Comparison of four gravity models

Inertial navigation systems must utilize a valid gravity model in order to accurately mechanize the navigation equations. Four commonly used gravity models are compared and their fundamental differences are summarized in this paper. The 4 gravity models (listed with increasing accuracy and/or validity) are as follows. 1. Low altitude gravity model is an approximation to the normal gravity vector. 2. J₂ gravity model uses an approximation to the normal graviational potential (an infinite series of spherical harmonics) and generates the gravity vector in the Earth-Centered-Earth-Fixed (ECEF) frame. These components are transformed to the local level navigation. 3. Normal gravity model generates a gravity vector normal to the reference ellipsoid on the ellipsoidal surface by definition; above the ellipsoid, the gravity vector has a nonzero north component. 4. General gravity approximation model uses a multiple-term approximation to the true gravitational potential (a double sum of infinite terms). All three components are nonzero at or above the ellipsoidal surface. The components depend upon longitude. This paper gives an introduction to the concept of gravity potential and normal gravity potential. Gravity vector direction comparison for these four gravity models are made at all latitudes, longitudes and altitudes. Coordinate dependency of the gravity components is also examined