LABORATORY UWB GPR SYSTEM FOR LANDMINE DETECTION

Transcription

1 LABORATORY UWB GPR SYSTEM FOR LANDMINE DETECTION B. Schees, Y. Plasman, M. Piee, M. Acheoy ELTE De., Royal Miliay Academy 3, Avenue de la Renaissance B- Bussels A. ande os Hyeféquences U.C.L., Belgium ABSTRACT In his ae, he design and he modelling of an indoo imulse UWB GPR sysems (GHz-5GHz), buil in he scoe of he HUDEM ojec, is esened. Fo an imulse UWB sysem, a ime-domain modelling is an obvious choice. We exlain how he anennas can be chaaceised by hei nomalised imulse esonse. By consideing he anenna as a convoluion oeao, we ge a mechanism fo modelling he whole ada sysem as a cascade of linea esonses, which gives a lo of advanages and ossible alicaion. In ou eseach i is used o exess he ada ange equaion in he ime-domain, o oimise he anenna configuaion and o une signal-ocessing algoihms. The deconvoluion of he signal souce and anenna imulse esonses is an ill osed oeaion. In his ae we esen a mehod fo decomosing an A-scan in a linea combinaion of waveles, using he Coninuous Wavele Tansfomaions - by oely choosing he mohe wavele. This echnique can also be used o educe he amoun of daa fo fuhe ocessing. Finally, esuls obained by ou UWB GPR sysem ae shown. Advanages and shocomings ae discussed. INTRODUCTION The Gound Peneaing Rada is a omising echnology fo deecion and idenificaion of buied landmines. Convenional GPRs usually wok a cenal fequencies below GHz (Daniels, 996). As landmines ae small objecs and ofen shallow laid, a lage bandwidh is needed fo a bee deh esoluion and deailed echo. The use of imulse wideband sysems involves some echnical oblems. Ciical oins ae he UWB anennas. In ou eseach on he deecion and classificaion of lasic AP mines by means of UWB GPR, we buil an indoo imedomain UWB GPR sysems (GHz-5GHz), o sudy he advanages and shocomings of such a sysem. The sysem comonens ae, exce fo he anennas, mainly off-heshelf equimen. I consiss of he following as: on he ansmiing side a Picosecond Pulse Labs se-geneao is used. The geneaed se has an amliude of ols, a ise-ime of 45 s and a high wavefom uiy. This se is hen ansfomed by an imulse-foming-newok o an imulse wih a maximal amliude of.5 ols and a FWHM of less han s. On he eceive side, a 6GHz digiising oscilloscoe is used o measue he backscaeed signal. The oscilloscoe has an inenal delay line, a 4 bi esoluion and can aveage u o imes, o obain a highe dynamic ange. The anenna ai consiss of wo TEM hon anennas and is mouned on a comueconolled xy-able of m by.5m and m high. In he scanning aea of he able, wo boxes ae laced,.5m by.5m each and.8m dee. The fis one is filled wih sand, he second one wih loam. The emiiviy of boh yes of soil is fully chaaceised in funcion of fequency and moisue conen. THE DIELECTRIC-FILLED TEM HORN ANTENNA The main effo was u in he develomen of diecive UWB TEM hon anennas (Schees e. al, a) ha can be used off-gound. In he design we ied o limi he dimensions and weigh of he anennas o guaanee a high degee of mobiliy. To educe he hysical size of he anennas and o imove he dieciviy, wihou educing oo much he bandwidh, he anennas wee filled wih a dielecic (ε 3 ). Anenna measuemens evealed indeed ha he anennas wee moe diecive, he fequency coveage moved owads he lowe fequencies comaed o he ai-filled anennas. The anenna laes wee eched on a ined cicui boad (PCB) and eminaed by a 5 Ohm load, by uing wo Ohm SMD esisos in aallel beween he anenna lae exemiies. We also elaced he anenna laes by a se of 4 wies. The disance beween he wies is oo small o influence he anenna chaaceisics, bu i foces he cuens o be adial and i limis he suface of conducing meal. The lae is vey imoan, when using he anennas in combinaion wih a meal deeco. The TEM hons wee designed o mach he

