I can’t answer all the questions but I can talk about RTK techniques. I recently used two $75 GPS receivers and two raspberry pis to create an RTK GPS system (real time kinematics). With RTK you have one receiver that is fixed - it does not move. The other receiver is on the thing whose position you want to measure, and the two devices are connected through the network. Because the system knows the fixed station is not moving, any drift in measurement represents the real time error of the current satellite constellation. The moving unit subtracts that error from its own measurements to get very precise figures for its position. In the system I built, it seems to have centimeter level repeatability. That is, if I am looking at the read out in real time and I stop moving the receiver, the drift in the measurement is a couple of centimeters. If I move it 20 centimeters that is pretty closely reflected in the read outs. Currently I don’t know the absolute accuracy of my system but the relative accuracy is very high!
