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New Concepts for a Carrier Phase Based GPS Positioning Using A Using a Virtual Reference Station to Compensate for Coordinate Transformations in GPS Surveying Bryan R. Townsend, Roberton Enterprises Ltd., Anna B. O. Jensen, National Survey and Cadastre, Denmark BIOGRAPHY datum, this not the preferred mode of operation. The Network is used to model the ionospheric, tropospheric, Bryan Townsend received his Master of Science degree in and orbit errors over a region. Using reference station 1993 from the Department of Geomatics Engineering at coordinates that are in another datum than WGS84 will the University of Calgary. Since then he has worked in reduce the ability of the Network RTK software to model several areas of GPS including GPS surveying, GPS these errors. receiver design and wide area reference systems. Most recently he is involved in the development of multi- This paper investigates using the VRS position to correct reference RTK systems. for local datum effects. With this method the reference station coordinates remain in WGS84 and the rover Anna B.O. Jensen holds an M.Sc. in surveying from the coordinates are maintained in the local system. This University of Aalborg in Denmark, and has since 1995 method allows the user to work in the local earth fixed been employed in the Department of Geodesy at the system for several years and not suffer any significant National Survey and Cadastre – Denmark (KMS). She is degradation in accuracy. presently doing her Ph.D at the University of Copenhagen and KMS. INTRODUCTION ABSTRACT The introduction of multi-reference station based RTK (Network RTK) offers several advantages to GPS The introduction of multi-reference station based RTK surveying. RTK coverage over regional areas is made (Network RTK) offers several advantages to GPS possible while using fewer reference stations, with more surveying. RTK coverage over regional areas is made ease to the user, and at a lower cost than by traditional possible while using fewer reference stations, with more single baseline methods. A regional network that can ease to the user, and at a lower cost than by traditional cover areas of several hundred kilometers square has single baseline methods. A regional network that can introduced new problems related to coordinate cover areas of several hundred kilometers square has transformations between WGS84 and the earth fixed introduced new problems related to coordinate regional reference system, which is often preferred by the transformations between WGS84 and the earth fixed user. regional reference system. This paper investigates this problem and presents a way it In single baseline RTK the position of the rover is can be solved using the Virtual Reference Stations calculated relative to the reference station. Therefore, it is (VRS’s). possible to enter the reference station coordinates in the local datum. As long as the baseline between the rover NETWORK RTK AND VIRTUAL REFERENCE and reference station is short, the coordinates output by STATIONS the rover can be used directly in the local datum without significant errors. Network RTK is an approach of using a network of reference receivers for centimeter level positioning In Network RTK the situation is more complicated. While [Raquet, 1998a and 1998b]. This method estimates the it is possible to have the reference stations in the local distribution of the differential carrier phase and datum, and thereby obtain rover coordinates in the local pseudorange errors over the area covered by the network. The errors are used to compute corrections for the The reference station data is corrected to compensate for reference station data based on the location of the rover the differential errors associated with the rover location. receiver. To do this the Network RTK software needs to know the approximate position of the rover. In MultiRefTM the rover Figure 1 is a functional diagram for the MultiRefTM position is reported back using the GPGGA message Network RTK software. MultiRefTM is a Network RTK format (see Figure 2). The GPGGA message is part of the software currently being developed by the University of NMEA 0183 Standard for Interfacing Marine Electronic Calgary and Roberton Enterprises Ltd. Devices [NMEA, 1997]. Most GPS receivers support this format. GPS RS Network Control Centre Rover Virtual reference stations (VRS’s) are often used in GPS Rx Network RTK. A VRS is virtual because it does not Two-way connection (eg Mobile Phone) physically exist at the location reported by its coordinates. VRS function at control centre. It is generated using data from a real reference station and therefore has the same characteristics as a real reference station. From a GPS positioning algorithm point of view, GPS Rx RS Communication Broadcast TCP/IP (eg ASC) it is not technically necessary to use VRS’s but using Serial Com Port VRS function at Rover. Dial-up VRS’s offers several advantages. Some of these are: 1) The number of VRS’s that can be generated is Figure 1: Network RTK functional diagram limitless. 