Development of a Network-Based RTK-GPS Positioning System using FKP via the Internet

Transcription

1 Paper Development of a Network-Based RTK-GPS Positioning System using FKP via the Internet Hiromune NAMIE * a), Osamu OKAMOTO **, Chunming FAN ***, and Akio YASUDA *** Abstract Real-time kinematic GPS (RTK-GPS) is a real-time satellite positioning system. Using RTK-GPS, the horizontal and vertical positions of an object can be determined to within a few centimeters. However, Japan currently lacks the essential infrastructure to support the consistent delivery of data. The authors developed and tested a FKP-method network-based RTK-GPS (FKP-RTK) positioning using the area correction parameter (FKP) via the Internet. We studied FKP-RTK positioning at a fixed point, while varying the position of the GPS reference station located inside the station network area. Both the precision and accuracy of the FKP-RTK positioning were confirmed to be independent of the position of the primary GPS reference station (PRS). Using an Internet line, as compared with a public phone line, comparable results were achieved. The degradation in positioning precision when compared to RTK-GPS positioning (non network-based) is not especially pronounced, and we can see that the results of the network-based RTK-GPS positioning correction results are high. Keyword GPS, RTK-GPS, FKP, Positioning, Internet 1. Introduction Real-time kinematic GPS (RTK-GPS) is a real-time satellite positioning system. The distance to the satellite from a user GPS antenna (determined by measuring the phase of the carrier waves from the GPS satellites) is precise to the millimeter level. Thus, the horizontal and vertical positions of an object can be determined to within 1 to 4 cm. This system is easier to operate than a conventional survey system, such as Total Station, and numerous applications of RTK-GPS have been investigated[1]-[14]. Although RTK-GPS positioning has numerous benefits, the system has some usability problems. First, RTK-GPS requires a fast data communications link in order that the correction data, which may include the carrier phase, be transmitted from a reference station located at a known position to a user receiver. In Japan, low-power radio communications devices and cellular phone systems are typically used because these do not require a license to operate. However, with the former, the data transmission area is inevitably limited to a few hundred meters in radius from the transmission antenna. Also, continuous operation of the latter is very expensive. In addition, it is difficult for users to purchase expensive GPS receivers capable of receiving reference stations, and it is technically difficult to establish a reference station and determine its position to within 1 cm. Furthermore, the baseline is limited to approximately 15 km in RTK-GPS due to the non-uniformity *The National Defense Academy , Hashirimizu, Yokosuka, a) nami@nda.ac.jp **Ibaraki College of Technology 866, Nakane, Hitachinaka, Ibaraki, ***Tokyo University of Marine Science and Technology 2-1-6, Etchujima, Koto-ku, Tokyo, of the ionosphere and the troposphere. Therefore, users who require high precision and real-time positioning in Japan require a more flexible and less-expensive correction data dissemination service for a long period of time (more than 10 years)[15][16]. In the late 1990s, a carrier phase data dissemination service in RTK-GPS was tested experimentally in Japan. There are among 1,200 GPS-Based Control Stations (GPS-Based CS), collectively known as GEONET (Global Earth Observation Network), established by the Geographical Survey Institute (GSI) of Japan to be used to observe the Earth s crustal movement for seismic prediction. For approximately one year beginning in December 2000, the service was used experimentally by the network-based RTK-GPS[17]-[20] using several stations and three types of systems: MultiRef from Calgary University in Canada, the Virtual Reference Station (VRS TM ) (VRS-RTK) from Trimble Terrasat in Germany, and Referenznetz from Geo++ GmbH in Germany (known as the area correction parameter (FKP[16]: flachen-korrecturparameter (in Germany)) (FKP-RTK) system in the Tokyo area). The modes were well tested and evaluated by several groups of surveyors and navigation engineers using a cellular phone system, which is a