A Critical Analysis of Various Methods for Lunar Gravity Field Modeling

Paper number

IAC-07-A3.I.A.13

Author

Prof. Manoranjan Sinha, Indian Institute of Technology, India

Coauthor

Mr. N.S. Gopinath, Indian Space Research Organisation (ISRO), ISAC, India

Coauthor

Mr. N. K. Malik, Indian Space Research Organisation (ISRO), India

Year

2007

Abstract

The accurate knowledge of the gravity model is necessary for the long life of the satellite, which requires that the fuel spent onboard be minimum, which in turn depends on the number of maneuvers accomplished. An accurate knowledge of the gravitational model can reduce the maneuvers to be performed thereby saving the fuel. Secondly, the mapping of the surface using a camera requires altitude maintenance within certain limits in successive passes which can be achieved efficiently if the gravity model is known accurately. Study of the underlying density distribution and its geophysical implication requires a local gravity model which can be obtained on a global basis if the global gravity model is accurate enough. But there are few impediments in achieving the above objectives. These are (a) phase lock between moon’s spin and orbital motion, (b) presence of mascons beneath the center of all ringed maria, (c) accuracy of observational data limits the features to be extracted and (d) precision of the gravity model extracted depends on the Legendre’s coefficients in the gravity models equation which increases as the degree and order of the model is increased. 
This paper summarizes and provides a critical analysis of the historical developments of lunar gravitational models from the earliest use of ground based tracking systems of the Lunar Orbiter to the Lunar Prospector mission and how the above mentioned four problems have been addressed during course of development. It includes a comprehensive analysis of the various methods used in the estimation of the gravity coefficients and the processing of large batches of diverse measurements and data types. It has been shown that weakness exists in the current models of the lunar gravity field, which is primarily due to the lack of lunar tracking data information. The primary aim of this study is to assess today’s lunar gravity field models for the optimal estimation of the fuel required for the longer life of the satellite in the forthcoming lunar missions. Improvements to the current models are part of a continuing process and the latest model improvements and plans for the future systems are discussed. A numerical comparison has been presented showing the performance of all the contemporary lunar gravitational models used. Besides, all the processing techniques, till today, have been included. It is hoped that this critical review will benefit the researchers by presenting the state of the art in this field. 

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