The Issues of Practical Implementation of the ...

The Issues of Practical Implementation of the Commercial RTK Network Service I. Petrovski, S. Kawaguchi, H. Torimoto, DX Antenna Co.Ltd., Japan K. Fuji, Hitachi Ltd. , Japan M.E. Cannon, G. Lachapelle, The University of Calgary, Canada

BIOGRAPHY Dr. Ivan G. Petrovski is the Chief Researcher at the GPS Division of DX Antenna Co. Ltd. Prior to joining DX Antenna, he was working as an Associate Professor at the Moscow State Aviation University (MAI), and then as a Science and Technology Agency Fellow with National Aerospace Laboratory, Japan

development, considerations for the utilization of different data links , including a broadcast service, cellular phones and the Internet. It also contains an approach for choosing an RTK network algorithm as well as other issues. Concepts for the future development of the system are discussed including Internet-based globalization, GALILEO and GLONASS deployment, and precise ephemeris utilization. INTRODUCTION

Seiya Kawaguchi joined GPS Division of DX Antenna in 1998. He holds a Master degree in Earth Science from National University of Kyusyu. Hideyuki Torimoto is a General Manager of the GPS Division of DX Antenna. Before joining DX Antenna he had established Trimble Navigation Japan Ltd. and was working as an Executive Vice President of this company since 1986. Kenjirou Fujii holds MS from Waseda University. He is working at the Industrial Components and Equipment in Hitachi Ltd. Japan. He is the principal specialist in automatic control, robotics and GPS related system development. Dr. M.E. Cannon is a Professor in Geomatics Engineering at the University of Calgary where she conducts teaching and research related to GPS and integrated GPS/INS systems. She is a Past President of the Institute of Navigation. Dr. Gerard Lachapelle is Professor and Head of the Department of Geomatics Engineering where he is responsible for teaching and research related to positioning, navigation, and hydrography. He has been involved with GPS developments and applications since 1980. ABSTRACT The paper presents a commercial real-time kinematic (RTK) Virtual Reference Station (VRS) Network service in Japan and issues related to its implementation. A prototype of an infrastructure for the commercial RTK correction service was introduced for public in September 2000. The paper discusses guidelines for infrastructure

ION GPS-01/Salt Lake City/September 11-14, 2001

The idea of a real-time kinematic (RTK) network service has been around for many years, but only relatively recently has its implementation been started in some countries. The importance of an RTK network increases every year. Despite the fact that we now have a large percentage of GPS consumers, who do not require RTK accuracies, the ease of getting such a service in the future will accelerate the consumer market. With SA being turned off, differential services have difficulties to distinguish themselves from standalone GPS in terms of accuracy, so part of the former DGPS users can turn to RTK for improved performance. In addition, the situation with the availability of navigation satellites will drastically change in the future. The GLONASS constellation will hopefully be realized at its full potential, and GALILEO will appear. New civil frequencies will be introduced for GPS and GLONASS. All these factors altogether will ensure that reliable, instantaneous RTK will be readily available. The main problem, which RTK can overcome, is the necessity to have a reference station (RS) in the vicinity of the user. The distance from the RS when using RTK should be generally no more than 10km on average, which is significantly different from DGPS, where distances to RS can exceed several hundred kilometers. It means that if you want to provide an area of 1000 km2 with a reliable RTK service, you have to have install about 2500 reference stations. The only way to overcome this problem without sacrificing accuracy or time for initialization, is to apply an approach similar to WAAS, whereby corrections are averaged over the coverage area. It allows a service provider to decrease the number of RS drastically, for example down to 100-400 depending on the ionospheric conditions.

