ISKANDARnet: A Network-Based Real-Time Kinematic Positioning System in ISKANDAR Malaysia for Research Platform

N. S. M. Shariff1, T. A. Musa1, S. Ses1, K. Omar1, C. Rizos2, S. Lim2

1UTM-GNSS & Geodynamics Research Group, Faculty of Geoinformation Science & Engineering, Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor, Malaysia - tajulariffin@utm.my

2School of Surveying and Spatial Information Systems, University of New South Wales, Australia

ABSTRACT

This paper describes the establishment of ISKANDARnet, a research facility for GPS N-RTK positioning within the development region of ISKANDAR, Malaysia. Located in an equatorial region, ISKANDARnet suffers from distance-dependent errors due to the severe atmospheric delays which make the network ambiguity resolution process much more difficult. Additionally, the inter-station distances within the ISKANDARnet should be kept long enough to provide N-RTK corrections for the metro-area of ISKANDAR Malaysia. An essential step in establishing the network is the data quality analysis at every GPS reference station. This step is useful especially to detect any multipath effects at a reference station and to therefore judge the suitability of the station location. In this study, the multipath vulnerability of each station is analysed and field experiments have been conducted to test the communication links, data streaming and generation of N-RTK corrections. These activities have yielded valuable experience on the issues that have to be addressed in ensuring high accuracy network-based positioning for both research activities and to support the development of the region of ISKANDAR Malaysia.

Keywords: Network-based RTK, Multipath, Distance-dependent Errors

1. INTRODUCTION

A fast growing economic region of Southern Peninsular Malaysia known as ISKANDAR has required substantial activities to be undertaken, particularly in relation to construction and the provision of infrastructure. These activities require a high precision and reliable positioning service based on the Global Positioning System (GPS). Such a service could be provided by a single-based Real-Time Kinematic (RTK) technique that can achieve centimetre-level positioning accuracy in real-time. However, this technique is restricted to scenarios based on a short distance between a rover GPS receiver and a reference station due to the residual atmospheric delays and orbital error effects on GPS baseline solutions. These distance-dependent errors can be modelled by applying the Network-based RTK (N-RTK) technique. N-RTK is a carrier phase-based positioning technique that combines measurements from multiple reference stations and thus generates “network corrections” that can be applied to extended baseline lengths. The implementation of N-RTK requires a network of permanent receiver stations, known as “Continuously Operating Reference Station” (CORS), communication links for data streaming and correction broadcast, and a processing centre (i.e. control centre). In summary, GPS measurements are recorded at the CORSs and streamed to the control centre via Internet links. Data gathered at the control centre are processed to model distance-dependent errors, generate N-RTK corrections, which are then delivered to users.

There have only been a few independent N-RTK systems developed by universities and receiver manufacturers (e.g. Rizos et al., 2003). Hence, the GNSS and Geodynamics (G&G) Research Group, Faculty of Geoinformation Science and Engineering, Universiti Teknologi Malaysia (UTM) is motivated to establish a research-based N-RTK system. Currently, the so-called ISKANDARnet system is being established in the metro-area of ISKANDAR Malaysia. In this paper the initial steps of establishing the ISKANDARnet are described. It includes setting up a control centre and the reference stations. Simulations of the ISKANDARnet have also been conducted to test: (a) communication links and data streaming, and (b) network correction (N-RTK messages) generation. Considering the data quality, multipath effects are analysed for two stations: ISKANDARnet1 and ISKANDARnet2. Evaluation and interpretation of multipath effects at the candidate CORS site is by the analysis of values of multipath on L1/L2 and by means of polar plots.

2. NETWORK-BASED RTK DESIGN

Over the last decade the N-RTK technique has come to dominate the high precision positioning arena, and is implemented (or will be shortly) in many areas around the world by the three major high precision GPS manufacturers Leica, Trimble and Topcon, in such locations as in the Australian states of Victoria (GPSnet),

