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

development, considerations for the utilization of BIOGRAPHY different data links , including a broadcast service, cellular phones and the Internet. It also contains an Dr. Ivan G. Petrovski is the Chief Researcher at the GPS approach for choosing an RTK network algorithm as well Division of DX Antenna Co. Ltd. Prior to joining DX as other issues. Concepts for the future development of Antenna, he was working as an Associate Professor at the the system are discussed including Internet-based Moscow State Aviation University (MAI), and then as a globalization, GALILEO and GLONASS deployment, Science and Technology Agency Fellow with National and precise ephemeris utilization. Aerospace Laboratory, Japan INTRODUCTION Seiya Kawaguchi joined GPS Division of DX Antenna in 1998. He holds a Master degree in Earth Science from The idea of a real-time kinematic (RTK) network service National University of Kyusyu. has been around for many years, but only relatively recently has its implementation been started in some Hideyuki Torimoto is a General Manager of the GPS countries. The importance of an RTK network increases Division of DX Antenna. Before joining DX Antenna he every year. Despite the fact that we now have a large had established Trimble Navigation Japan Ltd. and was percentage of GPS consumers, who do not require RTK working as an Executive Vice President of this company accuracies, the ease of getting such a service in the future since 1986. will accelerate the consumer market. With SA being turned off, differential services have difficulties to Kenjirou Fujii holds MS from Waseda University. He is distinguish themselves from standalone GPS in terms of working at the Industrial Components and Equipment in accuracy, so part of the former DGPS users can turn to Hitachi Ltd. Japan. He is the principal specialist in RTK for improved performance. In addition, the situation automatic control, robotics and GPS related system with the availability of navigation satellites will development. drastically change in the future. The GLONASS constellation will hopefully be realized at its full Dr. M.E. Cannon is a Professor in Geomatics Engineering potential, and GALILEO will appear. New civil at the University of Calgary where she conducts teaching frequencies will be introduced for GPS and GLONASS. and research related to GPS and integrated GPS/INS All these factors altogether will ensure that reliable, systems. She is a Past President of the Institute of instantaneous RTK will be readily available. Navigation. The main problem, which RTK can overcome, is the Dr. Gerard Lachapelle is Professor and Head of the necessity to have a reference station (RS) in the vicinity Department of Geomatics Engineering where he is of the user. The distance from the RS when using RTK responsible for teaching and research related to should be generally no more than 10km on average, positioning, navigation, and hydrography. He has been which is significantly different from DGPS, where involved with GPS developments and applications since distances to RS can exceed several hundred kilometers. It 1980. means that if you want to provide an area of 1000 km2 with a reliable RTK service, you have to have install ABSTRACT about 2500 reference stations. The only way to overcome this problem without sacrificing accuracy or time for The paper presents a commercial real-time kinematic initialization, is to apply an approach similar to WAAS, (RTK) Virtual Reference Station (VRS) Network service whereby corrections are averaged over the coverage area. in Japan and issues related to its implementation. A It allows a service provider to decrease the number of RS prototype of an infrastructure for the commercial RTK drastically, for example down to 100-400 depending on correction service was introduced for public in September the ionospheric conditions. 2000. The paper discusses guidelines for infrastructure

