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Altitude incorrect

Discussion in 'Pro/Adv Discussion' started by djfxprogressive, Jun 7, 2015.

  1. djfxprogressive

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    I've noticed that after I've been flying for 10 min or so, my altitude will not be accurate anymore. It always says that I'm much higher than I actually am. I flew for a little while today, and decided to bring it low to the ground (15' or so), but the app said I was still 100' ft high. Similar thing happened a few days ago, when I was no more than 50' high, but the app reported I was 130' high. I've already calibrated the IMU, which didn't help. Because of this issue, I can not get the VPS to enable when trying to land.

    I also experienced this same altitude issue on my P2V+. I'm just wondering how many other people have this issue. Thanks!
     
  2. Trumple

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    Just went out and tested this issue, I seem to have the same problem. Assuming the altimeter in the Phantom 3 is based on pressure (aka a barometric altimeter), then these are the reasons I can think up as to why this happens:

    - These types of altimeters are sensitive to temperature. As the temperature rises (as the aircraft warms up) or as the temperature falls (as you get higher and the temperature around the aircraft falls), the altimeter will give different readings. So wind, altitude, or anything that affects temperature might also affect the reading

    - Sudden changes in air pressure (owing to wind, cold fronts, etc.) will cause your altimeter readings to vary wildly

    Today I noticed that my Phantom will actually sink if left a few metres above ground. I tested this by aligning the Phantom with a tree in the distance in my line of sight, and noted that sometimes when it dropped, it wouldn't climb back up to where it was before. Within a minute or two, it was just half a metre from the ground after sinking about 2 metres.

    However, in an earlier flight, I noticed that it remained pretty much accurate throughout the flight (since I happened to notice negative altitude readings as I began to fly it down a hill I was on, so it must have been more or less accurate). This particular flight was the second of two consecutive flights, which leads me to believe that it is based more on the aircraft warming up than anything else.

    An altimeter of this kind is not really supposed to give absolute readings - instead, its primary function is to detect relative changes, so the Phantom can tell if its going up or down. As such, you can expect the reading to be off after flying around for a while, as the error begins to build up and the environment changes.

    As for the VPS, it should always be activated, but it only starts to limit decent when it detects you are very close to the ground. It should override whatever the barometer says, and that's what I noticed today when my decent slowed right down about a foot from the ground, despite the app reporting I was 10m high.

    Regardless, if it bugs you, there's a way to zero it again. If you fly around, and let the aircraft temperature stabilize for a minute or so, then bring it down and land, the reading will be set to 0 and you can take off again with (hopefully) minimized error in the altitude readings. The reality of it is though, these machines are for aerial shots, so if you're a couple of metres out from what the app reports and you're a hundred metres in the sky, it's usually no biggie :)
     
  3. bbfpv

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  4. Hughie

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    If it is anything like the P2, turning it off an on does help rezero it but it will still drift again. I recently did this and noticed an 8m error at the end of the flight. Due to component cost, the transducer and height system was never intended give Radar Altimeter accuracy. The flight controller is at the bottom end of those sold by DJI.
     
  5. Skynet1

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    The OP mentioned his altimeter readings were significantly off, more than just a few meters, but more like 25. I've noticed that on a few occasions I have received a barometric error message on my app. which will prevent starting of the motors, and directs re booting of the battery.
    This same message recommended contacting DJI support should the issue continue, not sure what affect this may have during flight.
    I probably will make a record of the incidents to DJI. djfxprogressive may want to do the same.
     
    #5 Skynet1, Jun 8, 2015
    Last edited: Jun 8, 2015
  6. Hughie

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    Understood. What I quoted was anecdotal, from one time when I actually checked how far out mine was. I am sure I have had better and worse results.
     
  7. djfxprogressive

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    Thanks for the responses guys! I assumed it was from the Phantom warming up, my P2V+ did the same thing. I was only concerned because I initiated auto land, and it was coming down very fast, so much that it likely would have been a very rough landing, since the Phantom thought it was much higher. I wasn't sure if VPS was going to kick-in, so I canceled auto land and landed it myself, as I usually do.

    Assuming that I would have let it continue to auto-land, do you guys believe that VPS would have taken over, and slowed the Phantoms descent? Thanks again!
     
  8. Skynet1

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    [QUOTE="djfxprogressive, post: 408247

    Assuming that I would have let it continue to auto-land, do you guys believe that VPS would have taken over, and slowed the Phantoms descent? Thanks again![/QUOTE]

    Probably, if all was working as it should, but ya never know until after the fact.
    You may simply want to use auto land when you feel there isn't too much of a discrepancy in altitude and you are lower to the ground, just to play it safe, as you had. : )
     
  9. djfxprogressive

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    Yep, my thoughts exactly, which is why I cancelled it.
     
    #9 djfxprogressive, Jun 8, 2015
    Last edited: Jun 8, 2015
  10. Hasan Ahmad

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    This is why DJI never designed the return to home to land but to hover to compensate for the altitude
     
  11. Skynet1

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    Ah ha! very interesting.
     
  12. Ted Pretty

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    Has there been any fix to this?

    I have altitude readings that are way off also.

    Using 1v.1.9 firmware as I've been told not to update yet...for bug reasons.
     
  13. FCPhantom

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    My alt is always 20 meter too high, before landing. I just wondering, how the RTH ,(automatic landing) works, if the alt reading is not correct? I dont have VPS enabled...
     
  14. Trumple

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    This is purely speculation but: It probably just computes the rate of change of altitude rather than the absolute reading. That is, it measures the rate of descent and looks for when the rate of descent drops to 0, indicating the quad cannot go any lower and therefore must have landed
     
  15. leekiya

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    I have had plenty of barometer error messages on 1.9 which prevented the craft from starting, now on 1.3 the error has changed to " unknown error 42 please contact dji" all I have ever had to do was whip the battery out and then back in again and it starts. Anyone else had this sort of problem.
     
  16. flyNfrank

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    1.3.2 is worthy. Btw, I'm would be one of the 1st to tell users to not update. But 1.3.2 is good to go.
     
  17. flyNfrank

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    Btw, you guys seeing altitude height change, what firmware and app are you using?
     
  18. Dadcat

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    My P3 was typically showing it was at 45 to 47 feet height after landing (on FW 1.1.9). I don't remember the iOS app number but I had skipped at least one of those updates.

    I saw no improvement with iOS 2.0.0. But after the fw update to 1.3.20 and an IMU calibration, it's much better - maybe 6 feet off instead of 45. I made sure the P3 was cooled to about 65 deg. F before starting the IMU cal.

    I now see the "IMU warming up" message for a few seconds at startup. I had never seen that before. Don't know if that has anything to do with the reduced height error.

    My Vision+ had the same type of height error after 16 minute flights. No amount of IMU calibrating made any difference with that one.
     
  19. FCPhantom

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    On the A/C I have 1.3.20 and RC I have 1.4.1. Android DJI GO 2.01
     
  20. nhoover

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    I have had this same problem several times (out of 75+ flights), typically at the end of a long flight and running 1.1.9 FW using the old 1.0.10 app. Planning to upgrade next week so we'll see how it goes.