Research Toward Wireless Internet-Based DGPS
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1 Research Toward Wireless Internet-Based DGPS Z. Liu and Y. Gao Department of Geomatics Engineering The University of Calgary Calgary, Alberta, Canada T2N 1N4 Tel: Fax: BIOGRAPHY Mr. Zhe Liu is a M.Sc. student in Geomatics Engineering at the University of Calgary. He received a ME and a BE from Beijing University of Aeronautics and Astronautics. He then worked for two years on GPS receiver design at Beijing Research Institute of Telemetry. He currently conducts research on GPS/GIS/Wireless Integration. Dr. Yang Gao is an Assistant Professor of Geodesy and Navigation in the Department of Geomatics Engineering at The University of Calgary. He has been involved in GPS research since 1990 and his research expertise includes both theoretical aspects and practical applications of satellite positioning and navigation systems. His current research focuses on high precision GPS positioning, wide and regional area differential GPS networks and mobile information management. ABSTRACT Data transmission using the Internet has become increasingly fast, reliable and cost-effective in the past several years. Commercial wireless data networks like GSM (Global System for Mobile), and CDPD (Cellular Digital Packet Data) are also emerging to form worldwide data service networks. An integration of the Internet and commercial wireless data network technologies with the Wireless Internet has the potential to support a large number of reference stations on-line to form much dense differential GPS networks in the near future. Based on this approach, the reference stations will send differential data to the Internet that can then be accessed by virtually unlimited number of mobile users. This innovative communication method has no limitation on effective range, as long as the reference stations and the mobile users are covered by either the Fixed Internet or the Wireless Internet, and could be an efficient alternative to current differential GPS network methods. Such a Wireless Internet-based differential GPS network can serve both local and wide area DGPS users from meter level to centimeter level positioning and navigation applications. Moreover, the wireless Internet-based differential GPS network supports precise relative positioning with mobile reference stations, thus it can open new applications using wireless Internet-based differential GPS Networks. Several field tests were conducted at The University of Calgary using CDPD approach and the results show that the time delay for the correction message from mobile reference stations to the mobile users was around 2.5 seconds, and the RTK positioning accuracy for low dynamic test (velocity less than 20 km/h) was at the centimeter-level with baselines over 10km. The tests also showed that UDP/IP could support Wireless Internet-based RTK positioning with mobile reference stations better than TCP/IP. 1. INTRODUCTION Conventional differential GPS (DGPS) such as Real-Time Kinematic (RTK) positioning is conducted based on a pair of navigation receivers and a pair of radios at each end of the baseline with one serving as the reference station at a precisely known location and the other as the remote station with unknown coordinates. The data collected at the reference station need to be transmitted via the radio to the remote station in order to derive centimeter level position solutions in real time [Zhodzishsky et al., 1998]. Due to the fast advances in wireless communication technology, the Internet has become one of the most important communication methods today [Nguyen, 2001]. Various investigations are currently under way to use the Internet to support GPS applications. In Muellerschon et al. (2000 and 2001), Hada et al. (2000), and Gao et al. (2001) the Internet has been proposed for distributing global GPS-based differential corrections. Lee et al. (2000) have investigated an Internet-based differential GPS system for mobile communication users. It is expected that the Internet will be increasingly used in the future as an efficient communication alternative to conventional radio-based methods. The Internet can also be an excellent communication method to support multiple mobile reference stations for high precision mobile to mobile positioning and