NSW (CORSnet) and Queensland (SunPOZ), Germany (SAPOS), Denmark (REFDK), Hong Kong (SatRef), Japan (GEONET), Singapore (SiReNT), and in the Malaysian MyRTKnet. These networks have diverse CORS distribution and density design. According to Rizos and Han (2003), the typical distance between reference stations is in the range 50-100km, however it depends on the geographic location of the network and the level of ionospheric activity. For instance, the SiReNT the high ionospheric activity requires inter-receiver distances of less than 40km to ensure successful network ambiguity resolution. Additionally, Willgalis et al. (2002) indicated that such networks usually cover only densely populated areas or an important economic region. This is due to the expensive infrastructure involved. The network-based RTK positioning technique can be realised by establishing at least three CORS. One of the reference stations can be treated as the ‘master station’, that is usually selected as the nearest to the roving user receiver (Musa, 2007). Each reference station streams observation data to a control centre via an Internet link. The control centre is responsible for receiving the data streams from the CORS, monitoring the data integrity as well as processing network corrections. The network users then may receive the corrections via several data formats such as the de facto industry standard Radio Technical Commission for Maritime (RTCM), or (quasi-) proprietary formats such as Trimble’s Compact Measurement Record (CMR). Generally, there are two components of the data communication system: (a) between the control centre and the various reference stations, and (b) communication between the control centre and users (Rizos et al., 2004). An overview of the architecture of network-based positioning is given in Figure 1.

3. ON-GOING PROCESS OF ESTABLISHING ISKANDARnet

ISKANDAR metro-area significantly requires advanced positioning techniques to support high precision applications. Three CORS stations; ISKANDARnet1 at Universiti Teknologi Malaysia (UTM), ISKANDARnet2 at the Port of Tanjung Pelepas (PTP) and ISKANDARnet3 at Community College of Pasir Gudang, have been designed to be deployed within inter-station distance about 24 km and up to 43 km to cover the ISKANDAR metro-area adequately as shown in Figure 2. At the time of writing, ISKANDARnet1 is the control center and is completely installed, whereas another two CORS stations; ISKANDARnet2 and ISKANDARnet3 are under installation. Overall, the network configuration of ISKANDARnet is planned and can be seen in Figure 3 which includes infrastructure, data streaming, and user practice.

3.1 Setting up the control centre The control centre of ISKANDARnet has been set up at the Faculty of Geoinformation Science and Engineering, UTM, in Johor Bahru. A server computer and internet connection were configured to collect all GPS measurements from the ISKANDARnet reference stations.

CORS 1 / Master Station Network

correction

CORS 3

CORS 2

User

Control Centre

Internet

Figure 2: Research coverage area of the ISKANDARnet (Source: Google Earth).

Figure 3: The configuration of the ISKANDARnet

Figure 1: The architecture of the network-based GPS positioning.

Communications between reference stations and the control centre are provided via the Transmission Control Protocol / Internet Protocol (TCP/IP), intended for real-time applications. The Networked Transport of RTCM via Internet Protocol (NTRIP) technology is then applied to enable data streaming through TCP/IP. NTRIP is implemented in three system software components: NtripClients, NtripServers and NtripCaster (Dettmering et al., 2006). The control centre utilises an NtripCaster to manage the GPS data streams. NtripCaster is configured with essential parameters such as mountpoints, a TCP port number, and user passwords. NtripCaster also maintains a source-table containing information on stations and data streams. At present the control centre has been successfully established with the infrastructure and the software setup, as can be seen in Figure 4.

Figure 4: ISKANDARnet control centre.

3.2 Setting up the reference stations A reference station, ISKANDARnet1, has been established at the Faculty of Geoinformation Science and Engineering, UTM. The ISKANDARnet1 is equipped with a dual-frequency Trimble 4700 receiver, micro-centred L1/L2 antenna, server computer, internet connection and Uninterrupted Power Supply (UPS). The antenna has been mounted on the Block C03 building. The antenna installation and cabling are shown in Figure 5.

Figure 5: Antenna installation and cabling works at a

reference station.

As far as software concerned, the UTM-G&G research group has a close collaboration with Satellite Navigation & Positioning (SNAP) Laboratory of the University of New South Wales in order to implement the N-RTK system in ISKANDARnet. The N-RTK processing software enables the generation of real-time network corrections using International GNSS Services (IGS) Ultra Rapid orbits, and via the Virtual Reference Station (VRS) approach. The NtripServer is used to broadcast measurements from a reference station to the NtripCaster at the control centre. The NtripServer includes settings of the receiver output port and the NtripCaster settings for the data output. At present ISKANDARnet1 is recording data for post-processing, and also streams the data to the control centre in real-time. However, there is only one ISKANDARnet station installed permanently at the time of writing this paper (July 2009), and therefore a simulation of ISKANDARnet operation has been conducted (see section 4). Installation of the other two CORS, ISKANDARnet2 and ISKANDARnet3, at their proposed locations will be carried out in the coming months.

