PERFORMANCE ANALYSIS OF LOW-COST SINGLE-FREQUENCY GPS RECEIVERS IN HYDROGRAPHIC SURVEYING

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1 PERFORMANCE ANALYSIS OF LOW-COST SINGLE-FREQUENCY GPS RECEIVERS IN HYDROGRAPHIC SURVEYING Mohamed Elsobeiey Departmet of Hydrographic Surveyig, Faculty of Maritime Studies, Kig Abdulaziz Uiversity P. O. Box Jeddah 21589, Kigdom of Saudi Arabia melsobeiey@kau.edu.sa KEY WORDS: Hydrographic Surveyig; Sigle-Frequecy; IHO; Precise Poit Positioig ABSTRACT: The Iteratioal Hydrographic Orgaizatio (IHO) has issued stadards that provide the miimum requiremets for differet types of hydrographic surveys executio to collect data to be used to compile avigatioal charts. Such stadards are usually updated from time to time to reflect ew survey techiques ad practices ad must be achieved to assure both surface avigatio safety ad marie eviromet protectio. Hydrographic surveys ca be classified to four orders amely, special order, order 1a, order 1b, ad order2. The order of hydrographic surveys to use should be determied i accordace with the importace to the safety of avigatio i the surveyed area. Typically, geodetic-grade dual-frequecy GPS receivers are utilized for positio determiatio durig data collectio i hydrographic surveys. However, with the evolutio of high-sesitivity low-cost sigle-frequecy receivers, it is very importat to evaluate the performace of such receivers. This paper ivestigates the performace of low-cost sigle-frequecy GPS receivers i hydrographic surveyig applicatios. The mai objective is to examie whether low-cost sigle-frequecy receivers fulfil the IHO stadards for hydrographic surveys. It is show that the low-cost sigle-frequecy receivers meet the IHO horizotal accuracy for all hydrographic surveys orders at ay depth. However, the sigle-frequecy receivers meet oly order 2 requiremets for vertical accuracy at depth more tha or equal 100m. 1. INTRODUCTION Typically, commercial GPS receivers vary accordig to their receivig capabilities. There are differet types such as siglefrequecy code receivers, sigle-frequecy carrier-smoothed code receivers, sigle-frequecy code ad carrier receivers, dual-frequecy receivers, ad triple-frequecy receivers. Siglefrequecy receivers access the L1 frequecy oly. Dualfrequecy receivers access both the L1 ad the L2 frequecies, while triple-frequecy receivers access L1, L2, ad L5 frequecies. The first type of sigle-frequecy GPS receivers, the siglefrequecy code receiver, is the cheapest ad the least accurate type of receivers. It measures the pseudorages with the C/Acode oly. The secod receiver type, the sigle-frequecy carrier-smoothed code receiver, uses the L1 carrier phase to produce high-precisio C/A-code measuremets. Siglefrequecy code ad carrier receivers output the raw C/A-code pseudorages, the L1 carrier phase measuremets i additio to the avigatio message. Dual-frequecy receivers are the most sophisticated ad most expesive receiver type. They measure GPS data o both L1 ad L2 frequecies. Triple-frequecy receivers, o the other had, are able to produce measuremets o the legacy frequecies L1 ad L2 ad the moderized frequecy L5. Ulike dual-frequecy receivers, however, ioosphere delay represets a major challege for sigle-frequecy receivers. There are two mai approaches to correct sigle-frequecy data from the ioosphere delay (Cai et al., 2013). The first techique is to use ioosphere models to correct for the ioosphere delay. These models may be empirical models such as Klobuchar model, which ca accout for up to 60% of the delay at midlatitudes (El-Rabbay, 2006). Klobuchar model coefficiets are trasmitted as part of the avigatio message ad ca be improved by extedig the eight parameters origial Klobuchar model to te-parameters to accout for the ioosphere variatio durig the ight time (Wag et al., 2016). Correctios from regioal or global etwork may be estimated ad the applied to sigle-frequecy receivers. A example for the global ioosphere correctios is the Global Ioosphere Maps (GIMs) produced by the Iteratioal GNSS service (IGS). Aother optio to correct for the ioosphere delay for sigle-frequecy receivers i real-time is to broadcast ioosphere correctios from Space Based Augmetatio Systems (SBAS) (Arbesser- Rastburg, 2002; Hofma-Wellehof et al., 2008). The secod techique to accout for ioosphere delay is to form ioosphere-free liear combiatio usig both code ad carrier phase observatios o L1 from the sigle-frequecy receiver. This techique is based o the Group ad Phase Ioosphere Calibratio (GRAPHIC) (Cai et al., 2013; Schüler et al., 2011; Shi et al., 2012; Sterle et al., 2015). Sice the availability of low-cost sigle-frequecy receivers, several attempts have bee carried out to reduce the cost ad icrease the accuracy which ca be obtaied from such Authors CC BY 4.0 Licese. 67

