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Altitude -(negative) while flying?

Discussion in 'Pro/Adv Discussion' started by antdon, Jul 24, 2015.

  1. antdon

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    Sorry if this has been brought up before. Today I was flying and noticed the altitude drop to -1. Another time during flight I noticed I was about 20ft up in the air it was showing 3-5ft in the Pilot app. And unless my mind was playing tricks on me, I did feel like the drone was somewhat unstable at this point but not anything too noticeable. I had already been flying maybe 15 minutes on my 2nd battery when this happened.

    Has anyone else experienced this before? I calibrate before every flight and also did a IMU calibration before leaving the house today.

    Also I did use a level to the best of my ability today when calibrating but does it always have to be EXACT or can you just calibrate the drone as close as possible?

    And if the Pilot app shows NO calibration needed, is this usually accurate? The reason I ask is because there are times when I'm driving and I feel I hit a big enough pot hole that'll cause the drone to shake up a bit and still the app would show NO calibration needed? Just want to know if I should go strictly by what the Pilot app is telling me.

    Picture below:

    Screenshot_2015-07-23-19-42-00.png
     
  2. TAZ

    TAZ

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    In regards to your altitude.... It would not be an issue with the IMU, or its calibration. There are two systems that measure your altitude, the sonar of the VPS (which is the far right reading in your included display, active when within 3M of the ground) and an internal barometric altimeter (which is separate from the IMU). The barometric altimeter is simply reading air density changes from your take off point (lower pressure, the higher it thinks you are, higher pressure, the lower). Temperature and relative air density changes can affect the barometric altimeter. All things being equal, when you gain in altitude, air density decreases. However, relative humidity (the more water in the air) can also affect air density. In addition an increase in temperature can decrease air density. So, over the course of a flight, if the humidity in the air changes, the temperature changes...or a low pressure or high pressure front is moving through the air, it is conceivable that the barometric altimeter can report an altitude difference from reality.

    In general barometric altimeters are usually very accurate in reading changes. However, if you are seeing a variance, it is telling you that at that current point of your flight, if you're displaying an altitude reading that is lower than you expect...either the humidity is higher, the ambient air pressure is higher, or the temperature is lower (or some combination of all three) then when your barometric altimeter was zero'd at its takeoff pressure.
     
    #2 TAZ, Jul 24, 2015
    Last edited: Jul 24, 2015
  3. antdon

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    Thank you Taz, for your well detailed response.

    Another issue I noticed on my first/second flight was the exact opposite.. although I was on the ground fully idle, the screen was showing that I was up 10ft+?

    If this does happen in future flights, do I have anything to worry about? Does it affect the flight/stability at all? I feel like when these numbers are off the drone is unstable, but that could just be me being a newb and in my mind. I still don't have the nerve to fly more than 100~ ft away if that.. lol. Hopefully I get used to it and grow some b****.

    Just to be cautious before flying today, I did an IMU calibration although the App said it wasn't needed.. I tried making sure the Drone was as leveled as possible. Does this have anything to do with IMU?

    Thanks again!
     
  4. TAZ

    TAZ

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    Well... The simple answer is no, you've nothing to worry about, because you're always flying by sight, right... Who cares what you're altimeter says, just don't crash it into the ground. In true practice, as you push the envelope...perhaps you are flying over a body of water, filming close to water for example...too far away to know your true altitude. Doing such a thing, you've got to recognize your altimeter may be off. VPS also reportedly doesn't work well over highly reflective surfaces such as water...so that may not be useful. Filming at distance, over highly reflective/sonically absorptive surfaces at low altitudes, might create issues if you rely strictly on your on screen display.

    The one area I would be cautious of though is automatic landing. If you have VPS turned off ( which many people recommend on current firmware) and your barometric altimeter is off...perhaps an automatic landing could cause a hard landing. Personally I never utilize the automatic landing feature, as I know my eyes are better than any altitude reading measurement could be...so I don't worry about it.

    When you say "Does this have anything to do with IMU?" I'm not sure what the 'this' is you refer, so don't know how to answer. Although I would never do an IMU calibration in the field (you don't want to just try for a level surface...you want to ensure it). Do that at home. Compass calibrations are the only thing you'd do in the field, if required.
     
  5. antdon

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    Roger that.

    So, the IMU calibration you recommend using an actual level? Does it have to be level EXACTLY or as close as you can get it?

    I have VPS turned on currently... are there issues with the current firmware? This is the first time I'm reading this.

    Tony
     
  6. TAZ

    TAZ

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    "although I was on the ground fully idle, the screen was showing that I was up 10ft+"

    In this case, I'd just turn off/on the bird. That should re-zero the altimeter.
     
  7. TAZ

    TAZ

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    In general, if you're not experiencing stability issues (and not all are) when within 3M of the ground...which is the operational limit of the VPS...leave it on, otherwise shut it off until the new firmware.

    And yes, LEVEL IMU calibrations are important.
     
  8. RedHotPoker

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    It's also been highly recommended to do your IMU calibration when the Phantom is as cool as possible, perhaps at night, set the Phantom outside for half an hour, with the battery removed. When ready, have your radio and device on and the calibration page in the app open. Ah, that way as soon as you place the battery in, turn it on and begin the calibration when they connect.
    This reduces the IMU warm up time significantly, for the many who are concerned over this lost minute of flight time. ;-)

    RedHotPoker
     
  9. DiluxEdition

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    Guess I have another reason to increase rth height even further. gonna have to get close to 400ft now...
     
  10. N017RW

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    HaHa that's funny. :)
    Maybe in the "Great White North", in S. Fla. its 85 at night here now! ;)
     
  11. antdon

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    Yeah, I know on my P2 Vision+ I would get the IMU is too hot error.

    On the P3, I don't recall seeing this and can just proceed with the IMU calibration regardless of how long the unit has been turned on?
     
  12. RedHotPoker

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    Well, I think the idea is to do a cold calibration, so no, the Phantom should be at rest. While in a colder room or even outside in the evening, get everything ready. When you are ready to calibrate, with radio on and Pilot app on the IMU calibration page, bring the Phantom 3 in, pop in the intelligent battery, turn it on, and perform the IMU calibration then.
    This will also get you flying a little bit sooner as the IMU, doesn't need to warm up as long... It's just a time saver really, & I'm not sure if it actually improves upon the actual calibration, or what ever.
    Are you breaking in your batteries using the suggested gentleman's approach, or just running them down, each & every flight?
    Some here say that by bringing your batteries down to the 50% level the first ten flights, helps setup the battery correctly. While others just fly them down to 20-30% from the beginning.
    Also, After the first twenty flights, bring those battery levels down to 8% capacity, then recharged to 100%.
    I'm Just curious enough to ask, how others are treating these expensive intelligent batteries?

    RedHotPoker