TOWARDS THE DIFFERENTIAL GLOBAL POSITIONING SYSTEM SERVICE FOR ISKANDAR MALAYSIA ABSTRACT

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1 TOWRD THE DIFFERENTIL GLOBL POITIONING YTEM ERVICE FOR IKNDR MLYI Wan nom W.. & Musa. T.. UTM-GN & Geodynamics Research Group, Dept. of Geomatics Eng., Faculty of Geoinformation cience & Eng., Universiti Teknologi Malaysia (UTM), kudai, Johor, MLYI Tel: ; Fax: BTRCT There is already a trend in many countries for the establishment of GP COR network to support a range of positioning applications. research-based GP COR network known as IKNDRnet is planned to provide DGP service within IKNDR Malaysia region. In this study, the reference station of IKNDRnet DGP is planned to transmit differential corrections in a standard format via the internet or radio link. This paper will be discussing the quality of reference station s coordinate and pseudorange measurement in order to compute the differential correction. Experimental tests on the DGP simulation have been conducted in order to evaluate the performance of differential correction. It is expected that the IKNDRnet DGP service will enable the user to achieve better positioning results for navigation and range applications within this area. 1.0 INTRODUCTION Global Positioning ystem (GP) measurements consist of biases and noises that affect the position accuracy especially the use of single GP receiver. Code-based Differential GP (DGP) is a technique that improves the navigation solution accuracy by providing satellite measurement corrections which is easy to be implemented (Kaplan & Hegarty, 2006). The DGP utilizes a reference station at a known location to estimate error sources such as satellite clock, atmosphere and orbit biases (Hoffman-Wellenhof et al., 2008) for each receiversatellite range measurements. These estimated errors are formulated into a pseudorange correction (PRC) by differencing the geometry range and measured pseudorange between satellite and the reference station. It can be noted that the reliability of the known coordinate and the quality of data measurement at the reference station are significant to produce better PRC. In navigation application, the PRC are needed in real-time and can be transmitted to the user via a communication link in a Radio Technical Commission for Maritime ervice (RTCM) standard format (El-Rabbany, 2006). The user can apply these PRCs to their measurements and the position is a differently corrected. The DGP is designed to provide less than 10m navigation service (95 percent) (Navigation Centre, 2008), and better accuracy as the user is located near the DGP transmitting station. This study focuses on the implementation of DGP service for IKNDR Malaysia region. In this paper, computation of the DGP is provided in ection 2. The structure of the IKNDRnet DGP and the set-up will be explained in ection 3. The coordinate determination and data quality checking at the reference station are comprised in this section. Next, the experimental work, results and analysis of PRC provided from the reference station will be discussed in ection 4. Finally, future work and concluding remarks are given in ection 5. 1

2 2.0 DGP LGORITHM ll the computation involved in this algorithm is using GP code-based (coarse-acquisition) measurements. 2.1 Observables The code pseudorange measured by GP receiver at reference station to satellite (any satellite in view) at epoch t can be modelled as (Hofmann-Wellenhof et al., 2008); R ( t) = ρ ( t) + ρ ( t) + ρ ( t) + ρ ( t) (1) where R is the measured code pseudorange from satellite to station, ρ is the geometric range between the satellite and the station, ρ denotes as range biases due to radial orbital error and atmospheric refraction effects, ρ is purely satellite dependent, and ρ is purely receiver dependent. The geometry range can be computed by ρ 2 2 ( t) = ( X X ) + ( Y Y) + ( Z Z ) 2 where, (X, Y, Z ) is satellite coordinate and (X, Y, Z ) is station coordinate at epoch (t) (2) 2.2 Differential Correction The PRC for satellite at reference epoch (t ), is generated by s PRC ( t) = ρ ( t) R ( t) = ρ ( t) ρ ( t) ρ ( t) (3) By adapting Equation 1 at epoch (t), so that the code pseudorange measured at the another GP receiver (i.e., rover station) B can be modelled by R ( t) = ρ ( t) + ρ ( t) + ρ ( t) + ρ ( t) B B B B (4) The rover B will be applying the PRC (t) in Equation 3 to correct its R B (t), and yields the corrected pseudorange, which can be expressed as RB ( t) corr = RB ( t) + PRC ( t) (5) In the case of the moderate distance (short baseline), satellite-receiver specific biases at both stations are highly correlated (Misra & Enge, 2004). ubstituting Equation 3 and 4 into Equation 5 provides, R ( t) = ρ ( t) + [ ρ ( t) ρ ( t)] + ρ ( t) ρ ( t)] B corr B B B (6) Thus, the satellite dependent error is eliminated and the influences of satellite-receiver dependent errors are significantly reduced. 2

