Test Results of a Digital Beamforming GPS Receiver in a Jamming Environment Alison Brown and Neil Gerein, NAVSYS Corporation

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1 Tst Rsults of a Digital Bamforming GPS Rcivr in a Jamming Environmnt Alison Brown and Nil Grin, NAVSYS Corporation BIOGRAPHY Alison Brown is th Prsidnt and CEO of NAVSYS Corporation. Sh has a PhD in Mchanics, Arospac, and Nuclar Enginring from UCLA, an MS in Aronautics and Astronautics from MIT, and an MA in Enginring from Cambridg Univrsity. In 1986 sh foundd NAVSYS Corporation. Currntly sh is a mmbr of th GPS-III Indpndnt Rviw Tam and Scintific Advisory Board for th USAF and srvs on th GPS World ditorial advisory board. Nil Grin is a Product Managr for NAVSYS Corporation s Rcivrs Group and is rsponsibl for th managmnt and dvlopmnt of NAVSYS nxt gnration of GPS rcivrs. H is currntly complting his M.Sc. in Elctrical Enginring and holds a BSEE in Elctrical Enginring from th Univrsity of Saskatchwan. ABSTRACT NAVSYS High Gain Advancd GPS Rcivr (HAGR) uss a digital bam-string antnna array to provid additional gain in th dirction of th GPS satllit signals. This incrass th rcivd signal/nois ratio on th satllits trackd and also improvs th accuracy of th psudo-rang and carrir-phas obsrvations. Th dirctivity of th digital bams cratd from th antnna array also rducs th ffct of jamming and intrfrnc. This papr dscribs th opration of th HAGR digital bam string array in a jamming nvironmnt and includs tst data collctd from th HAGR at th Elctronic Proving Grounds, Fort Huachuca, that dmonstrat th prformanc of a digital, bam-string rcivr against intrfrnc sourcs. INTRODUCTION Th suscptibility of th GPS signals to intrfrnc is of concrn to th GPS usr community. Bcaus of th low rcivd powr of th GPS signals, outags can asily occur du to unintntional intrfrnc, and vn low powr jammrs can dny GPS opration ovr significant aras of opration. Currnt gnration GPS military Usr Equipmnt (UE) uss nulling lctronics to incras th Jammr/gnal (J/S) margin undr which thy can oprat. Ths analog lctronics us a Controlld Rcption Pattrn Antnna (CRPA) to crat an adaptiv pattrn which provids nulls in th dirction of a dtctd GPS jammr. With a 7-lmnt antnna array, such as th GAS-1 antnna in us by th DoD, nulls can b placd on up to 6 diffrnt jammrs. In this papr, an altrnativ GPS jammr protction solution is dscribd whr digital bam-string is usd to apply gain in th dirction of th GPS satllits. This improvs th J/S margin for ach of th GPS satllits trackd by applying optimizd antnna wights on a channl-by-channl basis. Digital bam-string has th following advantags for GPS anti-jam applications. 1. Incrass GPS satllit signal powr. Th bamstring provids gain in th dirction of th GPS satllits incrasing thir ffctiv C/N0. 2. Improvd GPS masurmnt accuracy. Th incras in C/N0 on th GPS satllits rducs th psudo-rang and carrir-phas masurmnt nois improving th navigation solution accuracy. 3. Improvs satllit covrag factor. With nullstring lctronics, significant sgmnts of th sky ar blankd out whn a jammr (or jammrs) ar dtctd. This will caus th GPS UE to los lock on multipl satllits whnvr jammrs ar dtctd, rducing th satllit covrag factor. With th bam-string approach, th antnna pattrn is optimizd to incras th satllit gain. This improvs th amount of tim that four or mor Procdings of ION GPS 2001, Salt Lak City, Utah, Sptmbr 2001