2) The virtual reference station can be placed anywhere The raw GPS data from the reference stations is within the network of reference stations. transmitted in real-time to a central processing computer 3) For large networks, the VRS can be tailored for a (the Control Centre in Figure 1). The differential specific broadcast area. ionospheric, tropospheric, and orbit errors are calculated 4) The virtual reference station can be used for for the region covered by the network. The RTK data is coordinate transformation between WGS84 and other sent to rover receiver via wireless communications, and is geodetic systems. corrected based on the location of the rover. Figure 3 shows a VRS placed relative to the rover receiver The communication with the rover can be bi-directional within a network of reference stations. (one rover per connection) or broadcast (multiple rovers receive the same data). Each method has their own advantages and these will be discussed later in the paper. RS1 RS2 Almost all RTK receivers use the RTCM standard for receiving reference station data [RTCM, 1998]. Figure 2 shows a diagram representing the data interface between the Network RTK system and the rover GPS receiver. RTCM GPS Rover Receiver GPGGA VRS RS3 RS5 Figure 2: Interface to GPS RTK receiver The RTCM messages contain the L1 and L2 pseudorange and carrier phase measurements for a single reference RS4 station plus the reference station coordinates. MultiRefTM uses message types 18 and 19 for the measurement data and message types 3 and 22 for the VRS position. Figure 3: Virtual reference stations (VRS) There are several methods for VRS placement. Some determined from the broadcast ephemerides are given in Network RTK software place the VRS directly on the user WGS84. position. This has a disadvantage because the rover receiver will think the reference station is very close and When performing absolute GPS single point positioning therefore may be over optimistic about solving using broadcast ephemerides the position will therefore ambiguities. As a result, more missed fixes (i.e. wrong also be given in WGS84. ambiguity solutions) are likely. ITRF The MultiRefTM software places the VRS on the line between the rover position and the nearest reference The International Earth Rotation Service (IERS) is in station. Using the distance to the nearest reference station charge of providing realizations of worldwide celestial as a guide, the distance between the VRS and the rover is and terrestrial reference systems for instance the calculated by the equation, International Terrestrial Reference System (ITRS), which is considered to be the best reference system defined to DVRS = DRS/α (1) date. The ITRS is realized in the form of reference frames (ITRF) whenever necessary. where DVRS is the distance between the rover and VRS, DRS is the distance between the rover and nearest RS, and When GPS is used for geodetic and geodynamic purposes α is between 1.0 and 3.0 depending on the atmospheric where high accuracy is required, the ITRF is used as the conditions. reference frame, and the relative GPS positions are determined using precise orbit information e.g. from the REFERENCE SYSTEMS International GPS Service where the satellite positions are given in the ITRF. By using precise orbits and reference The advantage of using VRS’s to correct for coordinate station coordinates given in the ITRF at the same epoch in transformations is the main subject of this paper. Before time, geometrical misalignments are eliminated in the going further a discussion of reference system is positioning process, whereby the positions of the rover necessary. An increasing number of reference systems are will be free of any errors originating from reference frame being used for GPS positioning, and the following 5 mix-ups. sections describe the systems relevant for this paper. TRANSFORMATION BETWEEN WGS84 and ITRF WGS84 In 1996 and 1997 NIMA carried out a number of The World Geodetic System 1984 (WGS84) is a investigations with the purpose of the determining of set conventional terrestrial reference system defined to be of transformation parameters between ITRF94 and used in connection with GPS. The definition and WGS84 [Malys et. al, 1997a, and 1997b]. They came up realization of WGS84 is described in National Imagery with two sets of parameters for a 7-parameter datum and Mapping Agency (2000). transformation. The authors of the papers were however not completely satisfied with the results, and in the 1997b- WGS84 was originally realized, i.e. coordinates for a paper they are recommending that any transformation number of physical reference stations were determined in between the reference frames should be avoided because the reference frame, using the TRANSIT system in 1987. of the lack of statistical significance in the determination This realization of WGS84 was used until 1994 when a of the parameters.
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