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Altitude Issues

Discussion in 'Phantom 2 Vision + Discussion' started by RCRobman, Apr 4, 2015.

  1. RCRobman

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    I have seen many posts where people report that the altitude showing on the DJI App on their phones is incorrect. I too have this very problem in that my Phantom 2+ reports a negative altitude when the copter is on the ground. Some suggest that the answer is to power off/on the bird so that it is acclimatized temperature wise but that doesn't seem to work for me. I was wondering why the altitude is not being provided by the GPS? Location and altitude are 2 of the main factors provided by GPS and since the copter is in contact with 6 or more satellites (you only need 4 for altitude) why is that information not being fed back to the the DJI App? Seems strange to me that since 4 satellites will give you fairly accurate altitude info then 6 or more should just about pinpoint it for you?
    Thanks for any feedback!
     
  2. msinger

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    GPS is only used to find the position (latitude & longitude). It's not able to report the altitude. There is a barometric sensor inside of the Phantom that is used to find the altitude.
     
  3. N017RW

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    You might want to investigate some of your assumptions regarding GPS altitude accuracy.
     
  4. Mal_PV2_Ireland

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    Could you be getting mixed up with needing more than 6 satellites for ATTITUDE or ATTI mode and nothing to do with altitude.
     
  5. Meta4

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    GPS is very good for positional accuracy but very poor for altitude .
    That's why the Phantom does not use GPS for altitude.
    Here's a short comment from Garmin confirming this ...
    https://support.garmin.com/support/...caseId={66f1b0a0-4cd6-11dc-4733-000000000000}
    Or a more detailed examination of GPS altitude inaccuracy
    http://www.xcmag.com/2011/07/gps-versus-barometric-altitude-the-definitive-answer/

    If the Phantom used GPS for altitude you would crash into the ground every second flight.
    Your altitude issue has nothing to do with GPS.
     
  6. Scott848

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    What's your typical negative altitude?

    You might have a defective barometer like I did. The first Phantom I received showed about -300 feet as soon as I could get into the Vision app and kept counting down until it stabilized around -1700 feet.

    The replacement Phantom I received works great and about a minute after powering up shows around +4 feet.

    More about the problem I experienced at http://www.phantompilots.com/threads/incorrect-altitude.36273/page-2#post-340363

    - Scott
     
  7. RCRobman

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    Typically about -35 ft or so - it measures ok - at least it seems about right - just can't see why it isn't set to 0 or at least give you a way of zeroing it? Just to note that GPS altitude is accurate to 50 ft so my -35 is within the margin of error - the simple answer is to have the app be able to 0 the readout to where you are standing - all vertical height that we care about is from that point up.
    Thanks to all for your answers.
     
  8. Scott848

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    From my experience with my faulty barometer it appears the Phantom sets the altitude to 0 when powered on. Then any changes in altitude readings are based solely on the barometer and the GPS is not used at all in altitude calculations.

    In your case the barometer appears to be faulty but not nearly as bad as my original Phantom. When sitting on the ground is the altitude stable at about -35 feet?

    It seems DJI could make a couple of changes in firmware/software:

    1. Reset altitude to 0 a second time when the motors are started. This might be an effective fix for the barometer problem you seem to be experiencing.

    2. Use changes in GPS altitude to cross check the barometer-based altitude. This would have allowed the Phantom to detect and report the faulty barometer in my original Phantom.

    - Scott
     
  9. flyNfrank

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    Ken's "DJI Ultimate Flight w/GS" is set to zero altitude when motors start.

    But your altitude reading should not be much more then -3. Calibrating the compass has always cured such an issue for me. After you calibrate reboot the quad to lock new settings in memory.
     
  10. BlackTracer

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    You do know that the compass is for direction and the barometer is for altitude don't you? If calibrating the compass somehow fixes your altitude, it is purely coincidence.
     
  11. flyNfrank

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    Yes I do know that. In your case I would need to screen record a before and after calibration if you have never done it yourself. Btw, have you ever replaced a pressure sensor on a vPlus? If so, where was it located?
     
  12. N017RW

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    The baro. sensor is part of the IMU located in the Naza.

    http://wiki.dji.com/en/index.php/IMU
     
  13. RCRobman

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    Yes the altitude is fairly stable at -35 ft give or take a foot or two. As I said it's not a big problem since I just add 40 ft to whatever it tells me and that gives me good enough indication of the altitude. Again a slight change in the software would fix this - just being able to 0 it would do the trick. I know that another poster mentioned another application "Ultimate Flight with G/S" 0's the altimeter so there is no reason why DJI couldn't do something similar - except of course you wouldn't want to 0 the altimeter if you where setting a new home point while in flight - but then again since the copter will continue to descend at 2 m/sec until it stops moving it wouldn't be that much of a problem anyway. I am waiting for the new software to be fully finished and released before deciding to purchase it - want to make sure that Follow Me is working properly as is the Ground Station!
     
  14. Scott848

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    If you don't start the motors how long does it take to stabilize at -35 feet?

    My theory in case you want to test it... your faulty barometer starts at 0 when powered on but as it warms up it stabilizes at -35. Before starting the motors if you then power it off and a few seconds later power it on it would then stay stable at around 0.

    - Scott
     
  15. lignow

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    This works for me.- Take phantom out of pack. Turn on for about 40 to 60 sec. Turn off, Finish assembly . The altitude is within 5 to 8 feet. This is within acceptable limits for the phantom.
     
  16. Julius717

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    mine sometimes shows +/- 15 ft or so, usually I do a IMU calibration before any major flight and it seems to solve the problem,

    when im just screwing around in the neighborhood or something I usually don't worry about it.