Altitude inaccuracies during longer flight

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So I recently created a fairly detailed waypoint mission for litchi to test out some POI shooting and then at the end do some reveals and some other effects for learning purposes. I created one big mission with everything in there. I figured it would be some time before I got to test it out since the weather here has been horrible. Well yesterday the rain let up for a little bit so I took the P3S to the park and ran the mission. I recoded the screen from Litchi so I could watch it and see how it did. The winds where fairly high at 11 mph with gusts up to 18 mph - but I was surprised how well the drone flew and remained calm.

The first part of the mission flies around the park and points at different areas at 150 feet. I was watching the drone and the iPhone to ensure it stayed at the same altitude. The later part of the mission goes down to 50 feet at one point and then 10 feet for another. During the first part at 150 I noticed it seemed to be rising at one point (see video at around 2:35). The iPhone altitude didn't show any height rising and I figured the wind just blew it off it's course and it went back and corrected itself. But when it came for the lower parts of the mission, you can tell the altitude was way off. I tested it when landing it from where it took off and on landing it showed that it was at -15 feet.

I know the P3S is not that sophisticated and basically puts takeoff point at 0 and used GPS to determine height. But how can it be 15 feet off when it returns? Also, are the newer drones (Mavic and P4) better at lower level altitude since they have ultrasonic sensors? Still waiting for my Mavic to arrive and wanted to know if this issue would be there as well.

 

Meta4

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I know the P3S is not that sophisticated and basically puts takeoff point at 0 and used GPS to determine height. But how can it be 15 feet off when it returns?
For a start it doesn't use GPS for altitude.
If it did, the errors would be a lot more than what you observed.
Like all DJI and most other drones, it's using a barometer for altitude and measuring changes in atmospheric pressure.
Changes in temperature for the Phantom can affect the barometer.
To improve barometer accuracy try recalibrating the IMU on a perfectly level surface.
 
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That's good to know. I did a IMU calibration when I first got it, how often should I do one?

Also do drones with the downward
Sensors get better reading at lower altitudes?


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That's good to know. I did a IMU calibration when I first got it, how often should I do one?

Also do drones with the downward
Sensors get better reading at lower altitudes?


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All these little drones use air pressure IE: "30 inches of mercury" style barometric pressure to determine altitude. Given that, they can not get it down to inches. The air pressure, via barometric pressure, changes a microscopic amount from 5 feet to, say 15 feet, and they just do not have scientific grade barometers. As others have noted, GPS altitude is even less accurate. I would expect any atmospheric conditions to affect it, probably including wind. It is pretty good at telling you whether you are at 400 feet or 10 feet, and your speedometer in the car does not read out in decimal places either. Just don't cut it too close concerning altitude and you will be fine.
 
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So is the altitude measure with new drones more accurate at lower levels because of the sensors.

I normally don't plan things to start two low, but two of the shots I was testing called for starting at 3 m.


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So is the altitude measure with new drones more accurate at lower levels because of the sensors.

I normally don't plan things to start two low, but two of the shots I was testing called for starting at 3 m.


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It is my understanding that all those sensors are pretty much to tell the bird whether it is about to hit something, rather than take a measurement. Flying 10 feet from anything is for gamblers. Especially in Autonomous mode.
 
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Most of my flights I stay above 150. But some shots needed a lower start. But I always have my finger of the mode switch to take over in case litchi does something to endanger the drone.


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RodPad

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So is the altitude measure with new drones more accurate at lower levels because of the sensors.

I normally don't plan things to start two low, but two of the shots I was testing called for starting at 3 m.


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Looks like @Kirby Johnson , gave you pretty good answer.
Maybe not related but it has been said, the P3S, has the smoothest self landings, better than all with bottom sensors, haven't hanging around the Mavic forum much, so what the hear / say is on it?

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In fact GPS with more than 4 SATs can and will give altitude. But it is innacurate.
The P3S uses barometric pressure same as the traditional Altimeter in aircraft. its not the individual reading that gives the resulting height solution - but the change of pressure from initial start up based on standard change per foot / metre.

The other models do have more accurate VoS system but relies on solid works under to work best - why many people switch it off low over water.

Nigel
 

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