2 5 Ω diving cable. To avoid eflecion of an unbalanced cuen comonen on he coax feedline exeio, a wideband balun was inegaed. The incile of his balun is based uon an elecosaic easoning (Rumsey, 966). A ae in he boom lae ovides a gadual ansiion fom an unbalanced se-u (ue anenna lae on a gound lae), owads a balanced configuaion (wo symmeical anenna laes). We can see ha he dielecic-filled TEM hon anenna ai is caable of adiaing and eceiving vey sho, bu sill clean ulses, which is of couse imoan fo his alicaion. The elaively small 3dB beamwidh of he anennas, 3 in he H-lane and 65 in he E-lane, and he small dimensions (*6*cm), makes hem suiable fo using off-gound in hand-held alicaions. ANTENNAS AS CONOLUTION OPERATORS A common way of descibing anennas in he ime domain is by means of hei imulse esonse (IR). Diffeen yes of IRs can be defined. We oed fo he nomalised imulse esonse (nomalised IR), i.e. an imulse esonse inegaing all fequency deenden anenna chaaceisics (Schees e. al, b; Fa e. al, 998). In his way, he ime domain anenna equaions, exessed in ems of he nomalised IR, become vey simle and accuae o use. No assumion abou fequency deenden ems has o be made. To simlify he exessions, we only conside anenna efomance fo dominan linea olaisaion of he E-field. The exension o he moe geneal case is ossible. Fis conside he ime-domain anenna equaion fo he ansmiing anenna. The adiaed field in he fa field is given by: E ad Z (, θ, ϕ, = hn, Tx ( θ, ϕ, Z π c c ds( / c) d () h N (,,, Tx θ ϕ is he nomalised IR of he ansmiing anenna in he diecion ( θ, ϕ ), S ( is he exciaion volage a he anenna feed in a 5 Ω load, Z c he imedance of he feed cable, and Z he imedance of fee sace. The co-odinaes oigin = is aken in he viual souce of he anenna, i.e. an aaen oin in he anenna fom which he adiaed field degades by a faco / (Fig.). The eceived volage ec (, in a 5 Ω load, a he feed of he eceiving anenna fo an incoming field E inc (, evaluaed in he viual souce of he anenna, is calculaed by: Z ec ( Z N, Rx c = h ( θ, ϕ, E ( () inc The wo ime-domain anenna equaions () and () ae defined in such a way ha h N, Tx = h N, Rx fo wo idenical anennas. I is seen ha wihin he 3dB beamwidh of he anennas, he nomalised IR can be exessed as h ( θ, ϕ, k. h (,,, wih h N (,, he nomalised N = N IR on boesigh of he anenna and k he eak volage aen of he anenna in he diecion ( θ, ϕ ). Hence he anennas ae comleely chaaceised by he wo lae. The nomalised IR on boesigh is easy o measue, using wo idenical anennas and a veco newok analyse (Schees e. al, b). Fig. shows he nomalised IR on boesigh of he anennas designed fo he laboaoy UWB GPR. nomalised IR [m/ns] Fig.: Nomalised IR of TEM hon anenna One of he advanages of consideing he anenna as a convoluion oeao is ha we ge a mechanism fo modelling he whole ada sysem as a cascade of linea esonses. RANGE PERFORMANCE OF THE SYSTEM The ada ange equaion is a useful desciion of he facos influencing ada efomance. Descibing efomances of an "imulse" sysem, using his equaion in he fequency domain, has some dawbacks: i conains fequency deenden ems and does no secify he naue of he ansmied signal. Fuhemoe, i is moe convenien o sae he minimum deecable signal of a ime-domain sysem as a eak volage. Exessing he ada ange equaion as a cascade of imulse esonses would be moe acical. The backscaeed field fom he age, chaaceised by an IR Λ v, v (, can be descibed as: Esca( ', θ ', ϕ', ' ) = Λv, v ( θ ', ϕ', ' ) Ead( ' ' / c) 4π ' (3) The co-odinae sysem '= is conneced o he age (Fig. ). Λ v, v ( only akes ino accoun he backscaeed

