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Altimeter issue

Discussion in 'Phantom 2 Vision + Discussion' started by Perdidoflyer, Dec 24, 2014.

  1. Perdidoflyer

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    Hello fellow Phantom Pilots - I have a Phantom 2V+, purchased in late October, and really am enjoying the aircraft, and the experience, with many successful flights. I have seen lots of posts on this and the DJI forum(s), where one poses a question that suggests that one has not "done their homework". In other words, read the manual, find instructional videos to watch, do all (not just some) of the firmware/software/app updates, go through the pre-flight checklist each and every time you fly, and have done all of that, so I am not that guy.

    The ONLY problem that I am having is with regard to altitude, as displayed on my iPhone. This problem is a bit random, in that on some days, the altitude appears to be accurate, and on other days, no matter how long I wait (after all other systems are "go", compass orientation, gps satellites, home lock") before takeoff, I am showing -200 ft altitude or more. I can hover nearby, my Phantom needs occasional help in maintaining altitude in that hover, but it never seems to correct itself. The biggest issue for me (I tested this today, flying in NAZA mode), was when I switched S1 to Failsafe, with the Phantom visually at about 50 ft. altitude, it climbed dramatically. This suggests to me that it was correcting for the -200 ft. to attempt to get to the RTH 66 ft. No telling how it might go about landing!

    Any suggestions, known issues, etc.?

    Thanks!
     
  2. rtg

    rtg

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    Try performing an IMU calibration again.

    Do the advanced. Also get set up to perform right after powering the unit on. I found this to get the altimeter at least closer to a reasonable altitude.

    Happy Holidays.

    Rob
     
  3. Perdidoflyer

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    Rob - interesting. I have done the advanced cali, but your idea of less time being better than more is worth a try.

    Happy Holidays to you as well,

    Mack
     
  4. Perdidoflyer

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    Rob - thanks again. This is really interesting for me, in that it is the only real issue that I am having with my Phantom. I flew today, and have really "tightened up" the pre-flight checklist (too many points to mention, but correct power up sequence of RC, WiFi, then Phantom, then wait for all the important green led signals, then compass calibration, then connect to WiFi in iPhone, which was kept 15 ft. away from Phantom, confirm # of satellites, etc) and still displayed altitude is -300ft.

    I am not insecure in flying at that point, in that I have 6-9 satellites, don't fly out of LOS, but I have to wait 3-5 minutes of flight, hovering nearby, and ultimately, altimeter becomes realistic. Alternately, I could test awaiting altimeter correction while sitting on the ground, but it still seems to me that it should not take that long for the altimeter to establish itself.

    I am on the Florida panhandle, so essentially at sea level. Don't quite know what to make of it, and again, it only causes a problem in terms of my observance of the 400 ft. altitude limit, (Phantom is clearly confused as to that) and certainly in terms of RTH.

    Thoughts? Again, I think I have a really good P2v+, and trust it and how I am handling it, but this is puzzling!
     
  5. pdantoni

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    I'm having the same exact problem. Performed both the Basic and Advanced Cali. Mods are all within thresholds. If I monitor the altitude while the P2V+ is sitting on the kitchen table, the Altitude will slowly rise and fall between -125 and +125.
     
  6. Perdidoflyer

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    To be honest, I am a bit surprised that my clearly technical question (after doing EVERYTHING correctly pre-flight), altimeter, as displayed on my iPhone, always starts out at -300-ish ft. altitude, but after 5-10 minutes of flight, constantly watching altimeter reading, it becomes more or less accurate. As I have contended, this is important for a variety of reasons, including observing the 400 ft. ceiling, your RTH altitude setting, Failsafe, etc.

    Perhaps I am the only one having this problem? Otherwise, I have had a totally satisfactory experience with my Phantom, so perhaps it is just me and my Phantom?
     
  7. Mako79

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    The imu has a pressure sensitive barometer that helps calculate altitude. Barometers work by pressure. Heat affects pressure. Therefore the cold phantom will have a different reading to warm phantom.
     
  8. Perdidoflyer

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    Thanks for the info...so simply allowing Phantom to sit on the ground, after GPS/home established, compass orientation, will allow it to warm up, or do I do what I have been doing, which is hovering nearby, wasting battery, until altitude becomes more realistic. In other words, does Phantom/the IMU warm up pre-flight, or only during flight?
     
  9. rwskinner

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    Power cycling the bird zero's the altitude so what I've been doing is turning on the bird, then give it a few minutes while doing all my preflight stuff, then I power cycle the bird and reconnect and the altitude is much more realistic. Pain in the rump, yes, but it seems to work. Some act like this is normal but I sure never noticed it until after I performed my first advanced calibration. I must admit, my first few flights, I was more concerned about watching the Phantom than the telemetry data so maybe I just didn't notice it.
     