2 satllits will b trackd by th UE in th prsnc of jamming. 4. Maintains cod and carrir phas prcision. For high accuracy applications, such as Joint Prcision Approach Landing Systm (JPALS), it is ssntial that th cod and carrir phas prcision b maintaind. Th phas shifting applid by th analog null-string lctronics can dgrad th masurmnt accuracy whn jammrs ar dtctd. With digital bam-string, th cod and carrir masurmnt accuracy is maintaind. HIGH GAIN ADVANCED GPS RECEIVER NAVSYS High-gain Advancd GPS Rcivr (HAGR) 1 was usd to collct GPS masurmnts to obsrv th digital bam-string prformanc in th prsnc of jamming. Th HAGR componnts ar illustratd in Figur 1. With th currnt gnration analog CRPA antnna lctronics in us by th DoD, a singl composit RF signal is gnratd from th combind antnna inputs, adaptd to minimiz any dtctd jammr signals. With th HAGR digital bam-string implmntation, ach antnna RF input is convrtd to a digital signal using a Digital Front-End (DFE). In th currnt HAGR configuration, up to 16 antnna lmnts L1 and L2 can b supportd. Th 16-lmnt phasd array usd to support th bam-string tsts is shown in Figur 2. Each DFE board in th HAGR can convrt signals from four antnna lmnts (s Figur 3). Up to 16 Antnna Elmnts DFE Modul DFE Modul DFE Modul DFE Modul To All Moduls Local Oscillator 6 to 12 Procssing Channls Array Wights Logic Procssing Channl Antnna Elmnt Output Bus Wights & CorrlatorControl Sampl Clock and Rfrnc Clock to All Circuits Corrlator Logic Procssing Channl Procssing Channl Calibration Logic Control Computr I/Q Data N C B Figur 1 P(Y) HAGR Systm Block Diagram Attitud Snsor Th digital signals from th st of th antnna inputs ar thn providd to th HAGR digital signal procssing cards. Each card can handl th procssing for six GPS satllits, L1 C/A and L1 and L2 P(Y) whn oprating in th kyd mod. Th digital signal procssing is prformd in firmwar, downloadd from th host computr. nc th digital spatial procssing is uniqu for ach satllit channl, th wights can b optimizd for th particular satllits bing trackd. Th digital architctur allows th wights to b computd in th HAGR softwar and thn downloadd to b applid prcorrlation to crat a digital adaptiv antnna pattrn to optimiz th signal tracking prformanc. Figur 2 xtn Elmnt HAGR Antnna Array Figur 3 Digital Front End (DFE) Board DIGITAL BEAM-STEERING Th digital signal from ach of th HAGR antnna lmnts can b dscribd by th following quation. 2

3 Equation 1 y ( t) = k Ns s ( x, t) + n ( t) + Nj i k k i= 1 k = 1 j ( x, t) whr s i (x k,t) is th ith GPS satllit signal rcivd at th kth antnna lmnt n k (t) is th nois introducd by th kth DFE j j (x l,t) is th filtrd jth jammr signal rcivd at th kth antnna lmnt Th GPS satllit signal at ach antnna lmnt (x k ) can b calculatd from th following quation. Equation 2 2π T s i ( x k, t) = si (0, t)xp{ i 1i x k ) = si (0, t) sik λ whr s i (0,t) is th satllit signal at th array cntr and 1 i is th lin-of-sight to that satllit sik ar th lmnts of a vctor of phas angl offsts for satllit i to ach lmnt k Th combind digital array signal, z(t), is gnratd from summing th wightd individual filtrd DFE signals. This can b xprssd as th following quation. Equation 3 Ns Nj z( t) = w y( t) = w si ( t) si + n( t) + j j ( t) jl i= 1 l= 1 With bam-string, th optimal wights ar slctd to maximiz th signal/nois ratio to th particular satllit bing trackd. Ths ar computd from th satllit phas angl offsts as shown in th following quation. Equation 4 2π T xp{ i 1i x1) λ w BS =. = 2π T xp{ i 1i x M ) λ Th HAGR digital signal procssing implmntation also allows th bam-string wights to b adaptd in softwar basd on th dtctd jammr signal powr to furthr minimiz th Jammr/gnal powr aftr th wights ar applid. Th cross-corrlation matrix, R, can b usd to obsrv th jammr signal powr. This is computd from th raw signal digital outputs as shown in th following quation. Equation 5 T R = R = E[ y y ] = R + R + R J S Th signal-to-jammr gain can b computd basd on th following quation for a particular satllit signal 2. s N J j k Equation 6 w R S / J = w R w / tr[ R s i s i J w / tr[ RJ ] w Rs w / tr[ R ] i s ] w Rw / tr[ R] nc th satllit lin-of-sight is known, th numrator of this xprssion can b calculatd. Equation R / tr[ R ] = Ai / MAi = / M w w / M / J = w RJ w / tr[ RJ ] whr M is th numbr of antnna lmnts. GPS JAMMER TESTS AND DATA COLLECTION Jammr tsting, to valuat th digital bam-forming antijam prformanc, was conductd at th Army s Elctronic Proving Ground (EPG) at Ft. Huachuca, Arizona. Liv jamming tsts wr prformd using a 10 MHz wid nois jammr cntrd at L1. A singl jammr was usd which was locatd in a mountain canyon roughly NW of th tst location (s Figur 8 and Figur 4). During th tsts, GPS tracking loop masurmnts wr rcordd from a 16-lmnt HAGR antnna array (s Figur 5). Th HAGR was configurd to track using th L1 C/A cod signals (no P(Y)), using th convntional bam-forming wights drivd using mod of opration. Th tst rsults collctd wr compard with a SOLGR GPS rcivr at th sam location, which was usd as a rfrnc throughout th jammr tsts. During th jammr tsts, data was also collctd using NAVSYS Digital Storag Rcivr (DSR). Th DSR can b configurd to rcord data from up to 16 indpndnt antnna lmnts (s Figur 6). This allows logging of ral-world data from a digital phasd array, such as th 16-lmnt HAGR array shown in Figur 2. Th data rcordd from th multipl antnna lmnts can thn b playd back as a data library into our Advancd Hybrid GPS mulator (AGHS) 3 to valuat th prformanc of altrnativ digital spatial signal procssing algorithms post-tst. During th jammr trials, th DFE data was rcordd from four of th antnna lmnts and post-procssd to valuat th HAGR digital spatial procssing ffctivnss in diffrnt mods of opration. 3

