Engineering, Technology & Applied Science Research Vol. 8, No. 4, 2018, 3270-3275 3270 www.etasr.com Dammalage: The Effect of Multipath on Single Frequency C/A Code Based GPS Positioning The Effect of Multipath on Single Frequency C/A Code Based GPS Positioning Thilantha Lakmal Dammalage Department of Remote Sensing and GIS Faculty of Geomatics Sabaragamuwa University of Sri Lanka Belihuloya, Sri Lanka thilantha@geo.sab.ac.lk Abstract—The differential GPS (DGPS) technique is one of the most popular and comparatively accurate techniques available to enhance the positioning accuracy by minimizing most of the common errors. However, the ultimate accuracy of the user location depends on the remaining non-common errors (multipath, receiver clock, and noise), which occur at the points of observation and reference. Out of these errors, multipath is the most dominant and challenging error to predict and minimize. Single frequency C/A code based GPS receivers are popular due to their comparatively low cost compared to dual frequency (L1/L2) GPS receivers. This paper focuses on evaluating the effect of multipath error on single frequency C/A code based GPS positioning. For the analysis, 72,000 continuous GPS observations with one-second interval under four different multipath environments were conducted by utilizing three geodetic GPS units. Accordingly, the observations with more than 5cm on the 2D positional error, created by the effected multipath, were always less than 25%. Here, an average of 16% of observations exceeded 20cm in 2D positional error. Further, it was noted that the presence of multipath introduces significantly higher and comparatively lower 3D positional errors on DGPS observations. This could be due to the compensation of negative and positive effects caused by the multipath and other remaining non- common mode errors at the reference and user stations. In addition, C/A code based single frequency GPS observations were significantly influenced by multipath, not only by the close-by reflectors but also by the ground surface. The effect of multipath was about 50% of the total 3D positional error for the four tested multipath environments. Keywords-C/A code observations; DGPS; multipath I. INTRODUCTION Global positioning system (GPS) developed by the US Department of Defense, was first planned to have 24 satellites in operation. Each of the satellites continuously transmitted two high-frequency carrier waves L1 and L2, with frequencies of 1575.42MHz and 1227.60MHz respectively [1]. C/A-code (coarse-acquisition) is for civilian users and P-code (precision) for U.S. military or authorized users. The code signals are superimposed on the L1 carrier, while L2 carries only the P- code [2]. Both codes allow a GPS receiver to measure the signal propagation time from satellites to the receiver instantaneously using the distance from satellites to the receiver, (pseudo-range). Pseudo-ranges are then utilized for the estimation of GPS receiver position [3]. Basically, there are two forms of observations depending on the capability of the receivers to process C/A code and L1/L2 carriers, referred to as code and carrier range observations respectively [4]. Therefore, GPS based positioning accuracy directly depends on the accuracies of calculated ranges to at least four satellites [5]. Most of the presently available GPS receivers utilize almost all the state-of-the-art technical improvements in GPS hardware and processing algorithms. However, still these GPS receivers suffer from significant positioning errors due to signal propagation delays through ionosphere and troposphere, satellite and receiver clock errors, bias on ephemeris data, multipath, and receiver and measurement noises [6-8]. Hence, the standard positioning service (SPS) accuracy widely varies with time, place, and most importantly, GPS receiver performance [2]. These measurement errors are generally classified as either common or non-common mode errors. The common errors (ionosphere delay, troposphere delay, satellite clock, and bias on ephemeris data) have similar effects on all receiver measurements operating in a limited geographic area [7]. Non-common mode errors (multipath, receiver, and measurement noises) are distinctive and the amount of their influence depends on the surrounding obstructions at the observation site and the technical specification of receivers [9]. Numerous techniques are available to minimize the above- listed sources of errors from GPS position estimation. A user equipped with a dual frequency (L1, L2) GPS receiver can estimate the ionospheric group delay and phase advance