CHAPTER ONE
INTRODUCTION
Recent Earth Geopotential models, EGM 2008 (Earth Gravity Models) GRACE (Gravity Recovery and climate Experiment), and GOCE (Gravity field and Steady State Ocean Circulation Explorer) geoid models represent a great improvement to several applications related to gravity field modeling. These models offer higher accuracy and better resolution of gravity field. In this research these models are analyzed and compared over some points in Nigeria, aiming to know which one suits better in the country.
Planet Earth represents a complex dynamic system which may be described by geometrical and physical properties. Nowadays, geometrical positions can be determined by Global Navigation Satellite Systems (GNSS) with accuracy of some few millimeters. Such a level of accuracy is satisfactory for many industrial, commercial or even scientific purposes. On the other hand, physical properties of the Earth revealing important natural phenomena must not be neglected. One of the fundamental physical quantities is the Earth’s gravity field. Especially functions of gravity potential in the form of a geoid and gravity anomalies are of particular interest.
According to the classical definition, the geoid represents an equipotential surface (almost) coinciding with the mean ocean surface, and which is also extended beneath the topographic masses on continents. In geodesy, precise knowledge of the geoid allows determination of Orthometric heights by GNSS or realizing a global height reference system. In oceanography the geoid is of fundamental importance, because the deviations of the mean ocean surface from the geoid are a measure for ocean currents. In addition the regional and global structure of the gravity field can be used for exploring the mass distribution in the Earth’s crust and mantle, while temporal changes of the field give an indication of mass transport e.g. for hydrological or glaciological applications.
Global representation of the gravity field in the form of EGM 2008, GRACE and GOCE are usually based on spherical harmonics. From such models any functional of the gravity potential may be evaluated. Several Global Gravity Models (GGMs) have been made available to the scientific community at the International Centre for Global Earth Models (ICGEM, see http://icgem.gfz-potsdam.de/ICGEM).Significant improvement in modeling the global gravity fields have been made due to contributions of EGM08 and the satellite gravity missions GRACE and GOCE.
GOCE belongs to the core satellite missions of the European Space Agency [2] focusing on the static part of the global gravity field, GOCE is expected to provide a geoid model with an accuracy of 1cm and gravity anomalies with an accuracy of 1mGal at a spatial resolution of 100km. to achieve such a Stimulating Goal, satellite Gravity gradiometry (SGG) has been realized on board the low orbiting satellite. Three different approaches have been proposed to determine the spherical harmonic coefficients from GOCE. These are called the direct, the space-wise and the time-wise approach. GOCE-only GGMs from each of these three approaches have been provided to the user community. In addition there is one combined GOCE – GRACE model.
In this study, all these three models (EGM2008, GRACE and GOCE) are compared over some points in Nigeria to see which one suits better in the country.