Signal Processing 84 (2004) 969–979 www.elsevier.com/locate/sigpro AsymptoticalMLestimationofmultipleradartargets: performanceinthepresenceofmodelmismatch  MariaGreco a ; * ,FulvioGini a ,AlfonsoFarina b a Dept. di Ingegneria dell’Informazione, Via G. Caruso 14, 56122 Pisa, Italy b Chief Technical Oce, Alenia-Marconi Systems, via Tiburtina Km 12.4, 00131 Roma, Italy Received 27 September 2002; received in revised form 25 February 2004 Abstract This work deals with the problem of estimating the directions of arrival (DOA) and the complex amplitudes of multiple targetspresentinthesamerange-azimuthresolutioncellofasurveillanceradar.TheDOAasymptoticmaximumlikelihood (AML)isrstderivedbymaximizingtheasymptotic(largesamplesize)likelihoodfunction,assumingadeterministicmodel fortheunknowntargetamplitudes.TheperformanceoftheproposedestimatorandthatoftheMLestimatorareinvestigated and compared resorting to Monte Carlo simulation. In particular, here their performances are investigated in the presence of a model mismatch, simulating a scenario where the complex amplitudes randomly uctuate according to the Swerling I target model. The robustness of the AML and ML estimators is assessed by comparing their mean square estimation errors withtheCram er–Raolowerboundcalculatedbytakingintoaccountofthetargetamplitudesrandomness. ? 2004 Elsevier B.V. All rights reserved. Keywords: Radar targets; Multi-component signal; DOA estimation; Maximum likelihood; Performance analysis; Performance bound; CRLB 1. Introduction Inmostofmodernradarsystems,thetargetdirec- tionofarrival(DOA)isestimatedbythemonopulse technique [20, Ch. 4], which in principle can work with just a single pulse. In a monopulse system the estimate ˆ  TG ofthetargetDOAisafunctionofthera- tiooftwotightlymatchedreceivingchannelsoutputs, the sum () and the dierence () channels. When multipletargetsarepresentintherange-azimuthcell  ThisworkhasbeenfundedbyAlenia–MarconiSystemsS.p.A. ∗ Corresponding author. Tel.: +39-050-2217620; fax: +39-050-2217522. E-mail addresses: m.greco@ing.unipi.it (M. Greco), f.gini@ing.unipi.it (F. Gini), afarina@amsjv.it (A. Farina). under test, the monopulse system provides an erro- neous DOA measure, somewhere in the direction of the “power centroid” of all the targets [13,1,18,25]. Unfortunately, the position of any individual target maydiersubstantiallyfromthisaverageangularpo- sition and the measured statistic is not applicable to trackanyofthetargetsintheantennamainbeam. In the literature a few methods have been intro- ducedtoreducethedeleteriouseectsoftargetmul- tiplicity. Most of them propose the use of complex monopulseratios[18,17,2],ortheuseofbothin-phase andquadraturecomponentsofthesumanddierence channels [19]. Recently, in [3] a method to estimate theDOAofmultipletargetsusingonlyonereceiving channel has been proposed based on the maximum likelihood (ML) technique; it exploits knowledge of 0165-1684/$-see front matter ? 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sigpro.2004.03.001