Doppler shift:
The Doppler Shift is a change in the frequency of a wave when the source of the transmission and the observer are in relative motion to one another. This works with the Argos satellite, as when the satellite moves closer to a transmitter, the frequency of the transmitted waves will increase and then once again decrease as the satellite moves away. Every time that one of these satellites receives a message from a transmitter during the approximate 10 minutes that the satellite is in range of the transmitter, the satellite measures the frequency of the wave and tags the times of the arrival. Using the satellite's speed, position, and original frequency of the transmitter, the position of the transmitter can be calculated.