In 1985, GPS technology was formally introduced into Sweden when a working group was convened representing Stockholm's Royal Institute of Technology, the National Land Survey (NLS), the National Road Administration, the Swedish Maritime Administration, Onsala Space Observatory, the University of Uppsala, and the Lund Institute of Technology. During the 1987 - 1989 period, several single-frequency GPS receivers (later upgraded to dual-frequency capability) were procured to gain hands-on GPS experience through various pilot projects. Orbital injection of the first Block II NAVSTAR satellite in 1989 coincided with the purchase of second generation dual-frequency receivers (the Ashtech M-XII and Trimble's 4000ST). These more sophisticated devices allowed routine GPS measurements aimed at densifying the national triangulation network and establishing local control networks, plus test networks like the one located at the NLS observatory in MŒrtsbo.
      Also in 1989, NLS, in concert with members of Sweden's geodesy community, carried out forward-looking research into GPS usage during the upcoming nineties. The ensuing report, "Geodesy 90," was based upon interviews delving into the future needs of municipalities, the prospective requirements of surveying and engineering professionals, and the anticipated activities enumerated by certain government authorities. This report, considered a list of action items, outlined the following proposals and objectives:
      • That GPS technology in Sweden be actively sanctioned for those surveying practices deemed both applicable and cost-effective.
      • That NLS assume GPS coordination responsibility through the issuance of information and guidelines pertaining to GPS usage.
      • That a network of 50 reference points be established (though not permanently equipped with GPS receivers), and then tied to the national triangulation and leveling networks in order to facilitate densification of the triangulation network, and to interconnect with local control networks.
      • That a network consisting of four permanently equipped reference stations be set up on a priority basis in order to study its usefulness for various applications, to learn the operational routines for cadastral surveys tied to the national triangulation network, and to serve as control points for aerial photography and allied applications such as photogrammetry. Users would be invited to import data from this test network.
      • That a geoid applicable to the Swedish landform be computed at the earliest possible date in order to make height determinations with moderate (decimeter-level) accuracy.
      The objectives described in "Geodesy 90" have now been either implemented, are in some stage of implementation, or will be implemented shortly. The major thrusts of NLS activity are satellite navigation, aerial photography, cadastral surveying, and geodetic control surveying; baseline lengths in the latter discipline range from 0.5 to 20 kilometers, with control network accuracies (in the horizontal plane) of approximately 5 mm +1-to-2 ppm (1 sigma). In 1993 a guide, "GPS for Geodetic Surveying in Sweden," was published to illustrate methods used in different phases of GPS control surveying, with emphasis on network planning, data processing, computation and analysis, and data reduction based upon single baseline processing.
      Aerial photography, a major activity at NLS, improved markedly after GPS techniques were gradually introduced into the application. A software package, the Computer-Aided Aerial Photography System (CAAP) was developed during the Persian Gulf War. Military aerial photography in Kuwait and Iraq, augmented by GPS for both navigation and automatic shutter activation, demonstrated and verified GPS as an effective support to airborne photography. To gain a first-hand impression of GPS aerial capability, NLS carried out experiments using airborne Ashtech receivers during the period 1989 through 1992.
      The stated goals were:
      ¥ To inform the pilot of aircraft waypoints with respect to planned photostrips (i.e., for precise navigation).
      ¥ To enable automatic exposure(s) at preselected points and altitudes.
      ¥ To definitize and record the lens position at the instant of exposure.
      A satellite navigation project, initiated in 1988, was created to gain practical GPS experience in a variety of applications. This project, completed in 1992, led to a proposal for a Swedish network of permanent reference stations capable of deriving accurate positioning (via both real-time and post-processed data) for navigation purposes. Onsala Space Observatory, in conjunction with the Smithsonian Astrophysical Observatory, also advocated a network of Swedish stations dedicated to the study of vertical and horizontal crustal movements associated with postglacial rebound in Fennoscandia (i.e., the land recovering from the tremendous weight of glaciation during the past Ice Age). In 1991 the National Land Survey commenced a project called PREF, its goal to acquire hands-on experience in the operation of permanent reference stations, and secondarily to study the usefulness of such stations.
      The SWEPOS Program, a collaboration between Onsala Space Observatory, the National Land Survey, and a project called "GPS Resources in Northern Sweden," began in 1991. The SWEPOS network is comprised of 20 permanent reference stations (Figure 1). Today, 20 Ashtech Z-12 GPS receivers and 13 Allen Osborne Turbo Rogue GPS receivers are installed in these stations, which also include control centers and real-time/post-processing data distribution systems (Figure 2). In addition to doing more prosaic duty, certain SWEPOS stations are monumented with concrete pillars set in bedrock and surrounded by a miniature network dedicated to monitoring crustal movements.
