Originally Posted by
turbidus
The GPS system knows where the aircraft WAS...
The navigation message consists of 30-second frames 1,500 bits long, divided into five 6-second subframes of ten 30-bit words each. Each subframe has the GPS time in 6-second increments.
Once the GPS receiver has acquired a minimum of 4 satellites and collected a 30 second frame from each, it can monitor the carrier frequency of each GPS satellite. These are synced to the data frames, so once you know where each frame starts, you can compare the phase difference between the carrier signals continuously and not have to wait 30 seconds times 400 knots to calculate the position where you were. Admittedly an overly simplistic explanation of what the receivers do.