2 navigation. A mobile reference station can be a car, a truck, a boat or an aircraft and it can supplement to static reference stations in the development of DGPS networks. For instance mobile reference stations can be used to enhance the differential coverage of a wide or regional DGPS network without setup additional permanent reference stations. This paper investigates a Wireless Internet-based DGPS system that uses the Wireless Internet as the communication link between the mobile reference and remote stations. Based on a prototype system developed at The University of Calgary, the performance of a Wireless Internet-based system was tested with both static and mobile reference stations. In this paper, the characteristics of the radio-based and Wireless Internetbased DGPS systems are first compared followed by the analysis of the bandwidth supply and demand of the Wireless Internet-based DGPS systems. Then the implementation issues of a Wireless Internet-based DGPS system with mobile reference station based on currently available Wireless Internet access service are discussed. Numerical results and analysis are then provided to assess the operational performance of the system with respect to protocol selection, data transmission time delay, and positional accuracy. The final section presents conclusions and recommendations. 2. WHY INTERNET-BASED DGPS A radio data link has been used for communication between the base and rover stations in most DGPS systems. Radios that work in UHF Commercial Band ranging from 450MHz to 470MHz, such as Pacific Crest radio family, become the mainstream to provide data link for DGPS systems [Pacific Crest, 1998; Trimble Navigation Limited, 1996]. Radio based transmissions, however, have several significant disadvantages when used for DGPS positioning and navigation. First, radio has a very short data transmission range limited to line of sight (LOS) between the base and rover radios [McLarnon, 1997; Pacific Crest, 1998]. Second, the radio waves in UHF commercial band are crowded and noisy: the band is designed for public and there is lack of an effective frequency resource sharing strategy like cellular technology to avoid frequency collision. The channel separation in UHF commercial band is only 12.5 KHz that is much smaller compared to CDPD (Cellular Digital Packet Data) and GSM (Global System for Mobile) resulting in larger cross-channel interferences [Pacific Crest, 1998]. The interferences deteriorate the communication quality seriously, and lead to even smaller radio effective range than LOS. In urban area, where the radio wave environment is harsh, the radio effective ranges are usually limited to a few kilometers [Liu Z. et al., 2001]. Moreover, the data transmission in UHF commercial band has to give way to voice transmission according to FCC rules [Trimble Navigation Limited, 1996]. In addition to UHF commercial band, there are still some other bands in UHF and VHF available for DGPS data link. For example, Ashtech SSRadio works at UHF License-free Band ranging from 902MHz to 928 MHz [Magellan Corporation, 2000]. However, they usually suffer the similar problems as UHF commercial band: LOS limitation, small channel separation, lack of effective frequency resource sharing strategy, and inferior priority to voice. In the past several years, the Wireless Internet has evolved into an important communication and data distribution mechanism and it has many advantages over conventional radio data transmission methods. First, the Wireless Internet has a wide coverage. The Internet is a global network and its transmission range is not constrained by physical factors. The wireless telephony systems, AMPS (Advance Mobile Phone Service)/CDPD and GSM, two most common and dominant systems applied for the Wireless Internet in Europe and North America, keep expanding daily and are evolving into global networks. CDPD technology has been adopted in our prototype system development because currently the coverage of CDPD is larger than that of GSM [CTIA, 1999; GSM World, 2001; Virginia Tech 2000]. Second, the data transmission via the Wireless Internet is much more reliable than that via the wireless radio modem. For example, CDPD is a packet-switched data service that uses the existing AMPS network to transmit data at a rate of 19.2 kbps. Dense transmission tower network is built to keep a constant signal power and cellular technology is used to assign the neighbor transmission towers different frequencies in AMPS to avoid frequency collision. Moreover, the channel separation is set to 30KHz which is much larger than the channel separation in UHF commercial band to minimize the cross-channel interferences [Lin Y., 2001; Wong, P. 1995]. Although it can be assigned to a dedicated RF channel, CDPD's distinctiveness is that it transmits packet data over idle cellular voice channels, and automatically switches to another channel when the current channel is about to be assigned for voice usage, thus greatly improves the efficiency and reduce the costs for data transferring [Lin Y., 2001]. 3. BANDWIDTH SUPPLY AND DEMAND OF INTERNET-BASED DGPS SYSTEMS 3.1 Bandwidth Demand For Wireless Internet-Based DGPS operation there is a minimum wireless Internet bandwidth that must be satisfied. The required bandwidth depends on which
