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Altitude accuracy

Discussion in 'Pro/Adv Discussion' started by DEllis, Jan 20, 2016.

  1. DEllis

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    Been flying the p3p for a while. Love it with no issues. However sometimes when hovering at say about 6' the altitude on the app says 18' or so. Not always the same variance but just not accurate in my opinion. Have tried with vps on and off, but in pmode.

    Thanks
    David
     
  2. Rumbaar

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    yes, I get this too. mine often shows its lower than it's reading.
     
  3. bobmyers

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    Are you moving your Physical location like walking with the RC while flying and is the altitude difference reading consistent for the whole flight. How many Satillites showing?
     
  4. DEllis

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    No just notice it when hovering getting ready to fly and on return prior to landing. I am not moving and usually have 13-17 satillites showing.
     
  5. bobmyers

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    Strange-- have you checked it at different flying locations?
     
  6. DEllis

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    I've noticed it a couple of times but will pay more attention when I fly at different locations.
     
  7. DownUnder

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    It's uses barometric pressure I thought to calculate height as it's more 'accurate' than GPS
     
  8. PVFlyer

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    It's indicated in Mission Planner's Important Notes

    • Aircraft height (altimeter) may easily vary 15-20m (50-66ft) off the actual height after aircraft flying and heating up several minutes. You are recommended to let IMU warn up properly on the ground especially in cold weather, and set waypoint height minimum 25m (82ft). Cold IMU calibration trick is NOT recommended which might incur more discrepancy in altimeter reading.

    For example, if there is a tree 30m height and you set the first waypoint to 40m above that tree to maintain a 10m gap and the last waypoint is also set to 40m above the same tree after 10 minutes mission flight. The aircraft might reach the first waypoint well above the tree, but when it reaches the last waypoint 10 minutes later, it might hit the tree due to discrepancy of barometer though the altimeter is still reporting 40m height.
     
  9. Phantom751874

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    P3 is notorious for reporting the wrong height. All I could get out of P3 support on another forum was to land and reboot everything and try again to see if the barometer worked better. For me it's usuallaly somewhat accurate but sometimes it's way off by 30 or more feet. It's just how it is. You might try an IMU calibration to if that helps.
     
  10. Mark The Droner

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    Since there is no such thing as a GPS coordinate for height, the Phantom relies on the barometric pressure sensors in the aircraft. It knows it starts off at zero. Now, imagine you are in charge of writing the program that determines height at its first launch and hover.

    The props speed up, which increases the air pressure. The props push air down into the ground which increases pressure even more as the air hits the ground and moves in all directions. The increase in pressure normally indicates a drop in altitude, but you know the craft hasn't left the ground yet. So you don't necessarily write the software to show a drop in altitude at this point - at least not as much as would normally be indicated.

    Then you assume the craft is idling for a few secs so maybe you'll re-adjust the height reading closer to zero.

    A minute or two later the props speed up dramatically and you figure the aircraft is aloft. How high is it? You've still got increased air pressure from the props, so now you have to account for that. So you have to make a fuzzy assumption on height at this point. But it's just that. It's an estimate.

    From there, determing height is relatively easy. But I think the reason the height is not necessarily accurate is the fuzzy math that is done on the first few seconds after launch.
     
    stoneyb245 likes this.
  11. Carloss66

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    Not really. The props move the air and increase its speed, but that does not increase the barometric pressure. For the air pressure to increase, the air needs to be contained. Our air is contained by the atmosphere. Barometric pressure does not increase when the props spin up, the air is just moving from one place to another. Think of the air in you car tires, it is under pressure because it can not escape. Turn on a drone motor with a prop inside the tire. The air will just move but the pressure inside the tire will remain the same. The air pressure in the tire will increase if you add more air or if you increase the air temperature in the tire. Barometric pressure is based on the weight and density of the air. The higher the altitude the lower the air density and weight. Atmospheric conditions i.e. temperature affect barometric pressure, so calculating altitude using a barometer is always an educated estimate. Using GPS triangulation is more accurate since it is not affected by atmospheric conditions. The Phantoms use a barometer, so their altitude reading is always an estimate.
     
    #11 Carloss66, Jan 21, 2016
    Last edited: Jan 21, 2016
    Phantazmk and N017RW like this.
  12. Mark The Droner

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    Yes, I agree barometric pressure is a measure of air density. So if you're under the four props and they start up all around you, the air increases in density. That's why my altimeter reading jumps into the minus area when I start the motors and while it's sitting on the ground.