Target Detection by Multisite Ultra-Wideband Radar Systems with Information Fusion

Transcription

1 Targe Deecion by Mulisie Ulra-Wideband Radar Syses wih Inforaion Fusion V. CHERNYAK Moscow Aviaion Insiue (Sae Technical Universiy) , Volgina ul., Moscow, RUSSIA Absrac -Deecion characerisics of Mulisie Radar Syses (MSRSs) consising of Ulra-Wideband (UWB) radars are analyzed. Two ypes of UWB radars are considered: using pulses wih carrier frequencies and using shor carrier-free pulses. Targe radial diensions are assueo be uch greaer han range resoluion cells of UWB radars, so ha each radar receives range profiles of a arge. Noiceable advanages of MSRSs wih UWB radars have been shown; especially when range profiles of a arge ay be expeceo be equal wih respec o all radars. Key- words: Mulisie (ulisaic) radar syses, ulra-wideband radars. 1 Inroducion There is a growing ineres in Mulisie Radar Syses, MSRSs (Mulisaic Radars, Muliradar, or Need Radar Syses) during he las years for boh iliary and civilian applicaions (e.g., [1 5]). This ay be explained by any significan advanages of MSRSs as copared wih onosaic radars [6]. Aong hose advanages are: greaer arge deecion range, higher arge coordinaes easureen and racking accuracy in acive and passive odes, higher resoluion capabiliy and ohers. As i was shown in [6], any iporan characerisics of MSRSs srongly depend on signal bandwidhs: he wider bandwidh, he beer hese characerisics. In recen years an increasing aenion has been devoeo Ulra-Wideband (UWB) radars (e.g., [7 9]). Though no generally acceped, he following foral definiion of he UWB radar is used by any specialiss [7, 8]: η = ( f upper f lower )/( f upper + f lower ).5. (1) Here f upper, f lower are he upper anhe lower frequencies, respecively, of signal processed. Typical UWB radars corresponding o he above definiion are radars wih very shor carrier-free pulses. Advanages and drawbacks of such UWB radars are considered in any works (e.g., [7 9]). Here we noe only one significan drawback: because of very shor pulses, ransied energy is, as a rule, sall, so ha UWB radars of his ype have sall deecion range. Though i is no necessary follows fro he above definiion, he salien feaure of real UWB radars is heir large absolue signal bandwidh f s periing o resolve in range separae eleens (scaering ceners, flare spos ) of a arge. Received signals urn ou o be range profiles of arges. If his is acceped as a disincive feaure of UWB radars, hen anoher a well-known ype of radars ay be referred o as UWB radars. Radars wih absolue bandwidhs.3,.5 GHz (ha is wih range resoluion capabiliies c/ f s.5,.3 ) providing arge range profiles appeared as early as 3-4 years ago (e.g., [1, 11])). However, because radar range resoluion capabiliy does no depend on signal carrier frequency, all such radars have usually high carrier frequencies, so ha heir fracional bandwidh (he bandwidh o carrier frequency raio) is no large, as a rule, no ore han 1%. A large absolue bandwidh of such radars can be achieved also by using very shor odulaing pulses. The ain advanage of such radars is he absence of range sidelobes (as wih shor pulse carrier-free radars). However, hough such radars ay have anennas wih narrow direciviy paerns (unlike shor pulse carrier-free radars where such anennas are uch ore difficul o consruc), low ransied pulse energy leads o sall deecion range. Therefore, if a large deecion range is required, sufficienly long pulses are convenionally used wih frequency odulaion or phase coding inside each pulse (including frequency odulaion by rando noise [1]), and special echniques are eployeo iniize range sidelobes of copressed signals in receivers. Thus, UWB radars of boh ypes (wih carrier frequencies and carrier-free) have, as a rule, large absolue bandwidhs. Taking ino accoun ha soe principal characerisics of MSRSs depend essenially on he wavefor bandwidh, i is iporan o consider principal feaures of MSRSs based on UWB radars. I ay be expeceha such

