Originally Posted by
Gauges and Dials
GPS gives you a series of positions. Which means, as theory pedants like me will point out, GPS cannot tell you what your airplane (or ship, or cell phone) is doing in terms of course or velocity right now, all it can tell you is what your course and velocity were a little while ago.
This is wrong, anyway, in theory and in practice.
GPS
does calculate instantaneous velocity -- the velocity
right now -- not from a series of positions but by using the
doppler shift from each satellite.
In fact with just a single reading (from at least four satellites) a GPS receiver can calculate instantaneous position, velocity and time (PVT).
With multiple readings, very precise velocity estimates can be obtained by noting how the carrier phase changes between each reading. This is known as Time-Differenced Carrier Phase (TDCP).
All subject to the usual GPS errors (hence Kalman filters).