2 receivers compared with geodetic grade receivers. Highsesitivity low-cost receivers have thousads of correlators to reduce the search space of each correlator ad are able to acquire sigals with low decibel watt (dbw) (Schwieger, 2007). Hedgecock et al. (2013) itroduced the stadaloe relative localizatio system usig low-cost sigle-frequecy receivers. They foud that trackig the relative motio of the eighborig odes is a order of magitude better tha takig the differece betwee the absolute coordiates of each ode. Ambiguity resolutio, o the other had, is ot possible from sigle epoch data from sigle-frequecy receivers(odijk et al., 2012). However, ambiguity resolutio ca be achieved usig less tha 10 miutes of accumulated data, by which sub-cm ad a few cm levels ca be achieved for horizotal ad vertical directios, respectively (Odijk et al., 2014). The performace of low-cost sigle-frequecy receivers ca be improved by usig a geodetic grade atea istead of the low-cost sigle-frequecy atea (Takasu ad Yasuda, 2008). Low-cost sigle-frequecy receivers have bee used i umerous applicatios. A umaed aerial vehicle (UAV) ca be occupied by low-cost sigle frequecy receivers, which are cotrolled through radio commuicatio cotrol. The system works i RTK mode ad gets correctios from at least oe referece receiver (Stempfhuber ad Buchholz, 2011). Lowcost sigle-frequecy receivers ca be used alog with SBAS correctios for autoomous guidace of agricultural tractors. The pass-to-pass error i trajectories is withi 1m (Aloso- Garcia et al., 2011). It ca be used i Structure Health Moitorig (SHM) applicatios such as moitorig the displacemet. Averagig measuremets from several GPS receivers ca sigificatly reduce the oise level ad the dyamic displacemet respose ca be captured at 0.25m amplitude (Jo et al., 2013). Low-cost sigle-frequecy receivers ca be used also to moitor the sow liquid water cotet ad avalache predictio by measurig the chages of GPS L1 carrier stregth (sigal-to-oise ratio) (Koch et al., 2014). I this paper, u-blox NEO-7P low-cost sigle-frequecy GPS receiver, which fulfills accuracy requiremets for all RTK surveyig applicatios (Sioulis et al., 2015), is used to collect GPS data usig a hydrographic vessel. The mai objective of the paper is to evaluate the performace of such siglefrequecy receiver i hydrographic surveyig applicatios. 2. IHO POSITIONING STANDARDS The Iteratioal Hydrographic Orgaizatio (IHO) published several stadards that provide the miimum stadards for hydrographic surveys executio to collect data to be used to compile avigatioal charts. Such stadards must be achieved to assure both surface avigatio safety ad marie eviromet protectio (IHO, 2008). Accordig to the water depth, the IHO classifies surveys ito four orders, amely; special order, order 1a, order 1b, ad order 2. The first order is the special order which is the most rigorous amog all hydrographic surveys orders. It is used for areas where uder-keel clearace is critical ad where bottom characteristics are potetially hazardous to vessels such as critical avigatio chaels, harbors, ad berthig areas (geerally depth less tha 40 m). the secod order is order 1a which is for areas of depth more tha 40 m ad less tha 100 m where uder-keel clearace is less critical but ma-made or atural features o the seabed are of cocer to surface shippig may exist. Such areas like harbors, harbors approachig chaels, ad recommeded tracks. The third order is order 1b, which is for areas of depth less tha 100 m where uder-keel clearace is ot cosidered to be a issue for surface shippig i such areas. The fourth order is order 2, which is for areas deeper tha 100 m where the geeral descriptio of the sea floor is cosidered adequate ad the ma-made or atural features o the seabed will ot have ay impact o the surface avigatio. 2.1 Total Horizotal Ucertaity () Hydrographic surveys measuremets are affected by differet sources of ucertaities, icludig radom ad systematic errors. All sources of measuremet ucertaities are propagated, ad the ucertaities of the computed parameters are the calculated, which is kow as Total Propagated Ucertaity (TPU). The compoet of TPU i the horizotal plae is kow as the Total Horizotal Ucertaity (), which is a 2 Dimesioal () quatity expressig latitude ad logitude errors. 2.2 Total Vertical Ucertaity () It is a 1 Dimesioal () quatity that expresses the compoet of the TPU i the vertical dimesio. The depth ucertaity is affected by two errors, depth depedet ad depth idepedet errors. The ca be computed at cofidece level as follows (IHO, 2008): where 2 2 a ( b d) (1) a = the depth idepedet portio of ucertaity b = the coefficiet that represets the depth depedet portio of ucertaity d = the depth Table 1 summarizes the miimum stadards for hydro-graphic surveys orders. 3. FIELD TEST To test the performace of low-cost sigle-frequecy GPS receivers i hydrographic surveyig, raw GPS data was collected usig New-7P u-blox GPS receiver. The test area was Sharm Obhur, Jeddah, Saudi Arabia where Faculty of Maritime Studies (FMS) is located. The base statio was setup o the rooftop of FMS buildig usig Ashtech ProFlex 500 GNSS receiver ad the correspodig measuremets simultaeously collected ad employed to estimate the Real-Time Kiematic (RTK) solutio, which cosidered as the referece solutio. The legth of the collected data was about 2.0 hours usig Kig Abdulaziz Uiversity (KAU) vessel Hydrography 1, Figure 1. Figure 2, o the other had, shows the trajectory of the data collected i November, Authors CC BY 4.0 Licese. 68

