PULSE DOPPLER RADAR A pulse radar that extracts the doppler frequency shift for the purpose of detecting moving targets in the presence of clutter is either an M T I radar or a pulse doppler radar. The distinction between then is based on the fact that in a sampled measurement system like a pulse radar, ambiguities can arise in both the doppler frequency and the range measurements. Range ambiguities are avoided with a low sampling rate and doppler frequency ambiguities are avoided with a high sampling rate. However,in most radar applications the sampling rate, or pulse repetition frequency,cannot be selected to avoid both types of measurement ambiguities. Therefore a compromise must be made and the nature of the compromise generally determines weather the radar is called an M T I or a pulse doppler. M T I usually refers to a radar in which the pulse repetition frequency is chosen low enough to avoid ambiguities in range but with the consequence that the frequency measurement is ambiguous and results in blind speed. The pulse doppler radar, on the other hand, has a high pulse repetition frequency that avoids blind speeds, but it experiences ambiguities in rang, It performs doppler filtering on a single spectral line of the pules spectrum. One other method should be mentioned of achieving coherent M T I. If the number of cycles of the doppler frequency shift contained within the duration of a single pules is sufficient, the returned echoes from moving targets may be separated from clutter by suitable R F or IF filters.this is possible if the doppler frequency shift is at least comparable with or greater than the spectral width of the transmitted signal.It is not usually applicable to aircraft targets. but it can sometimes be applied to radars designed to detect extraterrestrial targets such as satellites or astronomical bodies. In these cases, the transmitted pulse width is relatively wide and its spectrum is narrow.The high speed of the extraterrestrial targets results in doppler shifts that are usually significantly greater than spectral width of the transmitted signal.
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