Doppler tracking

Doppler tracking is the most common method of tracking the position of vehicles in space. It involves measuring the Doppler shift of a radio signal sent from a spacecraft to a tracking station on Earth, this signal either coming from an onboard oscillator or being one that the spacecraft has coherently transponded in response to a signal received from the ground station. The second of these modes is more useful for navigation because the returning signal is measured against the same frequency reference as that of the originally transmitted signal. The Earth-based frequency reference is also more stable than the oscillator onboard the spacecraft.