from the measurements themselves, and virtually eliminate the ionosphere as a source of error [10, 11]. Besides, many different models have been proposed to quantify the effect of ionospheric and tropospheric delay on GPS measurements. The Klobuchar model is one of the popular empirical models, which uses the satellite broadcast parameters to estimate the propagation zenith ionospheric delay [12]. There is no dearth of tropospheric models, in particular, the Saastamoinen model, which was derived based on the gas laws and simplifying assumptions regarding changes in pressure, temperature, and humidity with altitude [13]. However, none of these models can mo loc dat em and po err ref uti wh the sta err ref the aff hig the the tw DG me tec dou dep sof DG the eff the bee mu sur nea tha ob hav ma tha of ele ins tre div its cum sig sur the sig L1 wh Engineerin www.etasr n precisely es ost of them a cal metrologic ta-processing mployed to ove Differential c d comparativ sitioning accu rors as a co ference station ilized to deter hich are then a ese errors by ations. The ex rors depends ference and us e assumption fect both rece ghly dependen e capability of e position solu o categories, GPS, where t eter-level res chniques are a uble differenc pend on the ftware [16]. GPS, the ultim e amount of re fect of non-co e point of obs en identified t ultiple receptio rrounding obje Multipath is ar the GPS an at follows th structions in ve higher pos any ways [18] at the ideal co the sky with evation and stance, due to ees, fences, an verse nature an mathematic mbersome [19 gnal is demons rface at distan e respective g ght carrier pha frequency canΦ , co here A and d ng, Technology r.com stimate the to always require cal data. Som software offe ercome these d correction (DG vely accurate uracy by minim mbined opera n, the position rmine the mag applied to othe assuming they xtent to which on various fa ser GPS receiv of common eivers. Furthe nt on the GPS f utilizing carri ution. DGPS t namely carri the accuracies spectively [4 available. For ce, which are capability o Whatever the mate accuracy esidual commo ommon mode ervation. Of th to be the mul ons of one sat ects. II. MUL caused by GP ntenna, which he direct path the surroundi ssibility to dim ]. Therefore, a nditions for G h no obstructio up. However Earth’s surfac nd cables, free nd complex be al representa 9]. The effect strated only by nce d from the geometry [15]. ase measureme n be representos denote the amp y & Applied Sci otal effect on e additional p me GPS rece ered at a price drawbacks. GPS) is one o e techniques mizing most of ation [6]. In of which is a gnitude of com er GPS observ y were equall h DGPS redu actors; mainly vers [14, 15]. I environmenta ermore, DGPS receiver type ier and/or cod technique has ier phase base s vary from 4]. Several r instance, sin e very comm of the receiv e processing of the user lo on mode error errors at the r he latter, the d ltipath, which tellite signal d LTIPATH ERROR PS signals refl h could be mis h from the s ing vicinity o minish the pos as a thumb-ru GPS observatio ons at least f r, in practic ce (ground an e observation ehavior of mul ations extrem of multipath o y using a plana e GPS antenna . Accordingly ent (Φ ) for sa ted as in (1): plitude (voltag ience Research Dammalage: measurement parameters su eiver-operation e, however, c of the most po to enhance f the common this techniq accurately know mmon mode e vations to min ly affected for uces common y the separati It, sometimes, al conditions w S accuracy is e, which varies de measuremen been classifie ed and code centimeter-lev DGPS proce ngle differenc on in practic ver and proce technique us ocation depen rs and the com reference poin dominant mod is named aft due to reflectio R lected from su staken as the satellite [17]. of the GPS an sitional accura ule, it is consi ons are a clear for about 5 de cal conditions nd water), buil sites are rare ltipath signals mely difficult on the carrier ar vertical refl a. Figure 1 pr y, the direct lin atellite “s” bas (1) ge) and the ph h V The Effect of M ts and uch as n and can be opular GPS mode que, a wn, is errors, nimize r both mode ion of limits which s also s with nts for ed into based vel to essing ce and e, yet essing sed in nds on mbined nt and de has ter the ons by urfaces signal Any ntenna acy in idered r view egrees s, for dings, e. The cause t and phase ection esents ne-of- sed on hase of the Fig. sign whe fact the whe frac sum con the whe sign sign whe fun dela freq min carr in r C/A effe wer mu neg and Vol. 8, No. 4, 20 Multipath on Si signal respect . 