Table Head Hydrographic Surveys Order Special Order Order 1a Order 1b Order 2 Depth (d) < 40m < 100m < 100m > 100m Area Characteristics Total Horizotal Ucertaity () Total Vertical Ucertaity ()

3 Table Head Hydrographic Surveys Order Special Order Order 1a Order 1b Order 2 Depth (d) < 40m < 100m < 100m > 100m Area Characteristics Total Horizotal Ucertaity () Total Vertical Ucertaity () Harbors, berthig areas, ad associated critical chaels with miimum uder-keel clearaces Uder-keel clearace is less critical but features of cocer to surface shippig may exist Uder-keel clearace is ot a issue for surface shippig expected to trasit the area Table 1. IHO Miimum Stadards for Hydeographic Surveys (IHO, 2008) geeral descriptio of the sea floor is cosidered adequate 2m 5m d 5m d 20m + 0.1d a = 0.25m b = a = 0.5m b = a = 0.5m b = a = 1.0m b = The real-time solutio from New-7P low-cost sigle-frequecy GPS receiver is recorded durig the surveyig sessio at 1Hz samplig frequecy. Moreover, the referece solutio from Hydrography 1 vessel is recorded i real-time at the same samplig rate. The sigle-frequecy solutio is compared to the referece solutio to ivestigate whether the low-cost siglefrequecy receivers satisfy IHO hydrographic surveys miimum stadards accordig to Table 1. Both ad of the lowcost sigle-frequecy receiver are computed at cofidece level the trasformed to cofidece level as follows (Mohamed El-Diasty ad Elsobeiey, 2015): i1 NR NSFS ER ESFS 2 2 i i (2) 2.44 (3) Figure 1. KAU Hydrography 1 Vessel i1 U U 2 R SFS i (4) 1.96 (5) Figure 2. Test Trajectory 4. RESULTS AND DISCUSSION Where = the total horizotal ucertaity of Northig ad Eastig positio error at cofidece level N ˆ R = the eastig coordiate of the referece solutio (the vessel itegrated solutio), Nˆ SFS is the Northig sigle-frequecy positio E ˆR = the Eastig coordiate of the referece solutio E ˆSFS = the Eastig sigle-frequecy positio is the total umber of epochs = the total horizotal ucertaity of Northig ad Eastig positio error at cofidece level = the total vertical ucertaity of the Up compoet at cofidece level = the total vertical ucertaity of the Up compoet at cofidece level Figure 3 shows the Eastig, Northig ad the horizotal error of the sigle-frequecy solutio compared with the referece solutio. I additio, Figure 4 shows the Authors CC BY 4.0 Licese. 69

