Direct Recovery of Mean Gravity Anomalies From Satellite to Satellite Tracking
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Date
1974-12
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Ohio State University. Division of Geodetic Science
Abstract
This study investigates the direct recovery of mean gravity anomalies from summed range rate (Ŕs) observations, the signal path being ground station to a geosynchronous relay satellite to a close satellite significantly perturbed by the short wave features of the earth's gravitational field. To ensure realistic observations, these were simulated with the nominal orbital elements for the relay satellite corresponding to ATS-6, and for two different close satellites one at about 250 km height, and the other at about 900 km height corresponding to the nominal values for Geos-C. The earth's gravitational field was represented by a reference set of potential coefficients up to degree and order 12, considered as known values, and by residual gravity anomalies obtained by subtracting the anomalies, implied by the potential coefficients, from their terrestrial estimates. The Geodyn orbit generation and parameter estimation program was used after modifying it to accept gravity anomalies as parameters. The standard deviation (std. devn.) of Ŕs observations was assumed as 0.08 cm/sec. based on an integration interval of 10 seconds. The recovery of mean gravity anomalies over 10° and 5° equal area blocks from Ŕs observations to close satellites at heights of about 900 and 250 km respectively were classified as recovery from strong signal. The recovery of 5° and 2°.5 equal area mean anomalies using the same close satellites were classified as recovery from weak signal. The anomaly recovery was considered over local or regional areas. The satellite state vectors could not be recovered from short individual arcs of 4 to 20 minutes duration, and were held fixed in this study to a-priori known values. It was found that gravity anomalies could be recovered from strong signal without using any a-priori terrestrial information, i.e. considering their initial values as zero and also assigning them a zero weight matrix. However, while recovering them from weak signal, it was necessary to use the a-priori estimate of the std. devn. of the anomalies to form their a-priori diagonal weight matrix. Without this a-priori information, the solutions from weak signal were unstable and not meaningful. The optimum density of observations was achieved by considering ascending and descending satellite arcs with spacing between adjacent arcs roughly half the size of anomaly block being recovered. If the observations along an arc were more closely spaced than the spacing between adjacent arcs, the std. devn. of the recovered anomalies was required to be multiplied by a scaling factor. The density of observations should be nearly uniform over the area. If the orbital inclination of the close satellite is about 45°, the area of investigation needs to be located in the form of a rhombus with its diagonals in the east-west and the north-south directions. A latitudinal extent up to 40° is advisable, and the area should lie either in the mid-latitudes or the equatorial region. [Some mathematical expressions are not fully represented in the metadata. Full text of abstract available in document.]
Description
Prepared for National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, Maryland: Grant No. NGR 36-008-161, OSURF Project No. 3210