The Virtual Reference Station (VRS) RTK Network, along with a number of other well-developed methods, allows the use of a moderate number of RS, providing the same full coverage (see Petrovski et al., 2000). A VRS has corrections calculated for it, rather than having a physical RS, and it is located near the user position. The user cannot use a real RS, because it is generally too far away. Over a distance greater than 10 km these errors could be comparable to the GPS signal wavelength and would interfere with the ambiguity resolution algorithm. The largest errors, which RS corrections intend to compensate, de-correlate with distance. Among these errors, the most significant are the ionospheric errors. Fortunately, these errors mostly decorrelate linearly with distance under normal operating conditions. Other errors, which decorrelate with distance are tropospheric, and orbital effects. Orbital errors usually are insignificant over a medium-length baseline, and tropospheric error distribution usually fluctuates on a medium-length baseline, and in some cases may be very significant. Using the distance-weighted errors at the known locations

of the real RS, one can find the approximate magnitude of the most probable error between them. In order to find this averaging error near the user position, we can use different criterions to optimize this estimate, using various algorithms. The difference between the results of these algorithms is not generally significant. A natural extension of the network concept is that RTK VRS increases the integrity and reliability of the service in contrast with a single baseline solution. Since 1999 a group of companies and universities started a project to create RTK VRS service in Japan. DX Antenna Co. Ltd. acts as a system integrator and in charge for the overall system. The Department of Geomatics Engineering at the University of Calgary provides VRS Software, adapted for real time applications by Roberton Enterprises. Hitachi Ltd. manufactures user equipment prototype and provides reference station (RS) network. Asahi TV, Keio University and WIDE project are involved in close cooperation related different parts of RTK VRS Network infrastructure.

As a result of this work, a prototype RTK VRS Network infrastructure was created and successfully introduced to the public in September 2000 in Tokyo. This attracted more than 300 representatives from different companies. A test that was conducted at that time demonstrated that the system can provide a 2-5 cm level accuracy over baselines longer than 30 km. (see Petrovski et al., 2000)). At the same time it demonstrated difficulties in providing continuously reliable solutions. These difficulties were caused by severe ionospheric conditions (reaching 15 ppm), which in turn had a negative impact on ambiguity resolution between the network RS. At the same time user hardware development was necessary. In order to ensure the reliability and quality of the service and resolve administrative issues, the service has been transferred to a test phase. The main service components are in use and are operating in a test mode. PRACTICAL CONSIDERATIONS The RTK VRS service is planned to be operational in 2002. In order to determine the consumer market for this service, we need look at services, which are available now, and those that will appear in the near future. The size of the potential market is the governing factor in the service design. The main application in terms of the number of users serviced in Japan will be GPS enhanced cellular phone. It will be in common use, operating in real time, easy to access, and providing a rather low accuracy. This service is for location only. The other group of

services is correction broadcast services (see Hada at al.,2000). They are widely used, operate in real time to provide a differential accuracy, and are mainly used for navigation. The Internet-based correction service has some advantages over a broadcast service, but now it operates on a small scale. It is a real time service, with differential accuracy, and the potential for RTK. Finally, the geodetic-quality Geographical Survey Institute (GSI) network, which can provide precise data for post mission, and real-time data, which can be used as the foundation for RTK services. Despite the very high density, the network cannot provide full coverage for RTK applications due to the above mentioned reasons. The services listed fully cover such applications as emergency location, car navigation, off-shore navigation, and surveying. However one can find a niche market for VRS. It is a real-time, good coverage, cm level service for construction, rapid surveying, and GIS. Some navigation applications like automatic parking, ship docking and etc. also require cm-level accuracy. This service fills the gap between demanding mm-level geodetic services and meter-decimeter-level differential services. The availability, continuity and convenience of the service will depend on components as data links, whereas accuracy depends on the algorithm, network configuration, data latency etc. An RTK Network incorporates a number of essential elements, each of which is essential in terms of estimation of the overall specifications, investment and running costs. Below we consider the elements from the point of view of potential users and service providers. For the service provider the main considerations are investment vs. running cost, source of revenue, such as equipment vs. service fees, and scalability of service. For the user, the most important issues are cost for equipment vs. the service fee, availability, accuracy, customization, continuity of service, and convenience. The basic requirements and characteristics of the RTK VRS network are based on the Japanese Geographical Survey Institute (GSI) specifications. The competitive