horizotal error ad the vertical error. Figures 3 ad 4 show that the maximum absolute horizotal ad vertical errors are about 3.12m ad 3.69, respectively. however, the mea error is about 1.26m ad -0.

Northig, Eastig, ad the horizotal Errors of Low-cost Sigle-frequecy Receiver 5.

4 horizotal error ad the vertical error. Figures 3 ad 4 show that the maximum absolute horizotal ad vertical errors are about 3.12m ad 3.69, respectively. however, the mea error is about 1.26m ad -0.62m for both horizotal error ad vertical error, respectively. Moreover, Table2 summarizes the ad values at ad cofidece levels. Figure 3. Northig, Eastig, ad the horizotal Errors of Low-cost Sigle-frequecy Receiver 5. CONCLUSIONS This paper ivestigates the performace of real-time precise poit positioig with low-cost sigle-frequecy GPS receivers i hydrographic surveyig applicatios. A sessio of two hours legth is used to collect sigle-frequecy data usig Kig Abduaziz Uiversity vessel, Hydrography 1. The vessel solutio, which cosists of the RTK itegrated solutio is used as a referece for compariso. To check if the sigle-frequecy receivers fulfil the IHO requiremets, both ad are estimated at cofidece level. It is show that the total horizotal ucertaity from the solutio of low-cost siglefrequecy receivers is about 1.69m, which meet the IHO stadards for all hydrographic surveys orders at ay depth (special order, order 1a, order 1b, ad order 2). However, the total vertical ucertaity from the solutio of sigle-frequecy receivers is foud to be 2.512m, which meets oly order 2 requiremets for vertical accuracy at depth more tha or equal 100m. i fact, ublox sigle-frequecy GPS receivers accoutig for troposphere ad ioosphere errors by applyig the correctios trasmitted from SBAS. Such correctios are limited to a accuracy of about 0.2m, which affects the receiver solutio, especially the height compoet. So, further ivestigatios are required to improve the height compoet of sigle-frequecy receivers i real-time solutio, especially tropospheric delay, which highly correlated with ellipsoidal height. ACKNOWLEDGMENTS The data set used i this research was collected usig Kig Abdulaziz Uiversity vessel, Hydrography 1. The Author, therefore, ackowledges with thaks the techical stuff from hydrographic surveyig departmet, Faculty of Maritime Studies. Figure 4. horizotal Errors ad Vertical Errors of Lowcost Sigle-frequecy Receiver Error Parameter Low-cost Sigle-frequecy Solutio Table 2. Summary Statistics of the Low-cost Sigle-frequecy Solutio Our aalysis showed that the is 1.69m at cofidece level which meets all hydrographic surveys types accordig to the IHO stadards (Table 1) at ay depth. However, the is 2.512m at the same cofidece level, which oly meets hydrographic surveys order 2 at 100m depth. REFERENCES Aloso-Garcia, S., Gomez-Gil, J., Arribas, J., Evaluatio of the use of low-cost GPS receivers i the autoomous guidace of agricultural tractors. Spaish Joural of Agricultural Research 9, Arbesser-Rastburg, B., Ioospheric correctios for satellite avigatio usig EGNOS. Proc. of XXVII-th URSI Geeral Assembly, Maastricht. Cai, C., Liu, Z., Luo, X., Sigle-frequecy Ioospherefree Precise Poit Positioig Usig Combied GPS ad GLONASS Observatios. The Joural of Navigatio 66, El-Rabbay, A., Itroductio to GPS : the Global Positioig System, 2d ed. Artech House, Bosto, MA. Hedgecock, W., Maroti, M., Sallai, J., Volgyesi, P., Ledeczi, A., High-accuracy differetial trackig of low-cost GPS receivers, Proceedig of the 11th aual iteratioal coferece o Mobile systems, applicatios, ad services. ACM, pp Authors CC BY 4.0 Licese. 70

