Multipath Rejection Through Spatial Processing

Transcription

1 ultipath Rjction hrough Spatial Procing Alion Brown, NAVSYS Corporation BIOGRAPHY Alion Brown i th Pridnt and CEO of NAVSYS Corporation. Sh ha a PhD in chanic, Aropac, and Nuclar Enginring from UCLA, an S in Aronautic and Atronautic from I, and an A in Enginring from Cambridg Univrity. In 986 h foundd NAVSYS Corporation. Currntly h i a mmbr of th GPS-III Indpndnt Rviw am and Scintific Adviory Board for th USAF and rv on th GPS World ditorial adviory board. ABSRAC h dvlopmnt of low cot, mall iz antnna array and digital antnna lctronic ha rultd in mall, affordabl GPS phad array and digital rcivr that ar uitabl for commrcial application. NAVSYS Highgain Advancd GPS Rcivr (HAGR product includ th capability to patially proc ignal from up to 6- antnna lmnt. Prviou papr hav prntd th HAGR capability to prform bamforming, in both tatic and mobil nvironmnt, providing ovr 0 db of additional ignal gain on ach atllit trackd. hi capability rult in highly accurat cod and carrir tracking providing uprior prformanc for high accuracy application, uch a kinmatic GPS. In thi papr, a furthr nhancmnt to th HAGR digital ignal procing i dcribd. A patial ignal procing algorithm ha bn dvlopd that allow multipath ignal to b dtctd and an adaptiv antnna array pattrn i thn ud to minimiz thir rcivd ignal powr whil till applying gain in th dirction of th dirct GPS ignal. t data i includd howing th bnfit of thi mthod in rducing th multipath rror on both th pudo-rang and carrir-pha obrvation. INRODUCION ultipath rror ar caud by th rcivr tracking a compoit of th dirct GPS ignal and GPS ignal rflctd from narby objct. h rulting pha rror i a function of th pha offt btwn th dirct and multipath ignal and th rlativ ignal trngth. For a fixd intallation, th rror appar a bia, changing only a th lin-of-ight to th atllit chang du to th atllit motion. In a mobil application, th multipath rror will alo chang du to th motion of th antnna. GPS rcivr commonly u broad antnna gain pattrn o that all of th atllit abov th horizon can b trackd. Unfortunatly, thi incra th ucptibility of GPS rcivr to multipath rflction of th GPS ignal from narby objct. ultipath ignal from blow th rcivr antnna can b rmovd uing antnna with good low lvation ignal rjction, uch a a chok ring, but ignal arriving from lvation abov th antnna cannot b rjctd without alo liminating th atllit ignal that ar ndd to obtain high accuracy GPS olution. If th GPS rcivr cannot b locatd abov all potntial multipath ourc, ignal arriving from building or narby vhicl can cau ignificant multipath rror that will not b rjctd by th antnna pattrn Signal procing tchniqu hav alo prviouly bn dvlopd for rducing th ffct of multipath rror on th tracking loop. h tchniqu includ mthod that apply tmporal filtring of th multipath rror, multipath corrction from it calibration or corrlation hap corrction uing multipl digital corrlator. Whil many of th mthod hav provn uccful in rducing th ffct of multipath on th cod tracking loop, tmporal filtring and multi-corrlator ignal procing hav no ffct on th multipath carrir pha rror. h patial procing dign dcribd in thi papr minimiz th multipath ffct on both th cod and carrir tracking loop within th GPS rcivr. Until rcntly, howvr, only military GPS rcivr hav takn advantag of patial ignal procing to improv th robutn of th GPS ignal through th u of Procding of ION GSP 000, Salt Lak City, Utah, Sptmbr, 000