3 3.0 DGP FOR IKNDR MLYI 3.1 The Location The development region of IKNDR Malaysia is located in the southern part of Peninsula Malaysia (see Figure 3.1). everal activities in this region such as marine, logistic sector, and transportation make use of GP navigation. IKNDRnet1 station (i.e., as part of IKNDRnet GP Network) was selected as IKNDRnet DGP reference station. This Continuously Operating Reference tation (COR) was established at Universiti Teknologi Malaysia (UTM) and planned to provide differential correction in real time and post process modes. Figure 3.1 IKNDR Malaysia Location & IKNDRnet DGP Coverage (Map is obtained from Google Earth, 2009). 3.2 The Project Design The system architecture of IKNDRnet DGP is illustrated in Figure 3.2. The IKNDRnet1 is responsible for data reception, processing, and data distribution. Basically, it is equipped with GP antenna and receiver to provide information on GP ephemeris data, and raw measurement. The data is logged by a computer server and formatted into Receiver IndepeNdent EXchange (RINEX). dditionally, the receiver also calculates the PRC. In real-time mode, the IKNDRnet1 is planned to provide two ways of data communication via radio link and/or internet basis. For radio link transmission, the receiver will be connected to radio modem and broadcast the PRC in RTCM standard format through specified frequency. Thus, the correction can be applied at user GP device by setting the same frequency. 3

4 Figure 3.2 ystem rchitecture of IKNDRnet DGP. Meanwhile, for the internet communication involves Network Transportation RTCM via Internet Protocol (NTRIP) system. The NTRIP consists of three components: server, caster and client (see Figure 3.3). The IKNDRnet1 server will used Transmission Control Protocol (TCP) and Internet Protocol (IP) via NTRIP erver software to enable for data streaming to the caster (see Figure 3.4). The caster can be treated as bridge connection between the server and the client. The client (i.e., the user) will retrieve the data from the caster by utilize the NTRIP client s software namely Global Navigation atellite ystem (GN) Internet Radio (see Figure 3.5) via compatible Hypertext Transfer Protocol (HTTP 1.1) (Lenz, 2004). The client will receive the PRC and display corrected position in RTCM and National Marine Electronics ssociation (NME) and Format, respectively. Figure 3.3 NTRIP Concept for IKNDRnet DGP. 4

5 Figure 3.4 NTRIP erver oftware. Figure 3.5 NTRIP Client oftware. 3.3 The et-up The IKNDRnet1 uses the Trimble Reference tation (TR) ystem for the data collection. In this study, Trimble 4700 receiver is set to track satellite in 5 seconds epoch, at 10 O elevation mask. Figure 3.4 shows the set up of IKNDRnet1 and the existing infrastructures. Meanwhile, Figure 3.7 illustrates the outdoor and indoor configuration. ll data from the GP receiver were automatically recorded on a 24 hour basis and can be accessed via the GN Internet Radio. Figure 3.6 IKNDRnet1 Reference tation. 5

6 Figure 3.7 Configuration of IKNDRnet Coordinate Determination for Reference tation The coordinate of IKNDRnet1 was updated by ULIG Online GP Processing ystem (UPO) (via total of 24 hours observation data for 8 days were processed as batch solution. The processing uses final orbit from International GP ervice (IG) and data from nearest IG stations (BKO, NTU and XMI) (see Figure 3.8 and Table 3.1). Realization of the coordinates and its Root Means quare (RM) are based on the International Terrestrial Reference Frame (ITRF) 2005, which is provided by the IG cumulative solution (Dawson et al., 2005). Table 3.2 and 3.3 show the coordinate of IKNDRnet with RM value from per day and batch solution, respectively. The RM and offset in coordinates of per day relative to the batch solution results are plotted in Figure 3.9. Figure 3.8 IG tations ( Table 3.1 Information on IG stations in this study. tations Location Latitude Longitude Height(m) bako Indonesia ntus ingapore xmis Indonesia

7 Table 3.2 IKNDRnet1 Geodetic Coordinate and RM per day based on ITRF 2005 reference frame. ession Latitude Longitude Height RM 21 st PRIL nd PRIL rd PRIL th PRIL th PRIL th PRIL th PRIL th PRIL Table 3.3 IKNDRnet1 Geodetic and Cartesian Coordinates from Batch olution (8 days observation) based on ITRF 2005 reference frame. Latitude RM Longitude RM Height RM m m m X RM Y RM Z RM m m m Figure 3.9 Variation of coordinate residual per day relative to the coordinate from batch solution. From the Figure 3.9, it can be seen that the RM demonstrates small fluctuation in between 7.1 to 9.4 mm. The results from Table 3.3 were used to calculate the DGP corrections for IKNDRnet DGP. The simulation of IKNDRnet DGP is presented in the ection Reference tation Quality Check Park et al. (2002) found that the geometry between GP satellite constellation and the reflector is repeating each sidereal day. Thus, daily behaviour of multipath error for each satellite can be determined. He also found that the RM of multipath errors for 70 sites of Korean GP COR has value 0.20 m to 0.30 m and 0.4 m to 4.0 m for MP1 and MP2 respectively. In general, 50 percent of 7

