Applications of spaceborne GPS to Earth science

With the recent completion of the Global Positioning System constellation and the appearance of increasingly affordable spaceborne receivers, GPS is moving rapidly into the world of space flight projects. Indeed, owing to the great utility and convenience of autonomous onboard positioning, timing, and attitude determination, basic navigation receivers are coming to be seen as almost indispensable to future low earth missions. This development has been expected and awaited since the earliest days of GPS. Perhaps more surprising has been the emergence of direct spaceborne GPS science and the blossoming of new science applications for high performance geodetic space receivers.

Applications of spaceborne GPS to Earth science include centimeter-level precise orbit determination (POD) to support ocean altimetry; Earth gravity model improvement and other enhancements to GPS global geodesy; high resolution 2D and 3D ionospheric imaging; and atmospheric limb sounding (radio occultation) to recover precise profiles of atmospheric density, pressure, temperature, and water vapor distribution. Figure 1 offers a simplified summary of the Earth science now emerging from spaceborne GPS.

Conventional single- and dual-frequency GPS tracking device have been flown in space for basic navigation and (increasingly) attitude determination on a number of recent missions. Consistent with these different uses, there has developed in recent years a two-tiered user community for GPS in space: those seeking basic, moderate-performance GPS navigation, timing, and (in some cases) attitude determination, and those pursuing the more demanding science activities requiring the highest performance dual-frequency receivers. As the mission-dependent requirements within each group are diverse, a variety of receiver models for space use has emerged.· While that healthy situation is likely to continue, from the standpoint of the scientists it may be hoped that in the future the high end instruments will reach levels of size, cost, and generality of function that will allow them to serve both user classes economically, thus converting the most utilitarian satellites into potentially powerful science instruments. As GLONASS becomes established as a reliable navigation system we can expect to see considerably more commercial resources devoted to developing the technology for both the ground and space. A high performance spaceborne GPS/GLONASS receiver for navigation and science applications is currently under development by the European Space Agency and may fly within two years.

The utilitarian spaceborne GPS applications represent, in essence, a fulfillment of the GPS vision. They exploit GPS( tracking device), sometimes in clever ways, for purposes for which it was expressly intended. For the growing class of high-precision spaceborne science users surveyed here, the same cannot be said. GPS was not conceived with such uses in mind (indeed, their feasibility was generally recognized only after GPS deployment was well underway), and has not been altered in any way to accommodate them. Within these diverse scientific enterprises we find many examples in which GPS innovators have, through ingenuity and industry, coaxed a reluctant system to perform unexpected feats, thereby expanding the GPS mission. In the face of the seriously confounding security features known as selective availability and anti-spoofing, they have extracted from GPS levels of performance undreamed of by its architects.

More GPS Tracking Solution at http://www.jimilab.com/ .