2 Controlld Rcption Pattrn Antnna (CRPA. NAVSYS pionrd th firt commrcial rcivr to includ patial ignal procing, th High-gain Advancd GPS Rcivr (HAGR. hi rcivr u digital patial procing to combin th ignal from up to 6 antnna lmnt, which allow ovr 0 db of gain to b applid to ach GPS atllit trackd, whn oprating in a bamforming mod [,, 3 ]. h rprogrammabl digital patial procing approach adoptd by th HAGR alo nabl adaptiv bam and null-forming to b applid which can b ud to rduc th ffct of multipath rror. GPS ULIPAH ERRORS ultipath rror ar caud by th rcivr tracking a compoit of th dirct GPS ignal and rflctd GPS ignal from narby objct uch a th ground or narby building. ULIPAH SV SIGNALS DIREC SV SIGNALS ANENNA ULIPAH SV SIGNALS GPS RECEIVER Figur GPS ultipath Error h rulting pha rror i a function of th pha offt btwn th dirct and multipath ignal and th rlativ ignal trngth. For a fixd intallation, th rror appar a bia, changing only a th lin-of-ight to th atllit chang du to th atllit motion. rflction ar rlativly clo to th antnna (compard with th C/A cod chip lngth of 93 m and that R(τ from th DLL i approximatly qual to on for both th dirct and multipath ignal. A ~ τ = A τ hi valu giv th approximat maximum cod pha rror for a -chip Early/Lat corrlator auming that th multipath dlay ar l than ½ chip. Blow, th carrir pha rror that i caud by th multipath rror i drivd a a function of th rcivd multipath ignal trngth and th carrir pha offt from th dirct path. ~ A = A + A ~ θ = ( A + A θ = θ θ θ A θ In Figur, th ffct of th multipath ignal on th C/N0 nvlop i hown a a function of th multipath ignal powr computd uing th abov quation. In Figur 3, th multipath pha angl rror nvlop i hown for th ca whn th multipath amplitud i btwn 3dB and 0dB blow th ignal A =A (0 db, (A =A/, and (A =A/0. A th pha of th multipath ignal chang rlativ to th dirct ignal, th obrvd ignal C/N0 and th carrir pha rror will ocillat btwn th rror bound hown in Figur and Figur 3. ( t = AC( t + τ in( ωt + θ + A ( tˆ( t = AR( τ ˆ τ in( ωt + θ + A ~ ~ = ( A + Ain( ωt + θ + θ R( τ C( t + τ in( ωt + θ ˆ τ in( ωt + θ A i th amplitud of th dirct ignal and A i th amplitud of th rflctd multipath ignal τ i th cod pha offt for th dirct ignal and τ i th cod pha offt for th multipath ignal θ i th carrir pha offt for th dirct ignal and θ i th carrir pha offt for th multipath ignal. Figur ultipath Amplitud Effct h abov quation can b olvd for th pudo-rang rror that will b obrvd with th DLL tracking loop and th pha rror that will b obrvd by th PLL tracking loop in a GPS rcivr. hi implifi to th following xprion, if it i aumd that th multipath

3 whr i (x k,t i th ith GPS atllit ignal rcivd at th kth antnna lmnt n k (t i th noi introducd by th kth DFE j (x k,t i th jth multipath ignal rcivd at th kth antnna lmnt h combind digital array ignal, z(t, i gnratd from umming th wightd individual digitizd GPS ignal. hi can b xprd a th following quation. N N z( t = w y( t = w i ( t i + nk ( t + i= j= j ( xk, t Figur 3 ultipath Pha Angl Error If th multipath ignal ar 0 db blow th dirct ignal, thn th carrir pha rror will b blow cm. If th multipath ignal can b maintaind 0 db blow th dirct ignal, thn th carrir pha rror will b blow 3 mm. ADAPIVE BEA SEERING AND ULIPAH NULLING ALGORIH o minimiz th ffct of th multipath ignal, th digital antnna array i ud to dynamically adjut th antnna array pattrn to provid gain toward ach of th atllit trackd, and alo dtct and plac null in th dirction of obrvd multipath ignal ourc. hi approach ha th following advantag ovr prviou multipath rduction tchniqu mploying fixd antnna pattrn or tmporal ignal procing mthod. Array lctronic timat th prnc of multipath ourc. h multipl ignal from th antnna lmnt ar procd to dynamically timat th prnc of multipath. hi provid a maur of th magnitud and ffct of thi rror ourc that can b ud to optimiz iting of GPS rfrnc tation to achiv high accuracy prformanc. Array lctronic adapt th antnna pattrn to optimiz th rlativ dirct ignal to multipath ignal powr. h bam tring i ud to provid antnna gain to rinforc th dirct ignal and to apply nulling to minimiz th ffct of th rcivd multipath ignal. A maximum liklihood algorithm wa lctd a our tchnical approach for implmnting th bamtring/multipath minimization algorithm. h digital ignal output from ach lmnt in th GPS antnna array can b dcribd by th following quation. y ( t = k N ( x, t + n ( t + i k k i= j= N j ( x, t k A paramtric approach i ud to timat th t of multipath ignal ourc. hi i computd bad on th cro-corrlation matrix computd from calibration ignal ( c gnratd within th digital array logic that obrv th ridual ignal on ach of th antnna lmnt aftr th bt timat of th dirct GPS ignal ha bn rmovd. P = E[( ˆ ˆ (( ˆ ˆ c A c A ˆ ˆ ( B ˆ N xˆ = ( CB xˆ π = xp{ ˆ λ U U i x x L h t of likly multipath ignal ourc ar calculatd a th t of valu (ξ that minimiz th following quation. min P Pˆ( ξ ξ Pˆ( ξ = ( N j= Aˆ j jθˆ j ˆ j ( N j= SV SV Aˆ j jθˆ j h dirction of th multipath ignal i ud to comput th optimal wight to apply bam-forming to th dird GPS atllit, and plac a null on th timatd multipath ignal dirction. h wight rquird to minimiz th antnna gain in th dirction of th multipath ignal and maximiz th gain in th dirction of th dird GPS atllit i thn computd from th following quation. R N = Iσ n + j= S S R w = R S Aˆ j ˆ j ˆ j h ignal-to-multipath gain improvmnt from th patial procing aftr th multipath nulling wight hav bn applid can b computd from th following quation 4. ˆ j

