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GPS1. Maintenance of Global Geodetic Networks, Terrestrial Reference Frame, and Earth Orientation Parameters

The accuracy of global geodetic networks advances by about a factor 10 per decade, with submillimeter-scale reference-frame accuracy likely in the near future. Continued improvements in accuracy are critical to a number of the recommendations of this report, from the study of sea-level change and improved gravity-field measurements to the detection and characterization of land surface change. The Internet and high-speed computing have recently been harnessed to provide near-real-time global GPS positioning and time transfer, of significant potential benefit to onboard satellite and airborne data reduction and natural hazards disaster management.

2. Precise Orbit Determination (POD)

As is well-recognized by its international partners, NASA's Solid Earth Science Program supports the development, maintenance, and continuing refinement and enhancement of computer software for modeling and computing satellite orbits. This precise orbit
determination (POD) capability is essential for the global space geodesy enterprise. GPS observations have documented that deformation across the Cacadia subduction zone in the Pacific northwest is governed by two processes acting on very different time scales.

3 . Coordination, Validation, and Calibration

While space-based measurements are inherently global in coverage, in many instances they are limited in their temporal or spatial sensitivity. For example, the spatial resolution of quantities derived from potential field measurements or their gradients will always be limited by the distance of a satellite from Earth's surface. In many instances, gaps in spatial and temporal coverage can be filled by utilizing
other measurement platforms. Fine spatial resolution is often best achieved with land or seafloor arrays installed either semi-permanently or on a campaign basis.

In the past, these supporting measurements have often been neglected or eliminated to accommodate budget shortfalls. Lack of such supporting data, however, reduces confidence in space-based measurements and in some instances can lead to incorrect inferences due to inherent measurement inaccuracy. Greater attention to validation and calibration is urged for all future mission planning.

Integrated Program for Solid Earth Science: Supporting Frameworks, Observational Strategies, Education, Information Systems, Research & Analysis, Technology Development  
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