How does the ATTI mode on the P3 actually, technically, work? If I take off at an altitude of 0 (of course) from a hill, and I then go lower, is there a way to use ATTI mode to warn me when I am almost at ground level (which could be -10 for example)? I like to fly low to the ground, but on the display it's hard to see if you are too low. With ATTI mode, can the P3 somehow measure the distance to the ground with the VPS? Or will it still display the altitude as in GPS mode using minus?