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).