3 signal in he same olaisaion as he incoming field and is he ime equivalen of he squae oo of he age ada coss secion. z=-.4 Tx Anenna θ x= Rx Anenna x ( S iual souce iual souce E inc ( ( E ad Λ v, v ( ) z= E T ans θ Teans d R Zai Z gound ec ( ( E sca z Fig. : Elecomagneic configuaion Fig. 3: Configuaion of UWB GPR Fuhe some addiional losses have o be inoduced. In he scoe of demining alicaions, using anennas off-gound, we will only ake ino accoun he ansmission losses a he ai-gound ineface and he oagaion loss in he gound. The ansmission losses a he ai-gound ineface ae given by he wo oblique inciden ansmission coefficiens ans and T eans on he ineface. The oagaion losses in he gound ae no so easy o inoduce. The bes way o handle wih hese losses is eesening he gound as a low-ass file. The ansfe funcion of his file, eesening a oagaion of d mees in he gound, is given by ( α+ jβ ) d H ( ω ) = e, whee α is he aenuaion consan d [N/m] of he medium and β he hase consan [ad/m]. Boh consans ae funcion of fequency and comlex ' '' emiiviy ε j ε. Fo a given soil, i.e. exue, densiy and moise conen, and fo a given wo-way ah lengh d in he gound, he imulse esonse ( of he soil, eesening he oagaion losses, can be calculaed. Subsiuing equaion (), () and (3), and inoducing he addiional losses, he GPR ada ange equaion in he imedomain is found as: ec g d T. T ans eans ( =. R R c g h d ( ) N, Tx ( θ, ϕ, ) Λ v, v ( ) 8π Wih R : R : h N, Rx ds( ( θ, ϕ, d T (4) The oal ah lengh fom ansmiing anenna o he age The oal ah lengh fom eceiving anenna o he age The losses due o he off-boesigh osiion of he age ae aleady aken ino accoun by ( θ, ϕ,. Fig. 4 shows he ange efomance of ou laboaoy UWB sysem fo a ficiious meallic age in a lossy ice field soil. The soil, coming fom Cambodia, has a exue comosiion of 69% of sand, 4% of sil and 7% of clay. This sandy soil eesens a egula agiculue soil wih which demines ae confoned. The comlex emiiviy of he soil was measued in funcion of he moise conen ove a lage fequency band. The age is a ficiious meallic objec wih an IR Λ ( =,34. δ ( ), h N v, v comaable o an IR of a meallic shee wih a adius of 5 mm, aking only ino accoun he suface scaeing. In he imlemenaion, we inoduced some aoximaions and simlificaions, wihou loss of genealiy: he bisaic RCS of he age is aken indeenden of he bisaic angle, he anennas ae always focussed on he age and he ansmission coefficiens suose a fla ineface, no losses in he gound and a olaisaion aallel o he ineface. The viual souces of he anennas ae a 4 cm above he gound and he wo anennas ae seaaed by.8 cm (Fig. 3). The ai-gound ineface is a z=. The minimal deecable eak amliude of ou eceive, limied by is noise efomance and he anenna couling, is abou m (-47 dbm). This means ha he maximal deh of he age o be deeced in a % moise soil is cm. I can be seen ha he efomance of an UWB sysem is limied by he moise conen of he soil. Noe ha he diving souce S ( of he laboaoy sysem has a maximum amliude of only.5 and ha he eceive has a 4 bi esoluion and no ime vaying gain. Anohe imulse geneao and eceive could enhance he ange efomance.

