How does GPS work?
The Doppler effect
Do you know
What The Doppler Effect is?
The very first satellite, Sputnik 1, carried radio transmitters. Researchers at John Hopkins University in the USA soon noted that its radio broadcasts were varying in frequency because of the Doppler Effect. Just as the frequency of a siren moving towards you appears higher in pitch, so as Sputnik approached a receiving station, its radio waves compressed, reducing the wavelength and increasing their perceived frequency.
By analyzing the data from Sputnik 1 and its successor Sputnik 2, scientists were able to predict the path of each satellite from a known location and thus anticipate its orbit. By applying the same principle in reverse, you can use satellites to work out the position of a receiver on earth.