4. SIMULATION TEST

A simulation test was conducted for nine days from 7-15 January 2009 on the UTM campus. The ISKANDARnet1 (ISKA1) has been installed permanently at Block C03, Faculty of Geoinformation Science and Engineering, and another two temporary stations, ISKANDARnet2 (ISKA2) and ISKANDARnet3 (ISKA3), were set up at Tun Fatimah College and at Tun Hussein Onn College, UTM respectively. The main objectives was to test the stability of the communication links and data streaming between the reference stations and the control centre, as well as to generate network corrections from the simulated ISKANDARnet system. 4.1 Test 1: Communications link and data

streaming The GPS data streams in RTCM format have been tested using two types of Internet connections: Local Area Network (LAN) and Third Generation (3G) wireless broadband. The ISKANDARnet1 uses LAN connection, whereas both ISKANDARnet2 and ISKANDARnet3 stations employ 3G wireless broadband links. The data from a reference station is sent out to the control centre via NtripServer and data retrieval can be done via an NtripClient. Retrieved data then will be used to generate network corrections. In this test, Figure 6 shows that the NtripServer and the NtripClient applications being operated in ISKANDARnet1, ISKANDARnet2 and ISKANDARnet3 for retrieving data. The stability of the data acquisition then has been monitored.

(a)

(b)

Figure 6: Data stream via (a) NtripServer and (b) NtripClient.

It has been found that the communication links influence data streaming. Communication links via LAN are more stable for data streaming than the 3G wireless broadband because the availability of the 3G service depends on the service coverage and the instantaneous status of the bandwidth sharing. Thus, LAN is preferred in order to improve the ISKANDARnet performance. 4.2 Test 2: Generate network corrections In order to generate network corrections, the Multiple Reference Station (MRS) function in the Network-RTK software has been used. The ISKANDARnet1 (ISKA1(M) in Figure 7) was selected as the master station.

Figure 7: Network corrections for simulated

ISKANDARnet.

Figure 7 shows the network corrections that have been generated on a satellite-by-satellite basis in the MRS RTK window. In this case, PRN 29 is automatically set as the reference satellite as it is the highest elevation

satellite. The result shows that network corrections were only generated for satellites 12, 5, and 30. This is due to the less-than-ideal locations of the ISKANDARnet2 and ISKANDARnet3 stations (i.e. near buildings and trees). These locations are susceptible to multipath effects and decreased satellite visibility (leading to poor satellite geometry). Thus, the ambiguity resolution process is having difficulties, and this affects the generation of the network corrections. During this test, the multipath effects also contribute to the generation of poor quality network corrections. The data quality and the multipath effect must be analysed before setting up reference stations; an essential step in order to reduce the possibility of obtaining poor data.

5. DATA QUALITY ANALYSIS

Data quality is particularly affected by the multipath effect, and hence it is necessary to study the suitability of the station location. The analysis can be done by computing the so-called Root Mean Square of multipath on L1 (RMS mp1) and on L2 (RMS mp2). The “Translation, Editing, and Quality Check” (TEQC) program was used to derive the RMS mp1 and RMS mp2 values. The TEQC software also provides output files such as signal-to-noise ratios (SNR) on L1 and L2, satellite elevation and azimuth, ionospheric magnitude, and the derivative of ionospheric delay. The multipath files containing RMS mp1 and RMS mp2, satellite elevation and azimuth information can be used to further examine the multipath effects with “Teqcspec”. Teqcspec is MATLAB code for creating colourized polar plots (Ogaja & Hedfors, 2007), which enable interpretation of the multipath effect at the site. In this study, the multipath effect at ISKANDARnet1 and ISKANDARnet2 stations were analysed.

5.1 Case 1: Analysis of multipath effects at

ISKANDARnet1 a) RMS mp1 and RMS mp2 The multipath effect at ISKANDARnet1 is analysed by taking one-month RINEX data: from 1-30 April 2009 (i.e. Day of Year 91 to 120). The observations are logged at 15-second intervals during 24 hours, and the satellite elevation cut-off angle was set to 15 degrees. The results of RMS mp1 and RMS mp2 which computed using TEQC are shown in Figures 8 and 9.

Figure 8: Multipath effects on L1 at ISKANDARnet1

station.

Figure 9: Multipath effects on L2 of ISKANDARnet1

station.

