Principles of Modern Radar - Volume 2 1891121537

464 CHAPTER 10 Clutter Suppression Using Space-Time Adaptive ProcessingFIGURE 10-3Spatial beampatternfor uniform andTaylor weightedULAs, includinginfluence of arrayerrors.3020Antenna Illumination PatternTaylor WeightingUniform WeightingTaylor Weighting, 0.5 dB/3° errorsdB100−10−200 5 10 15 20 25 30 35 40Azimuth Angle (Degrees)where L array is the array length in meters (substitute the array width into (10.26) forelevation beamwidth). The 3 dB beamwidth isB 3−dB = a swλ oL arraydegrees (10.27)a sw is a beam-broadening factor; typical values are in the range of 58 ≤ a sw ≤ 70 [1].10.2.2 Temporal Sampling and Doppler ProcessingMTI radar systems transmit and receive a sequence of N pulses as a means of temporallysampling the signal environment in the slow-time or Doppler dimension. The analog-todigitalconverters (ADCs) sample in the fast time, or range, dimension. The radar signalprocessor coherently combines the N pulses in a manner very similar to spatial beamforming;this operation is commonly called Doppler processing or Doppler beamforming.Doppler beamforming separates the target signal from stationary ground clutter returns atthe same angle and also enables estimation of target radial velocity.Consider a length-N periodic pulse train, with a pulse repetition interval (PRI) of T .Further assume the presence of a point scatterer, initially at range r o from the antenna phasecenter reference point. Due to the relative motion between the scatterer and radar platform,an interpulse change in range of r is present. The coherent dwell is kept sufficiently shortto maintain a constant radial velocity between radar and scatterer, so the interpulse changein range is constant-valued. With this in mind, the corresponding fast-time delay resultingfrom the motion between platform and scatterer isτ n = 2r o + n2rc(10.28)