Geopotential Research Mission Research Articles

Global gravity survey by an orbiting gravity gradiometer

Three dedicated space missions proposed for the 1990s promise to provide data for recovering the Earth's gravity anomaly with unprecedented accuracy and resolution: the Geopotential Research Mission (GRM), the Aristoteles Mission, and the Superconducting Gravity Gradiometer Mission (SGGM). SGGM, the most ambitious of the three, aims at recovering the global gravity field to a precision of 2–3 mGal with a resolution of 50 km. Such accuracy and resolution are required to answer many key questions in geophysics. The SGGM instrument package is a three‐axis superconducting gravity gradiometer which is integrated with a six‐axis superconducting accelerometer and a six‐axis shaker for active platform control. The intrinsic sensitivity of the gradiometer is 10−4 E Hz−½ (where E represents the unit of gravity gradient (1 E = 1 Eötvös = 10−9 s−2)) and that of the accelerometer is 10−13 gE Hz −½ in linear acceleration and 10−11 rad s−2 Hz−½ in angular acceleration. While precise attitude control of the Experiment Module is essential to mission success and is also technically the most challenging, pointing accuracy and disturbance isolation requirements of SGGM are less stringent compared to that of other missions such as the Hubble Space Telescope (HST) and Gravity Probe‐B (GP‐B). Thus they are within the reach of technologies of the 1990s. In the recently completed Phase A study, the SGGM Study Team addressed the problem of scientific requirements and mission feasibility. Marshall Space Flight Center is continuing with mission and spacecraft design, while mission simulation is being done at Goddard Space Flight Center, Greenbelt, Md. At the University of Maryland, prototypes of the three‐axis gradiometer and the six‐axis accelerometer are being fabricated, improved, and tested. A Shuttle flight test of the Experiment Module with the actual instrument package is suggested for 1994–1995. The full science mission hopefully will take place before the year 2000.

Geopotential orbit variations: Applications to error analysis

The position and velocity errors of a near‐circular orbit due to geopotential uncertainty are developed in Fourier series. Current satellite gravity models generate position uncertainties of up to 50 m alongtrack for close satellites (∼1000 km altitude) in shallow resonance (periods less than 5 days, from coefficients of order 13–14). Smaller radial errors are due to resonance and to first‐order terms poorly seen in current satellite geopotentials. Correlation of errors is strong by order and the same degree parity. Application of the development is made to (1) the prediction of errors for a future altimeter satellite, (2) the capability of earth‐based tracking of a close Mars orbiter in resolving that planet's gravity potential, and (3) improvement of the resolution of the low‐degree geopotential from the Geopotential Research Mission using tracking data from the Global Positioning System.

Improved gravitational recovery from a Geopotential Research Mission Satellite pair flying en echelon

In the standard mission the low‐low Geopotential Research Mission (GRM) pair fly close coplanar orbits in tandem at nearly constant distance and with a fixed mutual orientation for extended periods of time. Sensitivity to the gravity field is predominantly along‐track at a single altitude. Increased geopotential accuracy and discrimination can be obtained in other configurations for which radial and cross‐track information are also sensed strongly. One such configuration which is only a small change from the standard has the pair flying en echelon in slightly different orbit planes. The addition of cross‐track information in this approach is shown to yield considerable improvement in the geopotential, especially in near‐sectorial terms. Overall field accuracy is estimated to almost double with plane separations of only a few tenths of a degree (permitting fixed antenna operation). Many near‐sectorial terms show gains of up to an order of magnitude over the equivalent standard mission. These benefits are particularly important for low‐degree terms (l < 6) expected to show significant changes in the GRM lifetime due to past and present glacial activity.

Simulation of a Geopotential Research Mission for gravity studies

Simulation of a Geopotential Research Mission for gravity studies

Time variations in the Earth's gravity field detectable with geopotential research mission intersatellite tracking

The Geopotential Research Mission (GRM) is a proposed system of two artificial satellites in tandem, drag‐free polar orbits of low altitude (∼160 km) designed for a detailed global mapping of the earth's gravitational and magnetic fields. The mission should provide a large amount of new information about the gravity field, particularly over continents, resolving spatial variations in the gravity field as short as 100 km (half wavelength). However, GRM should also perform a much more difficult task, the detection of time variations in the gravity field. Among the effects which produce changes in the field that are large enough to be detected by the intersatellite millimeter wavelength (∼ 1 μ/s precision) GRM sensor are the following: (1) rebound from ancient deglaciation, (2) very large dip‐slip earthquakes, (3) detailed ocean tides, and (4) the seasonal growth and decline of continental glacial fields. In particular, many nonzonal as well as zonal components affected by postglacial rebound should be detected in a 6‐month GRM mission. Repeat passes over the fault zones of earthquakes similar to Alaska (1964), Chile (1960), or Sumbawa (1977) should reveal signal changes due to their earth movements. The effect of ocean tides on the GRM signal should be readily detectable over a broad band of frequencies even if all but 10% of the tides are correctly modeled. Continued geophysical interest in these now largely unobserved phenomena warrants extension of the GRM mission to somewhat higher altitudes.