1. Geometric The carrier ph nals of the samΦ , c ere / tor) and ′ is t phase shift du∆ ere is the f ctional shift. With referenc m of distance nverting this d phase shift (cos ere, is the w nal at the ante nal: Φ Φ Φ ere, R is the nction of , ay which is quency (L1, nimise the com By solving th rier phase mulΨ , t However, the representation A-code as longΔ Furthermore, ect based on c re derived in ltipath and gligible compa d (9) respecti 018, 3270-3275 ingle Frequency tively. cal representation hase measurem me satellite cancos , , is the at the amplitude ue to reflection frequency, ∆ ce to Figure 1 AB and BC, distance into pϕ) can be calcs wavelength of t enna is then th Φ cos ϕ e resultant carand and Ψ a function o L2) and sat mplexity of rep he previously d ltipath delay ctan e pseudo-range as in (7), and g as the approp a derivation carrier phase a n [20], takin noise on ca ared to those ively represen 5 y C/A Code Bas n of multipath effe ment (Φ ) bas n be written as 0 ttenuation (am of the reflecte n. is the time d , the total mu , which equal phase cycles an culated as the carrier sign he sum of the d Ψ rrier phase vΨ is the carrie of and . tellite ID (s) presentation. derived (4) an an be verified e multipath er d it is valid fo priate chipping n for pseudo and pseudo-ra ng advantage arrier phase of pseudo-ran nt the pseud 3271 sed GPS Positi fect by a vertical s sed on the refl s: 1 (2) mplitude redu ed signal, and (3) delay, and i ultipath delay i ls to 2 cos nd then to rad (4) nal. The comp direct and refl (5) voltage which er phase mult The notation are neglecte nd (5), the resu as; (6) rror ( ) is si or both P-code g period is use (7) o-ranges mult ange measurem of the fact measurments nge. Equation do-range mult ioning surface lected uction is is the is the . By dians, posite lected is a tipath ns for ed to ultant imple e and d. tipath ments that s are ns (8) tipath bas wh ph mu sig Th per pse be rec zer cor cod del of pse env env Ho me 5cm hig osc dif mu car GP bas cod car kil GP low Co of inv cor sta me eff fre suc ran sta the mi thi sin ma cod Engineerin www.etasr sed on L1 and here, K1 and ase and incl ultipath effect gnals and can herefore, K1 a riod of few ho eudo-range mu A reflected s censored aut ceiver, where ro for delays rresponds to a de and 50m fo layed by less delay and s eudo-range m vironment to vironment. In owever, the easurements is m [2]. It has b ghly variable cillations with fficulty in mod ultipath error rrier phase me PS receivers, sed DGPS co de based DGP rrier phase DG lometers [25]. PS receivers a w cost com onsidering the a multipath er vestigated the rrections that ation. They easurements f fect ( ), whi equencies as r ccessful attem nge (C/A code ations. The ex e method pro itigation from is paper is to ngle frequency A field expe ade to analyze de based GPS ng, Technology r.com d L2 frequencieΦΦ K2 are funct lude the unk is considered n be averaged and K2 can b ours [21], and ultipath and re signal, delayed tomatically by the auto-corre s longer than about 500m of or P-code signa than 1.5 chips signal amplitu measurements o more than n some cases, correspondin s typically two been found tha in the time d h a period of s deling it [21, is almost ten easurements [2 from low-cos orrections. Bes PS is several h GPS operation In addition, s are common in mpared to du practical adva rror on C/A co mitigation o are generated used a car for the deriva ch was formul represented by mpt was made e) multipath e xperiment pres oposed in [8 observations. analyze the y C/A code bas III. FIEL eriment was d e the multipath S positioning. y & Applied Sci es. Φ KΦ tions of the m known integer to be a combi d out to zero e estimated b d the result is eceiver noise. d by more tha y the correlat elation for the n 1.5 chips [ f increased pat al. The effect s would depen ude. Typical varies from n 5m in a it could reac g error in o orders of ma at the error cau domain showi several minute 23]. The mag n times greate 24]. Neverthele st to very exp sides, the oper hundreds of k ns are limited single frequen n use due to ual frequency antages and th ode measurem f the multipat d by a perman rrier phase ation of pseu lated in [20], b y (8) and (9) to precisely e error at perman sented in this ] for multipa . However, th effect of the sed GPS positi LD EXPERIMEN designed, and h effect on sin Three dual-f ience Research Dammalage: K (8)K (9) multipath on c r ambiguities ination of harm over a few h by averaging o the combinati an 1.5 chips, w tion process o C/A-code is n 22]. Such a th length for a of a reflected nd upon the am multipath err 1m in a b a