Quick word of ADVICE

cahutch said:
The barometric sensor is sensitive to temperature so it's common for the altitude reading to increase as the Phantom warms up on the deck.
There is no relationship at all between the Compass and the altitude reading. AFAIK.

How many times have you went through the sequence I described?
 
cahutch said:
The barometric sensor is sensitive to temperature so it's common for the altitude reading to increase as the Phantom warms up on the deck.
There is no relationship at all between the Compass and the altitude reading. AFAIK.


No doubt.

The compass is a magnetometer.
Provides the direction the 'front' of the a/c is pointing.
 
cahutch said:
The barometric sensor is sensitive to temperature so it's common for the altitude reading to increase as the Phantom warms up on the deck.
There is no relationship at all between the Compass and the altitude reading. AFAIK.

yep and it also gets affected by wind or air currents that result in slight pressure changes within the shell since, it is after all a sensitive air pressure measuring device ;)
Anyone know if alt readings are always based on the Barometric sensor or does it change to GPS alti values when enough sats are locked? Same for alt holding in GPS mode is it holding according to the barometric sensor or the GPS data?
 
From: http://gpsinformation.net/main/altitude.htm


"GPS altitude measures the users' distance from the center of the SVs orbits. These measurements are referenced to geodetic altitude or ellipsoidal altitude in some GPS equipment. Garmin and most equipment manufacturers utilize a mathematical model in the GPS software which roughly approximates the geodetic model of the earth and reference altitude to this model. As with any model, there will be errors as the earth is not a simple mathematical shape to represent. What this means is that if you are walking on the seashore, and see your altitude as -15 meters, you should not be concerned. First, the geodetic model of the earth can have much more than this amount of error at any specific point and Second, you have the GPS error itself to add in."

It is doubtful (but not certain, to me) that DJI has implemented the scheme or similar to the highlighted above.
Thus altitude of these a/c is [most likely] pressure based only.
 
Does anyone really know what DJI uses the compass for and why they want us to calibrate it. It seems to me that the calibration is to take into account any thing that we might have added to the bird that would affect the local magnetic field. After we have calibrated the compass in the middle of a large field we are at liberty to fly over buildings with lots of rebar etc so what does the compass do ? If it's a matter of the earths magnetic field why don't they use the GPS data and a look up table to see what declination to use. So if as I first suggested that it's to take into account any new item we have added then we should only have to calibrate when we add or move some item. If they don't use a lookup table for the declination then the only other reason to calibrate is if we move many many miles away from our normal place of operation. If we use one of the many maps showing how the declination varies over the world we can see that we could move several thousand miles and not have to re calibrate unless we have modified the phantom in some way.
So does anyone really know why they have the compass. If you do then please speak up as I would love to know.
Thanksj
 
The compass identifies the nose position along the vertical axis in reference to earths magnetic field and it is needed for the controller to hold position and to fly to waypoints in conjunction with the GPS data since the GPS data alone cannot identify the nose position. E.g. Multirotor with good compass calibration is on position hold in the air, Nose position is North, GPS data shows the unit moved to the South, controller will rev up the rear two rotors to correct the unit forward, unit moves forward back to it's original position, controller happy. Lets say the compass is now out of calibration it and says the nose is pointing north but in fact it is pointing more towards the West, again the GPS data says that the unit moved out of position to the South, controller thinks that the nose is pointing North therefore revs the rear two props up, since the nose is actually pointing west the unit will now move to the west, controller will see the GPS data out of position to the west thus will fire up the left props to move to the east, but in fact will move to the North,... the story goes on and on causing the unit to circle around the point it is supposed to be at.
 
It will also use the compass input along with ground track (from the GPS) to determine the wind correction angle to maintain its requested course.
 
I calibrated my compass once and when it came out of the box. I check the compas values in the assistant software now and then if it is between 1400 1600 i'am happy and it always is! Didn't even recal when i was in Italy and i live in Holland. No problems whatsoever!
 
Panamon Creel said:
... E.g. Multirotor with good compass calibration is on position hold in the air, Nose position is North, GPS data shows the unit moved to the South, controller will rev up the rear two rotors to correct the unit forward, unit moves forward back to it's original position, controller happy. Lets say the compass is now out of calibration it and says the nose is pointing north but in fact it is pointing more towards the West, again the GPS data says that the unit moved out of position to the South, controller thinks that the nose is pointing North therefore revs the rear two props up, since the nose is actually pointing west the unit will now move to the west, controller will see the GPS data out of position to the west thus will fire up the left props to move to the east, but in fact will move to the North,... the story goes on and on causing the unit to circle around the point it is supposed to be at.
Thank you Panamon for that explanation. That's the bit of information i was looking for to explain a crash I had that could only have been caused by the compass. I had the TBE initially and when I tried to rotate it she spiraled away and veered hard into the ground. In the aftermath I was left wondering how it's possible that the nose direction being off could cause this. Now I understand.

If only I had the foresight to switch to Atti mode as soon as I saw the TBE, I could have avoided the crash.
 

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