meter-level real-time positioning. These customers are involved, for example, in the offshore oil industry, precision agriculture, and certain marine applications, which require high reliability and global availability. Operators of earth-orbiting imaging satellites require rapid and precise geolocation of their images in order to provide rapid service to their customers. The global nature of many of these applications requires the products to be accurate in a well-defined and stable terrestrial reference frame.
The global geodetic infrastructure also contributes to improvements in the Global Positioning System (GPS). For example, geodetic research has led directly to the addition of a third GPS frequency and to the laser retroreflectors that may be added to future GPS satellites. In addition, the NASA Global Differential GPS (GDGPS) System2 uses the global GPS network to perform integrity monitoring and situational assessment of GPS in real time for the U.S. Department of Defense (NRC, 1995b). The GDGPS is also the basis for the real-time orbit improvement for the Advanced Control Segment, an Air Force-sponsored project that will improve the accuracy of GPS.
Satellites now provide a range of crucial services, including weather forecasts, communications, and land-use monitoring. By simply including a GNSS/GPS receiver on any satellite, it is possible to determine where that satellite is in its orbit. When the highest accuracy is required, it is necessary to supplement GNSS/GPS data with information from the global geodetic infrastructure, including the International GNSS Service network and the International Terrestrial Reference Frame (ITRF). In addition, models of the Earth’s gravity field based on geodetic observations, as well as geodetic observations on the location of the Earth’s rotation axis and rotation rate, are needed to determine the gravitational forces on the satellite (see Chapter 3). The existing geodetic infrastructure makes it possible to accurately position satellites for a wide range of applications; this capability is crucial to many of the proposed “Decadal Survey” missions, especially radar and laser altimetry missions (for example, SWOT, LIST, and ICESat-II), radar imaging missions (for example, DESDynI), and gravimetry missions (for example, GRACE-II) (NRC, 2007a).
In addition to applications focused on the Earth, geodesy has played and will continue to play an important role in the exploration of the solar system and regions beyond. Systems that prove successful on the Earth can be applied to other planetary bodies. For example, the GRAIL (Gravity Recovery and Interior Laboratory) project uses an approach for determining the moon’s gravity field that was pioneered by the GRACE (Gravity Recovery and Climate Change) project focused on the Earth.
Until we actually dig into the Earth or another planet, we must rely on information derived from surface observations, such as seismic and geodetic measurements, to learn about the interior structure. Zumberge et al. (2009) provide the example of Mars, which has had the precession of its rotation axis measured, and its gravity field and terrain mapped, using geodetic techniques. These observations have led to estimates of the size, mass, and physical state of the Martian core and to inferences about the seasonal variability of mass in the Martian polar icecaps.
In addition, the geodetic infrastructure is needed to track the location of spacecraft from Earth. As spacecraft get farther and farther away, the demand on the angular resolution of the tracking
NASA Global Differential GPS (GDGPS) System Website: http://www.gdgps.net