service in Japan should satisfy these requirements. As a result of analysis of such requirements we come up with the following specifications. The coverage area should be without gaps and require distance between RS from 30 to 50 km. Accuracy are not less than 5 cm + 1-10 ppm. The service provider should broadcast to the user either re-corrected corrections (VRS data) or corrections together with area correction parameters with an update rate of 1-2 Hz using a standard corrections format. The correction format should be one of RTCM (V2.1/2.2 or higher) Type 1, Type 18/19, Type 3, Type 20/21, Type 59 for area correction parameters. The baud rate should be limited to range of between 2400-4800 baud. Latency effects should be minimized by correction prediction. The system should have the potential to implement GLONASS, GALILEO and extra frequencies without major hardware modifications . Japanese users are interested in GLONASS due to obstructed visibility, although GLONASS implementation is difficult due to the frequency bias between channels, and the few number of satellites available. The system should provide integrity monitoring for detection and exclusion of erroneous data. The system ought to provide a capacity for data storage and possibility for data retrieval for post-processing, and the data should be retrievable through mobile telephone or Internet. The system should not apply any requirements to the rover receiver, except that the rover has to be RTK capable. 1. VRS RTK NETWORK COMPONENTS VRS RTK Network consists of a network of reference stations (RS), each of which sends correction data through a data link to a control center, which process all these data, and sends redefined corrections to a user through another data link. The rover side, which is usually, but not necessarily a part of the system, applies if necessary some operations to the corrections and uses them for RTK. This concept allow a user to

enjoy RTK, when the baseline to the nearest RS is from 10 to 50 km or greater. For a thorough discussion of the VRS concept see add a reference here. The system specifications, cost and design as well as the requirements for the user equipment will depend on the numerous options we choose for the algorithm and components. We will discuss these options in detail below. 2. NETWORK REFERENCE STATIONS A distinction should be made between a prime reference station (RS) that supplies the user with the main correction stream and all others RS that function to provide information on error decorrelation. These socalled secondary RS supply users with less urgent information related to a relative correction distribution around the network in relation to a primary RS. A latency of up to a few minutes is tolerable for the secondary RS corrections, because these corrections are the result of the ionosphere, which is generally not very dynamic. The primary RS, in contrast, supplies a user with absolute corrections, which are valid in its vicinity. Corrections from the prime station are critical and should be supplied with minimum latency. A second of latency in the prime station corrections may result in approximately 1 cm error in the user position. We started with a WIDE network of six Internet-based RS, see (Hada et al.,2000). For the test network at present we use six OEM-4 NovAtel receivers as the RS, which provide full compatibility with UC (University of Calgary) VRS software. The network configuration is depicted in Figure 15. The system will be implemented on this privately-owned network, which will be extended in the future. For the official GSI test , which is scheduled later this year, the GSI RS network will be used. The possibility of using data from a public network gives instantaneous advantages, because the infrastructure already exists. To use a public network will require certification, which in turn will make the service more attractive for users. On the other hand, it can put some restrictions on commercialization. The use of a privately owned network will cause no problems with certification and this network is easy to control. However it is expensive to install. 3. CONTROL CENTER A Control Center incorporates a core engine that calculates a correction grid, and when using the VRS concept, it can also calculate the VRS data stream. The amount of calculations required at the Control Center depends on the data link between a user and the Control Center. If the data link is bi-directional like a cellular phone, for example, the Control Center can calculate all the necessary data for the GPS receiver based on the user’s approximate location. This means that the number

of calculations will vary directly with the number of users. If the data link works in one direction like a radio broadcast, the user should make most calculations at their end, but the load for the Control Center processor will be less. The Control Center operates as follows: It receives correction data from a network of secondary RS, resolves ambiguities in real time over the network, calculates residual errors at the RS locations and re-calculates these errors into a correction grid. The Control Center has the following advantages against a rover receiver to resolve ambiguities over the long baseline. The position of the RS are static and known with mm accuracy, and the time for initialization is available. We use the NetAR software of the University of Calgary to resolve ambiguity over the network in real time (see Raquet et al., 1998). Commercial postprocessing software packages are available and used to validate the results (e.g. Bernese). DX Antenna has also developed a software for post-processing analysis, demonstration and validation (see Figure 4). The software incorporates simplified versions of algorithms presented on Figure 1, except a block that provides true ambiguity over a network in real-time. To help the network ambiguity resolution process, one may consider using estimates of ionospheric error, mapped in real time, and precise ephemeris. For the sake of simplicity without sacrificing accuracy, we can consider resolving ambiguities from a key RS somewhere in the middle of network to all RS one by one (see Figure 2). If RS N 2 is the key RS, we need to find ambiguities form RS 2 to RS 1 and RS 3, without resolving ambiguities between RS 1 and RS 3, which would be redundant. It will create a star configuration instead of a closed loop. It also brings the advantage that if there is a miscalculation in one baseline ambiguity, this error will not be forced onto all other ambiguities by network constraints. Carrier phase measurements are expressed as follows: Φ = 1/λ (ρ + T + I + O + U) + N, Where ρ is distance from satellite to the receiver (m), N is integer ambiguity (cycles), T is tropospheric error (m),