The Virtual Reference Station (VRS) RTK Network, of the real RS, one can find the approximate magnitude of along with a number of other well-developed methods, the most probable error between them. In order to find allows the use of a moderate number of RS, providing the this averaging error near the user position, we can use same full coverage (see Petrovski et al., 2000). A VRS different criterions to optimize this estimate, using various has corrections calculated for it, rather than having a algorithms. The difference between the results of these physical RS, and it is located near the user position. The algorithms is not generally significant. A natural user cannot use a real RS, because it is generally too far extension of the network concept is that RTK VRS away. Over a distance greater than 10 km these errors increases the integrity and reliability of the service in could be comparable to the GPS signal wavelength and contrast with a single baseline solution. would interfere with the ambiguity resolution algorithm. The largest errors, which RS corrections intend to Since 1999 a group of companies and universities started compensate, de-correlate with distance. Among these a project to create RTK VRS service in Japan. DX errors, the most significant are the ionospheric errors. Antenna Co. Ltd. acts as a system integrator and in charge Fortunately, these errors mostly decorrelate linearly with for the overall system. The Department of Geomatics distance under normal operating conditions. Other errors, Engineering at the University of Calgary provides VRS which decorrelate with distance are tropospheric, and Software, adapted for real time applications by Roberton orbital effects. Orbital errors usually are insignificant over Enterprises. Hitachi Ltd. manufactures user equipment a medium-length baseline, and tropospheric error prototype and provides reference station (RS) network. distribution usually fluctuates on a medium-length Asahi TV, Keio University and WIDE project are baseline, and in some cases may be very significant. involved in close cooperation related different parts of Using the distance-weighted errors at the known locations RTK VRS Network infrastructure.

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 As a result of this work, a prototype RTK VRS Network construction, rapid surveying, and GIS. Some navigation infrastructure was created and successfully introduced to applications like automatic parking, ship docking and etc. the public in September 2000 in . This attracted also require cm-level accuracy. This service fills the gap more than 300 representatives from different companies. between demanding mm-level geodetic services and A test that was conducted at that time demonstrated that meter-decimeter-level differential services. the system can provide a 2-5 cm level accuracy over baselines longer than 30 km. (see Petrovski et al., 2000)). The availability, continuity and convenience of the At the same time it demonstrated difficulties in providing service will depend on components as data links, whereas continuously reliable solutions. These difficulties were accuracy depends on the algorithm, network caused by severe ionospheric conditions (reaching 15 configuration, data latency etc. An RTK Network ppm), which in turn had a negative impact on ambiguity incorporates a number of essential elements, each of resolution between the network RS. At the same time user which is essential in terms of estimation of the overall hardware development was necessary. In order to ensure specifications, investment and running costs. Below we the reliability and quality of the service and resolve consider the elements from the point of view of potential administrative issues, the service has been transferred to a users and service providers. For the service provider the test phase. The main service components are in use and main considerations are investment vs. running cost, are operating in a test mode. source of revenue, such as equipment vs. service fees, and scalability of service. For the user, the most important PRACTICAL CONSIDERATIONS issues are cost for equipment vs. the service fee, availability, accuracy, customization, continuity of The RTK VRS service is planned to be operational in service, and convenience. 2002. In order to determine the consumer market for this service, we need look at services, which are available The basic requirements and characteristics of the RTK now, and those that will appear in the near future. The VRS network are based on the Japanese Geographical size of the potential market is the governing factor in the Survey Institute (GSI) specifications. The competitive 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