3 messages are being generated at the reference station, and the message period. The slowest rate at which one should send RTCM messages for a RTK DGPS system is once every 5 seconds. The remote receivers can fix integers with base station data arriving once every 5 seconds or faster [Magellan, 1998]. The position error will grow with an increase of data transmission latency, so a fast rate to send RTCM messages will help improve the RTK positioning accuracy. A rate of ten times per second was chosen in the prototype system since it was found that the server and the client can exchange data with the GPS receivers most reliably at this rate. The RTCM message types 18 and 19 are the most important messages for RTK positioning. They are usually set to output every second. The RTCM message types 3 and 22 are also used for RTK positioning. They are usually set to output every minute [Ashtech, 1998]. Therefore the RTCM messages for RTK positioning are mainly composed of types 18 and 19 so that the bandwidth requirements are mainly determined by the above two types of messages. Table 1 lists the message size for the RTCM messages 18 and 19. For RTCM, the data is packed in 6/8 format. The required number of bits is 8/6 times the number of bits in the message. The size of the TCP (Transmission Control Protocol) header is different from that of the UDP (User Datagram Protocol) header. For TCP, the header size of a packet is 40 bytes that include a 20 bytes TCP header and a 20 bytes IP header. If the IP packet size is set as 512 bytes, the required number of bytes is 512/472 times the number of bytes in the RTCM messages. For UDP, the header size of a packet is 28 bytes that include a 8 bytes UDP header and a 20 bytes IP header. If the IP packet size is set as 512 bytes, the required number of bytes is 512/484 times the number of bytes in the RTCM messages. Table 2 lists the minimum bandwidth requirements for Internet RTK positioning to send RTCM type 18 and type 19 messages every second (T). Number of Satellites Number of RTCM words in Message type 18 (30bits/word) Number of RTCM words in Message type 19 (30bits/word) 7 GPS+7 GLONASS (2+1+2*7)*2 = 34 (2+1+2*7)*2 = 34 9 GPS+9 GLONASS (2+1+2*9)*2 = 42 (2+1+2*9)*2 = GPS+12 GLONASS (2+1+2*12)*2 = 54 (2+1+2*12)*2 = 54 Table 1: Message Size for RTCM Messages 18 and 19 Number of Satellites Minimum Bandwidth Requirement for TCP (T = 1s) Minimum Bandwidth Requirement for UDP (T = 1s) 12 total GPS+GLONASS 4800 bps 4800 bps 14 total GPS+GLONASS 4800 bps 4800 bps 18 total GPS+GLONASS 4800 bps 4800 bps 24 total GPS+GLONASS 9600 bps 9600 bps Table 2: Minimum Bandwidth Requirements for TCP and UDP 3.2 Bandwidth Supply The bit rate of CDPD at static mode is around 14 kbps. Since the data transmission on CDPD is packet based and each packet includes a packet head, the real data throughout of CDPD at static mode is around 9.6 kbps [Lin Y., 2001; Wong, P. 1995]. When the CDPD modem moves from one serving area to another serving area, it registers itself for the upcoming serving MD-IS (Mobile Data Intermediate System) via
4 the registration service. The home MD-IS that is currently serving the CDPD modem will delete its link with the previous serving MD-IS and build a new link with the upcoming serving MD-IS [Lin Y., 2001]. Figure 1 shows the registration procedure of CDPD modem. Since the registration needs time, the real data throughout of CDPD at mobile mode will be less than 9.6 kbps. The field tests showed that the real data throughout of a CDPD modem located in a moving vehicle is around but less than 9.6kbps when it communicated with a socket program running on a computer with an Internet access to LAN (Local Area Network). Generally speaking, the real data throughout of CDPD at mobile mode can support RTK positioning while the reference station has an Internet access to LAN. When the reference station is mobile and both the reference station and the remote station communicate with CDPD modems, the real data throughout of CDPD will be less than 9.6 kbps. The field tests showed that the real data throughout of CDPD was about 4 or 5 kbps and was not stable. According to the tests, the data throughout of CDPD in this case could not support high dynamic (velocity more than 50 Km/h) RTK positioning, but could support low dynamic (velocity less than 50 Km/h) RTK. In order to support high dynamic RTK positioning, RTCM compression technology has to be applied. 4. IMPLEMENTATION OF A WIRELESS INTERNET-BASED RTK SYSTEM In this section, a Wireless Internet-based RTK system developed at The University of Calgary is described. The system consists of two high-precision RTK receivers for position determination and two CDPD modems for realtime data transmission. Equipped with CDPD modems, the base and the rover receivers are able to link together by the Wireless Internet. The system configuration of the Internet RTK system is depicted in Figure 2. The base station consists of a receiver capable of generating RTK differential corrections and a laptop, and a CDPD modem. The rover station consists of a rover receiver, a laptop, and a CDPD modem capable of receiving differential corrections from the base station via the Socket program throughout Wireless Internet. Figure 1: CDPD Modem Location Registration Procedure Figure 2: Configuration of an Internet-based RTK Positioning System
