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