service in Japan enjoy RTK, when the baseline to the nearest RS is from should satisfy 10 to 50 km or greater. For a thorough discussion of the these VRS concept see add a reference here. The system requirements. As a specifications, cost and design as well as the requirements result of analysis for the user equipment will depend on the numerous of such options we choose for the algorithm and components. We requirements we will discuss these options in detail below. come up with the following 2. NETWORK REFERENCE STATIONS specifications. The coverage area A distinction should be made between a prime reference should be without gaps and require distance between RS station (RS) that supplies the user with the main from 30 to 50 km. Accuracy are not less than 5 cm + 1-10 correction stream and all others RS that function to ppm. The service provider should broadcast to the user provide information on error decorrelation. These so- either re-corrected corrections (VRS data) or corrections called secondary RS supply users with less urgent together with area correction parameters with an update information related to a relative correction distribution rate of 1-2 Hz using a standard corrections format. The around the network in relation to a primary RS. A latency correction format should be one of RTCM (V2.1/2.2 or of up to a few minutes is tolerable for the secondary RS higher) Type 1, Type 18/19, Type 3, Type 20/21, Type 59 corrections, because these corrections are the result of the for area correction parameters. The baud rate should be ionosphere, which is generally not very dynamic. The limited to range of between 2400-4800 baud. Latency primary RS, in contrast, supplies a user with absolute effects should be minimized by correction prediction. The corrections, which are valid in its vicinity. Corrections system should have the potential to implement from the prime station are critical and should be supplied GLONASS, GALILEO and extra frequencies without with minimum latency. A second of latency in the prime major hardware modifications . Japanese users are station corrections may result in approximately 1 cm error interested in GLONASS due to obstructed visibility, in the user position. although GLONASS implementation is difficult due to the frequency bias between channels, and the few number We started with a WIDE network of six Internet-based of satellites available. The system should provide integrity RS, see (Hada et al.,2000). For the test network at present monitoring for detection and exclusion of erroneous data. we use six OEM-4 NovAtel receivers as the RS, which The system ought to provide a capacity for data storage provide full compatibility with UC (University of and possibility for data retrieval for post-processing, and Calgary) VRS software. The network configuration is the data should be retrievable through mobile telephone depicted in Figure 15. The system will be implemented or Internet. The system should not apply any requirements on this privately-owned network, which will be extended to the rover receiver, except that the rover has to be RTK in the future. For the official GSI test , which is scheduled capable. later this year, the GSI RS network will be used. The possibility of using data from a public network gives 1. VRS RTK NETWORK COMPONENTS instantaneous advantages, because the infrastructure already exists. To use a public network will require VRS RTK Network certification, which in turn will make the service more consists of a network of attractive for users. On the other hand, it can put some reference stations (RS), restrictions on commercialization. The use of a privately each of which sends owned network will cause no problems with certification correction data through a and this network is easy to control. However it is data link to a control expensive to install. center, which process all these data, and sends re- 3. CONTROL CENTER defined corrections to a user through another data A Control Center incorporates a core engine that link. The rover side, which calculates a correction grid, and when using the VRS is usually, but not concept, it can also calculate the VRS data stream. The necessarily a part of the amount of calculations required at the Control Center system, applies if depends on the data link between a user and the Control necessary some operations Center. If the data link is bi-directional like a cellular to the corrections and uses phone, for example, the Control Center can calculate all them for RTK. This the necessary data for the GPS receiver based on the concept allow a user to 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 broadcast, the user should make most calculations at their end, but the load for the Control Center processor will be less. I is ionospheric error (m), The Control Center O is error due to orbit degradation (m), operates as follows: It receives correction data from a U is a receiver noise and multiphase (m). network of secondary RS, resolves ambiguities in real time over the network, calculates residual errors at the RS Correspondingly, double differenced carrier phase locations and re-calculates these errors into a correction measurements can be expressed as below. grid. The Control Center has the following advantages against a rover receiver to resolve ambiguities over the ∆∇Φ = 1/λ (∆∇ρ + ∆∇T + ∆∇ I + ∆∇O + ∆∇U) + ∆∇N, (2) long baseline. The position of the RS are static and known with mm accuracy, and the time for initialization is where ∆∇ Φ is carrier phase double differences, available. We use the NetAR software of the University ∆∇N is true double differenced ambiguities, of Calgary to resolve ambiguity over the network in real ∆∇ρ is known double differenced ranges. time (see Raquet et al., 1998). Commercial post- ∆∇T is error due to the change tropospheric conditions processing software packages are available and used to over the baseline, validate the results (e.g. Bernese). DX Antenna has also ∆∇I is error due to the change ionospheric conditions developed a software for post-processing analysis, over the baseline, demonstration and validation (see Figure 4). The software ∆∇O is error due to orbit degradation, incorporates simplified versions of algorithms presented ∆∇U is noise and multiphase. on Figure 1, except a block that provides true ambiguity over a network in real-time. Corrections calculated at the control center relative to the key RS at the secondary RS position can be expressed as To help the network ambiguity resolution process, one follows . may consider using estimates of ionospheric error, mapped in real time, and precise ephemeris. For the sake ∆∇υ = ∆∇ Φ - 1/λ ∆∇ρ -∆∇N , (3) of simplicity without sacrificing accuracy, we can consider resolving ambiguities from a key RS somewhere The calculated corrections can be given by: 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 ∆∇υ = 1/λ(∆∇T + ∆∇I + ∆∇O + ∆∇U) , (4) 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, (1)

