1. A system for monitoring an aircraft, comprising: a first means aboard the aircraft for receiving navigation signals and deriving a position vector therefrom, a second means aboard the aircraft for receiving an input representative of altitude of the aircraft above ground level, a third means aboard the aircraft for sampling said position vector at a selected sample rate and outputting a succession of samples of said position vector, said third means being responsive to the second means to increase the sample rate in response to the altitude of the aircraft being less than a predetermined value, a fourth means aboard the aircraft for receiving the succession of samples of the position vector and generating data packets that contain respective samples of the position vector, and a fifth means aboard the aircraft for transmitting the data packets to a ground-based receiving station.