(1)

I is ionospheric error (m), O is error due to orbit degradation (m), U is a receiver noise and multiphase (m). Correspondingly, double differenced carrier phase measurements can be expressed as below. ∆∇Φ = 1/λ (∆∇ρ + ∆∇T + ∆∇ I + ∆∇O + ∆∇U) + ∆∇N, (2)

where ∆∇ Φ is carrier phase double differences, ∆∇N is true double differenced ambiguities, ∆∇ρ is known double differenced ranges. ∆∇T is error due to the change tropospheric conditions over the baseline, ∆∇I is error due to the change ionospheric conditions over the baseline, ∆∇O is error due to orbit degradation, ∆∇U is noise and multiphase. Corrections calculated at the control center relative to the key RS at the secondary RS position can be expressed as follows . ∆∇υ = ∆∇ Φ - 1/λ ∆∇ρ -∆∇N , (3) The calculated corrections can be given by: ∆∇υ = 1/λ(∆∇T + ∆∇I + ∆∇O + ∆∇U) , (4)

We believe that finding the correct double differenced ambiguities in real time is one of the main challenges in the creation of the RTK network software, because these ambiguities should be recalculated instantaneously in case of cycle slips, etc. The source of the problem is that the true double differenced ambiguities should be obtained from the very same equation, however the RS coordinates are known a priori If one can decrease the residual ∆∇υ values down to half of a wavelength, it will assist real time ambiguity resolution over the network. In this sense, some approaches can be proposed. The effect due to the orbital error decorrelation ∆∇O can be estimated from 0.5 (optimistic) to 2 cm (pessimistic) for a 50 km baseline. This error can be eliminated with use of the GSI predicted orbits instead of broadcast orbits, which brings ∆∇O down to 5 mm. GSI plans to employ precise orbit prediction in real time for regional use. We also propose to decrease ionospheric error in (2) through real time ionosphere mapping as a function of the station coordinates and satellite elevation. Est(∆∇I) = ∆∇ Est( Fsat (el) Fst(φ,Λ) ,

(5)

where φ is latitude, Λ is longitude, el is satellite elevation. The following mapping function has an advantage that although it is less accurate than a satellite-to-station estimate, it does not depend on a particular satellite and therefore ∆∇I in (4) can be quickly restored without resolving the ambiguity. The ionospheric mapping function can be estimated and then assist to ambiguity resolution, which in turn is used for ionospheric estimation. The algorithm requires time for initialization when implementing in real-time. Based on the corrections ∆∇υ calculated for the RS position, the Control Center can either recalculate them into the grid points in the case of a unidirectional link to the user, or keep as it is for later recalculation into the user position when using a bi-directional link. The Control Center algorithms will be different for different types of communication links between the Control Center and a user. In the case of a bi-directional communication link, the user gives its approximate position and allows

the Control Center to calculate the VRS, or corrections from the VRS in the vicinity of the user. In case of a unidirectional link, the user calculates the VRS data using the grid data. The proposed VRS RTK uses an algorithm that lies between these two methods and utilizes two VRSs. The first VRS is calculated at the Control Center and is placed in the middle of a predefined area, while the second VRS is calculated by the user. This allows for easy expansion of the Control Center service for a larger area, which originally required two or more primary VRSs. This algorithm also incorporates options for uniand bi-directional communication. A bi-directional communication algorithm, generally speaking, can put some extra constraints on the number of users. In Japan, the potential number of users could reach tens of thousands for one service area, which requires special equipment and extra time to connect an unknown number of users to the control center server at the same instant. This situation can cause increased latencies, which is a very critical issue for RTK methods. As a core engine for the control center, the MultiRef RTK method is used, which was proposed and developed by the University of Calgary and Roberton Enterprises (see Racket et al (1998)) and Townsend et al (1999). The Control Center encodes the correction grid into a modified RTCM format, which had been proposed as a draft for RTCM v.3 Network RTK message (see Townsend et al, 2000). Besides the main server, the Control Center will accommodate a backup server and another GPS receiver as a monitor station. At present the control center is collocated with the primary RS for test purposes. 4. ROVER SIDE A user can be equipped with a standard off-the-shelf single or dual frequency receiver capable of performing RTK positioning, a personal computer and an appropriate modem or data receiver. The rover software can be also incorporated into the GPS receiver firmware. The required equipment should implement standard RTCM