type of dual-direction communication system. The FKP mode is now recognized as the standard configuration for satellite positioning services, such as SAPOS. Currently, most GPS-Based CS receivers use propriety messages (unless they use RTCM Version 3). However, Japan currently lacks the essential infrastructure to support companies that deliver data consistently. Previously, we have used a satellite-based communication line[25] and TV broadcasting[26] to attempt to construct measurement systems. However, because satellite-based communications places an emphasis on convenience, whereas TV broadcasting worked to switch from analog broadcasting to digital broadcasting, attempts to bring this infrastructure to fruition have been unsuccessful, respectively. 9

2 In the present study, we examined Internet lines and the construction of a network-based RTK-GPS positioning system used for the transmission of the area correction parameters (FKP) (FKP-Network RTK-GPS). In addition, at the same fixed positioning point, an experiment was conducted comparing the commonly used method of transmitting data via public phone lines while changing the primary GPS reference station (PRS) and performing positioning. Using the Internet to transfer the correction data, some of the above-mentioned usability problems can be solved. 2. Internet Line Communication Protocol TCP (Transmission Control Protocol) connections and UDP (User Datagram Protocol) connections are well-known protocols used in communications over Internet lines. TCP is a protocol that is oriented toward creating a connection for reliable communications and is commonly used. On the other hand, processes to ensure connectivity and reliability are not necessary when using UDP. Therefore, UDP allows for faster transmission speeds and improved transmission efficiency compared to TCP..Moreover, UDP can unilaterally transmit data, regardless of whether there are requests from the system. Therefore, UDP is well suited for broadcast data and has the benefit of having lower transmission costs than TCP. From the perspective of a moving type of RTK-GPS positioning, it is important that it occurs in real-time, and data transmission reliability must be high. However, rather than TCP, which requires two-way transmissions, UDP was adopted for the present experiment for its high-speed transmission capability and suitability for broadcast transmission, despite the fact that the reliability of UDP is lower than that of TCP. Table 1 shows the principle source of the data transfer system used by the authors until now to transfer correction data for network-based RTK-GPS positioning. With Internet lines, even during peak-usage hours, performance equal to satellite communication lines and television sound multiplex data broadcasting can be achieved. In addition, regarding the transmission lag of Internet lines, it has been reported that, during peak usage hours, maximum delays were 520 ms and average delays 502 ms[30]. We investigated the impact of this lag on network-based RTK-GPS positioning. Table 1 Specifications of various data transmission systems System Data rate Delay Internet Line 56.0 kbps ms Satellite Line 56.0 kbps ms TV-ASC 8.0 kbps 740-1,320 ms area correction parameters (FKP), RTCM++ data, and FKP, are transmitted to the user stations via a data communication line. The primary reference station refers to the use of carrier phase raw data from the reference station s data. The FKP system uses the single phase difference between the primary reference station and other reference stations. We use RTCM (Radio Technocal Commission for Maritime Services Special Committee 104) Type 59 (Proprietary message) to transmit the FKP at 1 Hz from the data center to the users. RTCM Type 59 requires the approximate location of a user station to convert the last correction data V relation to a nearby user station (X U, Y U ) from the FKP data. The other parts of the network-based system are similar to normal RTK-GPS. The conceptual diagram for the creation of the correction data (FKP), which is the correction data for the FKP-RTK positioning system that we have created, is displayed in Figure 2. For simplification purposes, the conceptual diagram is explained here in two dimensional terms. Using the carrier phase data V k from the reference station k (X k, Y k ) that makes up the