4 -.8 cm -5 x= x θ = - -5 E 5 cm.5.5 Max signal amliude [dbm] Min. deecable eak amliude Rice sand: % moise Rice sand: 5% moise Fee sace loss Rice sand: % moise θ d Tans Teans 6cm Fig. 5a: Anenna configuaion X [cm] Fig. 5b: synheic B-scan z [cm] Fig. 4: Peak amliude of he eflecion on he age [dbm] in funcion of z [cm] fo %, 5% and % moise conen. ANTENNA CONFIGURATION In he design of he laboaoy UWB GPR, a sudy was made o oimise he osiion and oienaion of he Tx and Rx anennas. To educe he couling beween he wo TEM hons, hey wee u side by side wih a common H-lane. In incile anenna couling is no ciical and can be comensaed fo, bu if he inging beween he wo anennas lass oo long, i can inefee wih he useful backscaeed signal. The heigh of he anennas above he gound is chosen o be aound 5 cm. An imoan aamee fo he Tx-Rx anenna configuaion is he combined anenna aen i.e. he aen of he wo anennas consideed as one anenna. The esuling 3dB beamwidh of his combined anenna aen is obviously a funcion of he offse angle θ as eesened on Fig. 5a. A lage 3dB beamwidh is no a ioi unfavoable. When scanning ove a oin age wih he GPR, he B-scan will show a hyebolic sucue in he eflecion (Fig. 5b). A lage 3dB beamwidh oduces lage hyebolas in he B-scan, and can heefoe incease he deecabiliy of objecs. As a cieion fo he oimisaion of his offse angle θ, we have consideed he oal enegy found in he hyebolic shaed esonse of a oin age. This oal enegy eesens in some sense he execed efleced enegy of he oin age, afe enhancing he B-scan by an oimal migaion mehod. Fo his eason, we simulaed diffeen synheic B-scans (Fig. 5b) of a oin scaee a 6 cm in he gound, fo diffeen values of θ. The ficiious oin scaee is eesened by, = δ (. Fo each osiion a bisaic imulse esonse Λ v v x of he anenna ai (Fig 5a), he backscaeed signal ec ( was calculaed using he ada ange equaion (4). The oal enegy in he hyebolic shaed esonse of he age is calculaed as Eo = ec( d. dx. Fig. 6 shows x he oal enegy in he hyebolic esonse of he oin age as a funcion of he offse angle θ. The maximal enegy in he hyebola is found fo an offse angle of, which fo his configuaion (objec a a deh of 6 cm) coesonds o he angle ha focuses he anennas on he age, aking ino accoun he efacion. In ealiy he deh of he objec is a ioi unknown, bu is execed o be beween and cm. Theefoe, in he design of he laboaoy UWB GPR, an angle θ of was chosen. Enegy in hyebola x θ [deg] Fig. 6: Toal enegy in he hyebolic shaed esonse of a oin age a 6 cm of deh. DECOMPOSITION OF A-SCAN IN WAELETS In ada alicaions, i is well known ha he backscaeing fom a comlex age can be aoximaely modelled by a discee se of scaeing cenes (Tininalia e.al, 997). A mehod o exac he scaeing cenes of he age, and heeby enhancing deh esoluion, is by deconvolving he backscaeed signal. Because of he band limied naue of he emied wave and he effec of noise, deconvoluion of he backscaeed signal is an ill osed

5 oeaion. An addiional oblem when exacing he scaeing cenes in GPR signal is he boadening of he efleced signal due o he disesive behaviou of he gound. In his secion we esen a mehod fo exacing he scaeing cenes, ha is moe obus o noise and ha aially comensaes fo he disesive behaviou of he gound. The mehod, based on he Coninuous Wavele Tansfomaion (CWT), uses he scaling oeies of he waveles o couneac he disesive behaviou of he gound. The knowledge of he exac souce signal and of he anenna IR is basic in his mehod. If k scaeing cenes aoximaely model a scenaio of comlex ages and fla inefaces, he eceived signal can be wien as a linea combinaion of k waveles: ec ( B h τ, (5) k = a, ( whee he wavele h ( a, τ models he backscaeed signal on scaeing cene and B is amliude. Accoding o he ada ange equaion (4), and neglecing fo he momen he influence of he gound, he h backscaeed signal mus have he shae of ds ( hn, Tx ( hn, Rx( d, (6) which, in he case of ou laboaoy UWB GPR, looks like a deivaive of a Gaussian. Theefoe, we designed a nomalised mohe wavele o fi his shae: needed o accuaely eesen he oiginal eceived signal. Fig. 7 shows he esul of his mehod on an A-scan, coesonding o a PMN mine in loam a a deh of 5 cm. Aml [] x -7 B.. - Oiginal eceived signal Reconsuced signal Fig. 7: To: Aoximaion of A-scan as a sum of waveles. Boom: Coefficiens B of he scaeing cenes. The mehod can also be used o educe he amoun of daa fo fuhe ocessing. I is