Figure 8 indicates the RMS mp1 for ISKANDARnet1 station is mostly 0.06m. There are five days during the observation week that result in an RMS mp1 of 0.07m. Figure 9 indicates the RMS mp2 values range from 0.12m to 0.15m, which is higher than RMS mp1. The high value in RMS mp2 indicates that there are multipath errors with respect to the linear combinations of the carrier phase and pseudorange observations. According to Bruyninx et al. (2003), 75 percent of the European reference stations in the EUREF Permanent Network (EPN) have RMS mp1 values below 0.57m and RMS mp2 values below 1m. Furthermore, 50 percent of the International GNSS Service (IGS) stations around the world have RMS mp1 under 0.4m, and 75 percent have less than 0.5m. Meanwhile, the RMS mp2 value for 50 percent of the IGS stations are less than 0.6m and 75 percent are less than 0.75m. It seems that the multipath effect at ISKANDARnet1 is in the acceptable range. b) Polar plots The polar plots of ISKANDARnet1 have been created by taking 24 hours observation data recorded on 26th April 2009. The satellite elevation cut-off angle was set at 15 degrees. The polar plots were divided into four windows based on Coordinated Universal Time (UTC); 00:00:15 – 06:00:15, 06:00:15 – 12:00:15, 12:00:15 – 18:00:15, and 18:00:15 – 23:59:15. Figures 10 to 13 show each time window of 6 hours length. The main reason for using separated windows is to aid the analysis and visualisation of multipath effects at the site. The azimuth angle (0о to 360о) corresponds to the satellite azimuth, and the black dashed circle indicates the satellite elevation angle (0о to 90о) from horizontal. The coloured lines show the path of each satellite during the window period, where the satellite numbers are specified at the end of each satellite track.

Figure 10: Polar plot of first time window.

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Figure 11: Polar plot of second time window.

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Figure 12: Polar plot of third time window.

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The red circles in Figures 10 to 13 indicate the expected multipath effect. In this case, high multipath effect appeared in the south-east direction and at satellite elevation angles of 15о up to 50о. All polar plots showed a repeated multipath effect when satellites reached about 130о azimuth and 35о elevation. It is significant that the multipath effect also occurs for this area even though the satellite elevation increases up to about 50о (see satellite S30 and S18 in Figure 11). The reason for this is evident in Figure 14 as it is clear that a high-rise building (a mosque) is situated to the south-east of ISKANDARnet1. Although the distance between ISKANDARnet1 and the mosque is about 200m, the tall dome and tower most probably contribute to the multipath. The mosque’s tower can be seen in Figure 15a.

Figure 14: Map of ISKANDARnet1 environment.

Figure 15: Site environment of ISKANDARnet1.

Strong multipath effect also often occurred in azimuth direction 180о to 190о especially when satellites rise at about 30о above the horizon. By referring to Figure 14, there is a building at about this azimuth. A site inspection reveals that the Faculty of the Built Environment (FAB) building is situated only about 20m from the GPS antenna in that azimuth direction. Figure 15a shows the site environment with the FAB building being at almost the same height as the C03 (FKSG) building where the GPS antenna is mounted. Figure 13 shows that there is also high multipath in the west direction. From the site inspection there is a communication tower (see Figure 15b) located to the west of ISKANDARnet1. Other directions at ISKANDARnet1 have only small multipath effects, typically caused by low elevation satellites.

5.2 Case 2: Analysis of multipath effects at ISKANDARnet2

a) RMS mp1 and RMS mp2 ISKANDARnet2 has been temporarily established at PTP in order to study the suitability of the site. The test was conducted on 3rd to 6th April, 2009, on the rooftop of the PTP building, Gelang Patah, Johor. All GPS data were recorded at 15-second intervals with 15 degree satellite elevation cut-off angle. The analysis of the multipath effect was again conducted by calculating the RMS of mp1 and mp2 via TEQC. The results of RMS mp1 and RMS mp2 are shown in Figures 13 and 14 respectively.

Figure 16: Multipath effects on L1 at ISKANDARnet2

station.

Figure 17: Multipath effects on L2 at ISKANDARnet2

station. From Figure 16, the RMS mp1 values for ISKANDARnet2 vary from 0.15-0.17m over the four days. Meanwhile, Figure 17 shows the RMS mp2 varies from 0.22-0.25m. However, the RMS mp1 and RMS mp2 values are still within the acceptable range (by referring to the quality status of other CORS and IGS

GPS Antenna Mosque’s

tower FAB

C03, FKSG

(a) (b)

Figure 13: Polar plot of fourth time window.

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54о

18о

N

EW

S

stations mentioned earlier). Figures 16 and 17 also show that the pattern of RMS mp1 and RMS mp2 are almost the same. It seems that the same factors are affecting the L1 and L2 signal. Further investigation on the source of the multipath errors was conducted with the aid of polar plots and site inspections.

b) Polar plots

For ISKANDARnet2 24 hours of data from the 5th April 2009 was used to create polar plots. The procedure used and the multipath interpretation process is identical to Case 1.