network, by performing a simple linear interpolation, if the reference station nearest to the user station (X U, Y U ) is installed, then the carrier phase data V (correction data) that should be measured will be created. The data center takes the selected GPS reference station ( the primary reference station (PRS)) (X A, Y A ) as the center of the vector (the difference) of Fig. 1 Basic conceptual diagram of the network-based RTK-GPS positioning system ASC: Audio Subcarrier Channel 3. FKP-RTK Positioning Figure 1 shows the basic concept of the network-based RTK-GPS positioning system. The system includes three or more reference stations, a Data Control Center (Data Center) and user stations. Figure 2 shows the correction diagram of the FKP-RTK positioning system using FKP area correction parameters. The carrier phase data VR from the three reference stations (X R, Y R ) are transmitted to the data center. After the data is converted using the simple linear complementary method, the Fig. 2 Correction diagram of the FKP-RTK (FKP-method network-based RTK-GPS) positioning using FKP area 10

3 the single phase difference computational process (between reference stations) and creates the FKP from the carrier phase data observed in three or more reference stations. Then, this data, the coordinates of the primary GPS reference station (PRS), and the carrier phase raw data of the PRS are transmitted to the user, and the correction V for the user side is calculated. The specific method to realize FKP is shown in the following. For each reference station, the correctly observed carrier phase for each satellite is V i k, then k (A, B, C, D) is the number of reference stations. In addition, there are i (1, 2, 3, ) satellites. Calculating the difference in the observed carrier phase raw data and the four reference stations and the PRS and other reference stations as ( Δ X Ak, Δ Y Ak ) yields the following[32][33]: There are i vectors for each satellite. Taking the difference in coordinates between the primary GPS reference stations (PRS) and the other reference stations vector as A, we have[31]-[33] stations (PRS). However, the carrier phase error in the reference stations is not the same due to the effect of the ionosphere and the troposphere. Therefore, changing the PRS will have an affect on the positioning results. 4. FKP-RTK Positioning Experiment Using FKP via the Internet 4.1 Outline of the Experiment Figure 3 shows the arrangement of GPS reference stations and the fixed point of a user station for the positioning experiment. In this experiment, personal reference stations in Setagaya, Fujisawa, Ageo (Saitama Pref.), and Chiba were used. In addition, the data from each reference station was collected at the data center (Yokosuka GIS Center in Yokosuka Research Park, Kanagawa Pref.) via Internet lines (which is beyond the scope of the present study), and the FKP was created at the data center. Table 2 shows the coordinates of the GPS reference stations. Figure 4 shows the experimental configuration, and Figure 5 shows the GPS antenna at the fixed point of a user station at the Tokyo University of Marine Science and Technology (TUMSAT) in Koto-ku, Tokyo. At the user positioning point of Tokyo, Koto-ku, a signal from a GPS antenna (NovAtel GPS600) was distributed using distributors, each of which was connected to a NovAtel OEM4 GPS receiver. The antennas were installed on a 5th-floor rooftop (approximately 22 meters above sea level). The positioning times were Jan 20th, 21st, and 22nd, 2004, for However, note that (ΔX Ai, ΔY Ai ) = (X i X A, Y i Y A ). The FKP ( a b c ) T for each satellite can be found as follows: This data and the coordinates of the primary GPS reference stations (PRS) are sent to the user. At the user station, the user single (stand alone) positioning results (XU YU ) are used in and the correction for each satellite V i is calculated. However, (ΔX U, ΔY U ) = (X U X A, Y U X A ). Therefore, the user can use this FKP to perform RTK-GPS positioning from anywhere within the network of the reference stations. Several reference stations receive radio waves from all receivable GPS satellites simultaneously so that it is possible to correct the effect of the ionosphere over a wide area (i.e., when there is approximately 60 km between reference stations). Thus, it is possible to fix positions