for airspace restrictions in comps, in which case the Flytecs already give the correct information. I believe they can be set to display this information to the pilot as well.

    Those with only gps height or QNH will need to calibrate their instrument to the height at launch and calculate the height of the flight level restriction. OR those with manual calibration could set their instrument to 1013.2hPa and fly with PA/Flight Levels (I think this is correct). Those with gps height only, need to know on warm days the gps will read high.

    Those checking tracklogs will need to know what height data they are looking at. Having the dual data from the flytecs will help to give the gps – barometric differences at height.

    None of this is a problem if there is no height restriction of course.

    Cheers all




    I have added decoding of pressure altitude from the Brauniger, Flytec and Flymaster GPS. The question now is where in the .kml file do I put the data. As for now I have put the data just after and at the same level as <SecondsFromTimeOfFirstPoint>, and called the block <PressureAltitude>. Another question is wether the data block shall be included if no pressure altitude is available.



    Hello Stein

    I don’t know whether Stein-Tore has any plans for implementing a way of viewing both sets of height data.

    It is great to have them both available anyway, thanks.



    I have had a reply from Garmin about the auto calibrate function on the units with pressure sensors.

    They calibrate the altimeter constantly.

    The track log that is recorded is always the altimeter altitude (can be displayed on the unit by selecting the ‘elevation’ field). The altimeter is more sensitive to rapid changes in height than the gps.

    If auto calibrate is on then the altimeter is adjusted to gps height, slowly but continuously.
    If the auto calibrate is off then the altimeter calibration will remain the same as at launch (assuming it is not changed in flight).
    If the altimeter is switched off then gps height will be recorded.

    Unfortunately there is no way to know from the download what is being recorded nor the status of the calibration.

    I have tried changing the height in flight on my 76S and have the tracklogs if anyone is interested. The change is instant. So changing by 100m with a 5s tick rate shows a 20m/s climb rate which is detectable by looking at the file in excel or compegps. Smaller changes will be harder to detect on stronger days.


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