highly ref ch even 100m the carrier agnitude small used by multip ing quasi-sinu es creating ex gnitude of C/A er than that o ess, a wide ran pensive, offer rational distan kilometres whi d to several te ncy C/A code their compara y GPS rece he significant ments, authors th effect on D nent GPS refe and pseudo- udo-range mul based on L1 a ). Based on t estimate the ps nent GPS refe paper also u ath estimation he primary foc multipath err ioning. NT d observations ngle frequency frequency geo h V The Effect of M carrier . The monic hours. over a ion of would of the nearly delay a C/A- signal mount ror in benign flected m [15]. phase ler (1- path is usoidal xtreme A code of the nge of r code nce of ile the ens of based atively eivers. effect in [8] DGPS erence -range ltipath and L2 this, a seudo- erence utilizes n and cus of ror on were y C/A odetic- typ gro illu 03, con leng an obs D3= use obt sele sate 360 flat bod pro ben mu mu obs obs gen refl GP obs con obs cha met loca thro due of how mai to obs con GP dist com obs coo Vol. 8, No. 4, 20 Multipath on Si e GPS receiv ound control ustrated in Figu Fig. 2. G A Trimble 57 and the obs nducted using gths between Electronic Di served distanc =19.9590m. T ed as ground tained through ected had mini ellites at very 00 undisturbed t ground. The dies and concr oximity. Hence nign multipath ltipath effect ltipath envir servation for t servations wi nerating additi lectors. The la S 01 station servations wi nstructed at th servations, and anged for day tal sheets at ation, size, a oughout day 1 e to the change day 4 were c wever, the GP intained uncha The contribut minimum to servations. The nducted by as S 01 and 02 a tances, this a mmon mode servations. He ordinates are th 018, 3270-3275 ingle Frequency vers were mou points to ure 2. GPS receiver confi 700 L1/L2 rec servations at two LEICA each ground c istance Measu ces were, D1 These precise truth to valid hout this exp imum obstruct low elevation d open sky vie ere were no b rete, mettle, or e, this site co h environment analysis, thre ronments we three consecu ith 1-second ional multipath ast set was ob . Figure 3 i ith reflectors. he outset for th d then the sur y 2 and 3 obs the direction and orientatio 1, 2, and 3 ob e of multipath conducted wi PS receiver an anged. IV. RESU tion of commo analyze the erefore, C/A c suming GPS as users/rovers accuracy anal errors rema ence, the possi the effect of n 5 y C/A Code Bas unted over thr accomplish figuration in the fi eiver was utili stations GPS – System 500 control points urement (EDM =20.0295m, D ely known m date the accur periment. Th tions for satell ns (~100). Th ew and was sit buildings, elec r wood surface onfiguration as t for GPS obs ee different ar ere introduce utive days. Fo interval we th by concrete bserved witho illustrates the . The concr he first set of rface of the s servations by of GPS rece on were main bservations, to h environment. thout any ref ntenna height ULT ANALYSIS on mode errors e effect of m code based DG 03 as the refe s. Due to the v lysis assumed ain on differ ible inaccurac non-common m 3272 sed GPS Positi ree precisely the configur ield experiment ized at station S 01 and 02 0 instruments. were measure M) instrument. D2=19.8930m measurements racy of the re e experiment lite signals eve e selected site tuated on elev ctric cables, w es found at th ssumed to be servations. Fo rtificially desi ed for GPS our-sets of 20 ere conducted e, wood, and m out any reflect field set up ete reflector f 20-hours of d same reflector placing wood iver. The refl ntained uncha minimize the . Last, observa flector at GPS and location S s has to be red multipath on GPS processing ference station very short bas d that no res rentially corr cies of the res mode errors, o ioning fixed ration n GPS were . The ed by . The m and were esults t site en for e had vated, water he site as a or the igned S 01 -hour d by metal tor at p for was day 1 r was d and flector anged e bias ations S 01, were duced GPS g was n, and seline sidual rected sulted out of Engineering, Technology & Applied Science Research Vol. 8, No. 4, 2018, 3270-3275 3273 www.etasr.com Dammalage: The Effect of Multipath on Single Frequency C/A Code Based GPS Positioning which the multipath effect is the most significant. According to the arrangement of the observation site, the possible source of multipath error at GPS01 was the reflector and/or the ground surface. Fig. 3. Reflector configuration at GPS 01 station At GPS02 and 03, the only possible source was the ground surface. A Trimble 5700 receiver was used at GPS03 with a Zephyr Geodetic L1/L2 