8 IG stations around the world have 0.4 m and 0.6 m for MP1 and MP2, respectively. Meanwhile, another 70 percent have 0.6 m for MP1 and less than 0.75 m for MP2. The current IG station information can be viewed via This section will be evaluating the effect of pseudorange multipath at IKNDRnet1. The IKNDRnet1 uses micro-centred antenna (see Figure 3.10) to reduce the effect of reflective environments. In addition, GP data quality at IKNDRnet1 has been checked by using Translation, Editing, and Quality Checking (TEQC) software to quantify the effect of multipath for IKNDRnet1 station. Based on the TEQC process, the RM of multipath for all satellites from Day 1-8 are shown in Table 3.2. Result of MP1 and MP2 exhibits m and m, respectively. Figure 3.10 Trimble 4700 receiver with TRM GP Microcentered antenna. Table 3.2 The list of multipath RM within 1 week observations. Day of Observation Date Mean MP1 Mean MP2 Day 1 (21/04/09) m m Day 2 (22/04/09) m m Day 3 (23/04/09) m m Day 4 (24/04/09) m m Day 5 (25/04/09) m m Day 6 (26/04/09) m m Day 7 (27/04/09) m m Day 8 (28/04/09) m m verage m m The investigation found that the value of MP1 and MP2 at IKNDRnet1 are small and acceptable. Further study on MP2 was conducted by plotting the variation of multipath effect from each satellite. Two days data were analyzed to see the repeatable of multipath effects (see Figure 3.11 and 3.12). From these figures, the darker colour in the scale bar represents the effect of multipath. Both figures demonstrate having same pattern of multipath characteristic along the day. 8

9 Figure 3.11 Variation of Multipath at L2. Figure 3.12 Variation of Multipath at L2 (on the next day). The multipath sky plots for MP2 were produced to study the environment of GP station at IKNDRnet1 site. The sky plots were analysed using the QC2KY software, which revealed the geographical oriented of the multipath effect. Figure 3.13 indicates the scale bar of MP2 range between 0.0 m and 2.0 m. Figures 3.14 and 3.15 show the sky plots for multipath effect according to observation on Day 1-8. The curve lines represent each satellite s path during the observations. Figure 3.13 QC2KY Colour cale. 9

10 Figure 3.14 Multipath sky plot at L2. Figure 3.15 Multipath sky plot at L2 (on the next day). The multipath effects MP2 are dominant in the southeast orientation, which is indicated by the orange-yellow colour (refer scale bar in Figure 3.13). This might be caused by an object nearby to the GP antenna. Figure 3.16 shows the GP antenna set up on the rooftop of the faculty building (left) with the reflector on top of the nearby building (right) located in the southeast orientation from the GP antenna. Figure 3.16 Undefined reflector located proximity to the GP antenna. 10

11 4.0 IMULTION OF IKNDRnet DGP In this section, an experiment was conducted to examine the performance of DGP correction provided by IKNDRnet1. The experiment was designed to compare the result with and without DGP from two different baseline lengths. 4.1 The etup The study area is selected within part of IKNDR Malaysia region. ccording to Figure 4.1, UTM Helipad station was selected for short baseline from the IKNDRnet1 station. Meanwhile, tation MyRTKnet located at Kukup is selected for medium baseline. These two stations were treated as rover. For evaluation purposes, the precise coordinates of the rover stations were determined from the previous static GP observation. Figure 4.1 tudy rea (part of IKNDR Malaysia region). The GP observation was conducted on pril 28th, 2009 from 16:00:00 to 17:10:30 UTC time. The observation interval was set at every 15 seconds. Figure 4.2 show the GP sky plot and Dilution of Precision (DOP) of the study area. The RTCM Message Type 1 was decoded by using NTRIP RTCM decoder containing the information of PRC from the IKNDRnet server (see Figure 4.3). 11

12 Figure 4.2 ky Plot and DoP during the observation. Figure 4.3 NTRIP RTCM Decoder. 12

13 Figure 4.4 illustrates the process of IKNDRnet DGP simulation utilized MTLB programming. t the rover station, the PRC from IKNDRnet1 are applied to common satellite. t this stage, a set of corrected pseudorange (see Equation 6) can be used to calculate the rover position. For comparison analysis, the rover also calculates its position without applying the PRC. s the rover is located at a known point, the result of point positioning and DGP techniques can be assessed. Figure 4.4 The Process of IKNDRnet DGP imulation (Wan nom W.. et al, 2008). 4.3 Results & nalysis The experimental results for a set of one hour observations are summarized in Figure 4.5, 4.6, 4.7, 4.8 and 4.9. Figure 4.5 shows the availability of PRC provided by IKNDRnet1 during the observation. The pseudoranges with and without PRC at both rover stations can be illustrated by Figure 4.6 and 4, respectively. The grey bar represents uncorrected pseudorange. Technically, the common satellite at rover and IKNDRnet1 has a higher possibility to be corrected. In this simulation, at least four corrected pseudoranges results will be used to compute the final DGP position. 13

