A global positioning system receiver for use to determine accurate position during flight based on data received from GPS satellites, the receiver including a clock with high short term accuracy, to allow determination of position even when data is received from less than four satellites, wherein the receiver is able to: extrapolate a trajectory forward for a period of a few seconds, and calculate a predicted reachable set (PRS) related to the extrapolated trajectory; and while receiving data from four satellites or more, to calibrate the clock in synchrony with the GPS clock.