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Incorrect altitude after flight

Discussion in 'Phantom 2 Vision Discussion' started by hook3m, Jan 21, 2014.

  1. hook3m

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    I calibrate my Phantom just about every flight and when I take off the altitude shows close to 0. After each flight the altitude is incorrect when I land. Sometimes off by 30 feet or so. Anyone else having this issue? Wondering if incorrect altitude will cause the Phantom to crash land if Return to Home is activated.
     
  2. Pull_Up

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    The barometer in the Vision will be affected by local changes in air pressure. A rise/drop of one millibar in atmospheric pressure will reduce/increase the relative height shown at ground level by around 30ft... Having said that if it's happening every flight then it's more likely to just be the margin of error from the barometer.

    No worries about crashing on RTH - the autoland feature means the aircraft just keeps coming down slowly until it registers it's touched the ground, it doesn't just assume its down when the relative altitude is the same as it was on take-off.
     
  3. AnselA

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    Is it allways same direction? Above or below the take off level?
     
  4. hook3m

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    The phantom has a barometer? I thought it gets it altitude from GPS. It always shows I’m higher after a flight. The last time I flew it showed +38 Feet after I landed but was 0 to 1-2 feet when I took off. Seems the higher I go on a flight the more inaccurate it is when I land. I did wait for a full minute after landing to see if it corrected but it still stayed at about 38 feet.
     
  5. dkatz42

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    GPS altitude is relatively crappy because it can't triangulate very well (the satellites you really need are hidden by the earth). WAAS correction makes it closer to the accuracy of baro altimeters (so you can do Cat I instrument approaches with it), but I doubt that the P2V has a WAAS receiver (though they're getting pretty cheap). Even baro altimeters aren't terribly accurate in terms of absolute altitude, though they're pretty repeatable as long as the atmospheric pressure isn't changing (not down to inches, however).
     
  6. Pull_Up

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    DJI quote height hold accuracy as +/- 0.6m - definitely baro territory, not GPS-derived. In my experience it holds it's height very well indeed, too.
     
  7. frank1023

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    I have the same issue. After a flight it is off by about 30ft. The problem I see with this issue is that if the phantom goes into RTH mode and is off by 30ft it will not climb up to 65ft before returning home. So if there is an obstacle in the way and you need it to go to it's pre-determined minimum height it will be off by 30ft. The big problem would be it will be 30ft lower so instead of 65ft it will be at 35ft. :?
     
  8. themosttoys

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    Yup, my altitude is always off by 10 to 30 feet when I land. The funny thing is that if I power down (Ex: to swap in a new battery) and fly again, the altitude tends to be more accurate the 2nd flight. The not so funny thing is that in addition to the RTH altitude being off (as stated by frank1023) the P2V will not auto land, it will keep hammering itself into the ground until it crashes. It seems to bounce like flubber (getting higher every cycle) trying to break through to get to it's landing altitude (30 feet under ground).
     
  9. Pull_Up

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    I see a lot of people worried about the RTH height algorithm and the baro being off. If it was me, and I knew there were obstacles between the stored home point and my Vision, I would not fly lower than that obstacle, plus a margin for error. If the obstacles were less than 60ft tall, I would fly higher than 60ft and lock in that safety height. Don't forget it you are higher than 60ft when return to home initiates the aircraft will maintain current height. With a 5.8GHz control system you really need to be line of sight anyway to get the best out of it - I don't understand why people keep wanting to fly behind trees and things! :)
     
  10. frank1023

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    Unfortunately some times you can't stay above the highest obstacle in the area you are trying to get a picture or video. Would try to make sure that nothing over 65 ft stood between the phantom and the home point. Its not always a perfect world out there. I was falling back a little on that fact that it would get back up to 65ft before returning home.
     
  11. garygid

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    The point of questioning the landing AGL value of 30 feet is that it might NOT be at
    65 feet AGL when doing an RTH, but actually at only 35 feet AGL, crashing into
    the 36 foot high trees that you carefully planned to avoid at 65 feet AGL.