5. DATA LINK BETWEEN CONTROL CENTER AND USER Basically, the general requirements for the DGPS data link include low data latencies, good mobile performance, inexpensive user equipment, and nationwide coverage. The data link between the network RS and a Control Center differ in terms of requirements from the data link between the Control Center and a user.

V2.1/2.2 message Type 1, Type 18/19, Type 20/21, Type 3, and if required, Type 59 for area correction parameters. The baud rate should be no less than 2400-4800 bps. Theoretically, in case of bi-directional communication, some rovers can be used as they are, provided that the control center calculates appropriate corrections for the rover position. In this case, the corrections have to be applied to the user position instead of into grid points at the Control Center. Then Control Center encodes the latest data into RTCM 2.2 format and feeds them directly into the GPS receiver. In practice, the rover receiver tries to take into account the known baseline length, which is undesirable because RTK network corrections emulates a short, or zero,-baseline case. Moreover, the rover can reject corrections on the basis of a long baseline. The rover side software decodes RTCM v.3.0 messages from the Control Center, and recalculates the raw data for each satellite for a given VRS location. It calculates corrections for the current location, based on the correction grid in the RTCM Network RTK message. Recalculated corrections at the user position could be as simple as RS corrections weighted with inverse distance to reference stations (see Figure 3).To ensure better accuracy, we use a more sophisticated NetAdjust software from the University of Calgary. Equipment that provides the user with the required data link and a microprocessor for VRS calculations, have been developed and produced by Hitachi Ltd. (see Figure 14). In one housing it contains an ASC receiver, a GPS receiver (NovAtel OEM-4 board) and the microprocessor to conduct VRS-related calculations (Figure 13).

A characteristic of the data link between the Control Center and a user defines the main features of the entire system. For example, a bi-directional data link puts the main calculation load on the server side rather than on a rover. We implement both by-directional and unidirectional data links. The main data stream which is supposed to cover most users, is transmitted by a unidirectional TV-broadcast system, developed to broadcast differential and RTK GPS/GLONASS corrections. These data are encoded onto a TV audio subchannel signal (ASC). This data link is intended for mobile users such as car navigation systems. Apart from a GPS receiver, the user is required to accommodate an ASC receiver (see Figure 7). For the car navigation system the ASC receiver, besides RTK and DGPS/DGLONASS corrections, provides an extra data channel for weather and traffic information. We also use an alternative method to provide a bidirectional data link between the Control Center and a user. DX Antenna has developed a special DGPS Data Receiver (see Figure 10) that allows the user to automatically dial the nearest Control Center and to establish a link through a cellular phone between it and the Control Center. Usage of this device allows one to establish a stable and reliable connection over the cellular phone with latencies between two and three seconds (see Figure 11). In the case of the ASC data link, latencies are on average one second higher (Figure 12). A cellular phone allows bi-directional communication between a user and a Control Center. Therefore the user can transmit its approximate position to the Control Center, which permits the transfer of the VRS calculation to the Control Center. Bi-directional communication also allows the transfer of only required information through the optimal channel. The user can select a subset of the correction information that is needed for its particular purpose. On the other hand, a bi-directional data link will increase the requirements to the Control Center in terms of the number of simultaneous VRS calculations needed to support different locations. These requirements could be difficult to meet in the case of large numbers of users. There is also a possibility to use the Internet as a data link between the Control Center and the user. At present the Internet does not provide connections reliable enough for

