Lambert Wanninger

Ingenieurbüro Wanninger, Germany
E-Mail: wanninger@wasoft.de
URL: http://www.wasoft.de

Introduction

This column provides the web-based GPS resources and their technical background information. Its purpose is to inform the reader about the data, software, and electronic documents that are available on-line. This column is coordinated by Dr. Jinling Wang, The University of New South Wales, Sydney. Comments and suggestions are appreciated (jinling.wang@unsw.edu.au).

In this issue, the column provides a brief introduction to differential GPS based on virtual reference station (VRS) techniques and presents addresses of several websites containing information on either software to generate VRS observations or networks providing VRS positioning. The sources are selected by Dr. Lambert Wanninger, who is a software developer and consultant for precise GPS applications. He proposed the VRS technique for network RTK in 1997. Since then he has published several papers on this topic. 

Background

Differential GPS techniques, whereby a station with known coordinates is used as a reference station, are the key to sub-meter or even centimeter-level positioning. Using single-frequency code observations as primary observable, this technique is known as DGPS (Differential GPS). A much higher accuracy can be obtained with dual-frequency carrier phase observations and by fixing their double-differenced ambiguities to the correct integer values. This technique is commonly referred to as RTK (real-time kinematic). Both differential techniques degrade with increasing distance to the reference station due to distance-dependent biases such as ionospheric refraction, tropospheric refraction and, to a smaller extent, orbit errors. DGPS systems with a positioning accuracy on the meter or sub-meter level are limited to inter-receiver distances of a few hundred kilometers, whereas centimeter-level RTK systems are restricted to distances in the order of 10 km.

In order to provide a differential positioning service for a larger area, several reference stations have to be set up. In contrast to DGPS, these reference stations should not be considered as independent, with each station covering a specific part of the area. However, the observations of a network of stations should be pre-processed in order to generate models of the distance-dependent biases. Based on the model parameters and the user's approximate position, individual corrections of a virtual reference station (VRS) can then be predicted which enable differential positioning.

This kind of network approach for DGPS positioning is known as wide-area DGPS (WADGPS). Whereas for some systems new data formats were created to be able to relay the model parameters to the users equipment, which then predict corrections for their approximate positions (e.g., WAAS, or EGNOS, or the Active VRS-service of Thales LandStar-DGPS), other systems utilize the data formats originally created for single reference station systems to provide VRS corrections (Cell VRS-service of Thales LandStar-DGPS, see http://www.landstar-dgps.com/).

With network RTK, the differential errors caused by ionospheric and tropospheric refraction, and satellite orbit errors are precisely estimated based on dual-frequency carrier phase observations of a local or regional network of reference stations. Correction model parameters are determined to allow the prediction of the differential errors for the baseline between a master reference station and the user's position. Applying these corrections to code and carrier phase observations of the master reference station, VRS measurements are generated for RTK positioning of the rover receiver.

Currently, no standardized data format exists for broadcasting of correction model parameters. Furthermore, there is not even an agreement on the parameterization of the correction models. Therefore, the most common form of network RTK nowadays uses VRS measurements to be relayed to the users. The advantages are obvious: using the VRS technique, existing standardized data formats and standard off-the-shelf receivers are capable to work in network RTK mode.

However, the currently used network RTK format also has some disadvantages. First, two-way communication links are required, since the users have to transmit their approximate positions to the processing center which in return sends the users the observations or corrections of an individual VRS. Second, the baseline processing software inside the rover receiver is not able to notice that it processes virtual reference data. It considers the data to originate from a single reference station located very close to the rover position and thus in some cases it may not come to optimal decisions in the baseline processing. Nevertheless, as long as no standardized network RTK format exists, transferring VRS observations will stay the preferred method of providing pre-processed network information to the users.

Selected websites on GPS VRS or network RTK

Several research groups working in this field have published a lot of papers on this topic, which can be founded in relevant journals and conference proceedings. Extensive information, however, is provided on the web by companies, which either produce software packages for network RTK and VRS generation or provide an RTK positioning service based on VRS technique.
 

–	http://www.trimble.com/vrs.html is part of the website of Trimble Navigation Limited, Sunnyvale, California. The basic principles of the VRS technique are described and Trimble's Virtual Reference Station System, which was developed by Trimble Terrasat, Höhenkirchen, Germany, is presented.
–	http://www.geopp.com/gnsmart/page1.html is part of the website of Geo++ GmbH, Garbsen, Germany, presenting GNSMART, a software for controlling networks of reference stations. Many publications on network RTK are found on the same website (http://www.geopp.com).
–	http://www.wasoft.de/e/virt is part of the website of the author of this column. It presents WaV, a post-processing software for VRS generation. Access to publications on the VRS technique is provided as well.

The list of websites presenting network RTK positioning services given in Table 1 is not intended to be a complete one. Representative websites have been selected according to their information content. Since these websites are intended for local users they often do not provide an English version. Most of the existing network RTK systems have been installed in the densely populated areas of central Europe. Many more RTK networks are being established, but no websites have been created yet.

Table 1.  List of websites presenting network RTK positioning services

Website address	Website languages	Number of network stations	Covered area
http://www.swisstopo.ch/de/geo/swiposgisgeo.htm	German, French	29	Switzerland
http://www.swissat.ch/	German, French, English	23	Switzerland
http://www.lgn.de/produkte/sapos/	German	60	Lower Saxony, Germany
http://www.hkvv.hessen.de/dienstleistung/sapos/index.htm	German	30	Hesse, Germany
http://www.ascos.de/	German	120	Germany
http://www.gpsnet.dk/	Danish, English	25	Denmark
http://gps.wallonie.be/	French	23	Wallonia, Belgium
http://www.flepos.be/	Dutch	38	Flanders, Belgium
http://www.06-gps.nl/	Dutch	21	Netherlands
http://www.jenoba.jp/	Japanese	200	Parts of Japan