5 Data transmission latency is the time difference between the transmission time of the differential RTK data at the base station and the receiving time of the same data at the rover station. A short data transmission latency is desired for high precision DGPS positioning system such as RTK, and the data transmission latency is close related to the RTK positioning accuracy, so it should be assessed. The latency of the differential data, namely the time difference between the transmission time of the differential RTK data at the base station and the receiving time of the same data at the rover station, should be assessed. To minimize the data transmission latency and its subsequent influence on the RTK positioning accuracy, the Internet protocols, which defines how the data are transmitted through the Internet, should be carefully selected. The Internet currently uses a Transmission Control Protocol/Internet Protocol (TCP/IP) to connect all networks, organizations and users across the world. Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) are two important transport protocols that have been widely used for Internet applications. The TCP provides a stream delivery and virtual connection service to applications through the use of sequenced acknowledgment with retransmission of packets while the UDP provides a simple message delivery for transaction-oriented services with no acknowledgement of delivery. The TCP is able to provide highly reliable data transmission since it ensures reliability, flow control, and connection maintenance. The TCP requires an acknowledgement of data arrival and any lost data must be sent again. As a price for the reliability, the data efficiency of the TCP is not as high as that of the UDP. The UDP is able to provide faster data transmission, however, there may be loss of data or data sequence change using the UDP. To testify which protocol is better for a wireless Internet-based RTK system, two sets of tests have been conducted at The University of Calgary and the test results based on TCP/IP are compared with those based on UDP/IP. 5. FIELD TESTS AND DATA ANALYSIS In this research, the Cellular Digital Packet Data (CDPD) service has been used to support the wireless Internet access for the developed Internet-based RTK system. The CDPD modems used during the field tests were two Merlin Wireless PC Cards from Novatel Wireless Inc. and the Internet access service was provided by the Telus TM Corporation. 5.1 Internet-based RTK Positioning with a Mobile Reference Station Using TCP/IP A static zero-baseline test was conducted in Calgary on June 3, Two Javad Legacy dual frequency GNSS receivers were used as the base and rover receivers. One Javad Legant antenna and a signal splitter were adopted to provide GNSS signals for both the reference receiver and the remote receiver. During the test period, the reference receiver was connected to a Laptop and the differential corrections were distributed over the Wireless Internet using a CDPD modem. The rover receiver was also connected to a Laptop and the differential corrections were received over the Wireless Internet using a CDPD modem. TCP/IP protocol was adopted to build data link between the reference station and the remote station. In order to maximize the RTK performance, it is important to minimize the data transmission latency. To evaluate the latency over the Internet, a program was developed to monitor the time taken to transmit the differential data from the base station to the rover station. The server computer at the base station sends a signal to the client computer at the rover station and the signal is then sent back to the server computer from the client computer. The server computer can then calculate the transmission and reception time difference to determine the total time or total latency taken for the round trip of transmitting this signal. The forward route latency is not necessary the same as the backward route latency because they are dependent on the network traffic flows over the Internet when the signal goes through the network and the forward route is possibly different from the backward route. Despite of this, the round trip latency divided by 2 can be used to provide an estimate of the data latency or the time taken to transfer the differential data from the base station to the rover station. The latency results from the static RTK test are shown in Figure 3. The data transmission latency, from the time that the data was collected from the reference receiver to the time that the data was received by remote receivers, which included the time waiting for transmission, grew linear approximately with the time passed on. The reason was likely because the real data throughout of the CDPD modem at the reference side wasn t enough to transmit the correction data out of the CDPD modem collected from the reference receiver. As a result, the data was accumulated at the reference side. Because the data transmission delay kept growing with time when using the TCP/IP protocol, TCP/IP is not adequate to support Wireless Internet-based RTK Positioning with mobile reference station at this case because of the bandwidth limitation.
