dwallersv
lvl.3
Flight distance : 1843015 ft
United States
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Aardvark Posted at 2017-1-2 11:31
Barometric Altimeters are inherently inaccurate, just google and you'll come up with many examples like This explaining the potential error.
^This.
Everyone here is expecting more from the sensors than they are capable of.
The accurate elevation (NOT altitude, BTW) sensor on the P4 is the ultrasonic rangefinder on the bottom, which is good to +/- centimeter, thereabouts. However, its range is limited to something like 33 feet, so it isn't used by the bird for elevation measurement, only positioning and avoidance.
The other ways to sense altitude -- which tared to starting altitude becomes elevation, the datum displayed in the DJI GO app -- is with a barometer. However, since barometric pressure is always changing, it is only really "accurate" for changes in altitude, not absolutely altitude measurement. The latter requires periodic calibration to update for constant barometric changes caused by atmospheric dynamics.
This means that, the longer you fly after takeoff, the greater the potential error in elevation. With volatile weather conditions, ambient BP can easily change enough in 10 minutes to register the equivalent of 10-30 feet of error.
"Volatile" can simply mean windy, BTW. Instability indicates pressure gradients in the atmosphere, which means pressure's going to change as that airmass moves over you.
The other way to sense altitude is via GPS, but this too lacks the +/-1ft accuracy that everyone here (understandably) expects.
So, bottom line, don't look at the "altitude" (actually elevation) telemetry with any more accuracy than +/- 10ft or so.
Now, all that said, consistent errors to one side or the other, again and again, are suspicious. Justifies a deeper look. However, to be "consistent" a group of errors like this must be taken days apart, under different weather conditions ideally. If you simply do several flights in a row on the same afternoon and get the same trend in elevation error, its more likely because the pressure is changing consistently rather than you have a consistent error.
Finally, if you have consistent climbing/descending behavior relatively quickly (like, 3-5'+/min), this is definitely something "broken". |
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