      SWEPOS data was used to study crustal dynamics during the IGS campaign (June-October 1992). Onsala Space Observatory currently computes daily baseline solutions using the SWEPOS network. To date, a series of 500 daily baseline solutions have been recorded in support of the study of crustal movements. Most SWEPOS applications take advantage of relative GPS observations by allowing one or more remote receiver(s) to gather data that undergoes real-time correction via transmitted GPS data received from observables at the reference station's known location. This type of data exchange can be done concurrently in the field via real-time processing firmware, or in the office through the use of computerized post-processing software. Curiously, post-processing has been difficult to implement for single frequency C/A-code receivers, a problem thought to arise due to the incompatible data formats of receivers produced by different manufacturers. Combinations of GPS data for single-frequency receivers are most readily processed in real-time by using the RTCM-104, Version 2 data format, which seems to work well with all types of reveivers. Here, ordinary modems are connected to the differential (serial) output ports of the Ashtech Z-12 receivers so that correction data can be called up by ordinary telephone landlines, or cellular phones. This is a relatively simple method of simultaneous (differential) data distribution.
      Thus far, Swedish users testing Differential GPS (DGPS) techniques have adopted this system. At NLS, cellular phones were used to test DGPS for both cadastral surveying and airborne photography. In December 1993, a Swedish company, TERACOM, collaborated with NLS in conducting a test that used existing FM radio stations and the RDS (Radio Data System) to transmit pseudorange corrections from SWEPOS Permanent Reference Station Lovš (see Figure 1) with promising results. TERACOM has since decided to provide a nationwide differential service based upon pseudorange corrections from the SWEPOS network linked by RDS called the EPOS-service.
      The more expensive, high-end GPS receivers (Ashtech, Geotracer, Leica and Trimble) usually offer more consistent formats. These receivers universally support the RINEX data format, hence combining sophisticated devices of this type with reference station data in a post-processing environment does not present a problem. For post-processing purposes, the following SWEPOS observables are now provided by NLS: C/A-code pseudoranges modulated on the L1 carrier frequency; P-code (encrypted to Y-code) pseudoranges modulated on the L1 and L2 carriers; phase measurement data derived from both carriers; Doppler redshift (time) corrections, and predictive satellite ephemerides and almanac information.
      A very important use of the SWEPOS stations is in cadastral surveying. In Sweden's less densely populated districts, many of which lack control points, the connection of cadastral surveys to the national control network is often accomplished via orthophoto maps or land-usage maps. Unfortunately, this makes these measurements inappropriate for use in the geographical databases being generated at NLS and elsewhere. Processing GPS data relative to the reference stations is considered the most cost-effective method of connecting cadastral surveys to the national control network in those areas lacking control points.
      The SWEPOS permanent reference network's main purpose is to provide continuous data (carrier phase observations, plus pseudorange corrections) for both real-time data processing and/or post-processing, and to do so for as many GPS applications as possible. During 1991 and 1992, users carried out a number of experiments with the aim of broadening the range of such applications. The Swedish University of Agricultural Sciences worked with test spots in forests that were situated via post-processed, carrier-smoothed pseudoranges. The SKANSA/Swedish Building Trade Research Foundation has post-processed carrier phase observables in order to target drilled holes for geotechnical investigations. Sweden's Board of Forest Remote Sensing has positioned test spots for soil investigation by post-processing carrier phase-smoothed pseudoranges.
      Experiments involving the present distribution of 20 SWEPOS stations have amply demonstrated a high probability of obtaining decimeter-level, or better, point-position accuracies within the reference system for the Swedish triangulation network, as well as a likelihood of attaining centimeter-level accuracies from the SWEPOS stations themselves. One question remains: Can such accuracies be achieved in the field using real-time processing, or only by post-processing retrieved data? NLS is very interested in the proposed revisions to RTCM SC-104 that may standardize radio-linked, real-time systems and result in the facile attainment of decimeter-level 3D position accuracies. That this is achievable was proven to the satisfaction of NLS by Ashtech's Precision Navigation software package (PNAV), a program that demonstrated very promising results in the processing of cadastral survey data.
      NLS has also followed the EUREKA Project's "System for Wireless Infotainment Forwarding and Teledistribution" (SWIFT) as an aid in studying the feasibility of using a service based upon high data-rate broadcasting over the existing FM radio network, and thereby obtaining highly accurate, real-time positioning. This service has been designated the Data Radio Channel (DARC).

About the Author:
Bo Jonsson is GPS program manager in the Geodetic Research Division of Sweden's National Land Survey located in GŠvle. Bo has acquired practical experience ranging from geodetic astronomy to TRANSIT observations, and holds a B.A. degree from the University of Lund. His work with GPS applications began in 1985. He may be reached at +44 26 15 30 00 (phone) or +46 26 61 06 76 (fax).

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