A Multiple-receiver Synthetic-aperture Active Sonar Imaging System

Abstract

Synthetic aperture iechniqucs have not been widely used in sonar imaging duc to problcms with son:u platform motion, slow mapping iatc, ;ind lncdiuln instnhility. A time-varying medium or nonuniform son:bplntform trajectory will cause ph:ise errors in the soi i ;~ ' rct1111is th:lt dcgradc image quali(y. In :I sufficicntly stA>lc rnctfiuin and with motion compcnsntion. Ihc m:isiinurn possihlc unambiguous mapping rate o f ;I single rcccivcr systcln is approximatcly cqu:rl to one-fourth the propag;ition spccil i n [he medium tinics the width o f the tr:insiniltcr. Even :if this rate, the resulting synthetic :ipcrtiirc im:igcs suffer from specular noise. We present a multiple rcccivcr syslcm for synthetic apcilure sonar imaging nnd :I rohusl algorithm th:ll c:iil pcrfonn thc ncccsssry cstiinalion ml col-rcclion ol' motion emors and phasc errors ;ind c:in map ;it :I faster rate than ;i single receiver syslciri. The synlhclic apcilrirc imaging system consists of a lii~ti-rcsol~itioii s dcsc:in s y tcm which has been rnotlitictl to timiismit over :I n;u-row apei-rure, resulting i n a broader hcaiiiwidlh. which innkcs synthetic apcrture imlging possible. Thc ;rlgorithm u t i lizcs the acoustic returns at niulliplc receivers to foim ;I coin plex val ue d prcl i in i nary i m:igc for c:ic t i trans in i t h u I'S I. Each prclirninary i m q c is translated and rot:itcd relntivc to the previous one by an :tmount detcrmincd by the motion of the s o n x pkitforin. Pcilurlxitions o f thc sonar plntform velocity also C:IUSC i-an?c-tlcpcndcnt phxc errors hctween successive preliminary irnligcs. Translation nnd rotation arc estiimtcd Ihrouyh coi-rcl:ition Ixisctl tcchniqucs, then comectcd Ihrougli i m q c rcgistr:ifioii, The rcgislcrcd images are [hen aligned i n ph;isc h y applying :I rxigcdependent phasc correction. Fin:illy the registered pti:isc corrected imagcs x e coticrcntly superimposed to lorin ;I high-resolution synthetic apcrtirrc s o i w iiiwgc. t Chest cr laggin s i s wait t i Son nl ecli , I11 c.. S;in I n I3a I-IXIIII. ('n I i 10 iii i ;i 93111. The tcchniquc involved i n forming the complexv:ilucd preliminary iixigcs :ind the motion compensation :ind p h x c con'cction :ilgorithms will be presented. The rclnlivc tr:tdc-offs hclwccn coherent superposition to form high-resolution synlliclic :rpcrturc images with specular noise. ant1 magnitude siipcrposilion to foim multiple-look physical :ipcilur-c im:rgcs with less specular noise, will be discussed. We present results of cxperiinents at sea with the prolotypc syiiltictic :ipcrtiirc soii'x system.