4 S / i = w w w w h prformanc of th bam-formr/multipath minimization algorithm i illutratd in Figur 4 and Figur 5 for imulatd data auming a ingl pointourc multipath rflction at az/l [70,0]. h figur how that th multipath minimization algorithm appli ovr 40 db of attnuation on th multipath ignal in ach of th ca. orovr, th gain from th bam forming incra th C/No by 8 db of gain on ach atllit trackd. h rult how th idal ituation with a clan multipath ourc. In th ral-world nvironmnt multipl ourc will b prnt which will rduc th null-dpth poibl. DIGIAL BEAFORING GPS RECEIVER (HAGR h GPS patial procing algorithm for multipath nulling ha bn dvlopd for implmntation in NAVSYS High-gain Advancd GPS Rcivr (HAGR 5. h rprogrammabl digital patial procing capability inhrnt in th HAGR allow GPS ignal to b combind from a many a 6 antnna and crat a multi-bam antnna pattrn to apply gain to up to ight GPS atllit imultanouly. Currntly a Prci Poition Sytm (PPS vrion of th HAGR (th HAGR-00 i alo in dvlopmnt with channl L and L capability. h 6-lmnt antnna array i hown in Figur 6. A mall 7-lmnt antnna configuration can alo b ud which lvrag NAVSYS miniatur antnna array (ini- Array tchnology 6. t data howing th bam-tring prformanc of th ini-array and HAGR in a mobil nvironmnt uing th ini-array i includd in rfrnc []. Figur 4 Singl ultipath Sourc - SV at 90 dg lvation Figur 6 HAGR 6-lmnt antnna array Figur 5 Singl ultipath Sourc - SV at 30 dg lvation

5 6 Antnna Elmnt DFE odul DFE odul DFE odul DFE odul o All odul Local Ocillator 8 Procing Channl Array Wight Logic Procing Channl Antnna Elmnt Output Bu Wight & CorrlatorControl Sampl Clock and Rfrnc Clock to All Circuit Corrlator Logic Procing Channl Procing Channl Calibration Logic Control Computr Figur 7 HAGR Sytm Block Diagram I/Q Data N C B Attitud Snor h HAGR ytm architctur i hown in Figur 7. h ignal from ach antnna lmnt i digitizd uing a Digital Front-End (DFE a hown in Figur 8. h bank of digital ignal i thn procd by th HAGR digitalbam-tring card to crat th compoit digital bamtrd ignal input for ach of th rcivr channl. h wight for ach channl ar dynamically downloadd through oftwar control. h wight ar adaptivly computd in th HAGR hot computr uing th corrlation rult gnratd in th calibration logic to timat in ral-tim th dirction of multipath ignal ourc, a dcribd in th prviou ction. accuracy of th multipath dtction and null dpth would b xpctd to improv whn procing longr window of data in th HAGR oftwar. In Figur 0 to Figur 3 th pot-procd data i plottd howing th dtctd ignal amplitud on ach individual lmnt, and from th phad array olution combining th lmnt in th array. hi how that thr i ignificant variation in th dtctd amplitud lvl (up to 5 db indicating th prnc of trong multipath ignal. In Figur 4 to Figur th dtctd dirct ignal and multipath ignal ar plottd from th array. A xpctd, thr ar multipl multipath ourc vidnt. In th figur, th antnna gain pattrn i alo plottd which how that around db attnuation i placd in th dirction of th multipl ourc. h null dpth in th dirction of th jammr will alo improv whn uing mor than 0 mc of data in th timation proc. h patial procing will rduc th multipath rror on both th cod and carrir tracking loop. In Figur 9 th pak pha rror that can b xprincd from multipath i plottd a a function of multipath/ignal trngth. Sinc th multipath ignal trngth i alway blow th dirct ignal, vn bfor th patial procing ha bn applid, th additional attnuation providd by th patial procing will rduc th multipath ffct on th pha rror to blow mm. W ar currntly working on implmnting th multipath dtction and minimization algorithm in th HAGR oftwar to allow ral-tim opration. 6 lmnt DFE modul Broad-band Data Loggr Figur 8 Data Loggr for 6 Elmnt Antnna Pak pha rr (m ULIPAH INIIZAION ES DAA t data wa collctd with our GPS digital torag rcivr (DSR to analyz th prformanc of th multipath minimization algorithm. hi rcord th broad-band GPS ampld ignal from ach of 6 antnna lmnt at a rat of 40 p (Figur 8. h data wa thn playd back through th HAGR ignal procing to valuat th ffct of th patial procing algorithm on rducing th cod and carrir multipath rror. Sinc th DSR data i rcordd at a rat of 80 byt/c, only a hort window of data wa collctd for pot-tt analyi. h multipath dtction and minimization algorithm wr computd uing a 0 mc ampl of th data. h ultipath Attnuation (db Figur 9 ultipath Pak Pha rror v Attnuation (db