RTK. At present there is no Internet Protocol, which is sufficient for our purpose. We use an Audio Sub-Carrier data Channel (ASC). ASC is a data channel multiplexed onto TV broadcast signal. One of the main ASC advantages is that the required infrastructure for a TV broadcast system exists already and has good nationwide coverage. Terrestrial TV systems are widely used over the world in general and therefore can be used for DGPS correction broadcast almost worldwide. After a period of laboratory and field tests, the ASC was adapted for mobile reception in Japan. In August 1999 Asahi National Broadcasting Co. Ltd. (TV Asahi), in cooperation with DX Antenna Co. Ltd., started GPS/GLONASS DGPS/RTK corrections broadcast on one of audio sub-carriers. The Japanese Ministry of Posts and Telecommunications (MPT) had set up an advisory committee to legislate the system as one of the Japanese TV Standards. At present, the ASC is officially adopted by the MPT for broadcasting GPS and GLONASS DGPS and RTK corrections in Japan. Today ASC is the only service certified to broadcast GLONASS corrections. Area coverage for the ASC data channel for differential service is different for Yagi and Whip types of ASC antennas. On average, the broadcast distance for an RTK VRS is up to 70 km. Asahi TV has a network of TV Stations that covers all of Japan. These TV Stations, along with satellite stations for mountainous areas, comprise a nationwide infrastructure. In the VRS RTK Network approach, calculated VRS data from a Control Center are transferred through a 64 Kbps dedicated line to Asahi TV facilities. These data are encoded onto TV audio signals by a modulator, combined with a video signal in an audio transmitter, which is transferred to an Asahi TV tower and then is broadcast to a user. In Japan, NTSC TV operates in two frequency ranges, which are very high-frequency VHF from 90 to 222 MHz and ultra high-frequency UHF from 470 to 770 MHz.

Both VHF and UHF waves use line-of-sight propagation. In this type of propagation very high frequency signals are transmitted in straight lines directly from antenna to antenna. This type of propagation comes with a few shortcomings. This type of signal is strongly affected by a multipath. Also, if high buildings are located between the transmitting tower and a rover, the signal can be lost. Generally this type of transmission requires the transmitting antenna to be tall enough in order not to be affected by the earth curvature. Asahi TV use the Tokyo Tower (shown on Figure 6) for its broadcast. The height of the Tokyo Tower is 300 m, with a service area that ranges from 40 to 100 km, depending on the antenna type. There is a lot of areas in Japan that are blocked for TV signals due to the mountain topography, which could make it impossible to receive the DGPS signal in the areas of interest. TV Asahi uses 98 Satellite Relay to cover such areas and to make it possible for TV viewers to receive the TV signal and data. Every satellite relay station viewer can receive the ASC signal. A TV signal consists of a video and an audio signal (see NTSC spectrum on Figure 8). On Figure 2 fa=209.75 MHz signifies the frequency of the audio carrier, which is located at 4.5MHz higher than the video carrier (fv), and has a 0.5 MHz bandwidth. The ASC data channel uses Frequency-Division Multiplexing (FDM) in the audio band. The FDM technique allows for the combination of different signals with bandwidths smaller than the data link bandwidth into one composite signal that can be transported by the link. Carrier frequencies are separated by enough bandwidth to accommodate the modulated signal. These bandwidth ranges are the channels through which the various signals travel. The channels are to be separated by strips of unused bandwidth (guard bands) to prevent signals from overlapping. The data channel carrier frequencies measured in reference to the TV synchronization frequency (fH) are 4.5fH and 7.5fH (see audio band spectrum on Figure 9). The deviation level to the audio carriers is as follows: 70.804KHz ± 3KHz and 108.007KHz ± 6KHz correspondingly for the 1st and 2nd frequency. The audio frequency is modulated with audio signals using Frequency Modulation (FM). For data modulation, the ASC uses Differential Quadrature Phase Shift Keying (DQPSK) which is a modification of a usual phase modulation technique Phase Shift Keying (PSK). In PSK, the phase is varied to represent binary 1 or 0, which is a binary PSK. In DQPSK, instead of utilizing only two variations of a signal, each representing one bit, one can use four variations and each phase shift will represent two bits. This allows one to get a higher bit rate for the same bandwidth. So while the baud rate will be the same for PSK and DQPSK for the same bandwidth, the DQPSK bit rate can be two times greater. The error correction mechanism is capable of MultipleBit Error detection and correction. Up to 11 error per