antenna and Trimble Stealth ground plane technology, which could reduce most of the multipath created from the ground surface. Therefore, the negative influence that could be introduced by multipath on DGPS corrections generated at GPS03 reference station is assumed to be minimum. To evaluate the effect of multipath on C/A code based GPS positioning, 2D and 3D positioning accuracies at GPS1 and GPS2 were comparatively analyzed by utilizing the accurately measured baseline distances with GPS3 reference station. A. Effect on 2D Positioning The multipath effect is analyzed by evaluating the 2D positional accuracy of 72,000 observations recorded for 20- hours with a 1-second interval. Table I presents the number of observations as a percentage of total records, within four different 2D positional error limits, less than or equal to 5cm, greater than 5cm, and greater than or equal to 20cm and 50cm (<5cm, >5cm, >20cm, and >50cm). The effect of multipath is calculated for both baselines, BL1 (GPS 03 – GPS 01) and BL2 (GPS 03 – GPS 02), as reported in Table I. TABLE I. OBSERVATION PERCENTAGE AT 2D POSITIONAL ERROR CAUSED BY MULTIPATH Observation 2D positional error due to multipath <5cm >5cm >20cm >50cm Day1 BL1 78.4% 21.6% 16.0% 7.6% BL2 80.1% 19.9% 04.1% 1.8% Day2 BL1 79.1% 20.9% 16.1% 6.8% BL2 89.9% 10.1% 03.6% 0.5% Day3 BL1 75.4% 24.6% 17.3% 9.3% BL2 84.7% 15.3% 3.8% 1.5% Day4 BL1 79.3% 20.7% 06.7% 0.4% BL2 84.4% 15.6% 02.6% 0.7% Observations with the 2D positional error of less than or equal to 5cm could be considered as observations that are comparatively low affected by multipath. An average of about 78% and 85% were recorded with minimum multipath error for baselines BL1 and BL2 respectively. In addition, an average of about 22% and 15% observations were affected by higher multipath errors (more than 5cm of 2D positional errors for baselines BL1 and BL2 respectively). According to the condition of the observation site, the possible source of error for BL1 is the multipath at GPS01 created by the reflector and/or the ground surface. For BL2 it is the multipath at GPS02 created by the ground surface. Therefore, the percentage difference between BL1 and BL2 represents the additional multipath introduced by the artificial reflector separately from the ground. Accordingly, the artificial reflectors introduced both lower and higher than 5cm multipath errors on average of 7%. The highest difference percentage was observed for 2D positional error limit more than or equal to 20cm. When compared to the observations without reflector at GPS 01, the artificially-generated multipath diminished. However, for 2D positional error limit of more than or equal to 20cm, the number of observations were improved by 9.3%, 9.4%, and 10.6% over the observations without a reflector for concrete, wood, and metal, reflectors respectively. This confirms that the amount of multipath effect changes with the material of the reflector and is significant for higher 2D positional errors of greater than 20cm on C/A code based GPS observations. B. Effect on 3D Positioning Further analysis was conducted to investigate the effect of the multipath error on 3D positioning with single frequency C/A eff po wit cal pse Mu mi the po mu BL ref mu mi res pse sim ref of BL can non mi Fig mit ver rec Ho sho for rem mu cha non be Th cal sig acc Engineerin www.etasr A code based fect of multipa sitional error th a 1-second lculated befor eudo-range ob ultipath residu itigated from G e methodology sitional errors ultipath mitiga L2 (GPS 03 – ference statio ultipath mitiga itigation. The spective day o eudo-range m milarity to eac flector. Howev 3D positiona L1 and BL2 w n be conclude n-common m itigation were g. 4. 3D posit tigation The common ry short baseli ceiver at both owever, the t own any sign r both BL1 maining 3D po ultipath mitiga anging with t n-common mo the possible s he comparison lculated befo gnificant dev curacies. For ng, Technology r.com d GPS observ ath generated b was calculate d interval. Al re and after bservations be uals were calc GPS 01, 02, a y proposed in s are presented ation for baseli – GPS 02), w on. The red ation and the b time-series o of observation multipath mit ch other, even ver, after mul l errors for ea were significan ed that the rem mode errors fo almost similar tional accuracy n reference st ine observation h user station time-series of nificant correla and BL2. Th ositional errors ation are not time and day ode errors, bot sources of thes n of time-ser ore multipath viation betwe BL2, the erro y & Applied Sci vations. In or by the reflecto ed for 10 hou lso, the 3D p the