Calibration/validation methods for GRACE

Abstract

The upcoming GRACE (Gravity Recovery and Climate Experiment) satellite mission is expected to map Earth's global gravitational field with unprecedented accuracy, especially at the long wavelengths. The measurements are line-of-sight range rates between two co-orbiting, low-altitude satellites that can be transformed into differences of geopotential and horizontal gravitation. One method to attempt the calibration and validation of GRACE data is to compare these in situ measurements using very accurate local terrestrial gravity fields that are upward continued to satellite altitude. This paper presents a rudimentary analysis of the requirements in gravity anomaly data spacing, accuracy, and extent to compute differences in disturbing potential and horizontal gravity disturbance at GRACE altitude. These studies show that to achieve the anticipated in situ accuracy of 0.01 m(2)/s(2) in potential and 0.01 mgal in gravitation the terrestrial spacing and accuracy requirements are rather loose, e.g., 0.5 degrees spacing and 0.25 mgal accuracy. Data extent, on the other hand, is much more important, where caps with radius larger than 10 degrees are required, and then only to validate wavelengths smaller than 1200 km (30-deg reference field). Furthermore, it is shown that terrestrial data requirements are significantly less stringent in the case of computing horizontal gravitation differences at altitude rather than potential differences.