packet can be corrected. The ASC uses Cyclic Redundancy Check (CRC), which is the most powerful redundancy checking technique available. CRC is based on binary division. A sequence of redundant bits (CRC remainder) is appended to the end of a data unit so that the resulting data unit becomes exactly divisible by a second, predetermined number. At the receiver the incoming data unit is divided by the same number. If at this step there is no remainder, the data unit is assumed to be intact and therefore accepted. A CRC generator can be represented as an algebraic polynomial. The generator polynomial for a data packet is given by: G(X)=X82+X77+X76+X71+X67+X66+X56+X52+X48+ X40+X36+X34+X24+X22+X18+X10+X4+1 , (7) The generator polynomial for mode control is given by: G(X)=X10+X8+X5+X4+X2+X+1, (8) The ASC frame structure has a small transmission delay. The bit rate for each ASC data channel is 16,000 bps and the effective bit rate after correction is 9,600 bps. Therefore, its transmission time is 0.577sec, and with encoding and decoding time, it is 1.154 sec. ASC monitoring block has two separate channels for the reference station and a monitor station. Correction data

from the reference station and the computer are fed to the ASC modulator, one for each sub-carrier (see 4.5fh DQPSKMOD and 7.5fh DQPSKMOD on Figure 5). The 4.5fH modulator is used basically to encode weather and traffic data, and 7.5 modulator for code/phase differential GPS/GLONASS corrections. The 4.5fH modulator is also thought to be used as an extra data channel for GALILEO. The ASC signal is fed into an audio transmitter that combines the audio and video carriers and delivers them to the tower using a dedicated 64K line. The user equipment was initially developed as a part of a car navigation system . This equipment includes a GPS receiver, an ASC receiver, and a cellular phone. The cellular phone allows the integration of a car into the Internet based ITS. An ASC receiver measures 195 by 25 by 132 mm (see Figure 7). It incorporates a TV turner, an SH7709(SH-3) CPU and a PCI memory card. The device allows the user to assign up to four channels in diapason 19.6608 – 78.643 MHz. The interface includes an RS-232 serial port, an interface for DRAM memory, PCI card slots, and an antenna input. A new waterproof modification of the ASC receiver also includes an power source, a magnetic shield, a booster to improve reception, and a filter to separate the signal. This is interesting for the user as it can include such signals as the code or carrier-phase corrections, from GPS or GLONASS.

data through a bad data-link, because an acknowledgement of arrival is required, and the lost data should be sent again. Consequently, TCP is more suitable for guaranteed, but not real-time, service. We use TCP for the Secondary RS to Control Center data link, because it has low latencies. In contrast, UDP is suitable for realtime services but it is not guaranteed. Depending upon the communication conditions, UDP can be chosen for the RTK data transfer to a user. There are different options for a user to connect to a reference station, such as a cellular phone, wireless LAN, wireless public radio network, even wired phone or network in case the user’s location allows it.

6. DATA LINK BETWEEN SECONDARY RS AND CONTROL CENTER The different types of data links are used between the RS and the Control Center in the test system. The data links to the GSI RS Network are Integrated Services Digital Network (ISDN) lines that connect each GSI RS to the Japan Association of Surveyors Computerized Interface, which provides public access to the GSI Network. The GSI network has been used for a short time for test purposes. At present, our RTK Network RS is connected to the Control Center through the Internet. Two of the RS are connected to the Internet using OCN line with 128 Kbps, others using ISDN with 64 Kbps. Our Control Center is connected to the Internet through a dedicated 128 Kbps telephone line. For the data link between the secondary RS and the Control Center we use the Transmission Control Protocol/Internetworking Protocol (TCP/IP). IP is the transmission mechanism for TCP/IP protocol. It is a besteffort protocol that provides no error checking or tracking. Arrival of the data is not guaranteed. Therefore, a transmission could be destroyed for a number of different reasons. There are two transport protocols in TCP/IP: TCP and UDP (User Datagram Protocol). UDP provides transportation when reliability and security are less important than size and speed. As soon as the arrival of data is not guaranteed under UDP, an application itself should detect the data arrival and quality. In the Internet, most applications usually require reliable end-to-end delivery and therefore use TCP. The drawback of TCP for real-time service is that it takes a long time to transfer

