International Network of Global Fiducial Stations Science and Implementation Issues (1991) / Chapter Skim
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3. Operational Considerations
3. Operational Considerations
Pages 63-88
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From page 63... ...
Mentioned in previous chapters are several global networks using different technologies that were deployed to meet different objectives. In this section, we shall delve into technologicalaspects in slightly greater detail, examine the geographical distribution of existing sites, and compare briefly the contributions of various techniques.
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From page 64... ...
64 ~ ~ i.- -- 1 -_ ca d O Cal ~7 P
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From page 65... ...
. · Earth orientation The technique is sensitive to the direction and magnitude of the Earth's angular velocity vector with respect to the reference frame defined by the adopted reference site coordinates (polar motion)
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From page 66... ...
. Planned for the Earth Observing System in the late 1990s is the Geoscience Laser Ranging System (GLRS)
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From page 68... ...
As illustrated by Tables 3 and 4, no single satellite target meets the complete range of geodetic needs from reference frame to gravity field, even though laser ranging measurements are sensitive at some level to all of them. Because gravity decreases with altitude, the lower-altitude satellites are more sensitive to field variations and have therefore made the greatest contributions to our knowledge of the gravity field.
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From page 69... ...
While it is theoretically possible to use one satellite to determine the gravity field, experience has shown that highly correlated coefficients and geographical distribution of stations will adversely influence such determinations. In summary, the demonstrated sensitivity of laser ranging techniques to geodetic parameters is as follows:
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From page 70... ...
· Inertial orientation Only high satellite ranging, especially LLR, is adequately sensitive to an inertial frame defined by the orbital characteristics of the Earth about the sun. There is no direct sensitivity to the quasar reference frame, only an indirect link through common terrestrial sites of laser ranging and VLBI.
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From page 71... ...
as soon as the system is declared fully operational (end of 1992~. Provided that sufficient dwell time is allowed at each site that is, long enough to average the dithering effects of SA—selective availability is not expected to impede significantly most geodetic applications, which involve site occupations of several hours.
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From page 73... ...
PRARE is self-contained, with two-way transmission of all relevant data between the satellite and the ground stations on the tracking loop. The space segment has sufficient memory to store tracking data and corrective data transmitted from the global network stations for transmission to a master ground station during overflights.
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From page 76... ...
Future Systems: GeoBeacon, Geoscience Laser Ranging Systems Among future space-geodetic systems that might contribute significantly to a global network of fiducial sites are the GeoBeacon concept and the Geoscience Laser Ranging System (GLRS) , mentioned earlier.
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From page 77... ...
normal points with subcentimeter accuracy. These range measurements combined with accurate GPS tracking of the EOS-B platform, are expected to yield station position estimates with a precision of a few millimeters within regional networks that are several hundred kilometers in extent.
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From page 78... ...
In the next two sections we discuss the organizational and physical implementation of an international global network of fiducial stations. Organizational Implementation The global nature of the geodetic network envisioned demands that an international entity be designated as the responsible governing body.
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From page 79... ...
The challenge facing us is to learn how to integrate these diverse stations into a single network that can meet the demands of all users, including those who may require great consistency and continuity across the network. Physical Implementation Users of global geodetic networks will fall into several categories,-ranging from those who require raw data to those whose interest is in derived products, such as GPS satellite orbits or Earth orientation information.
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From page 80... ...
At current levels of precision many can be approximated by either a linear change with time (plate motion, regional deformation, local site deformation) ; a quasi-periodic change with time (solid Earth tides, meteorological effects)
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From page 81... ...
For example, reference stations occupied frequently enough may provide some insight into desiccation cracking or expansive soil swelling in wet conditions. Some meteorological effects can be identified by their periodicity; temperature variations can produce diurnal and annual signals.
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From page 82... ...
Changes in the networks greatly complicate the solution of problems in that artificially induced changes must be separated from changes in the phenomena under study. Global networks have been and will be implemented to contribute to the solutions of specific scientific problems (and through them to the solutions of societal problems)
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From page 83... ...
A second important step is to maintain continuity in the position of the stations that make up the global network. It is essential that the relative position of an existing station and its replacement station be known to 1 mm in all three coordinates.
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From page 84... ...
For implementing change in global geodetic networks, especially the core network, the following recommendations are made: 1. Continue VLBI observations at key sites for the foreseeable future to provide the long vector precision required by many applications and to provide a reference system for comparison of newer techniques.
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From page 85... ...
to monitor regional or local crustal deformation or relative plate motions and (2) to supplement the core network, providing redundancy and therefore enhanced orbit control by serving as additional fiducial stations.
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From page 86... ...
Both regional and global ephemerides will need to be produced, with the former being available within a few days and the latter within a few weeks or less. In addition, global ephemerides based on the GPS core network could be computed within a few days with the goal of providing Earth orientation data to the IERS.
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From page 87... ...
For other applications, however, Earth rotation parameters obtained from IERS may be used in calculating GPS ephemerides. · Edited Tracing Data: The edited tracking data from the fiducial sites used in the ephemeris estimation process can be made available along with appropriate quality indicators.
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From page 88... ...
The establishment of archive centers with catalogs describing available data and their location is an essential element of the global network. Use of electronic networks should permit a user to browse the holdings of an archive, as well as determine the data holdings for raw and higherlevel data products stored at any other archive worldwide.
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