The polar plots from ISKANDARnet2 (Figures 18 to 21) show that the azimuth range 180о to 210о is prone to high multipath. This effect is caused by satellites at elevation angles of about 40о. Another location that is strongly affected by multipath at azimuth 290о and elevation 35о can be seen in all the plots. Additionally, there are also multipath effects in the azimuth range 120о to 180о, especially when satellites reach about 15о to 30о elevation.

The repetition of the multipath effect can be further explained by site inspections. The strong multipath in the azimuth range 180о to 210о is most probably due to the nearby the communications antenna, as can be seen in Figure 22a. In addition, the occurrence of the multipath effect at azimuth 290о is caused by the tall object shown in Figure 22b. Nevertheless, the multipath effect can be reduced by designing a taller supporting bracket for the GPS antenna.

Figure 18: Polar plot of first time window.

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Figure 20: Polar plot of third time window.

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Figure 21: Polar plot of fourth time window.

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Figure 19: Polar plot of second time window.

Other object

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6.0 CONCLUDING REMARKS & FUTURE

WORK In this paper the Network-RTK system known as ISKANDARnet – established by UTM-G&G Research Group in collaboration with SNAP-UNSW – is described. The infrastructure of the control centre and reference stations as well as communication links for data streaming have been configured. Furthermore, data streaming and the generation of network corrections has been tested by conducting a simulation of the complete ISKANDARnet system. Sufficient network corrections can be generated if raw data are streamed continuously and are of good quality. Essentially, the multipath effects were checked at every reference stations as part of the data quality and site selection evaluation process. It was found that the multipath effect at ISKANDARnet2 is higher than at ISKANDARnet1, however the multipath effects at the both stations are within acceptable limits. Selection of another two CORS sites will be carried out soon. The same procedure will be applied, but the

generation of the network corrections needs further testing over the actual network coverage area. It is expected that this work will provide an important research facility as well as an operational platform for high precision applications within the metro-area of ISKANDAR Malaysia. REFERENCES Bruyninx, C., Carpentier, G. and Roosbeek, F. 2003. Today’s EPN and its network coordination. EUREF Symposium, 4-6 June, Toledo, Spain. EUREF Publication, 13 (33), p.p 38-49. Dettmering, D., Waese, C. and Weber, G. 2006. Networked Transport of RTCM via Internet Protocol Example Implementation. Federal Agency for Cartography and Geodesy (BKG), Frankfurt, Germany. http://igs.bkg.bund.de/index_ntrip.htm (accessed 28th May 2009).

Musa, T.A. 2007. Analysis of residual atmospheric delay in the low latitude regions using network-based GPS positioning. Doctor of Philosophy thesis, School of Surveying & Spatial Information Systems, University of New South Wales, Sydney, Australia. Ogaja, C. and Hedfors, J. 2007. TEQC multipath metrics in MATLAB. GPS Solutions, 11, p.p. 215 – 222. Rizos, C. and Han, S. 2003. Reference station network based RTK systems-concepts and progress. Wuhan University Journal of Natural Sciences, 8 (2), p.p. 566-574. Rizos, C., Kinlyside, D.A., Yan, T.S., Omar, S. and Musa, T.A., 2003. Implementing network RTK: The SydNET CORS infrastructure. 6th Int. Symp. on Satellite Navigation Technology Including Mobile Positioning & Location Services, 22-25 July, Melbourne, Australia. Rizos, C., Yan, T.S. and Kinlyside, D.A. 2004. Development of SydNET permanent real-time GPS network. Journal of GPS, 3(1-2), p.p 296-301. Van Sickle, J. 2008. GPS for Land Surveyors, Third Edition. Boca Raton, Landon, CRS Press. Willgalis, S., Seeber, G., Krueger, C.P. and Romao, V. 2002. A real time GPS reference network for Recife, Brazil, enabling precise and reliable cadastral survey. FIG XXII International Congress, Technical Session 5.8. 19-26 April, Washington DC, USA. ACKNOWLEDGEMENTS The authors would like to express their gratitude to Ministry of Science, Technology and Innovation (MOSTI) Malaysia for their financial support throughout the project.

Other antenna GPS antenna

Figure 22: GPS antenna at the rooftop of PTP building with the other (a) communication

antenna and (b) objects nearby.