2 MSRSs have significan advanages over MSRSs wih convenional narrow-band radars and good prospecs in a wide range of applicaions. We consider in his paper only deecion characerisics of UWB MSRSs. Deecion Algorihs I is clear ha resulan deecion characerisics of UWB MSRSs are deerined by energy characerisics of UWB radars and by energy gain as a resul of join processing in MSRSs (inforaion fusion). As was enioned above, hanks o large signal bandwidhs f s of UWB radars, os arges urn ou o be exended ones (heir radial diensions are greaer or uch greaer han radar range resoluion cells c/ f s ). In his siuaion, deecion characerisics depend significanly on specific arge range profiles and received signal processing. Le us consider UWB radars of he seconype (wih large absolue and sall fracional bandwidh). The deerining feaure for signal processing a such radars is he fac ha he received signal w a v e f o r refleced by a poin- like arge is known because i is he sae as he wavefor of ransied signals. Opial processing consiss of coheren filraion acheo reflecions fro each flare spo of a arge ano incoheren inegraion along is range profile (e.g., [13]). When all or nearly all flare spos of a arge are resolved in range, received signals do no flucuae or flucuae very weakly. The oupu value (which is o be copared wih a hreshold) is a su of n uncorrelaed (under cerain condiions) rando variables wih Rice or Rayleigh probabiliy disribuions. Rice disribuion corresponds o hose range resoluion cells of a range profile where signals fro flare spos are presen, and Rayleigh disribuion corresponds o cells wih noise alone. Here n is he oal nuber of range resoluion cells in a range profile. We assue here for sipliciy and for obaining he bes possible resuls, ha n is known for all UWB radars. If i is no so, differen ulichannel srucures ay be used wih cerain energy losses [13, 14]. If n is large enough (especially when os of range resoluion cells are wih signals) hese oupu values ay be considered as Gaussian variables wih cerain eans and variances depending on he specific range profiles. I ay be expeceha wo opposie facors influence UWB radar deecion characerisics as copared wih narrowband radars. For fixed energy (or average energy) of received signals, he bandwidh widening leads, on he one hand, o energy gain (for large values of deecion probabiliy) because of flucuaions eliinaion, and, on he oher hand, o energy losses because of incoheren suaion anhe necessiy of higher hreshold for keeping fixed resulan false alar probabiliy. For radars of he firs ype (using very shor, carrier-free pulses), he deerining feaure for signal processing is u n c e r a i n y o f r e c e i v e d s i g n a l wavefor even fro a p o i n - like arge. Because of very large fracional frequency, signal wavefors change significanly in he process of ransission, propagaion, reflecion and recepion (e.g., [7, 8]). The opial deecion algorih (according o he generalized axiu likelihood crierion) for such radars was synhesized in [14]. The algorih akes ino accoun ha he Pulse Repeiion Period (PRP) is usually known. Besides, range profiles of a arge ay be considered as having he sae (hough unknown) for in several, for exaple, M, successive PRPs. The value of M depends on any facors (including he characer of arge oion) bu for sufficienly sall PRP, a leas M >1 ay be assued. (I should be noeha because of sall axiu range of such radars enioned above, heir PRPs are usually shor). The opiu algorih ay be wrien as follows: L = x( + + kt ) d r u () < where is he duraion of he expeced signal (range profile) in ie ( = n / f s ), M is he nuber of successive range profiles assueo have he sae for, x() is he overall received signal (signal plus noise or noise alone), d is he ie delay deerined by arge range, T r is he PRP of he radar. As can be seen fro Eq. (), he assupion of unchanged range profiles of a arge during successive M PRPs, has leo coheren suaion of M corresponding porions of inpu signals. The energy of his su is o be copared wih a hreshold. 3 Deecion Characerisics Typical deecion characerisics of a narrowband radar and UWB radars of boh ypes for he sae arge are shown in Fig. 1. These curves are calculaed on he assupions ha: 1) wo echoes (of