4 16 Digital Front Ends HAGR Procssor Data Loggr Figur 6 xtn Elmnt Digital Storag Rcivr Digital Data Collction Digital Data Storag Softwar mulation (Matlab) Digital Storag Rcivr (DSR) Figur 4 Elctronic Proving Grounds Jammr Tst t RF Rmodulation (SIGGEN) Digital Data Play-back (AGR) Figur 5 HAGR at tst sit Figur 7 Advancd GPS Hybrid mulator Architctur REAL-TIME BEAM-STEERING TEST RESULTS Figur 8 is a skyplot of th satllit positions during th tst, with th rlativ jammr position indicatd by th arrow. Th tst sit was locatd in a mountain canyon so many of th lowr lvation satllits wr maskd from viw. Figur 9 to Figur 20 show th HAGR C/N0 (grn), th SOLGR C/N0 (blu), and th jammr to signal ratio rportd by th SOLGR (rd). During th tsts th SOLGR was rporting 40 db to 45 db J/S valus on L1 P(Y) cod. Th highst jamming lvl occurrd in th first 10 minuts aftr th jamming startd at 1:00 pm local tim. Th gain of th digital bams cratd from th HAGR antnna array improvs th prformanc of th rfrnc rcivr. Th dirctivity of th digital bams also provids additional anti-jam capability whn th satllits ar not in th lin of sit of th jammr (SV 2 and SV 7). SV 24 was only in viw of th rcivrs at th nd of th tst squnc. 4

5 Figur 12 SV 4 SOLGR L1 P(Y) JSR Figur 8 Satllit positions during jamming tsts Figur 13 SV 7 C/N0 Figur 9 SV 2 C/N0 Figur 10 SV 2 SOLGR L1 P(Y) JSR Figur 14 SV 7 SOLGR L1 P(Y) JSR Figur 11 SV 4 C/N0 Figur 15 SV 8 C/N0 5