using the RTK-GPS based on carrier phase measurements over a much wider area surrounded by the reference station network. Ideally, with respect to each reference station, if the error in each carrier phase is the same, then the FKP should be the same, regardless of the position of the primary GPS reference Fig. 3 Allocation of the fixed positioning point of the user station at the Tokyo University of Marine Science and Technology (TUMSAT) in Koto-ku, Tokyo, and four GPS reference stations in Setagaya, Fujisawa, Ageo, and Chiba in the experiment on FKP-RTK (FKP-method network-based RTK-GPS) positioning Table 2 Spec ifications of various data transmission systems Name Latitude Longitude Height Setagaya m Ageo m Chiba m Fujisawa m 11

4 Fig. 4 Experimental diagram of the FKP-RTK (FKP-method network-based RTK-GPS) positioning system using FKP via the Internet and public phone lines Fig. 6 FKP-RTK (FKP-method network-based RTK-GPS) horizontal positioning distributions using FKP via the Internet and public phone lines using different primary GPS reference stations (PRS) at a fixed-point user station at the Tokyo University of Marine Science and Technology (TUMSAT) in Koto-ku, Tokyo Fig. 5 View of the GPS antenna at the fixed point of a user station at the Tokyo University of Marine Science and Technology (TUMSAT) in Koto-ku, Tokyo 23 hours continuously, from 2:00 AM to 1:00 AM (UTC) the following day. The primary GPS reference stations (PRS) were switched through Setagaya and Chiba on the 20th, Setagaya and Fujisawa on the 21st, and Setagaya and Ageo on the 22nd. FKP-RTK positioning was then performed on both groups simultaneously at 1 Hz. In all cases, from the FKP transmission from the data center to the user station measurement point, transmission was performed using public phone lines at a rate of 9,600 bps when the Setagaya reference station was the PRS and using Internet lines when any other reference station was used as the PRS. FKP-method positioning settings were performed with all kinematic (Whether kinematic or static mode) and in synchronization mode. Due to disconnections during positioning, only a small amount of data was collected during the 23-hour period. 4.2 Results of Positioning Experiments Figure 6 shows the horizontal positioning distribution by FKP-RTK. Figure 7 shows the height change over time. Table 3 lists the results obtained using the nearest GPS-Based Control Station (GEONET) as the reference station, and the average value of Fig. 7 FKP-RTK (FKP-method network-based RTK-GPS) vertical positioning temporal distributions using FKP via the Internet and public phone lines using different primary GPS reference stations (PRS) at a fixed-point user station at the Tokyo University of Marine Science and Technology (TUMSAT) in Koto-ku, Tokyo the results of VRS-method network-based RTK-GPS (VRS-RTK) positioning are adopted as the true value. These positioning results are 2drms (twice distance root mean square) at the horizontal positioning distribution. In addition, these results are better than 2.44 cm and are reliable. The same axis show that for each day of the experiment, the longitudinal offset was 10 cm, and the latitudinal offset was 20 cm, as shown in Figure 6. In addition, the vertical offset was 20 cm, as shown in Figure 7. The graph shows the results when Setagaya reference station was the primary GPS reference station (PRS) in blue, and the results when other reference stations were used as the PRS in green. No notable difference can be seen in the 12

5 Table 3 Results of FKP-RTK (FKP-method network-based RTK-GPS) positioning using the FKP via the Internet and public phone lines using different primary GPS reference stations (PRS) at a fixed-point user station at the Tokyo University of Marine Science and Technology (TUMSAT) in Koto-ku, Tokyo Fig. 8 Precision of FKP-RTK (FKP-method network-based RTK-GPS) positioning using FKP via the Internet and public phone lines using different primary GPS reference stations (PRS) at a fixed-point user station at the Tokyo University of Marine Science and Technology (TUMSAT) in Koto-ku, Tokyo Fig. 9 Discrepancy of FKP-RTK (FKP-method network-based RTK-GPS) positioning using FKP via the Internet and public phone lines using different primary GPS reference stations (PRS) at a fixed-point user station at the Tokyo University of Marine