6 Figure 3: Data Transmission Delay in Static Test 5.2 Internet-based RTK Positioning with a Mobile Reference Station Using UDP/IP A kinematic field test was conducted in Calgary on June 2, Two Javad Legacy dual frequency GNSS receivers were used as the base and rover receivers. Two vehicles were used to carry the reference station and the remote station respectively. During the test period, the reference receiver was connected to a Laptop and the differential corrections were distributed over the Wireless Internet using a CDPD modem. The rover receiver was also connected to a Laptop and the differential corrections were received over the Wireless Internet using a CDPD modem. UDP/IP protocol was adopted to build data link between the reference station and the remote station. During the field test, the vehicles were driven away from the campus, and the maximum baseline length between the reference antenna and the remote antenna was up to about 12 kilometers. In addition to the real-time RTK position outputs, the raw measurements from both the base and the rover receivers were saved for postprocessing High Dynamic (Velocity more than 50 km/h) Field Test The latency results of the high dynamic RTK test (velocity more than 50 km/h) are shown in Figure 4 and Figure 5. Figure 5 enlarges Figure 4 to show the detail. There were serious packet losses as shown by Figure 5. Serious packet losses directly resulted in serious data losses. The reason was likely due to the fact that the bandwidth wasn t adequate to transmit the correction data out of the CDPD modem form the reference receiver. Since there was no mechanism in UDP/IP to acknowledge the reference station if the data had been received by the remote station, the reference station would continue to send the data even there is not enough bandwidth for it, so the data losses is inevitable. Because of the serious data losses, UDP/IP is not adequate to support Wireless Internet-based RTK Positioning with mobile reference station for high dynamic at this case because of the bandwidth limitation Low Dynamic (Velocity less than 50 km/h) Field Test The latency results of the low dynamic RTK test (velocity less than 20 km/h) are shown in Figure 6. Test results showed that low dynamic RTK test could be supported by the prototype system. The reason was likely due to the same reason already indicated earlier that the data transmission rate was higher in low dynamics than in high dynamics using the CDPD modems. The positioning accuracy statistics for different coordinate components are shown in Figure 7. The results indicate that the RTK positioning errors were in the range a few centimeters up to about 20 centimeters for all coordinate components. 6. CONCLUSION The concept of Wireless Internet-based Real-Time DGPS with mobile reference station has been described in this paper. To validate the concept, a Wireless Internet-based RTK prototype system was developed to assess the feasibility of implementation. Field tests with the newly developed Wireless Internetbased RTK system were conducted using both TCP/IP and UDP/IP protocols. The test results demonstrated that currently TCP/IP could not support Wireless Internetbased RTK positioning well because of the bandwidth limitation. The success of TCP/IP for Wireless Internetbased RTK positioning is possible to be achieved by RTCM compression technology or 3G cellular technology.
