Genghis9
First Officer
United States
Offline
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Cetacean,
I wish I had a good electronics answer for you on this but I don't. However, I can provide a tiny bit of insight on altitude.
As you seem to be aware you have pressure altitude and absolute altitude. Pressure altitude is the height above MSL (Mean Sea Level) and is based on the standard deviation of pressure from MSL factor in standard day temperature. However, as you correctly noted, due to changing weather systems the atmospheric pressure fluctuates, going from high to low pressure systems and back again. In order to compensate for those changes a pressure altimeter must be frequently adjusted, in a manned aircraft this is done by setting the pressure as noted in inches of mercury reading for the area in use in what is called the Kollsman Window (i.e. setting 29.92 which by the way is the mean setting for sea level standard day & temp, and the setting all aircraft use once in the PCA FL180 and above). Since I have yet to see a way to set the pressure altimeter in a UAV from DJI, I doubt their altitude is determined solely by pressure. I believe that pressure only relates to VVI calculations at most.
That leaves absolute altitude commonly known as AGL, the height above the ground, typically measure in manned aircraft by a radar altimeter and is similar in nature to the downward VPS system on a Phantom. However, that does not answer the issue of what is determined to be the initial altitude for a phantom drone. It might be pressure, as total flight time is less than 30 minutes and it is very unlikely that pressure will change dramatically enough in that time to matter greatly. It could be mapping altitude, which depending on the mapping data used may or may not be accurate and it would not account for atmospheric pressure change either therefore introducing an error at the start, most likely. Since I'm not 100% sure, my guess is it uses a combination of pressure and map, although as you seem to indicate that is how you can end up with errors from true like a negative altitude. I don't think that a phantom can measure AGL past about 6 feet, accurately anyway, so that input can be eliminated directly.
Hope this helps, if not sorry for wasting your time. |
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