Science and Technology (TUMSAT) in Koto-ku, Tokyo average number of satellites or the average PDOP value. Therefore, it can be said that the effect of the satellite constellation on the positioning is less than 1 mm. F rom Table 3, no notable difference can be seen in the average number of satellites or in the average PDOP values in relation to the experiment day or primary GPS reference station (PRS). Therefore, we can conclude that the satellite constellation had no effect. Compared to the VRS-RTK results, which were obtained using the surrounding GPS-Based Control Station (GEONET), only discrepancies of within 1 cm in both the latitudinal and longitudinal directions can be observed. However, with respect to height, any discrepancy can be observed. In particular, when the Setagawa reference station was used as the PRS, the average offset was more than 4 cm. Therefore, we believe that personal reference stations have an offset of a few centimeters in height. Figure 8 shows the positioning precision in graph form, and Figure 9 shows the offset between FKP-RTK and VRS-RTK in graph form. The solid lines in the figures represent the first-order approximation line of the horizontal direction and vertical direction results using all plots. Figure 8 shows no notable difference in positioning precision caused by the primary GPS reference station (PRS) line length. In addition, Figure 9 shows that horizontal positioning is extremely stable at less than 1 cm. However, vertical positioning has an offset of more than 2 cm without exception. With respect to the RTK-GPS positioning, the impact of the ionosphere and the troposphere, which depends on the basic line length, varies depending on the day and therefore cannot be simply compared to another day s positioning results. However, from Table 3, in which the standard Setagaya reference station is used as the PRS, the variation in the daily results obtained using public p hone line correction data transmission is on the order of s everal centimeters. From this, we can convert the effect of the t roposphere and the ionosphere into positioning results and see that the effect of the troposphere and the ionosphere is on the order of a few centimeters. In both cases, the obtained results were comparable to those obtained with the Setagaya PRS when using public phone lines. The degradation in positioning precision as compared to RTK-GPS positioning is not especially pronounced, and we can see that the FKP-RTK positioning correction results are comparable (1 to 4 cm). 5. Conclusion In the present study, we used Internet lines for the transmission of correction data and created a FKP-RTK positioning system. In addition, we compared with the conventional FKP-RTK method using public phone lines and created a high-precision environment that was free from significant error. Comparable result were achieved using Internet lines and public phone lines. The degradation in positioning precision as compared to RTK-GPS positioning ( not network-based) is not especially pronounced, and we can see that the results of the FKP-RTK 13

6 positioning correction results are high accuracy and high International Symposium on GPS/GNSS 2008 in Tokyo, p.73, precision We hope that the present experiment will help to promote the practical use of the network-based RTK-GPS positioning system in Japan because the precise positioning of RTK-GPS must be improved in order to be easily used by the public. [12] Neil Brown and Paul Alves, New Method for Graphically Representing Residual Error in an RTK Network in Real Time, Program and Abstracts of International Symposium on GPS/GNSS 2008 in Tokyo, p.74, 2008 [13] Suqin Wu, An Investigation of Regional Atmospheric Acknowledgments The authors would like to thank Mr. Shinji Tanaka for his help. The present study was supported in part by the Scholarship & Education Foundation through a research grant provided by the Defense Academy of Japan. Models for Network RTK Positioning - A Case Study in Victoria, Program and Abstracts of International Symposium on GPS/GNSS 2008 in Tokyo, p.75, 2008 [14] Kazutoshi Sato, Hirotaka Obata, Akira Wada, Masayuki Kanzaki, Tetsuya Iwabuchi, Christian Rocken and Teruyuki Kato, Real-Time Kinematic GPS Analysis of the 2008 References [1] Osamu Okamoto, Hiromi Tsuboi, Hiromune Namie and Iwate-Miyagi Inland Earthquake Using by 