Satellite techniques for determining the geopotential of sea surface elevations

Spaceborne altimetry with measurement accuracies of a few centimeters has the potential for determining sea surface elevations necessary to compute three‐dimensional geostrophic currents from traditional hydrographic observations. The existing limitation in this approach is the uncertainty in our knowledge of the global and ocean geopotentials that produce satellite and geoid height uncertainties about an order of magnitude larger than the goal of about 10 cm. This paper begins with a description of the quantitative effects of geopotential uncertainties on processing altimetry data. This is followed by a review of existing models, which are shown to be inadequate. Potential near‐term improvements not requiring additional spacecraft are discussed. However, even though there would be substantial improvements at the longer wavelength, the oceanographic goal would not be achieved. The potential NASA Geopotential Research Mission (GRM) is described. This mission should produce geopotential models that are capable of defining the ocean geoid to 10 cm and near‐earth satellite positions significantly better. For completeness the state of the art and the potential of Spaceborne gravity gravimetry is described as an alternative approach to improve our knowledge of the geopotential.

Comparison of MAGSAT and low-level aeromagnetic data over the Canadian Shield: implications for GRM

Band-limited magnetic anomalies over the Canadian Shield, for wavelengths of ~500–2200 km, derived from the magnetometer satellite (MAGSAT) data correlate peak for peak with the corresponding anomalies derived from the aeromagnetic data. This suggests that the anomalies detected by MAGSAT are physically meaningful. A new potential field satellite, Geopotential Research Mission (GRM), has been proposed to fly at about 160 km altitude. The magnetic anomaly map that can be derived from the GRM data is expected to have a resolution about an order of magnitude higher than the one derived from MAGSAT data. A magnetic anomaly map of the Canadian Shield, which is expected to derive from the GRM data, is shown based on the aeromagnetic data. The map delineates many major geological features quite clearly and illustrates the importance of the GRM mission for global geological mapping.

Satellite-to-Satellite Range-Rate Measurement

The measurement of range rate between two low-orbiting spacecraft with a precision of better than 1 μm/s provides a means for sensing global gravity variability. An analysis is given to show the conditions under which this precision can be realized, with emphasis on the requirements of NASA's Geopotential Research Mission. Experimental results obtained with a millimeter-wave demonstration system are presented along with a proposed design for a spaceborne instrument.

Satellite Geodynamics

Satellite geodynamics concerns the use of space technology in the study of the dynamic processes and movements of the solid Earth. The study involves measurements of the Earth's gravity and magnetic fields, the distortions and relative motions of its tectonic plates and the orientation of, and rate of rotation about, its axis. These studies address fundamental questions of geophysics such as the causes of Earth movements and earthquakes, the nature of convection in the mantle, and the origin of the magnetic field. The use of space technology has helped advance our concepts and models in solid-Earth geophysics. Shortly after the dawn of the space age, tracking data were used to precisely define the pear-shaped Earth. Since that time, other data have given several up-to-date representations of the secularly changing magnetic field and global models of the gravity field. Using this technology, it is now possible to determine the contemporary rate of relative plate motion at the level of about 1 cm a year. There are exciting prospects for future missions in satellite geodynamics. The Geopotential Research Mission is designed to improve gravity and magnetic field models and new, highly portable geodetic systems based on the Global Positioning System and spaceborne laser systems should permit rapid determination of positions with 1-cm precision.

Geopotential Research Mission: Status Report

The Geopotential Research Mission (GRM) is a planned NASA mission that promises to produce significant improvements in our knowledge and understanding of the Earth's gravity and geomagnetic fields. This paper gives a description of this mission and discusses some of the engineering investigations that are being conducted to make improvements in the current spacecraft configuration.

Gravity Gradiometry from the Tethered Satellite System

Measurement of the gradient of the gravitational acceleration from a satellite platform is likely to provide the next improvement in knowledge of the Earth's gravity field after the upcoming Geopotential Research Mission (GRM). Observations from the subsatellite of a Tethered Satellite System (TSS) would increase sensitivity and resolution due to the low altitude possible. However, the TSS is a dynamically noisy system and would be perturbed by atmospheric drag fluctuations. The dynamic noise is being modeled in order to evaluate the feasibility of TSS gradiometry and to design methods of abating the error caused by this noise. The demonstration flights of the TSS will provide an opportunity to directly observe the dynamical environment and refine modeling techniques.

Gravitational and magnetic field variations

The GRM Science Conference on regional and global variations in the gravitational and magnetic fields of the earth was held October 29‐ 31 , 1984 at the University of Maryland, College Park, under the sponsorship of the National Aeronautics and Space Administration (NASA) and AGU. The title of the conference (“GRM”) is derived from the Geopotential Research Mission, a satellite system proposed to determine variations in the gravitational and magnetic fields to a resolution of about 100 km.

GRM: Observing the terrestrial gravity and magnetic fields in the 1990's

Beginning with the earliest days of space exploration, satellites have been used to chart the gravity and magnetic fields of the earth. As a continuation of these studies NASA is proposing to launch a new geopotential fields exploration system called the Geopotential Research Mission (GRM). Two spacecraft will be placed in a circular polar orbit at 160 km altitude. Distances between these satellites will vary from 100 to 600 km. Both scalar and vector magnetic fields will be measured by magnetometers mounted on a boom positioned in the forward direction on the lead satellite. Gravity data will be computed from the measured change in distance between the two spacecraft. This quantity, called the range‐rate, will be determined from the varying frequency (Doppler shift) between transmitter and receiver on each satellite. Expected accuracies (at the one sigma level) are: gravity field, 1×10−5 m s−2 (1 milliGal). 5 cm geoid height; magnetics, scalar field 2 nT, vector to 20 arc seconds (96 microradians), both resolved to less than 100 km. With these more accurate and higher resolution data we will be able to investigate the earth's structure from the crust (with the shorter wavelength gravity and magnetic anomalies) through the mantle (from the intermediate wavelength gravity field) and into the core (using the longer wavelength gravity and magnetic fields).