How does one determine what the "compass indicates" as you stated above.?
"One" doesn't, that's buried deep inside the flight controller! You typically only get to see that information after it's been processed. It may get logged, I don't know.
Actually for pretty much any drone using GPS for navigation, the GPS is added onto a inertial guidance system. How primitive this guidance system varies with different flight controllers. Smarter ones don't need the GPS to operate for most basic functions like flying to waypoints or hovering in place, but you don't see those that often.
At the risk of over-simplifying how this works, more or less it's using the compass to indicate direction the drone is facing and traveling in, the barometer to indicate height, the accelerometer to indicate speed, and the gyro to indicate inclination relative to the ground.
All of these factors can be determined much quicker and with greater localized accuracy by local sensors built into the flight controller than thru GPS.
BUT in the broader picture of the craft's surroundings, any errors from these sensors (no matter how minor) add up quickly. And this is where GPS comes in, it provides a cross check so if the inertial system 'thinks' it's heading due north at 100ft and going 20mph, any errors can be compensated for.
But because of the latency in the GPS readings, it's always checking what JUST happened, rather than what is happening now (which CAN be detected by the on board sensors).
Realistically GPS is only really used when hovering in place and during autonomous flights. If you are pushing the sticks around, only the gyro and accelerometer are really being used (and maybe the barometer as well), GPS is merely fed back to the pilot's display (and typically ignored) .
So GPS increasing the accuracy and precision of the inertial system is an important use, but not the only one. GPS also provides position in the real world that's outside the flight controller's rather limited concept of the world.
This is where autonomous flight starts getting useful, because the drone can now relate to the physical world.
Interestingly, the RealSense feature actually increases the intelligence of the inertial system, enough even that you shouldn't need GPS for most uses..