3 Fig. 1. Deecion characerisics of a narrowband radar (curve 1), an UWB radar wih carrier-free pulses (curve ), an UWB radar wih carrier (curve 3); M = ; P f a =1 3. wo neighbor periods) are joinly processed ( M = ); ) apliude flucuaions of narrowband received signals are subjec o Rayleigh probabiliy disribuion, anhese flucuaions are copleely correlaed in neighbor PRPs; 3) oal nuber of arge range resoluion cells for UWB radars n is known and equal o 3, so ha oal nuber of oupu signal saples is equal o n = 64; 4) all flare spos of he arge are resolved in range by UWB radars of boh ypes; 5) he range profile represen signals fro N f s = 16 flare spos wih he following disribuion of Signal-o-Noise Raios (SNRs): each signal fro flare spos has SNR equal o 1% of he oal SNR, each signal fro 8 flare spos has SNR equal o 7% of he oal SNR, each signal fro 4 flare has SNR equal o 4%, and each signal fro flare spos has SNR equal o % of he oal SNR; range side lobes are ignored; 6) he oal SNR (E s /N) for he UWB radars is equal o he average SNR ( E s a v /N) for he flucuaing narrowband signals; 7) he false alar probabiliy is P f a =1 3. Deecion characerisics do no depend on he specific arrangeen of resolved flare spos along a arge. I can be seen ha UWB radars wih carrier frequencies and wih shor carrier-free pulses have alos he sae deecion characerisics (for he assued paraeers). Coherence processing ached o signals refleced fro all flare spos is possible in radars wih carrier frequencies bu incoheren signal suaion afer envelope deecion is no so effecive. Unknown wavefor received fro each flare spo does no peri using ached filraion in radars wih shor carrier-free pulses bu his ay be copensaed by coheren suaion of inpu signals. For M = we have a balanced siuaion. 1 As was o be expeced, hese radars have energy gain over narrowband radars for high deecion probabiliies ( P d >.8). Energy loss for P d =.5 is abou 5 db. I is ineresing o noe ha for a arge wih doubled n and N f s, anhe sae oal SNR, addiional loss of.7 db akes place. I eans ha when all flare spos are resolved, so ha signal flucuaions are eliinaed, furher increase of range resoluion leads only o addiional energy losses. As was shown in [6], deecion characerisics enhanceen in a narrowband MSRS as copared wih a onosaic radar, depends significanly on correlaion degree of signal flucuaions a he inpus of spaially separaed saions. Join processing of signals wih copleely correlaed flucuaions leads o an energy gain caused by he increase of oal received signal energy. When UWB radars are used in MSRSs and all or nearly all arge flare spos are resolved in range, so ha received signals do no flucuae, energy gain as a resul of join signal processing (inforaion fusion) is deerined by he increase of oal received signal energy. When a MSRS consiss of wideband radars, sall baselenghs beween spaially separaed saions (copared wih expecearge range) ay be used. Such MSRSs are uch sipler han MSRSs wih large baselenghs. Under his condiion, signal energy received by several spaially separaed UWB radars wih equal characerisics ay be considered o be equal for arges wih approxiaely equal disances fro radars. This is he ore so, since resolvearge flare spos have usually broad direciviy paern. As far as specific fors of range profiles are concerned, we consider wo cases: equal range profiles and differen range profiles. When he baselenghs of MSRSs are sall enough o have h e sae (bu unknown) range profiles of a arge a all UWB radars, hen here is no difference beween signal processing a each saion and inersaion processing. Fro Eq. () we have opiu deecion algorih: 1 Coheren suaion of M successive signals fro a oionless arge is possible heoreically a UWB radars wih carrier frequencies oo. However, since a carrier frequency is usually a leas by an order greaer han he bandwidh, coheren suaion is uch ore difficul han a he UWB radars wih carrier-free pulses.