Where ρ is distance from satellite to the receiver (m), N is integer ambiguity (cycles), T is tropospheric error (m),

We believe that finding the Control Center to calculate the VRS, or corrections the correct double from the VRS in the vicinity of the user. In case of a differenced ambiguities unidirectional link, the user calculates the VRS data using in real time is one of the grid data. The proposed VRS RTK uses an algorithm the main challenges in that lies between these two methods and utilizes two the creation of the VRSs. The first VRS is calculated at the Control Center RTK network software, and is placed in the middle of a predefined area, while the because these second VRS is calculated by the user. This allows for ambiguities should be easy expansion of the Control Center service for a larger recalculated area, which originally required two or more primary instantaneously in case VRSs. This algorithm also incorporates options for uni- of cycle slips, etc. The and bi-directional communication. A bi-directional source of the problem communication algorithm, generally speaking, can put is that the true double some extra constraints on the number of users. In Japan, differenced ambiguities should be obtained from the very the potential number of users could reach tens of same equation, however the RS coordinates are known a thousands for one service area, which requires special priori equipment and extra time to connect an unknown number If one can decrease the residual ∆∇υ values down to half of users to the control center server at the same instant. of a wavelength, it will assist real time ambiguity This situation can cause increased latencies, which is a resolution over the network. In this sense, some very critical issue for RTK methods. approaches can be proposed. The effect due to the orbital error decorrelation ∆∇O can be estimated from 0.5 As a core engine for the control center, the MultiRef RTK (optimistic) to 2 cm (pessimistic) for a 50 km baseline. method is used, which was proposed and developed by This error can be eliminated with use of the GSI predicted the University of Calgary and Roberton Enterprises (see orbits instead of broadcast orbits, which brings ∆∇O Racket et al (1998)) and Townsend et al (1999). The down to 5 mm. GSI plans to employ precise orbit Control Center encodes the correction grid into a prediction in real time for regional use. We also propose modified RTCM format, which had been proposed as a to decrease ionospheric error in (2) through real time draft for RTCM v.3 Network RTK message (see ionosphere mapping as a function of the station Townsend et al, 2000). Besides the main server, the coordinates and satellite elevation. 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 Est(∆∇I) = ∆∇ Est( Fsat (el) Fst(φ,Λ) , (5) purposes. where φ is latitude, 4. ROVER SIDE Λ is longitude, el is satellite elevation. A user can be equipped with a standard off-the-shelf single or dual frequency receiver capable of performing The following mapping function has an advantage that RTK positioning, a personal computer and an appropriate although it is less accurate than a satellite-to-station modem or data receiver. The rover software can be also estimate, it does not depend on a particular satellite and incorporated into the GPS receiver firmware. The therefore ∆∇I in (4) can be quickly restored without required equipment should implement standard RTCM 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

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.