6 75 iv 80 iv SNR (db SNR (db im (mc Figur 0 SV 6 Amplitud from ach lmnt and array 75 iv im (mc Figur 3 SV 6 Amplitud from ach lmnt and array SNR (db im (mc Figur SV 7 Amplitud from ach lmnt and array 75 iv 3 Figur 4 SV 6 Dtctd Dirct and ultipath Signal Powr SNR (db im (mc Figur SV 3 Amplitud from ach lmnt and array Figur 5 SV 6 ultipath Nulling Antnna Pattrn

7 Figur 6 SV 7 Dtctd Dirct and ultipath Signal Powr Figur 9 SV 3 ultipath Nulling Antnna Pattrn Figur 7 SV 7 ultipath Nulling Antnna Pattrn Figur 0 SV 6 Dtctd Dirct and ultipath Signal Powr Figur 8 SV 3 Dtctd Dirct and ultipath Signal Powr Figur SV 6 ultipath Nulling Antnna Pattrn CONCLUSION h analyi and tt data collctd hav dmontratd that digital patial procing can b ud to dtct GPS multipath ignal and minimiz thir ffct by applying

8 gain toward th GPS atllit (bam-tring and null in th dirction of th dtctd GPS multipath ignal. Sinc th digital patial procing ha th ffct of rducing th multipath/ignal amplitud lvl bfor th corrlation proc i prformd, thi mthod of multipath rjction improv th tracking prformanc on both th cod and carrir tracking loop. h dynamic bam/null-tring patial procing capability dcribd in thi papr (patnt pnding i bing implmntd in our HAGR commrcial product. hi tchnology offr bnfit to high accuracy GPS application that rly on prci cod and carrir pha obrvation. h application includ providing GPS rfrnc data, kinmatic GPS poitioning and prci tim tranfr. REFERENCES A. Brown, K. aylor, R. Kurtz, H. ng, Kinmatic t Rult Of A iniaturizd GPS Antnna Array With Digital Bamtring Elctronic, Procding of ION National chnical ting, Anahim, California, January 000 A. Brown, J. Wang, High Accuracy Kinmatic GPS Prformanc uing a Digital Bam-String Array, Procding of ION GPS-99, Nahvill, nn, Sptmbr A. Brown, H. ng and R. Kurtz, t Rult Of A Digital Bamforming GPS Rcivr For obil Application, Procding of ION National chnical ting, Anahim, California, January D. Johnon and D. Dudgon, Array Signal Procing, Concpt and chniqu, Prntic-Hall, 993 pp5 5 Dr. Alion Brown, Randy Silva, Gnghng Zhang, t Rult of a High Gain Advancd GPS Rcivr, ION 55 th Annual ting, Cambridg, aachutt, Jun A. Brown and H. ng, iniaturizd GPS Antnna Array and t Rult, Procding of GNSS 000, Edinburgh, Scotland, ay 000