Amateur radio operators encounter the Doppler effect (or Doppler shift) when working with satellite transmissions. Satellites travel at very high speeds relative to a terrestrial observer - The Doppler shift is particularly pronounced at the higher VHF and UHF bands. The Doppler shift in satellite communications is also a function of the flight path. An approaching satellite is shift higher in frequency. As the satellite passes directly over the observer the frequency shift is zero. Then as the satellite moves away from the observer the frequency shifts lower.