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 sub- channel signal (ASC). This data link is intended for V2.1/2.2 message Type 1, Type 18/19, Type 20/21, Type mobile users such as car navigation systems. Apart from a 3, and if required, Type 59 for area correction parameters. GPS receiver, the user is required to accommodate an The baud rate should be no less than 2400-4800 bps. ASC receiver (see Figure 7). For the car navigation system the ASC receiver, besides RTK and Theoretically, in case of bi-directional communication, DGPS/DGLONASS corrections, provides an extra data some rovers can be used as they are, provided that the channel for weather and traffic information. control center calculates appropriate corrections for the rover position. In this case, the corrections have to be We also use an alternative method to provide a bi- applied to the user position instead of into grid points at directional data link between the Control Center and a the Control Center. Then Control Center encodes the user. DX Antenna has developed a special DGPS Data latest data into RTCM 2.2 format and feeds them directly Receiver (see Figure 10) that allows the user to into the GPS receiver. In practice, the rover receiver tries automatically dial the nearest Control Center and to to take into account the known baseline length, which is establish a link through a cellular phone between it and undesirable because RTK network corrections emulates a the Control Center. Usage of this device allows one to short, or zero,-baseline case. Moreover, the rover can establish a stable and reliable connection over the cellular reject corrections on the basis of a long baseline. phone with latencies between two and three seconds (see

Figure 11). In the case of the ASC data link, latencies are The rover side software decodes RTCM v.3.0 messages on average one second higher (Figure 12). A cellular from the Control Center, and recalculates the raw data for phone allows bi-directional communication between a each satellite for a given VRS location. It calculates user and a Control Center. Therefore the user can transmit corrections for the current location, based on the its approximate position to the Control Center, which correction grid in the RTCM Network RTK message. permits the transfer of the VRS calculation to the Control Recalculated corrections at the user position could be as Center. Bi-directional communication also allows the simple as RS corrections weighted with inverse distance transfer of only required information through the optimal to reference stations (see Figure 3).To ensure better channel. The user can select a subset of the correction accuracy, we use a more sophisticated NetAdjust software information that is needed for its particular purpose. On from the University of Calgary. the other hand, a bi-directional data link will increase the requirements to the Control Center in terms of the number Equipment that provides the user with the required data of simultaneous VRS calculations needed to support link and a microprocessor for VRS calculations, have different locations. These requirements could be difficult been developed and produced by Hitachi Ltd. (see Figure to meet in the case of large numbers of users. 14). In one housing it contains an ASC receiver, a GPS receiver (NovAtel OEM-4 board) and the microprocessor There is also a possibility to use the Internet as a data link to conduct VRS-related calculations (Figure 13). between the Control Center and the user. At present the

