Geodesy is the study and interpretation of the external shape of the Earth.
Knowledge of the superficial mechanisms also yields information about the internal
construction of the planet.
Geodetic techniques can be ground- or space-based. The latter include Global Positioning Systems (GPS), very Long Baseline Interferometry (VLBI), and Satellite Laser Ranging (SLR).
The Global Position System is a U.S. Department of Defence (DoD) developed satellite-based radio-navigation system.
It operates through a constellation of 24 operational satellites, orbiting
at 20200 km, which are used for navigation and precise geodetic position measurements.
Full Operational Capability (FOC) was met as of April 27, 1995.
A user can determine his instantaneous position at any time and anywhere on
the Earth, with an accuracy of 5 to 150 m depending on the sophistication of
the receiver. Still many geodetic applications, in particular for geodynamics,
require millimetre level of accuracies, which is beyond the capability of the
system. However by using the GPS signals in a differential mode, the accuracy
of the relative position determinations can be increased substantially.
This effect is bolstered by the addition of measurements of the phase of the carrier signals.
Because of the very short wavelength of these signals (~ 20 cm), the measurement precision can be refined to the millimetre level.
New developments in the data analysis technique, and improvement in the receivers has pushed the accuracy level of a few parts be billion and at the same time increased the capabilities to regional and even global networks.
The International GPS Service for Geodynamics (IGS) was established in 1989
with the purpose of providing common standards for acquisition of GPS observations
and the subsequent data analysis, and to generate precise ephemerides for the
GPS satellites together with other products, such as earth orientation parameters
and GPS clock information.
The benefits consist primarily of a tremendous improvement in the accuracy of the GPS ephemerides and the establishment of a rigorous global reference frame.
This universal operating frame has brought about the set-up of a number of international programs, such as the WEGENER-I and -II projects, a wide-spaced network of reference stations which are embedded in the global reference frame provided by IGS.
There has been incredible growth and expansion of GPS applications including the experiment networks for crustal deformation and tectonic motion; these are observed episodically and recur every one to two years.
Continuous operating dense arrays of receivers are also used to monitor motions within a smaller active region; indeed field measurements, such as of the already rather well surveyed area along the San Andreas fault, have yielded accurate and repeatable horizontal co-ordinates to ±4-6 mm and elevations to ±1-2 cm, over baselines up to 11 km long. Moreover uncertainties do not grow proportionally over longer distances. Indeed over baselines up to 225 km long, GPS can provide distances accurate to one part in 20 million.
Plate movements dynamics have also benefited by the Rapid Static Surveys (RSS). These observe many points along a profile for very dense sampling of precise surface positions. Another deployment of the GPS receivers is the measure of co-seismic displacements and post-seismic relaxation.
Seafloor geodesy, which aims to eventually measure ocean spreading centre rates, is indirectly helped by GPS too. The network is used to position sea-surface buoy instruments which concurrently measure precise distances to ocean bottom transponders.
The precision and accuracy of the GPS technique is at the root of the several
obstacles that block an equal increase in scientific results. Stability and
reliability of the benchmarks is one. Another obstacle is that geodetic-quality
GPS measurements require extensive and time-consuming analysis before the results
are in hand.
The major obstacle to widespread use of GPS geodesy is military security since
the U.S. DoD in 1990 has considered it necessary to intentionally degrade the
GPS signal (by A-S and Selective Availability ). Fortunately in 1998 the White
House announced that the military will end this practice within 4 to 10 years.
This will signify the end of expensive corrective techniques and equipment
to compensate for the fuzzy signal.
The space geodetic called Very Long Baseline Interferometry was developed
25 years ago by NASA as a radio astronomical tool for high-resolution mapping
of the structure of distant quasars.
VLBI is a geometric technique: it measures the time difference between the arrival at two Earth-based antennas of a radio wavefront emitted by a distant quasar.
