A Space/Fast-Time Adaptive Monopulse Technique
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A Space/Fast-Time Adaptive Monopulse Technique Yaron Seliktar,1 Douglas B. Williams,2 and E. Jeff Holder3 1 Jerusalem
College of Engineering, P.O. Box 3566, Jerusalem 91035, Israel of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0250, USA 3 Sensors and Electromagnetic Applications Laboratory, Georgia Tech Research Institute, 7220 Richardson Road, Smyrna, GA 30080-1041, USA 2 School
Received 28 October 2004; Revised 29 May 2005; Accepted 14 June 2005 Mainbeam jamming poses a particularly difficult challenge for conventional monopulse radars. In such cases spatially adaptive processing provides some interference suppression when the target and jammer are not exactly coaligned. However, as the target angle approaches that of the jammer, mitigation performance is increasingly hampered and distortions are introduced into the resulting beam pattern. Both of these factors limit the reliability of a spatially adaptive monopulse processor. The presence of coherent multipath in the form of terrain-scattered interference (TSI), although normally considered a nuisance, can be exploited to suppress mainbeam jamming with space/fast-time processing. A method is presented offering space/fast-time monopulse processing with distortionless spatial array patterns that can achieve improved angle estimation over spatially adaptive monopulse. Performance results for the monopulse processor are obtained for mountaintop data containing a jammer and TSI, which demonstrate a dramatic improvement in performance over conventional monopulse and spatially adaptive monopulse. Copyright © 2006 Hindawi Publishing Corporation. All rights reserved.
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INTRODUCTION
A tracking radar requires a high-precision angular measurement of a target’s azimuth (or elevation) which is traditionally achieved using monopulse processing. Historically, monopulse radars employed two separate feed horns on a single antenna element in order to generate two receive beams that were slightly offset in azimuth (or elevation) angle. Sum and difference outputs were formed by summing and subtracting the two beam outputs, respectively. The ratio of difference to sum output voltages, called the error voltage, was then used to determine the degree of correction necessary to realign the beam axis with the target. With the introduction of phased array technology, it became unnecessary to employ special hardware for monopulse processing, since the array itself can electronically form the multiple beams needed. A typical conventional monopulse processor for a phased array radar is obtained by appropriately phasing the individual array channels to obtain sum and difference outputs. The ratio of difference to sum outputs provides the measure by which the angle offset from the beam axis (i.e., look direction) is determined. The updated angle measurement is then used to recompute phases for the channels so as to realign the beam axis with the target. Mainbeam jamming occurs when the jammer signal is directly impinging on the radar’s receive beam. Spati
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