Internet does not provide connections reliable enough for

Both VHF and UHF waves use line-of-sight propagation. In this type of propagation 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 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 RTK. At present there is no Internet Protocol, which is areas of interest. TV Asahi uses 98 Satellite Relay to sufficient for our purpose. cover such areas and to make it possible for TV viewers to receive the TV signal and data. Every satellite relay We use an Audio Sub-Carrier data Channel (ASC). ASC station viewer can receive the ASC signal. is a data channel multiplexed onto TV broadcast signal. One of the main ASC advantages is that the required A TV signal consists of a video and an audio signal (see infrastructure for a TV broadcast system exists already NTSC spectrum on Figure 8). On Figure 2 fa=209.75 and has good nationwide coverage. Terrestrial TV MHz signifies the frequency of the audio carrier, which is systems are widely used over the world in general and located at 4.5MHz higher than the video carrier (fv), and therefore can be used for DGPS correction broadcast has a 0.5 MHz bandwidth. The ASC data channel uses almost worldwide. Frequency-Division Multiplexing (FDM) in the audio After a period of laboratory and field tests, the ASC was band. The FDM technique allows for the combination of adapted for mobile reception in Japan. In August 1999 different signals with bandwidths smaller than the data Asahi National Broadcasting Co. Ltd. (TV Asahi), in link bandwidth into one composite signal that can be cooperation with DX Antenna Co. Ltd., started transported by the link. Carrier frequencies are separated GPS/GLONASS DGPS/RTK corrections broadcast on by enough bandwidth to accommodate the modulated one of audio sub-carriers. The Japanese Ministry of Posts signal. These bandwidth ranges are the channels through and Telecommunications (MPT) had set up an advisory which the various signals travel. The channels are to be committee to legislate the system as one of the Japanese separated by strips of unused bandwidth (guard bands) to TV Standards. At present, the ASC is officially adopted prevent signals from overlapping. The data channel by the MPT for broadcasting GPS and GLONASS DGPS carrier frequencies measured in reference to the TV and RTK corrections in Japan. Today ASC is the only synchronization frequency (fH) are 4.5fH and 7.5fH (see service certified to broadcast GLONASS corrections. audio band spectrum on Figure 9). The deviation level to Area coverage for the ASC data channel for differential the audio carriers is as follows: 70.804KHz ± 3KHz and service is different for Yagi and Whip types of ASC 108.007KHz ± 6KHz correspondingly for the 1st and 2nd antennas. On average, the broadcast distance for an RTK frequency. The audio frequency is modulated with audio VRS is up to 70 km. Asahi TV has a network of TV signals using Frequency Modulation (FM). For data Stations that covers all of Japan. These TV Stations, along modulation, the ASC uses Differential Quadrature Phase with satellite stations for mountainous areas, comprise a Shift Keying (DQPSK) which is a modification of a usual nationwide infrastructure. phase modulation technique Phase Shift Keying (PSK). In PSK, the phase is varied to represent binary 1 or 0, which In the VRS RTK Network approach, calculated VRS data is a binary PSK. In DQPSK, instead of utilizing only two from a Control Center are transferred through a 64 Kbps variations of a signal, each representing one bit, one can dedicated line to Asahi TV facilities. These data are use four variations and each phase shift will represent two encoded onto TV audio signals by a modulator, combined bits. This allows one to get a higher bit rate for the same with a video signal in an audio transmitter, which is bandwidth. So while the baud rate will be the same for transferred to an Asahi TV tower and then is broadcast to PSK and DQPSK for the same bandwidth, the DQPSK bit a user. rate can be two times greater.

In Japan, NTSC TV operates in two frequency ranges, The error correction mechanism is capable of Multiple- which are very high-frequency VHF from 90 to 222 MHz Bit Error detection and correction. Up to 11 error per and ultra high-frequency UHF from 470 to 770 MHz.

packet can be corrected. The ASC uses Cyclic from the reference station and the computer are fed to the Redundancy Check (CRC), which is the most powerful ASC modulator, one for each sub-carrier (see 4.5fh redundancy checking technique available. CRC is based DQPSKMOD and 7.5fh DQPSKMOD on Figure 5). The on binary division. A sequence of redundant bits (CRC 4.5fH modulator is used basically to encode weather and remainder) is appended to the end of a data unit so that traffic data, and 7.5 modulator for code/phase differential the resulting data unit becomes exactly divisible by a GPS/GLONASS corrections. The 4.5fH modulator is also second, predetermined number. At the receiver the thought to be used as an extra data channel for incoming data unit is divided by the same number. If at GALILEO. The ASC signal is fed into an audio this step there is no remainder, the data unit is assumed to transmitter that combines the audio and video carriers and be intact and therefore accepted. delivers them to the tower using a dedicated 64K line. A CRC generator can be represented as an algebraic polynomial. The user equipment was initially developed as a part of a The generator polynomial for a data packet is given by: car navigation system . This equipment includes a GPS receiver, an ASC receiver, and a cellular phone. The G()=X82+X77+X76+X71+X67+X66+X56+X52+X48+ cellular phone allows the integration of a car into the X40+X36+X34+X24+X22+X18+X10+X4+1 , (7) Internet based ITS. An ASC receiver measures 195 by 25 by 132 mm (see Figure 7). It incorporates a TV turner, an The generator polynomial for mode control is given by: SH7709(SH-3) CPU and a PCI memory card. The device allows the user to assign up to four channels in diapason G(X)=X10+X8+X5+X4+X2+X+1, (8) 19.6608 – 78.643 MHz. The interface includes an RS-232 serial port, an interface for DRAM memory, PCI card The ASC frame structure has a small transmission delay. slots, and an antenna input. A new waterproof The bit rate for each ASC data channel is 16,000 bps and modification of the ASC receiver also includes an power the effective bit rate after correction is 9,600 bps. source, a magnetic shield, a booster to improve reception, Therefore, its transmission time is 0.577sec, and with and a filter to separate the signal. This is interesting for encoding and decoding time, it is 1.154 sec. the user as it can include such signals as the code or carrier-phase corrections, from GPS or GLONASS. ASC monitoring block has two separate channels for the reference station and a monitor station. Correction data