An Internet-based RS has many advantages including flexibility, availability and scalability. It also allows the creation of a low-cost infrastructure that could be used worldwide (see Petrovski at al., 2000; Hada at al., 2000). A bi-directional communication link between a server and a rover allows for the transfer of required information through the optimal channel. A user can select a subset of the correction information that they need for their particular purpose, and the propagation agent can select the best server for this user. The other advantage of this service is that it could easily grow. Everyone can create new reference station in the Internet without any license, which is necessary for radio-transmitters. The Internet-based reference station that consists of a GPS receiver and a PC-based workstation, which is connected to the Internet. We use an MMX-Pentium as the workstation with FreeBSD as an operating system. We chose FreeBSD because its high server performances. The server computer is connected to the Internet through an Ethernet for 24 hours a day. The server has two serial ports, each of which is connected to a GPS receiver. One port is used for the RTCM data logging and another for control and monitoring purposes. The server software is written in the C and Perl languages and works as a daemon. 7. OVERALL SYSTEM TEST RESULTS The tests for the different network configuration were conducted in 2000-2001 and partly presented in (Petrovski et al., 2000). See network configurations and scatter plots on Figure 16. SUMMARY The RTK Network infrastructure is under development to implement the Virtual Reference System Concept. This infrastructure provides a user with multiple options for data links, such as a TV Audio Sub-channel, the Internet, and a cellular phone. The hardware and software for the network, control center, data links and user are developed

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and tested. The system is operating in test mode in the Tokyo area and shows the desired level of accuracy. ACKNOWLEDGMENTS We would like to acknowledge the following companies and individuals. Roberton Enterprises Ltd., Calgary, as our main consultant on the VRS concept and which provides the real-time VRS software. Kouji Sasano of Asahi TV Media Strategy Office, who has developed the ASC data channel. WIDE project (the biggest Internet research group in Japan that includes over one hundred participants from different companies and universities), especially its leader, Professor of Keio University Dr. Jun Murai, and Dr. H. Hada, K.Uehara and Y. Kawakita of the InternetCAR research group.

REFERENCES 1.

I. PETROVSKI, S. KAWAGUCHI, M. ISHII, H.TORIMOTO, K.FUJII, K.EBINE, K. SASANO, M.KONDO, K.SHOJI, H. HADA, K.UEHARA, Y. KAWAKITA, J. MURAI, T. IMAKIIRE, B. TOWNSEND, M.E. CANNON AND G. LACHAPELLE (2000) New Flexible Networkbased RTK Service in Japan. Proceedings of

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GPS2000 (Salt Lake City, September), The Institute of Navigation, 1124-32. B.TOWNSEND, K.V.DIERENDONCK, J.NEUMANN, I.PETROVSKI, S.KAWAGUCHI, H.TORIMOTO (2000) A Proposal for Standardized Network RTK Messages. Proceedings of GPS2000 (Salt Lake City, September), The Institute of Navigation, 1871-78 H. HADA, H. SUNAHARA, K. UEHARA, J.MURAI, I. PETROVSKI, H. TORIMOTO, S. KAWAGUCHI (2000). DGPS and RTK Positioning Using the Internet. GPS Solutions Vol.4, N1, John Wiley & Sons, Inc. FORTES, L.P., G. LACHAPELLE, M.E.CANNON, G. MARCEAU, S. RYAN, S. WEE and J. RAQUET (1999) Testing of a Multi-Reference GPS Station Network for Precise 3D Positioning in the St.Lawrence Seaway. Proceedings of GPS99 (Session A4, Nashville, 14-17 September), The Institute of Navigation, Alexandria, VA. TOWNSEND, B., G. LACHAPELLE, L. FORTES, T.E. MELGARD, T. NØRBECH, and J. RAQUET (1999). New Concepts for a Carrier Phase-Based GPS Positioning System Using a National Reference Station Network. Proceedings of National Technical Meeting, The Institute of Navigation (January 25-27, San Diego, CA), 319-326. RAQUET, J. G. LACHAPELLE, and L. FORTES (1998) Use of a Covariance Analysis Technique for Predicting Performance of Regional Area Differential Code and Carrier-Phase Networks. Proceedings of GPS98 (Session A5 (Nashville, 15-18 September), The Institute of Navigation, 1345-54.