5 Hofma-Wellehof, B., Lichteegger, H., Wasle, E., GNSS - global avigatio satellite systems : GPS, GLONASS, Galileo, ad more. Spriger, Wie ; New York. IHO, IHO STANDARDS FOR HYDROGRAPHIC SURVEYS, Special Publicatio No. 44, 5th Editio, February 2008, Iteratioal Hydrographic Bureau, MONACO. Jo, H., Sim, S.-H., Tatkowski, A., Specer, B.F., Nelso, M.E., Feasibility of displacemet moitorig usig low-cost GPS receivers. Structural Cotrol ad Health Moitorig 20, Koch, F., Prasch, M., Schmid, L., Schweizer, J., Mauser, W., Measurig Sow Liquid Water Cotet with Low-Cost GPS Receivers. Sesors 14, Mohamed El-Diasty, Elsobeiey, M., Precise Poit Positioig Techique with IGS Real-Time Service (RTS) for Maritime Applicatios. Positioig 6, Odijk, D., Teuisse, P., Zhag, B., Sigle-Frequecy Iteger Ambiguity Resolutio Eabled GPS Precise Poit Positioig. Joural of Surveyig Egieerig 138, Odijk, D., Teuisse, P.G., Khodabadeh, A., Sigle- Frequecy PPP-RTK: Theory ad Experimetal Results, i: Rizos, C., Willis, P. (Eds.), Earth o the Edge: Sciece for a Sustaiable Plaet. Spriger Berli Heidelberg, pp Schüler, T., Diessogo, H., Poku-Gyamfi, Y., Precise ioosphere-free sigle-frequecy GNSS positioig. GPS Solut 15, Schwieger, V., High-Sesitivity GPS the low cost future of GNSS. FIG Workig Week, Hog Kog, SAR 13, Shi, C., Gu, S., Lou, Y., Ge, M., A improved approach to model ioospheric delays for sigle-frequecy Precise Poit Positioig. Advaces i Space Research 49, Sioulis, A., Tsakiri, M., Stathas, D., Evaluatio of low cost, high sesitivity GNSS receivers based o the ISO RTK stadards. INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES 6, pp Stempfhuber, W., Buchholz, M., A precise, low-cost RTK GNSS system for UAV applicatios. Iteratioal Archives of Photogrammetry, Remote Sesig ad Spatial Iformatio Sciece 38, 1-C22. Sterle, O., Stopar, B., Pavlovčič Prešere, P., Siglefrequecy precise poit positioig: a aalytical approach. J Geod 89, Takasu, T., Yasuda, A., Evaluatio of RTK-GPS performace with low-cost sigle-frequecy GPS receivers, Proceedigs of iteratioal symposium o GPS/GNSS 2008 Tokyo, Japa, pp , pp Wag, N., Yua, Y., Li, Z., Huo, X., Improvemet of Klobuchar model for GNSS sigle-frequecy ioospheric delay correctios. Advaces i Space Research. Authors CC BY 4.0 Licese. 71

HOW BAD RECEIVER COORDINATES CAN AFFECT GPS TIMING

HOW BAD RECEIVER COORDINATES CAN AFFECT GPS TIMING H. Chadsey U.S. Naval Observatory Washigto, D.C. 2392 Abstract May sources of error are possible whe GPS is used for time comparisos. Some of these mo