seen ha in mos of he cases an A-scan is accuaely eesened by a sum of 5 waveles. B,τ, a. Each wavele is chaaceised by one ile ( ) This daa educion becomes imoan fo C-scans. Fig. 8 shows a 3D eesenaion of a PMN mine a 5 cm of deh, by eesening only he scaeing cenes. h The delay a, τ ( = a τ π a e τ a (7) τ akes deals wih he wo-way avelling ime anennas-scaeing cene, and he scaling faco a aially comensaes fo he boadening of he eflecion on he scaeing cene. The coefficiens B ae found ieaively. Fis, The CWT of he eceived signal is comued (Hubbad, 996), using (7) as mohe wavele. B is found as being he maximum of he esuling CWT. To B coesond a scale faco a and a delay τ. Afewads B. h a τ ( ) is subaced fom he eceived signal. This, ocess is ieaed o geneae as many coefficiens as Fig. 8: 3D eesenaion of scaeing cenes

6 RESULTS The laboaoy UWB GPR sysem was buil o sudy he advanages and shocomings of UWB sysems. As a main shocoming we aleady menioned he limied ange efomance in we soils. As an advanage, we exec a bee deh esoluion and addiional infomaion on he ages due o he lage bandwidh. Theefoe some indoo ess wee efomed. Fig. 9 shows he aw daa of a B-scan of an Ialian S/5 AP mine buied a a deh of cm. I can be seen ha wihou any signal ocessing, he mine can easily be disinguished fom he ai-gound ineface. We also ied some signal-ocessing echniques on A- scans, essenially based on Pony mehods and on imefequency analysis, bu wihou much success. This is obably due o he low Q faco of he ages. None of hese mehods seems o be obus enough o be used fo classificaion uoses. We heefoe will oien ou wok in he fuue on B-scans and C-scans. esonses. An imoan a in his cascade is he modelling of he anennas by hei nomalised imulse esonse. The advanage of using he nomalised IR is ha he ime-domain anenna equaions become vey simle and accuae o use. We develoed he ada ange equaion diecly in he ime-domain. The ange efomance of he UWB GPR could heeby be exessed as a funcion of minimal deecable eak amliude of ou eceive and no in ems of a fequency deending signal owe as in he sandad ada ange equaion. I is seen ha he ange efomance deceases damaically wih moisue. To enhance he ange efomance, anohe eceive and moe insananeous owe in he imulse is needed. The imedomain model is also used o oimise he anennas offse angle. The oimal offse angle is he one ha focuses he anennas on he age. In his ae we also esened a ocessing echnique fo exacing he scaeing cenes fom he backscaeed signal, by decomosing his signal ino a linea combinaion of waveles. The mehod is obus o noise and akes ino accoun he disesive behaviou of he gound. The mehod can also be used o educe he amoun of daa fo fuhe ocessing..5 REFERENCES ime [ns] Daniels D.J., 996. Suface Peneaing Rada, IEE, London, UK os [cm] Fig. 9: B-scan of S/5 AP mine a cm deh CONCLUSIONS In ou eseach on he deecion and classificaion of lasic AP mines by means of UWB GPRs, we buil a elaively simle UWB sysem. The main effo was u in he develomen of he UWB anennas, ha can be used offgound. Indoo ess evealed he caabiliy of deecing shallow buied mines. When mines ae buied deee and he soil has a high aenuaion he deecion becomes almos imossible. Because of he low Q faco of he ages i is had o exac obus feaues ou of he A-scan, even fo an UWB sysem. We heefoe will oien ou wok in he fuue on B-scans and C-scans. Disosing of an accuae ime-domain model of he sysem uns ou o be essenial fo edicing sysem efomances and fo uning signal ocessing algoihms. The modelling is done by consideing he sysem as a cascade of linea Fa, E.G., Baum, C.E., 998. Time Domain Chaaceizaion of Anennas wih TEM Feeds, Senso and Simulaion Noes, noe 46, Ai Foce Reseach Laboaoy, USA. Hubbad, B.B., 996. The wold accoding o waveles, A K Pees, Ld., MA, USA, 996. Rumsey,., 966. Fequency indeenden anennas, Academic Pess Inc, New Yok, USA. Schees, B., Piee, M., ande os, A., a. Develomen of Dielecic-Filled TEM Hon Anennas fo UWB GPR, submied o IEEE AP- confeence, Swizeland, Ail. Schees, B., Acheoy, M., ande os, A., b. Time Domain Simulaion and Chaaceisaion of TEM Hons Using Nomalised Imulse Resonse, submied o IEE Poceedings - Micowaves, Anennas and Poagaion. Tininalia, L.C., Ling, H., 997. Feaue exacion fom elcomagneic backscaeed daa using join imefequency ocessing, Ula-Wideband, Sho-Pulse Elecomagneics 3, Plenum Pess, New Yok, 997.