1Hz Data, Program and Abstracts of International Symposium on GPS/GNSS 2008 Akio Yasuda, Application of RTK-GPS to Immersion of in Tokyo, p.83, 2008 Caisson, The Gravity, Geodesy and Marine 1996 [15] Hiromune Namie, Naoto Tanaka and Akio Yasuda, (GraGeoMar96), Vol.117, pp , 1997 RTK-GPS Positioning in Japan by GPS-Based Control Station [2] Osamu Okamoto, Hiromi Tsuboi, Akio Yasuda and via DMCA Mobile Radio Communication System, Hiromune Namie, Application of RTK-GPS in Construction Engineering in Japan, The 64th Federation Internationale des Geometres (FIG), International Symposium, 1997 [3] Hiromune Namie, Nobumi Hagiwara, Hakjin Kim, Shinji Nitta, Yoshinobu Shibahara, Tetsuro Imakiire and Akio Yasuda, RTK-GPS Positioning in Japan by Virtual Reference Station (VRS) System with GPS-Based Control Station, Proceedings of the 14th International Technical Meeting of the Satellite Division of the Institute of Navigation (ION GPS-2001), pp , 2001 [4] Hiroshi Tsuchiya, About Virtual Reference Station Mode of Network-Based RTK-GPS Positioning, Research Meeting Concerned with RTK-GPS Positioning - by Virtual Reference Station (VRS) Mode -, pp.1 11, 2001 (in Japanese) [5] Jun Isejima, Tomoji Takasu, Takuji Ebinuma and Akio Proceedings of 1999 National Technical Meeting, Institute of Navigation, pp , 1999 [16] Gerhard W ubbena, Andreas Bagge and Martin Schmitz, Network-Based Techniques for RTK Application, Text for GPS Symposium 2001, GPS Society, Japan Institute of Navigation, pp.53 66, 2001 [17] H. Marel, Virtual GPS Reference Stations in the Netherlands, Proceedings of the 11th International Technical Meeting of the Satellite Division of the Institute of Navigation (ION GPS-98), pp.49 58, 1998 [18] Hakjin Kim, Hiromune Namie and Akio Yasuda, VRS (Virtual Reference Station) Positioning Using a Multiple Reference Station in Japan, The 8th GNSS Workshop, pp.49 55, 2001 [19] Falin Wu, Nobuaki Kubo, Akio Yasuda, Hakjin Kim, Yasuda, Performance Evaluation of the RTK-GPS Positioning Hiromune Namie and Yoshinobu Shibahara, Marine with Communication Delay, The Journal of Japan Institute of Navigation (JIN), Vol.119, pp , 2008 (in Japanese) [6] Hiroyuki Hasegawa, Ikuo Kitagawa and Takashi Hayama, RTK-GPS Positioning in Tokyo Bay Using Virtual Reference Station (VRS) System, The ENC-GNSS2002, 2002 [20] Hakjin Kim, Hiromune Namie, Falin Wu, Nobuaki Kubo Evaluation on Area Correction Point (FKP) Survey, Journal and Akio Yasuda, RTK-GPS Positioning Using Virtual of Applied Survey Technology (JAST), Vol.15, pp.29 36, 2004 (in Japanese) Reference Station (VRS) System in Japan, The 9th GNSS Workshop, pp , 2002 [7] Hiroyuki Hasegawa, Toshitaka Miyazaki, Susumu Hattori, [21] Shusaku Imoo, Hiromune Namie and Akio Takashi Saito and Shigeo Ichinose, FKP and Forest Base Map Specification, Journal of Applied Survey Technology (JAST), Vol.19, pp.25 32, 2008 (in Japanese) [8] Tomoji Takasu, Evaluation of RTK-GPS Performance with Low-Cost Single-Frequency GPS Receivers, Program and Abstracts of International Symposium on GPS/GNSS 2008 in Tokyo, p.15, 2008 [9] Tatsunori Sada, Shigeyuki Murayama and Shogo Egami, Yasuda, 3-Dimensional Attitude Measurement of a Ship by RTK-GPS, Journal of Japan Institute of Navigation, Vol.98, pp.1 7, 1998 (in Japanese) [22] Osamu Suzuki, Takeshi Nakamura Hiromune Namie and Akio Yasuda, Real Time Anchor Watch by GPS, Journal of Japan Institute of Navigation, Vol.98, pp.25 32, 1998 (in Japanese) [23] Osamu Okamoto, Akio Yasuda and Hiromune Namie, Comparison of Accuracy of the RTK Solution and Experimental Study on the Observation of Landslide Initialization Time with Fixed and Roving Antenna, Program and Abstracts of International Symposium on GPS/GNSS 2008 in Tokyo, p.16, 2008 [10] Hee-Sung Kim, Position-Domain DD Hatch Filter to Maintain Float Solution Accuracy in RTK, Program and Abstracts of International Symposium on GPS/GNSS 2008 in Tokyo, p.18, 2008 [11] Nobuaki Kubo, Instantaneous RTK Positioning Based on User Velocity Measurements, Program and Abstracts of Movement by RTK-GPS, The Japanese Association of Surveyors Proceedings of the Applied Survey Technology, Vol.9, pp.55 62, 1998 (in Japanese) [24] Hiromune Namie, Akio Yasuda, Koji Sasano, RTK-GPS Positioning in Japan by TV Audio-MPX-Data Broadcast, The Application of GPS and Other Space Geodetic Techniques to Earth Sciences, The Earth, Planets and Space, vol.52, nos.10 and 11, pp , 2000 [25] Hiromune Namie, Osamu Okamoto, Chunming Fan, 14