4 L = i= 1 x( + + k Tr ) d < u. (3) Typical deecion characerisics are shown in Fig. for a MSRS wih = 3 he sae radars as in Fig. 1. For narrowband radars, flucuaions are assueo be copleely correlaed in ie and in space. A each saion wo repeiion periods are joinly processed (M = ). This siuaion is equivalen o a onosaic radar processing echoes of M = 6 successive periods. of 18 received signals. The corresponding deecion characerisics are presened in Fig. 3. I can be seen ha energy gain of he MSRS consising of UWB radars (as copareo a onosaic UWB radar, see Fig. 1) is greaer by abou 1 db for he radar wih shor carrier-free pulses and abou 5 db for he radar wih carrier frequencies. A noiceable energy advanage over he MSRS wih narrowband radars begins for deecion probabiliy exceeding.75. When a arge provides d i f f e r e n u n k n o w n r a n g e profiles relaive o spaially separaed saions, and cooperaive signal recepion does no used, opial signal processing is reduceo suaion of Fig.. Deecion characerisics of he MSRS wih = 3 he sae radars as in Fig. 1; he noaion is he sae as in Fig. 1; M = echoes are processed a each radar; equal unknown arge range profiles a each radar; P f a =1 3. I is seen ha UWB radars wih shor carrier-free pulses have beer deecion characerisics han UWB radars wih carrier frequencies because of coheren signal processing of all 6 received signals. Energy gain caused by inforaion fusion in he MSRS differs fro db for he UWB radar wih carrier frequency anhe narrowband radar up o 5 db for he UWB radar wih shor carrier-free pulses. Advanages over he narrowband radar begin fro deecion probabiliy P d.8. Much greaer energy gain ay be obained in a MSRS wih so called cooperaive signal recepion [6]. In his case all radars ay receive and process arge echoes when a arge is illuinaed no only by own bu by any oher radar (or ransiing saion). If range profiles are equal a he inpus of all he receivers, he opiu deecion algorih akes he for: L = x( + + kt ) d r u. (4) i= 1 k = < When = 3, and echoes of M = repeiion periods are processed a each radar, we have join processing Fig. 3. Deecion characerisics of he MSRS wih = 3 he sae radars as in Fig. 1 and cooperaive signal recepion; he noaion is as in Fig. 1; M = ; equal unknown arge range profiles a each radar; P f a =1 3. energy esiaes obained a all saions: i M 1 1 L = x i ( d i kt r ) i 1 N + + u. (5) < = i I eans ha inersaion processing becoes incoheren. This leads o energy losses. Typical deecion characerisics for he sae MSRS as in Fig. bu for differen unknown range profiles relaive o all he 3 saions are shown in Fig. 4. I can be seen ha energy gain of he MSRS consising of UWB radars wih shor carrier-free pulses is uch less han in he case of equal range profiles because of he incoheren inersaion signal processing. MSRSs wih UWB radars of boh ypes have alos he sae deecion characerisics. For cooperaive signal recepion, and no oo large baselenghs beween saions, a range profile received by he i-h saion when a arge is illuinaed by he j-h saion ay be assueo be equal o he range profile received by he j-h saion

5 Corresponding deecion characerisics for = 3 and M = are shown in Fig. 5. I is seen ha he deecion characerisic for he MSRS consising of radars wih shor carrier-free pulses akes an inerediae posiion in coparison wih characerisics of Fig. 3 and Fig. 4. Energy gain as copared wih a onosaic radar is of db. Advanages over he MSRS wih narrowband radars begin when deecion probabiliy is greaer han Fig. 4. Deecion characerisics of he MSRS wih = 3 he sae radars as in Fig. 1; he noaion as in Fig. 1; M = echoes are processed a each radar; differen unknown arge range profiles a each radar; P f a =1 3. when he sae arge is illuinaed by he i-h saion. Then we have M equal range profiles a each saion and M pairs of equal range profiles a ( 1)/ differen saions. For radars wih equal echnical characerisics he opiu deecion algorih akes he for: i L = x i ( + d i + kt r ) + i= 1 i j i= 1 j= i+ 1 1 ij k = x ( + dij + kt ) + x r ji ( + dji + kt r ) u. < (6) Fig. 5. Deecion characerisics of he MSRS wih = 3 he sae radars as in Fig. 1 and cooperaive signal recepion; he noaion as in Fig. 1; M = equal unknown arge range profiles a each radar and M = pairs of equal range profiles a = 3 differen radars; P f a = Conclusion Deecion characerisics of MSRSs consising of UWB radars have been analyzed. 1. Fro he poin of view of opiu signal processing for arge deecion, here is a significan difference beween wo ypes of UWB radars: 1) radars wih shor carrier-free pulses, and ) radars using ulra-wideband pulses wih carrier frequency (shor pulses or pulses wih inernal frequency odulaion or phase coding). For he seconype, he received wavefor fro a poin-like arge is known, anhis knowledge is used for a coheren porion of signal processing. For he firs ype, he received wavefor even fro a poin-like arge is unknown.. Deecion characerisics are obained for UWB MSRSs of boh ypes. I is shown ha eliinaion of received signal flucuaions leads o noiceable energy gain and energy loss a high and low deecion probabiliies, respecively, for boh onosaic UWB radars and UWB MSRSs, as copared wih narrowband radars and narrowband MSRSs. 3. Significan energy gain can be achieved by UWB MSRSs wih shor carrier-free pulses if arge range profiles received by differen saions ay be expeceo be equal (especially for cooperaive signal recepion) because of coheren inersaion processing. For differen range profiles a spaially separaed saions, inersaion processing us be incoheren, and deecion characerisics of shor carrier-free UWB radars near o hose of UWB radars wih carrier frequencies. References: [1] Vladiir Kubecek and Per Svoboda, Passive surveillance syse VERA, in Proc. of Fifh In. Conf. on Radar Syses (Radar 99), Oral session 1.5, Bres, France, 17 1 May 1999.