6 Figur 16 SV 8 SOLGR L1 P(Y) JSR Figur 17 SV 9 C/N0 Figur 18 SV 9 SOLGR L1 P(Y) JSR Figur 19 SV 24 C/N0 Figur 20 SV 24 SOLGR L1 P(Y) JSR POST-TEST JAMMING ANALYSIS Th rcordd DSR data from four of th antnna array lmnts was playd back post-tst to valuat th digital bam-string prformanc and to analyz th dtctd jammr signal powr. A data st rcordd on Fbruary 6 was analyzd. This was collctd immdiatly following th HAGR bam-forming tst data shown in Figur 21. During this tsting, th SOLGR was not tracking any satllits. Th data was post-procssd as a 4-lmnt phasd array. This provids 6 db of gain ovr th individual lmnt data which was also analyzd (s Figur 22). Th digital bam-forming antnna gain pattrns for ach of th satllit obsrvations ar shown in Figur 24 to Figur 33. Th dtctd jammr powr was also computd from th cross-corrlation matrix (R) and is shown in Figur 23. Th powr in th top lft cornr of Figur 23 is from th jammr. Th powr in th bottom lft cornr is a scondary rturn that may hav bn causd by a slight tilt in th antnna array or a rflction off a narby objct. Th post-corrlation dtctd satllit powr was also computd and is shown in Figur 34 to Figur 37. For th satllits 2 and 24, which wr at too low a powr lvl to b trackd, th powr shown is only from th jammr. For th rmaining satllits, th dtctd satllit signal powr is shown post-corrlation. In Figur 38 and Figur 39 th stimatd J/S margin from th digital bam-string gain is shown, for a 4-lmnt and a 16-lmnt bam-string array calculatd basd on th satllit locations ovr 24 hours and th stimatd jammr location from Figur 23. Th worst cas J/S valu ovr a st of N satllits was computd. To navigat, at last four GPS satllits (N=4) ar rquird. Th worst cas J/S lvl ovr th 24 hour priod is catalogd in Tabl 1. With a 4-lmnt array, th phasd array always providd a jammr attnuation of 12 db on at last on satllit. On avrag, th array providd 13 db of attnuation on at last four satllits. For a 16 lmnt, th jammr attnuation for on satllit was always at last 28 db. On avrag th 16-lmnt array providd at last 27 db of attnuation on four satllits 6

7 and th worst cas attnuation ovr 24 hours for four satllits was 13 db. Tabl 1 J/S Attnuation (db) Satllits ovr 24 hrs #SVs lmnt avrag 4-lmnt worst cas 16-lmnt avrag 16-lmnt worst cas Figur 23 Dtctd Jammr Powr 48 C/N0 db-hz Figur 21 HAGR Obsrvd Bam-string C/N0 (Fb 6th ) lmnt bam string SV 4 Figur 24 4-Elmnt Pattrn - SV Mag Tim (mscs) Figur 22 Individual Elmnt and Composit gnal Powr Figur Elmnt Pattrn - SV 2 7

8 Figur 26 4-Elmnt Pattrn - SV 4 Figur Elmnt Pattrn - SV 7 Figur Elmnt Pattrn - SV 4 Figur 30 4-Elmnt Pattrn - SV 9 Figur 28 4-Elmnt Pattrn - SV 7 Figur Elmnt Pattrn - SV 9 8

9 Figur 32 4-Elmnt Pattrn - SV 24 Figur 35 Post-Corrlation Powr - SV 7 Figur Elmnt Pattrn - SV 24 Figur 36 Post-Corrlation Powr - SV 9 Figur 34 Post-Corrlation Powr - SV 2 Figur 37 Post-Corrlation Powr - SV 24 9

10 1 "Tst Rsults from a Digital P(Y) Cod Bamstring Rcivr for Multipath Minimization," A. Brown, N. Grin, ION 57 th Annual Mting, Albuqurqu, NM, Jun D. Johnson and D. Dudgon, Array gnal Procssing, Concpts and Tchniqus, Prntic-Hall, 1993 pp51. 3 Advancd GPS Hybrid mulator Architctur, A. Brown and N. Grin, Procdings of ION 57th Annual Mting, Albuqurqu, NM, Jun Figur 38 Four-Elmnt Bam-String worst cas J/S Figur 39 xtn lmnt Bam-String worst cas J/S CONCLUSION Th tst data collctd dmonstratd th ability of th HAGR digital bam-string GPS rcivr to oprat in th prsnc of a jammr and provid improvd J/S margin ovr a GPS rcivr using a singl antnna lmnt. Th tst data rcordd was also usd to dmonstrat how th digital antnna array could dtct th jammr spatial profil. This data can b usd to crat an adaptiv bam/null-string digital array pattrn that would furthr improv th A/J prformanc. Currntly an adaptiv digital bam/null-string HAGR rcivr is bing dvlopd by NAVSYS that will b flight-tstd undr contract to th Air Forc nxt yar. ACKNOWLEDGMENTS Th authors would lik to acknowldg th support of Mr. Eric Fishr of NEWTEC and th staff of th US Army Elctronic Proving Ground (EPG) GPS tst program for th assistanc thy providd during ths tsts. REFERENCES 10