Report of GPS-99 Session 14
Innovative developments in GPS geodesy in support of the earth sciences

Contributions were sought which described work that highlighted new GPS methodologies, in particular those that were more flexible, more efficient and more cost effective than traditional GPS methodologies. Topics for this session included the feasibility of using single-frequency GPS instrumentation, mixing dual- and single-frequency GPS receivers, combined GPS and GLONASS results, the integration of GPS with other geodetic and geophysical instrumentation, high-rate GPS used in seismometer mode, real-time solutions, and so on. It was unavoidable that there was overlap with other sessions, hence many developments reported on in other sessions at the symposium were indeed "innovative". As a consequence only seven papers (six oral and one poster) were presented in this session.

The invited speaker for this session was James Stowell, director of the Reference Station Development Group of the Leica Geosystems company. (James was one of the very few representatives of commercial companies that made presentations at the symposium.) He introduced a prototype "Continuous Real-Time Network" (CRTN) system that was intended for local scale deformation monitoring of engineering structures by networks of GPS receivers. In essence, the CRTN consists of GPS sensors (there is no onboard memory) that can be networked to each other via any communications link, with real-time software running at a control facility that generates a true multi-station solution (as opposed to single baselines, as is the present capability of commercial off-the-shelf GPS systems). It is planned to release such a product in the new year.

Akio Kobayashi of Japan Meteorological Agency presented monitoring system of crustal activity in Tokai area, Japan. There are a lot of instruments to observed crustal deformation whose resolution is high ( 10 to the -9 to -11 ). The data observed by high resolution instruments are contaminated by local deformation around the instrument ( effect of rainfall, ground water ). GPS observation has advantage in the space coverage and in time domain stability. The combination of GPS and other borehole instrument would enable us to improve the detection capability for crustal deformation.

Chris Rizos reported on results of an experiment jointly carried out by the Satellite Navigation and Positioning (SNAP) group at The University of New South Wales (Australia) and the Geographical Survey Institute (Japan) involving the densification of part of the GEONET around Tsukuba using four single-frequency GPS receivers. The combined processing of the dual-frequency GEONET and the single-frequency data was carried out using an innovative technique developed at SNAP. It was demonstrated that for long observation sessions (8hr and 24hr) comparable horizontal accuracy was obtained to using expensive geodetic GPS receivers (with processing by the Bernese software), though the vertical results were somewhat weaker. In the case of one hour sessions, there was no statistically significant differences in the performance of the hardware and software.

Xiaoli Ding, of the Hong Kong Polytechnic University (Department of Land Surveying & Geo-Informatics), described a concept whereby a single GPS receiver could be attached to a number of separate antennas (the prototype system was capable of supporting six antennas), and data collected using a multiplexing technique. The system configuration is intended to be a low-cost solution to the problem of deploying GPS for local scale structural deformation monitoring. The issue of long antenna cables was not yet addressed.

Mike Stewart, from the School of Spatial Sciences (Curtin University of Technology, Australia), presented results of the processing of long Glonass baselines (up to 7000km). The data was collected during the recent (and continuing) International Glonass Experiment (IGEX) at a worldwide network of stations equipped with receivers capable of making both GPS and Glonass measurements. Several southern hemisphere baselines were processed using scientific software (with precise IGEX orbits held fixed) in order to gauge the potential of GPS and Glonass being used together for global crustal motion surveys. Preliminary results indicate that indeed sub-part-per-million relative accuracy was achieved, and that the results of combined GPS/Glonass processing is better than GPS-only.

Linlin Ge, of the SNAP group at The University of New South Wales, reported on an experiment undertaken with the assistance of the Meteorological Research Institute of Japan (using their earthquake "shaker truck") to determine the feasibility of using high-datarate GPS receivers as seismometers. A receiver capable of 20Hz real-time GPS solutions was used in the experiment. The results indicate that the velocity and acceleration signals were recovered (and compared with those measured directly by an accelerometer and a broadband seismometer) from the time series of GPS coordinates.

Shigeru Nakao, of the Earthquake Research Institute, Tokyo University, presented a poster showing results of two years of continuous monitoring of a region in the vicinity of Ito City (Japan), using a network of 11 single-frequency GPS receivers. In particular, the ground deformation associated with the earthquake swarm commencing 20 April 1998 was measured, demonstrating the feasibility of using dense, low-cost GPS receiver networks for studying local scale deformations.

Chris Rizos

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