The Earth's gravity field provides a record of the mass distribution within the system and can be used to understand the evolution and dynamics needed to maintain that distribution. For the fluid portions of the Earth, gravity measurements can be used to sense directly the motions of mass within the system. The inversion of the gravity signals to obtain the mass distribution and the dynamics that cause them is not a straightforward problem, but through a combination of spatial and temporal analyses, knowledge of the gravity field and its temporal variations can provide insights into the processes that control these dynamics. The reliability of these inferences depends on the accuracy, spatial resolution, temporal resolution, and duration of the gravity measurements. Within this report, we have summarized the expected performance for a series of generic, space-spaced, gravity-measurement missions and the expected signals that are likely to exist in the Earth's gravity field.
The generic missions fall into two basic classes: spaceborne gravity gradiometry (SGG) and satellite-to-satellite tracking (SST and SSI). The SGG class missions accentuate the high-spatial-frequency variations in the Earth's gravity field by making measurements of gradients in gravitational acceleration over very short distances (typically a few centimeters). To achieve the accuracy needed, current gradiometer technology requires cooling to very low temperatures to minimize thermal noise, so an SGG satellite will need to be equipped with a helium dewar. The loss of helium from the dewar limits the duration of this class of missions to less than one year, although improved miniaturization and cryogenic techniques may permit extended missions in the future (SGGE). The SST and SSI missions, which measure the differential accelerations between satellites separated by several hundred kilometers, are more sensitive to longer-wavelength features in the gravity field and do not need to be super-cooled. To meet accuracy requirements at shorter wavelength, both satellites need to be flown at a low altitude. A feature common to all these systems is that tracking Global Positioning Satellites (GPS) from the orbiting vehicle is feasible and inexpensive. The technology exists currently to fly the SGG and SST missions. Future technology using laser interferometry offers the potential for the development of the SSI mission, with its order-of-magnitude improvement in performance over the microwave differential tracking of the SST. In the remainder of this chapter, however, we will limit our attention to the two currently available technologies.
Both the SGG and SST missions would significantly improve our knowledge of the Earth's gravity for wavelengths longer than 200-300 km, depending on the altitude of the mission; the lowest-altitude missions (300 km) provide the shortest-wavelength resolution. Almost independent of altitude (within the 300500 km range), the SGG mission provides better results than SST for wavelengths shorter than 300 km. For long-wavelength signals (2,000 km), SST is expected to improve our knowledge of the gravity field by approximately 3 orders of magnitude, whereas the
SGG mission provides improvement by a factor of about 30 over current knowledge.
A long-duration mission (5 years) would enable temporal variations in the Earth's gravity field to be measured during a single mission. For example, at wavelengths of 2,000 km, the SST mission could provide measurements of changes in the gravity field of 10-4-10-5 mGal every 90 days. Successive snap shots of the Earth's gravity field could also be measured with a series of SGG class missions.
The gravity missions provide a means of studying a series of geophysical problems ranging from deep Earth structure to tracking mass redistribution on and near the surface of the Earth. Fields of study that would be significantly advanced by a dedicated gravity mission include oceanic dynamic topography and seafloor pressure variations, sea-level rise (separation of steric and nonsteric contributions), ice-sheet mass change (glacial waxing and waning of Antarctica and Greenland), continental water storage, and post-glacial rebound. Knowledge of the static portion of the field also provides a reference frame that can be used to provide absolute dynamic topography of the oceans and to re-calibrate terrestrial, marine, and airborne gravity surveys, thus providing higher spatial resolution in selected locations than obtainable purely from space-based gravity measurements. With any of the generic missions, it is expected that oceanographic currents on basin scales (>1,000 km) and on the scale of the Antarctic Circumpolar Current (500-1,000 km) can be resolved accurately, whereas the measurement of processes on the scale of the western boundary currents (50-100 km) will not be significantly improved. However, knowledge of the latter could be improved by accurate registration of ship gravity data in these regions.
Tracking mass redistribution on or near the surface of the Earth should be possible with the SST class missions. The sensitivity of the measurements should be such that changes in mass equivalent to 10 mm of water over an area of 500,000 km2 could be measured monthly. Annual signals of this size could be measured in areas about half this size. Some specific applications of these measurements would be: monitoring the secular change in the High Plains aquifer, global monitoring of soil moisture, which could provide a world wide inventory of the yield potential of agricultural lands; and studying evapotranspiration on large spatial scales. These measurements would also provide a means for monitoring the total mass of the world's continental ice sheets and the effects of their changes on global sea level. In a 5-year mission, SST should be able to resolve whether the masses of the Greenland and Antarctic ice sheets are increasing or decreasing—even that basic fact is not currently known. In general, SST will have sufficient spatial resolution to isolate the regions where change is occurring, but the exact process causing the change will not be identifiable from gravity alone. Ancillary data and synthesis of the gravity data into models of processes will be needed to exploit fully the information contained in the gravity field.
Satellite gravity measurements can provide unprecedented views of the Earth's gravity field and, given sufficient duration, its changes with time. Not only can they provide a truly global integrated view of the Earth, they have, at the same time, sufficient spatial resolution to aid in the study of individual regions of the Earth. In many cases, the gravity data will be exploited best by assimilating them, together with other geophysical data, into geophysical models of known processes. Examples of processes that require ancillary data to better interpret the gravity data are structure and evolution of the mantle and plumes, structure and evolution of the crust and lithosphere (including regional deformation of the seafloor and continents, structure of passive margins, and the continent-ocean transition), and ice-sheet mass balance.