What is the relationship between GPS and the compass and why should the phantom fly away if the compass is not well calibrated?
After all the other consumer based GPS equipment like car navigators and the like you never have to calibrate and they still working fine and correctly. The don't get confused by the magnetic interference of a car.
When a Phantom wants to fly away will tuning to Atti-mode prevent this behavior because I have red several treads that this will not work.
Consumed based GPS navigator don't have magnetic compass on board, so it calculates your cars heading by the movement of your car time to time ... Your car heading on the map won't be updated when your car is not moving (stoped).
It means, your car heading is the final calculation result, never been needed for further calculation. And if your cars GPS navigator doesn't have any compass, so there is nothing to be calibrated.
But in the term of drone, heading of the A/C must be identified first, before the flight controller take further calculation about which side to move its position (forward, backward, left or right) even when there was no command on the stick (hovering in place). It means, this positioning calculation is executed every seconds (or even miliseconds) just to find which way to go next. Why this calculation is still being executed while there was no command on the stick at all? Because it need to be stay in the correct GPS coordinate while it being draged by the wind (or any other external cause). So, the compass reading is needed to feed the flight controller data input. And if there is a compass, then it will need a calibration once it has large enough of magnetic reading deviation. And every device with digital compass onboard, can determine its heading right away even when it stays on its place (not moving as your car).
In some circumstances, your digital compass reading might have data reading deviation, just like your smartphone when you find your heading on the map is not matched to your real heading. That's why you need to calibrate by moving your phone in a "8" shape.
When phantom's flight controller aware that it has compass failure on the air, it will automatically switch to ATTI mode even you don't touch the mode switch at all, and it will give you a message telling about this situation on the App's screen, so you have to land imediatelly in ATTI mode to prevent any further risk of crash.
But you have to notice that in some old phantoms (before Phantom4), sometime this compass failure situation was not correctly determined by the flight controller, so it will run the positioning calculation in the P-GPS mode using the "bad data" feed from the compass. That's what causing "toilet bowl" effect, since it just like a dog chasing its own tail... Calculating it's position over and over that never been correctly found by the equation.
So, why this compass error reading would cause a "flyaway".
The answer is really simple. Read this illustration carefully.
GPS is the map papper on your hand, while the compass is your awareness about which way is north-south-east and west.
When you need to go to some other place from your current position, you need to determine your current position on the map, and define which way to move. Both of those information must be acquired and have to be calculated for your travel.
And now, imagine when you only got one of those informations, suppose you have a map on your hand but you don't know which way is south-north-east-west, what will you do? Otherwise, suppose you know which way is north-south-east-west but you don't have a map in your hand not even in your brains memory, what will you do?
After this illustration, you will understand why a phantom "don't know where to go" if it lack of valid GPS and/or compass data. So it need to be controlled manually in ATTI mode by the pilot as long as there is a connection established to the R/C. Otherwise, it will only keep the vertical position (altitude, using barometer reading), and have no idea on what to do about horizontal position, and soon it will draged by the wind. That's called a "fly away".
FYI: I tried to google "Phantom 4 toilet bowl effect" and could find any case about this. I assume DJI has a work arround on this problem very good on the new generation of phantoms.
As long as you're not on it or right next to it, you should be fine.