  10. Perdidoflyer

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    Interesting - I will give it a try, and thanks!
     
  11. N017RW

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    Unfortunately the barometer is not meant for accurate altitude reading over the course of a flight.

    It is intended to be used for attitude control/maintenance while hovering.

    DJI has tried to leverage this for use as an altitude reference for AGL or AHP (above home point) data to enhance the experience.
     
  12. Mopar Bob

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    I do the same thing and find the altitude goes from way off to off by 1 or 2 feet after the second power cycle.
     
  13. Perdidoflyer

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    There does seem to be something to this, and Mopar Bob, I agree with you that I didn't notice the issue until I did the Advanced Cali. So now pre-flight is:
    Turn on RC and WiFi
    Turn on Phantom
    Await indications of GPS and home (aft led's)
    Do the compass orientation dance
    Power cycle Phantom
    Await second confirmation of GPS/home (aft led's)
    Connect iPhone and confirm # of satellites
    Fly
     
  14. cahutch

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    The altimeter is not meant to be that accurate.
    A barometric altimeter is based on air pressure AND temperature which are both variables even when standing still.
    You cannot expect it to be consistent to within a few feet. It's meant to measure 10's or 100's of feet, not one or two.

    The flight controller uses it to try to maintain altitude as best as it can within the accuracy of the sensor, which is why even if you have an accurate reading at takeoff, hovering will still vary up and down by 5-10 feet and by the time you land it will be nowhere near the same reading it was at takeoff.
    The flight controller doesn't not use the altimeter to determine where the ground is. The actual reading doesn't matter, only that the reading changes as it goes up or down.

    So trying to get an accurate altitude calibration before takeoff is a complete waste of time and will have zero effect on flight control or safety.

    Oh, and this was probably already mentioned but failsafe mode will automatically and deliberately cause it to climb to a safe altitude before returning to home point. This is to (try to) avoid obstacles on the way back.
    I would suggest another reading of the manual and a look at this guide.
    viewtopic.php?t=23597
     
  15. Perdidoflyer

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    Umm...I don't think it is supposed to start out showing -325 feet at initial takeoff
     
  16. ThePhantom

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    My P2 V2 behaves the same and cannot maintain a hover without compensating a bit of throttle. I tried every possible fix which yielded no good results. Already sent it to DJI for repair/replacement.
     
  17. Mako79

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    Do an Adv IMU Calibration and DO NOT IGNORE THE MC TEMP WARNING. If you see this warning, do not proceed. The more you try, the longer it will take and the less accurate it will be. Need to remember that the IMU uses pressure to measure altitude. What affects pressure? HEAT!. Everyone lets her idle and then does the adv calibration. This is the mistake as the Phantom warms up in less than a minute without the props spinning.

    Anyways somes notes on doing a correct adv calibration:
    Do this on a surface levelled to the horizon.
    Do it when the phantom is cooled. If it has been on, charge the battery and let her cool for 30 minutes or so.
    Goto to the Phantom assist and goto the Adv IMU page and have it ready. Plug the usb in the phantom and computer and then turn on the phantom. DO THE CALIBRATION IMMEDIATELY when the phantom turns on. This will ensure accuracy because the phantom has not warmed up to affect the readings.
    Wait for the green tick. It should only a take 2 minutes or less.
     
  18. rwskinner

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    It's not meant to be accurate within a few meters but it is there for a reason. If you had one that read way off and had erratic readings you would noticed how the Phantom can not stabilize during a hover. It may not be dead nuts, but it's zero'd on power up and used as a reference, so yes, it is important or it wouldn't be on there.

    Richard

     
  19. cahutch

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    325 ft seems like a lot but I don't think it's unusual. That just means that the barometric pressure or more likely the temperature of the sensor is changing from when you turn it on to when you check the altitude reading. Like I said, the actual reading doesn't matter unless you're competing for an altitude record or something. All that matters is that it eventually stabilizes and that the reading increases and decreases when you go up or down.
    The more stable it is, the more accurately it will hover at a particular altitude of course, but temperature and barometric pressure are always changing, so hovering altitude can only be so stable.

    When you turn it on, it sets the current reading to zero, but then the internals start to warm up while going through the start up procedure and waiting for satellites.
    If it's cold outside and you just brought it out from the house or the car, the internals could actually be cooling off. The bigger the temperature differential, the larger the altitude change.
    When you start up the motors and actually take off, the props cool the system off again and your reading is thrown off again.

    The reason the re-start seems to work is because you're letting the temperature stabilize and then re-setting the system so it re-sets the altitude to zero after the temperature has stabilized.
    The problem is that once the motors start, the temperature will change again. Once you've gained any altitude or you're moving laterally at any speed, the temperature changes even more.
    When you land, your reading will be way off again.
     
  20. rwskinner

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    Agreed.