multipat efore DGPS b culated based and 03 observ [8]. The resu d in Figure 4 ines BL1 (GPS where, GPS 0 lines present blue ones the e of 3D position ns between BL tigation have n for day 4 ob ltipath mitigat ach day of ob ntly correlated maining un-mo or BL1 and B r. comparison befo tation used for ns and the use ns could be f 3D position ation between his provided s even after D systematic, th . The un-mod th at the refere se remaining 3 ries of the 3D h mitigation een BL1 and or after multip ience Research Dammalage: rder to analyz ors at GPS 01, urs of observ positional erro th mitigation baseline proce on (8) and (9 vations by ado ulted time-seri as before and S 03 – GPS 01 03 was used a t the error b error after mul nal errors for L1 and BL2 b e not shown bservations w tion, the time- bservations be d. Based on t odeled commo BL2 after mul ore and after mu r both of thes e of the same ty the main rea nal errors hav n the different evidence tha DGPS correctio hey are random deled common ence and user, 3D positional e D positional revealed an d BL2 posi path mitigation h V The Effect of M ze the , a 3D vations or was from essing. 9) and opting es 3D d after 1) and as the before ltipath r each before n any ith no -series etween this, it on and ltipath ultipath se two ype of asons. ve not t days at the on and m and n and could errors. errors nother itional n was alw BL erro mu pre 3D GP com mu refe sou surf tech mu The refe assu mu calc mit mo sam mu of eve Fig Fig rev pos obs pos con has on G Fig. code freq Vol. 8, No. 4, 20 Multipath on Si ways lower tha 1. Surprisingl or before mu ltipath mitiga sence of mult positional er S observatio mpensation of ltipath and oth erence and use According to urce of multip face. However hnology at th ltipath on its erefore, the erence station, umed to be mi ltipath error culated by sub tigation from deled commo me before and ltipath error e true multipath en after C/A c gure 5. The s gure 4 is utiliz ealed that mu sitional error servations. Th sition solution ndition of the s a significant GPS observati 5. Multipath e based DGPS ob This paper quency C/A c 018, 3270-3275 ingle Frequency an that before, ly, for some t ultipath mitiga ation. Therefor tipath not only rrors but also ns. This cou f negative an her remaining er stations. o the observa path at the re r, with the use he GPS03 re DGPS correct contribution , to the differe inimum. Base on C/A code btracting the 3D the one bef on and non-co d after multipa effected the us h error conce code based D same 10 hour zed for the cal ultipath contrib of single fre he magnitude n varies with t multipath env influence on s ions even at fa h error as a percen bservations V. CO presented th code based GP 5 y C/A Code Bas but this was n time intervals, ation was bet re, it can be y introduces s comparativel uld be caus nd positive e non-common ation setup, eference statio e of Trimble st eference stati tions could be of multipath entially correct ed on which, th e based GPS D positional e fore by assum ommon mode ath mitigation ser observation erning the tot DGPS processi rs of observa lculations. Co butes about 50 equency C/A of the multip the time of o vironment. In single frequen avorable multi ntage of total rem ONCLUSION he multipath PS positioning 3274 sed GPS Positi not always tru , the 3D posit tter than that concluded tha significantly h ly lower error sed by the effects cause n mode errors a the only pos on was the gr tealth ground p ion, the effec e easily minim h effected at ted observatio he real effect o observations rror after mult ming that the errors remain . The resulted ns. The percen al error rema ing is present ations presente omparative ana 0% of the tota code based path effect on bservation an general, mult ncy C/A code b ipath environm maining error afte effect on s g by comparat ioning ue for tional after at the higher rs on error d by at the ssible round plane ct of mized. t the ons, is of the s was tipath e un- n the d true ntage aining ted in ed in alysis al 3D GPS n 3D nd the tipath based ment. er C/A single tively Engineering, Technology & Applied Science Research Vol. 8, No. 4, 2018, 3270-3275 3275 www.etasr.com Dammalage: The Effect of Multipath on Single Frequency C/A Code Based GPS Positioning analyzing the 2D and 3D positioning accuracies after DGPS processing. Based on the analysis, averages of 22% and 15% of the tested 72,000 observations were recorded with higher multipath errors of more than 5cm on 2D positioning for all the tested multipath conditions of baselines BL1 and BL2 respectively. 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