7 The test results showed that UDP/IP could support Wireless Internet-based RTK positioning in low dynamic but not in high dynamic with a mobile reference station using the current CDPD network. The RTK positioning accuracy was in the order of about ten centimeter-level and the data transmission delay was about 2.5 seconds. The success of UDP/IP for Wireless Internet-based RTK positioning in high dynamic could be achieved by compressing the RTCM messages or using the 3G cellular technology. Figure 4: Data Transmission Delay in Static Test Figure 5: Data Packet Losses Figure 6: Data Transmission Delay in Kinematic Test
8 Figure 7: RTK Positioning Accuracy with a Mobile Reference Station ACKNOWLEDGEMENTS The research was partially supported by a NCE GEOIDE project held by the second author. The authors would also like to thank Minha Park for his assistance during the field test. REFERENCES CTIA, Cellular Telecommunications & Internet Association Web Site (1999), Coverage Maps, Gao, Y., Liu, Z. (2001), "Differential Satellite Positioning and Navigation over Internet, GEOINFORMATICS' 2001, May 23-25, 2001, Bangkok, Thailand GSM World (2001). GSM Association Web Site: m Hada, H., S. Yamaguchi, Y. Kawakita, J. Murai (2000). Design of Internet Based Reference Station Network for New Augmentation System, Proceedings of ION GPS 2000, Salt Lake City, Utah, September 19-22, Hada, H., H. Sunahara, K. Uehara, J. Murai, I. Petrovski, H. Torimoto, S. Kawaguchi (1999). New Differential and RTK Corrections Service for Mobile Users, Based on the Internet, Proceedings of ION GPS 1999, Nashville, Tennessee, September 14-17, Lee, Y., H. Kim, J. Hong, G. Jee, Y. Lee, C. Park (2000). Internet Based DGPS for Mobile Communication User, Proceedings of ION GPS 2000, Salt Lake City, Utah, September 19-22, Lin, Yi-Bing (2001), "Wireless and Mobile Network Architectures", 2001, John Wiley & Sons Ltd. Liu, Z., Gao, Y., and Liu, ZZ (2001), "Development of an Internet-Based Wireless Platform for Mobile Information Management and Service", ION National Technical Meeting 2001, Jan.22-24, Los Angles, USA. Magellan Corporation (1998), "GG24 OEM Board & Sensor GPS + GLONASS Reference Manual". Magellan Corporation (2000), "Ashtech SSRadio Specification", McLarnon, Barry (1997). "VHF/UHF/Microwave Radio Propagation: A Primer for Digital Experimenters", Proceedings of the 16th ARRL and TAPR Digital Communications Conference, Baltimore MD, October 10-12, Muellerschoen, R., W. Bertiger, M. Lough (2000). Results of an Internet-Based Dual-Frequency Global Differential GPS System, Proceedings of IAIN World Congress in Association with the U.S. ION 56th Annual Meeting, San Diego, California, June 26-28, Muellerschoen, R, Bar-Sever, Y.E., Bertiger, W.I., and Stowers, D.A. (2001). "NASA s Global DGPS for High- Precision Users". GPS World, Vol 12. No. 1. Nguyen, Lam (2001). "Mobile Internet: Internet Revolution or Mobile Revolution?", Diamond Cluster White Paper, ess.asp
9 Pacific Crest Corporation (1998), "RFM96 Radio Modem User s Guide", Revision 5.0, Pacific Crest Corporation, October Trimble Navigation Limited (1996), "RTK License Information Update for Dealers and Customers in the United States", Virginia Tech (2000), "ECPE6504: Wireless Networks and Mobile Computing", Spring, Wong, Peter & Britland, David (1995), "Mobile Data Communications Systems", 1995, Artech House, Inc. Zhodzishsky, M., M. Vorobiev, A. Khvalkov and J. Ashjaee (1998). Real-Time Kinematic (RTK) Processing for Dual-frequency GPS/GLONASS, Proceedings of ION GPS 1998, Nashville, Tennessee, September 15-18, 1998.
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