A geopause satellite system concept |
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Authors: | Joseph W Siry |
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Institution: | (1) National Aeronautics and Space Administration, Goddard Space Flight Centre, 20771 Greenbelt, Md., USA |
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Abstract: | The forthcoming 10 cm range tracking accuracy capability holds much promise in connection with a number of Earth and ocean
dynamics investigations. These include a set of earthquake-related studies of fault motions and the Earth's tidal, polar and
rotational motions, as well as studies of the gravity field and the sea surface topography which should furnish basic information
about mass and heat flow in the oceans.
The state of the orbit analysis art is presently at about the 10 m level, or about two orders of magnitude away from the 10
cm range accuracy capability expected in the next couple of years or so. The realization of a 10 cm orbit analysis capability
awaits the solution of four kinds of problems, namely, those involving orbit determination and the lack of sufficient knowledge
of tracking system biases, the gravity field, and tracking station locations.
The Geopause satellite system concept offers promising approaches in connection with all of these areas. A typical Geopause
satellite orbit has a 14 hour period, a mean height of about 4.6 Earth radii, and is nearly circular, polar, and normal to
the ecliptic. At this height only a relatively few gravity terms have uncertainties corresponding to orbital perturbations
above the decimeter level. The orbit s, in this sense, at the geopotential boundary, i.e., the geopause. The few remaining
environmental quantities which may be significant can be determined by means of orbit analyses and accelerometers. The Geopause
satellite system also provides the tracking geometery and coverage needed for determining the orbit, the tracking system biases
and the station locations. Studies indicate that the Geopause satellite, tracked with a 2 cm ranging system from nine NASA
affiliated sites, can yield decimeter station location accuracies. Five or more fundamental stations well distributed in longitude
can view Geopause over the North Pole. This means not only that redundant data are available for determining tracking system
biases, but also that both components of the polar motion can be observed frequently. When tracking Geopause, the NASA sites
become a two-hemisphere configuration which is ideal for a number of Earth physics applications such as the observation of
the polar motion with a time resolution of a fraction of a day.
Geopause also provides the basic capability for satellite-to-satellite tracking of drag-free satellites for mapping the gravity
field and altimeter satellites for surveying the sea surface topography. Geopause tracking a coplanar, drag-free satellite
for two months to 0.03 mm per second accuracy can yield the geoid over the entire Earth to decimeter accuracy with 2.5° spatial
resolution. Two Geopause satellites tracking a coplanar altimeter satellite can then yield ocean surface heights above the
geoid with 7° spatial resolution every two weeks. These data will furnish basic boundary condition information about mass
and heat flows in the oceans which are important in shaping weather and climate. |
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