Doppler effect

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The pitch of a sound emitted by a moving object appears to be higher the faster it approaches, and lower the faster it moves away. A good analogy is a stream into which leaves are thrown at regular intervals. As we move upstream towards the source, leaves will flow past us more often. The faster we go, the more leaves we will see. Conversely, as we move downstream away from the source leaves will flow past less often-to the point where we would only see a single leaf when moving at the same speed as the current. High-pitched sounds have high frequencies, which means we “meet” the sound wave often-as if we were moving towards the source (or it was approaching us); low-pitched sounds have lower frequencies-as if we were moving away. The same principle applies to light rays, which shift towards the longer wavelengths as they move away, and towards the shorter wavelengths as they approach. (Adapted from Evry Schatzmann, Les Enfants d’Uranie)
We can thus determine the velocity of a moving object emitting sound or light waves by measuring the shift between the transmitted and received frequencies. The DORIS system achieves precise orbit determination and location by measuring the Doppler shift in this way.