The Doppler Effect allows the distance between a satellite transmitting from space and a radio receiver on the ground to be measured by observing how the frequency received from the satellite transmitter changes as the satellite approaches, passes overhead, and moves away.
The frequency received can be displayed on a Doppler Curve graph relating frequency to time. As a satellite approaches and passes overhead, the received frequency appears to fall. However, the rate of change in frequency is not constant. At first, the frequency changes slowly. Then the change increases to its greatest rate at the time of closest approach. After passing overhead, the rate of frequency change slows as the satellite moves away.
- As a satellite approaches, the frequency of its transmitter appears to be higher than the actual transmission frequency.
- Overhead is the time of closest approach when the transmitted frequency and the received frequency are the same.
- As a satellite moves away, the frequency appears to be lower than the actual transmission frequency.
example of a Doppler Curve graph
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