6 [] Albero Moreira, Spaceborne SAR syses: fuure developens owards uli-saic configuraions, in Proc. In. Radar Syposiu (IRS 3), pp. 7 3, Dresden, Gerany, 3 Sepeber Ocober 3. [3] Werner Langhans, Waler Randeu, and Helu Schreiber, Wide Area ulilaeraion syses for enroue surveillance purposes in he Ausrian air raffic anageen, in Proc. In. Radar Syposiu (IRS 3), pp. 35 4, Dresden, Gerany, 3 Sepeber Ocober 3. [4] Andrew L. Hue and Chrisofer J. Baker, Need radar sensing, in P r o c. o f 1 C I E I n. C o n f. o n Radar, pp , Beijing, China, Ocober, 1 (see also I E E E A e r o s p a c e a n d Elecronic Syses Magazine, Vol.18, No., 3, pp. 3 6). [5] Magnus Herberhson, A Mulisaic radar nework for air raffic anageen, in P r o c. I n. Radar Syposiu (IRS 3), pp. 9 34, Dresden, Gerany, 3 Sepeber Ocober 3. [6] Vicor S. Chernyak, Fundaenals of Mulisie R a d a r Syses. Mulisaic Radars a n d M u l i r a d a r Syses, Gordon and Breach Science Publishers, [7] Igor Ya. Ioreev, Ulra-wideband (UWB) radar: principal peculiariies, differences fro radiional radar, Elecroagneic Waves and Elecronic Syses, Vol., No.1, 1997 (in Russian). [8] Jaes D. Taylor, edior, Inroducion o Ulra- W i d e b a n d Radar Syses, CRC Press. Boca Raon, Ann Arbor, London, Tokyo, [9] Jaes D. Taylor, edior, Ulra- Wideband Radar T e c h n o l o g y, CRC Press. Boca Raon, London, New York, Washingon D.C.. [1] Alexander A. Kuriksha, Ivan D. Oel chenko, Alexander B. Shelev, and Vladiir A. Yakunin, Locaion of space arges by radars, Radio - engineering Indusry, issue 1, 1995, pp , (in Russian). [11] R.K. Aven, J.D. Shelon, and P. Brown, The ALCOR C-band iaging radar, IEEE Anennas and P r o p a g a i o n M a g a z i n e, Vol. 38, No. 6, 1996, pp [1] Hongbo Sun, Yilong Lu, and Guosio Liu, Ulrawideband echnology and rando signal radar: an ideal cobinaion, IEEE Aerospace and Elecronic Syses Magazine, Vol. 18, No. 11, 3, pp [13] Yakov D. Shiran, Sergey P. Leshchenko, and Valery M. Orlenko, Advanages and probles of wideband radar, in P r o c. I n. C o n f. o n R a d a r ( R a d a r 3 ), pp. 15 1, Adelaide, Ausralia, 3 5 Sepeber 3. [14] Vicor S. Chernyak and Igor Ya. Ioreev, Deecion of exendearges by ulra-wideband radars, in Proc. In. Radar Syposiu (IRS 3), pp , Dresden, Gerany, 3 Sepeber Ocober 3.