Each receiving radio telescopes records this information, calibrated by atomic clocks. Using large numbers of time difference measurements from many quasars observed with a global network of antennas, VLBI determines the inertial reference frame defined by the quasar and simultaneously the precise position of the antennas. Because the time difference measurements are precise to a few pico-seconds, once the atomic clocks are matched and the records of two or more sites are compared, VLBI determines the relative positions of the antennas to a few millimetres in all three dimensions and the quasar positions to fractions of a milli-arc-second .
Since the antennas are fixed to Earth, their locations track the instantaneous orientation of the Earth in the inertial reference frame. Relative changes in the antenna locations from a series of measurements indicate tectonic plate motion, regional deformation, and local uplift or subsidence.
VLBI and satellite laser ranging were the two high accuracy geodetic techniques
adopted at the inception of NASA’s Crustal Dynamics Project (CDP) in
1979. Collocated measurements demonstrated that VLBI and SLR site position
measurements agreed to less than 1 cm in the horizontal and 2 cm in the vertical.
Horizontal velocities agreed at 2 mm/year.
The projects made the first contemporary measurements of the relative motions of the Earth’s tectonic plates. Other results enabled space geodetic measurements to distinguish between different plate motion models. The CDP demonstrated the internal stability of the continental and oceanic plates by measurements of motions less than 2 mm/year for baselines on the North America, Pacific and European Plates. Contemporary plate motions were first measured by the CDP.
From 1984 and 1990, the CDP occupied six Alaskan sites with mobile VLBI systems to measure deformation related to the Pacific-North American plate boundary.
Today NASA’s Space Geodesy Program (SGP) builds upon the 12-year legacy
of the CDP.
Geophysicists are pursuing research investigations sponsored by NASA’s Dynamics of the Solid Earth program using space geodetic measurements acquired by the VLBI, SLR, and the GPS techniques.
VLBI measures three-dimensional point positions in one-day sessions, and such measurements over time yield estimates of velocities. The positions and velocities form the terrestrial reference frame. Between 1979, when the Mark III system was first deployed, and early 1995, the VLBI database of observations grew to include over 1.5 million data points. This historical record provides a long-term database of well-calibrated, stable and consistently measured points to study phenomena that may not be detectable or even recognised over a shorter period.
VLBI's precision for a one-day session is as good as 1 mm in the horizontal and 3 mm in the vertical. At present, point positions are measured at least twice annually for 40 globally distributed VLBI antennas. The historical data base contains positions for 123 sites, with the smallest errors approximately 1 mm in horizontal, and 1-2 mm in vertical. There are VLBI velocity estimates for about 60 sites, and the best precision is better than 1 mm/year. Vertical rates have uncertainties typically two to three times larger.
The principal disadvantage of the VLBI technique is that it requires a fully
functional radio observatory, equipment that is neither easily portable nor
available to the average geodesist. However, monitoring of existing VLBI stations
provides a highly accurate, coarsely spaced framework for the shape and the
rates of deformation of the surface of the Earth.
There are ongoing detailed comparisons of the terrestrial reference frame as determined by GPS and by VLBI to validate the capabilities and accuracy of the relatively new GPS technique. While both GPS and VLBI use radio frequencies, some of their error sources are very different. Examining differences in results will lead to isolation of the true geophysical signals.
A program is underway to determine the optimum mix of VLBI and of GPS.
VLBI contributes its long history of measurements, several times better accuracy from a single day's measurement, stable terrestrial reference frame, and ability to measure the celestial reference frame, fundamental for all satellite tracking.
GPS contributes a lower operational cost, dense networks for deformation zone monitoring, flexibility to go where measurements are needed, and continuous temporal coverage.
The analysis complexities and immense data handling requirements that are necessary to integrate regional GPS into the global terrestrial reference frame on a regular basis is a future challenge for both VLBI and GPS.
Although advanced technologies promise easier data acquisition for VLBI by allowing small antennas to perform as well as the larger ones do today , still probably the best balance of accuracy, cost, and mobility is GPS, which may well become the standard for many geodetic and other applications in the near future.
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Various web sites: i.e. Goddard Space Flight Centre, NASA etc