14 Figure 4.5 PRC availability provided by IKNDRnet1 (during the observation). Figure 4.6 Pseudoranges with & without PRC at G11 (during the observation). Figure 4.7 Pseudoranges with & without PRC at tation MyRTKnet Kukup (during the observation). Figure 4.8 and 4.9 show offset of the rover position from the known point at G11 and tation MyRTKnet Kukup, respectively. In these figures, the red diamond shape represents the known position of the rover station. Meanwhile, the offset of rover position without and with PRC are plotted as blue and green cross marks, respectively. 14

15 Figure 4.8 set of user position at G11. Figure 4.9 set of user position at tation MyRTKnet Kukup. From Figure 4.8, it can be seen the blue crosses are scattered. Meanwhile, the green cross marks are precisely approaching to the known rover station. The results show that the delta longitude and latitude at G11 decreased for about 1-3 metres. Meanwhile, the blue cross marks in Figure 4.9 are spread and the delta latitude is increased. fter apply the PRC (green marks), the size of delta longitude delta latitude reduced for about 2 metres, considerably. From the two figures, the PRC has successfully reduced the errors in pseudoranges and thus, improved the rover positioning results. 15

16 5.0 CONCLUDING REMRK From ection 3, the coordinate of IKNDRnet1 was updated from precise GP processing. s a result, the precise coordinate computed by using IG final orbit has been set at the reference station to generate the PRC. The measured pseudorange at the IKNDRnet1 shows multipath effects still remain and the location of the reflectors have been verified. However, the level of multipath at IKNDRnet1 is still acceptable compared to other IG stations. In order to improve the quality of pseudorange measurement, the elevation mask of station is planned to be increased and subject to further study. In ection 4, from the analysis section, the PRC provided by IKNDRnet1 is able to reduce the satellite dependent and the satellite-receiver dependent errors at both rover stations. The accuracy of the position results from IKNDRnet DGP is improved for about 1-3 metres. The DGP results from the both rover stations demonstrate a same performance. This is because, during the observation, almost 75 percent from the number of visible satellite were fully corrected along the observation. The reference station must be capable to track and compute the PRC from all visible satellites. Thus, the rover receiver can achieve higher possibility to apply the PRC for its satellite constellation. It can be summarized that, the PRCs from IKNDRnet1 works well to improve the precision of the rover position. Further work will be conducted to evaluate the variation of PRC in relation to the behavior of error sources such as satellite clock, atmosphere and orbit biases within IKNDR Malaysia region. 6.0 REFERENCE El-Rabbany,. (2006): Introduction to GP, the Global Positioning ystem (second edition). rtech House Inc., Norwood, United tate. Dawson, J., Govind, R., and Manning, J. (2005): The ULIG Online GP Processing ystem (UPO), ccessed via ( ccessed: 5 th pril Hofmann-Wellenhof, B., Lichtenegger, H., Wasle. E. (2008): Global Navigation atellite ystems. GP, GLON, Galilieo & more. pringer Wien New York. Kaplan, E.D. and Hegarty, C.J. (2006): Understanding GP, Principles and pplication. rtech House Inc., Norwood, United tate. Lenz, E. (2004): Networked Transport of RTCM via Internet Protocol (NTIP) pplication and Benefit in Modern urveying ystems. FIG Working Week May, thens, Greece. Misra, P. and Enge, P (2004): Global Positioning ystem: ignals, Measurements and Performance. Ganga-Jamuna Press, Lincoln, Massahusetts. Navigation Center (2008): UCG Differential GP Navigation ervice. Downloaded via ( Park Kwang-Dong, Ki-Nam Kim, Hyung-Chul Lim, and Pil-Ho Park (2002). Evaluation of Data Quality of Permenent GP tations in outh Korea. ( ccessed: 2 nd June

17 RTCM pecial Committee No. 104 (2004): RTCM Recommended tandards for Differential GN Version 3.0. RTCM Paper /C104-TD. Radio Technical Commision For Maritime ervices. Wan nom, W.., Musa, T.., H. etan,., es (2008): UTMnav: n pproach of DGP Technique for Precise Navigation ystem. Proceeding of International ymposium and Exhibition on Geoinformation Putra World Trade Centre, Kuala Lumpur. 17