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Now consider radar from a moving platform, e.g. an aerial mounted on the underside of an aeroplane. Recall what you hear as an ambulance, police car or fire engine passes by. The pitch of the siren changes, being higher as the vehicle approaches the observer and lower as it recedes. This is due to the Doppler efJect. See Appendix 4 for a mathematical treatment of this effect. The change in pitch may be used to measure the speed of the source. The same phenomena occurs with stars; the red shift of the emitted light being used to determine the speed with which outer galaxies are receding from the Earth. Similarly for flying aircraft; the detected frequency of the radiation at a point in front of the aircraft is raised, compared with that emitted, and the frequency of the radiation detected behind the aircraft is lowered (Figure 6.9). An analysis of the timing and phase of the return signal when combined with the height and speed of the aircraft provides an accurate picture of the terrain and targets around the aircraft. Hence, by flying over the terrain, the aeroplane acts as a large aperture detector. This principle of synthetic radar is widely used

and is also incorporated into the design of satellites. For example, the European Research Satellite ERS-1 was launched by the European Space Agency in July 1991 to collect information about the oceans, sea surface temperature (see Plate 7)' coastal water and land use. B and C microwaves are used for synthetic aperture radar, the return beam being collected for a 800m long 100km wide strip with a resolution of 30m on the ground. When operating in such a mode ESR-1 produces 100 Mbits of data for the ground-based computers to process!