Principles of Modern Radar - Volume 2 1891121537

418 CHAPTER 9 Adaptive Digital BeamformingFIGURE 9-18Example of digitallyresteering beyondthe point where theoverlapped subarrayarchitecture canmaintain lowsidelobes.Relative Directivity (dB)0−5−10−15−20−25−30−35−40−45Δu = 0Δu = 0.05−50−1 −0.5 0 0.5 1Sine Space Anglearray combined into six 3:1 overlapped subarrays, where each subarray is composed of24 elements. Low sidelobes in resteered beams are maintained as long as the beam isn’tresteered to the point where the next lobe of the array factor begins to enter the mainlobeof the subarray pattern, as shown in Figure 9-18. The increase in the subarray size dueto overlapping results in subarray patterns that roll off more sharply, leading to greaterdirectivity loss as a function of the amount of resteer. This directivity loss is noticeable inFigure 9-17 and blatant in Figure 9-18.The degree to which the peak of the resteered beam is offset from the desired location,u 0 + u,is determined by the shape of the subarray pattern and the amount of resteer.Figure 9-19 shows resteered beams for the regular, irregular, and overlapped subarrayFIGURE 9-19Subarray-leveldigitally resteeredbeams comparedwith a fully steeredbeam.Relative Directivity (dB)0−1−2−3−4−5−6−7Regular resteeredIrregular resteeredOverlapped resteeredFully steered beam−8−9−100.2 0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3Sine Space Angle