7 Shinji Tanaka and Akio Yasuda, Development and Experiment Study of a Network-Based RTK-GPS Positioning System Using a Satellite Communication in Japan, Wiley Part 1, Vol.89, No.9, pp.1 9, 2006 [26] Hiromune Namie, Keiichi Nishikawa, Koji Sasano, Chunming Fan and Akio Yasuda, Development of Network-Based RTK-GPS Positioning System Using FKP via a TV Broadcast in Japan, IEEE Transactions on Broadcasting, Volume 54, Issue 1, pp , 2008 [27] Hiromune Namie, Nobumi Hagiwara, Nobuharu Nitta, Yoshinobu Shibahara, Masayuki Saito, Yoshinobu Kanai, Hakjin Kin and Akio Yasuda, Evaluation of RTK-GPS by Virtual Reference Station (VRS) System, The Communication Society, The Institute of Electronics, Information and Communication Engineers, vol.j84-b, no.12, pp , 2001 (in Japanese) [28] Osamu Okamoto. Evaluation of Precision of Network-Based RTK-GPS Positioning of VRS Mode, GPS Symposium 2002, GPS Society, The Japan Institute of Navigation, pp , 2002 (in Japanese) [29] Hiroshi Isshiki and Hiromune Namie, GPS Foundation of GPS, GPS Symposium 2002, GPS Society, The Japan Institute of Navigation, pp , 2002 (in Japanese) [30] Zhixing Liu, Akio Yasuda and Chunming Fan, The Evaluation of the Accuracy and Measurement of Latency of Internet-Based DGPS and RTK-GPS Positioning, The Proceedings of the 14th International Technical Meeting of the Satellite Division of the Institute of Navigation 2001, pp , 2001 [31] Shinji Tanaka, A Study on Network-Based RTK-GPS, Masters Paper, Tokyo University of Marine Science and Technology (TUMSAT), 2003 [32] Gerhard W ubbena, Andreas Bagge and Martin Schmitz, RTK Networks based on Geo++ GNSMART - Concepts, Implementation, Results, The Proceedings of the 14th International Technical Meeting of the Satellite Division of the Institute of Navigation 2001, 2001 [33] Gerhard W ubbena, Andreas Bagge, RTCM Message Type59-FKP for transmission of FKP version1.0, Geo++ White Paper Nr., 2002 [34] Hiromune Namie and Akio Yasuda, Evaluation of Data Transmission System of RTK-GPS, The Communications Society, The Institute of Electronics, Information and Communication Engineers, Vol.J82-B, No.1, pp , 1999 (in Japanese) since 1996 in Japan. Osamu Okamoto is a associated professor at Ibaraki College of Technology. He was previously with the Institute of Technology, Nishimatsu Construction Co., Ltd. He received his doctorate in engineering from Tokyo University Mercantile Marine in He is a member of the Steering Committee of the GPS/GNSS Society, the Japan Institute of Navigation (JIN-GPS) and a secretary of the Joint Research Committee on Advanced Utilization of Positioning and Geographical Spatial Information, the Institute of Electrical Engineers (IEE) of Japan. His specialty is RTK-GPS error correction for slope failure monitoring. Chunming Fan is a research associate at the Tokyo University of Marine Science and Technology (TUMSAT), Japan. He is a member of the Steering Committee of the GPS/GNSS Society, the Japan Institute of Navigation (JIN-GPS). His major interests are satellite communication and positioning systems, including GPS/GNSS applications, and the development of instruments for marine research. Akio Yasuda is an honorary professor at the Tokyo University of Marine Science and Technology, Japan. He received his doctorate in engineering from Nagoya University. He has been the president of the Institute of Positioning, Navigation and Timing (IPNT), since His major interests are satellite communication and positioning systems, including the applications of GPS/GNSS and the development of instruments for marine research. biography Hiromune Namie is a lecturer at the National Defense Academy (NDA), Yokosuka, Japan. He received his doctorate in engineering from Tokyo University of Mercantile Marine in He is a secretary of the Steering Committee of the GPS/GNSS Society, the Japan Institute of Navigation (JIN-GPS). and an assistant secretary of the Joint Research Committee on Advanced Utilization of Positioning and Geographical Spatial Information, the Institute of Electrical Engineers (IEE) of Japan. His research interests are r elated to the wide-spread use of highprecision RTK-GPS and indoor positioning and their applications. He has been the administrator of the Internet ing list GNSS ML 15