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Altitude is not always accurate.

Discussion in 'Phantom 2 Vision + Discussion' started by rveach67, Nov 2, 2014.

  1. rveach67

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    I am running into an issue where my altitude will be off 8-10 feet. Anyone else having this and is there a fix?

    I have updated everything and calibrated everything.
     
  2. AsiaFlyer

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    Yeah I have same problem, seems to be less accurate the warmer the bird gets
     
  3. wetauser

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    My Attitude was showing at - 200meters ( -656 feet) on my balcony 3 meters up , this never seemed to effect GPS flight, but I have have sent it back to the dealer for investigation , never dared to switch to atti mode just in case it went into space.
    I did all the usual IMU advanced calibration nothing worked.
     
  4. Mako79

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    The IMU (inertia Measuring Unit) also has a barometer which helps calculate altitude. To fix, do an Advanced IMU calibration. Its in the Phantom Assist under one of the tabs. Just make sure you get the green tick.

    Most of us do advanced calibrations when the phantom has been next to the computer for 15 minutes. Without the props on, the phantom does not get ventilation to cool the electronics. Heat changes pressure which affects the barometric readings.

    This is why when you do advanced IMU calibration, it must be done when the phantom is cold. The calibration is not only more accurate but also quicker. Just make sure you do it when your first turn on the phantom and do NOT leave it idling. And find a levelled surface.
     
  5. Mopar Bob

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    I have had the problem since day one. At ground level, it shows -10 feet, sometimes -5 feet. I found that if I power down the phantom and restart it, the error is reduced significantly. Like from -10 feet to -1 foot after the shutdown. And its consistent. I have done it may 100 flights and it always reduces the error after the power down. I called UAV Direct where I bought it and DJI and neither of them offered any solid reason for the problem. They wanted me to return it for repair (6 weeks minimum) but decided I would live with it. After wrestling with the problem for 2 months, I am convinced its a problem in the design. It uses barometric pressure (measured within the IMU). I think their sensor is not up to the task of measuring the minute amounts of pressure that determines the distance of a few feet. But that's just my opinion.
     
  6. Meta4

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    Next time you hand catch, note how still you have to hold the Phantom to shut down the motors.
    That's how sensitive the barometric sensor is. It detects variations of less than an inch.
    The calibration mentioned above sounds like the way to go.
     
  7. nhoover

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    +1 exactly.
     
  8. Mopar Bob

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    I doubt it can measure an altitude variation of 1 inch. Where on earth do you get your information? The most sensitive scientific barometric tools have difficulty discerning from 1 story to another. The atmospheric pressure at sea level is 29.92 and at 1000 feet its 28.86. That's a little over 1 psi over 1000 feet! Also barometric pressure is typically stated as an average. Weather changes, including temperature, causes it to fluctuate.

    I am serious about my question about where you get your information. Does DJI have a source for their design criteria? If so it would be very valuable to share with us so we can all have a better understanding of our aircrafts.
     
  9. wetauser

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    I suspect that when you hand catch the phantom is not level, and its the auto levelling that can be felt from the gyroscope not barometer
     
  10. N017RW

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    The NAZA is DJI's lowest tier MC (to date).

    The A2 is their top-tier MC and that boasts a +/-0.5 meter hovering accuracy.
    http://wiki.dji.com/en/index.php/A2

    I could not find similar info for the Naza but I doubt it's much different, certainly not better.
     
  11. aartsf

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    Absolute accuracy of the barometric calculated altitude is something else than the relative accuracy of a barometric measurement. If you forget about that, you get the confusing dicussions as above.
    Regards,
    FA
     
  12. Meta4

    Meta4 Moderator
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    Good point
     
  13. Bibos

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    This is what I've been experiencing as well. I started to do the the power-down and then power-up procedure and the altitude error reduces enormously. However, my error is in the opposite direction. Mine Shows the Phantom higher and higher than it is, which is even worse than yours. With your Variation you don't have to worry about clearance. I always have to add several Meters to be safe.

    But even after this and after flying one entire battery (Manual or autonomously) and landing the altitude is way off again by about 5 Meters (15ft.)

    Rgds,
    Kent
     
  14. Mopar Bob

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    You definitely want to give it a fair amount of extra altitude anyway. Just to be safe. In your case you need to give it an especially wide birth. The other problem I worry about is in the event of an auto land. If it thinks its 10 feet off the ground but its really 5 feet, will it drill its way back to china!
     
  15. N017RW

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    The barometer is used for attitude control as it is relative.

    The error(s) would only be manifested in RTH or other programmed heights.

    As far as auto-land(ing), again that is relative. Once there is no longer a sensed increase in pressure whilst decending the a/c then determines it is on the ground or other object.
     
  16. wetauser

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    I have got my phantom back from the dealer, they replaced the naza control, now altitude reads near enough correct.
    My theory was that the hole in the barometer was blocked, and the heat outside caused the pressure to rise giving the high negative altitude
     
  17. WessexWyvern

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    This raises the question for me at least - what altitude is the Phantom reporting -

    Above Mean Sea Level

    or

    Above Ground Level


    and is the barometric pressure gauge only correct at 29.92 hg?


    Without adjusting the barometric pressure gauge to the current local pressure setting or without setting off from MSL, I would have thought that the Phantom is unlikely to report the altitude to any real accuracy given that GPS altitudes are pretty inaccurate, +/-8-10ft sounds pretty good to me.
     
  18. jadebox

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    The altimeter measures the altitude from the ground. It adjusts itself to the ambient conditions when you turn on the quadcopter.

    Altimeters like this are accurate to within a few feet.

    -- Roger
     
  19. wetauser

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    I agree accuracy is only an approximation .it compares the difference in pressure from take of point, to pressure at the height of flight.

    The huge error that I had of -600 feet never caused me any problems flying ,except for very slow descent rates using failsafe,a friend with exactly the same phantom performed this procedure much quicker , we tried them out one after the other.his was measured in seconds mine needed a calendar .and I always had to abort the test to avoid a flat battery.
     
  20. WessexWyvern

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    Surely it would need to know either the current elevation or the barometric pressure in order to do this.