APPLICATIONS OF GLOBAL GRAVITY FIELD MODELS
IN GEODESY AND OCEANOGRAPHY
R. S. Nerem
NASA/Goddard Space Flight Center, Space Geodesy Branch
Code 926, Greenbelt, MD, 20771, U.S.A.
INTRODUCTION
S. M. K1osko and N. K. Pavlis
Hughes STX, Greenbelt, MD, 20770, U.S.A.
During the last decade, remarkable progress has been made in improving the
gravitational models of the Earth and the terrestrial planets. The determination of these
models has been accomplished using a wide variety of different measurement types and
solution techniques. The measurement types for measuring the Earth's potential field can
be divided into satellite tracking measurements, surface gravity measurements, and
satellite altimeter measurements; planetary gravity fields are determined exclusively from
tracking data. Gravity field models accompanied by accurate error estimates are used to
support a wide variety of applications including the orbit determination of spacecraft, a
variety of geophysical investigations, oceanographic investigations using satellite
altimetry, and for the definition of a unique vertical datum. The latter becomes
increasingly important because of its implications in sea level studies and in the
determination of orthometric heights and height differences from GPS positioning
without the need for leveling. This paper will briefly review the progress made in recent
gravity models, a spectrum of different applications of global gravity field models, and
describe their ability to meet current requirements. In addition, the characteristics of the
temporal variations of the gravity field will be reviewed with respect to obtaining a better
understanding of important geophysical processes ongoing in the solid earth-oceanatmosphere system. We will also review the current status of modeling planetary gravity
fields, and discuss the applications of these models for geophysical studies and orbit
determination. Finally, the prospects for improving current gravity models of the Earth
and the planets will be discussed.
Before reviewing the different applications of global gravity models, it is instructive to
summarize the major techniques that are used in their determination. In both near-Earth
and interplanetary contexts, satellite tracking data are used to measure gravitational
perturbations affecting satellite orbital motion, with the accuracy and spatial/temporal
distribution of the data being the most important factors in the resulting field accuracy
and resolution. The satellite orbit characteristics are an important factor, with gravity
signal strength attenuating with altitude and non-conservative force modeling needed to
isolate the gravity effects from those arising from principally drag and solar flux. Surface
gravity data provide a more direct measurement of the gravity field, but acquiring data
uniformly over the Earth has always been difficult. The development of precise GPS
positioning has led to increased interest in airborne gravity measurements, although these
measurements are not currently being used in the computation of global gravity models.
Because the sea surface largely conforms to the geoid, satellite altimetry provides precise
measurements of the marine gravity field, provided that satellite orbit errors and nongeoidal sea surface height variations can be adequately modeled. Comprehensive gravity
field solutions must incorporate and effectively exploit these disparate data types in order
to estimate mathematical parameters describing the gravity field, such as spherical
1
IN GEODESY AND OCEANOGRAPHY
R. S. Nerem
NASA/Goddard Space Flight Center, Space Geodesy Branch
Code 926, Greenbelt, MD, 20771, U.S.A.
INTRODUCTION
S. M. K1osko and N. K. Pavlis
Hughes STX, Greenbelt, MD, 20770, U.S.A.
During the last decade, remarkable progress has been made in improving the
gravitational models of the Earth and the terrestrial planets. The determination of these
models has been accomplished using a wide variety of different measurement types and
solution techniques. The measurement types for measuring the Earth's potential field can
be divided into satellite tracking measurements, surface gravity measurements, and
satellite altimeter measurements; planetary gravity fields are determined exclusively from
tracking data. Gravity field models accompanied by accurate error estimates are used to
support a wide variety of applications including the orbit determination of spacecraft, a
variety of geophysical investigations, oceanographic investigations using satellite
altimetry, and for the definition of a unique vertical datum. The latter becomes
increasingly important because of its implications in sea level studies and in the
determination of orthometric heights and height differences from GPS positioning
without the need for leveling. This paper will briefly review the progress made in recent
gravity models, a spectrum of different applications of global gravity field models, and
describe their ability to meet current requirements. In addition, the characteristics of the
temporal variations of the gravity field will be reviewed with respect to obtaining a better
understanding of important geophysical processes ongoing in the solid earth-oceanatmosphere system. We will also review the current status of modeling planetary gravity
fields, and discuss the applications of these models for geophysical studies and orbit
determination. Finally, the prospects for improving current gravity models of the Earth
and the planets will be discussed.
Before reviewing the different applications of global gravity models, it is instructive to
summarize the major techniques that are used in their determination. In both near-Earth
and interplanetary contexts, satellite tracking data are used to measure gravitational
perturbations affecting satellite orbital motion, with the accuracy and spatial/temporal
distribution of the data being the most important factors in the resulting field accuracy
and resolution. The satellite orbit characteristics are an important factor, with gravity
signal strength attenuating with altitude and non-conservative force modeling needed to
isolate the gravity effects from those arising from principally drag and solar flux. Surface
gravity data provide a more direct measurement of the gravity field, but acquiring data
uniformly over the Earth has always been difficult. The development of precise GPS
positioning has led to increased interest in airborne gravity measurements, although these
measurements are not currently being used in the computation of global gravity models.
Because the sea surface largely conforms to the geoid, satellite altimetry provides precise
measurements of the marine gravity field, provided that satellite orbit errors and nongeoidal sea surface height variations can be adequately modeled. Comprehensive gravity
field solutions must incorporate and effectively exploit these disparate data types in order
to estimate mathematical parameters describing the gravity field, such as spherical
1