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 real- time 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.

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 6. DATA LINK BETWEEN SECONDARY RS AND the best server for this user. The other advantage of this CONTROL CENTER service is that it could easily grow. Everyone can create new reference station in the Internet without any license, The different types of data links are used between the RS which is necessary for radio-transmitters. and the Control Center in the test system. The data links to the GSI RS Network are Integrated Services Digital The Internet-based reference station that consists of a Network (ISDN) lines that connect each GSI RS to the GPS receiver and a PC-based workstation, which is Japan Association of Surveyors Computerized Interface, connected to the Internet. We use an MMX-Pentium as which provides public access to the GSI Network. The the workstation with FreeBSD as an operating system. GSI network has been used for a short time for test We chose FreeBSD because its high server performances. purposes. At present, our RTK Network RS is connected The server computer is connected to the Internet through to the Control Center through the Internet. Two of the RS an Ethernet for 24 hours a day. The server has two serial are connected to the Internet using OCN line with 128 ports, each of which is connected to a GPS receiver. One Kbps, others using ISDN with 64 Kbps. Our Control port is used for the RTCM data logging and another for Center is connected to the Internet through a dedicated control and monitoring purposes. The server software is 128 Kbps telephone line. written in the C and Perl languages and works as a daemon. For the data link between the secondary RS and the Control Center we use the Transmission Control 7. OVERALL SYSTEM TEST RESULTS Protocol/Internetworking Protocol (TCP/IP). IP is the transmission mechanism for TCP/IP protocol. It is a best- The tests for the different network configuration were effort protocol that provides no error checking or conducted in 2000-2001 and partly presented in tracking. Arrival of the data is not guaranteed. Therefore, (Petrovski et al., 2000). See network configurations and a transmission could be destroyed for a number of scatter plots on Figure 16. different reasons. There are two transport protocols in TCP/IP: TCP and UDP (User Datagram Protocol). UDP SUMMARY provides transportation when reliability and security are less important than size and speed. As soon as the arrival The RTK Network infrastructure is under development to of data is not guaranteed under UDP, an application itself implement the Virtual Reference System Concept. This should detect the data arrival and quality. In the Internet, infrastructure provides a user with multiple options for most applications usually require reliable end-to-end data links, such as a TV Audio Sub-channel, the Internet, delivery and therefore use TCP. The drawback of TCP for and a cellular phone. The hardware and software for the real-time service is that it takes a long time to transfer network, control center, data links and user are developed

GPS2000 (Salt Lake City, September), The Institute of Navigation, 1124-32. 2. 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 3. 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. 4. 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. 5. 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 and tested. The system is operating in test mode in the Station Network. Proceedings of National Technical Tokyo area and shows the desired level of accuracy. Meeting, The Institute of Navigation (January 25-27, San Diego, CA), 319-326. ACKNOWLEDGMENTS 6. RAQUET, J. G. LACHAPELLE, and L. FORTES (1998) Use of a Covariance Analysis Technique for We would like to acknowledge the following companies Predicting Performance of Regional Area Differential and individuals. Code and Carrier-Phase Networks. Proceedings of GPS98 (Session A5 (Nashville, 15-18 September), Roberton Enterprises Ltd., Calgary, as our main The Institute of Navigation, 1345-54. 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 Network- based RTK Service in Japan. Proceedings of