The Need and the Solution
Existing models of the Earth's gravitational potential have clearly been useful for many of
the applications previously discussed. However, current models have weaknesses caused
by several factors including the lack of surface gravity data in some regions, the lack of
uniform coverage of the oceans with high resolution satellite altimetry, and the lack of
satellite tracking data at low « 35°) orbital inclinations. In addition, there is a clear lack of
precise tracking data for satellites at altitudes less than 700 km.
Developments in each of these areas in the past few years indicated that the determination
of a new potential model combining satellite observations, altimeter information, new
terrestrial gravity data, and improved elevation data sets, was a very desirable goal. To this
end, GSFC and DMA signed a Memorandum of Understanding for a Joint Gravity Field
and Geoid Improvement Project in April 1994 with the following objective: "The primary
goal (of the project) is to improve the Earth Gravity Model (EGM) and its associated global
geoid, to support terrestrial and extra-terrestrial scientific endeavors, as well as to meet the
mapping, charting and navigation requirements of both the civil and military sections." The
primary responsibilities for developing the new model reside at GSFC and DMA. This
work is supported by activities at the Naval Surface Warfare Center (NSWC) and The Ohio
State University.
The general direction for the new model development are defined by representative
persons from the organizations involved. More specific activities are carried out by the
following working groups: Working Group I: Combination Methods and High Degree
Expansions; Working Group II: Surface Gravity Data Preparations; Working Group III:
Evaluation of Altimeter Implied Gravity Anomalies; Working Group N: Satellite Gravity
Model Development. In the following sections, a brief description of the procedures and
data being used by each working group in the development of the 360 x 360 model is
given.
SATELLITE TRACKING DATA AND MODELING PROCEDURES
Over the last 30 years, a variety of different types of tracking data have been collected for
satellites in a number of different orbits. The gravity field is best determined by having
tracking data from satellite orbits with a wide range of inclinations and altitudes. Different
satellite inclinations are principally required in order to define the unique individual
coefficients of the model. Data from different altitudes are desirable since both the forces
of gravity and atmospheric drag vary inversely with altitude. Satellites at altitudes of 1
Earth radius, such as Lageos 1 and 2, are optimal for determining the long wavelength
gravity field since the effects of non-conservative forces, such as atmospheric drag, are
small. However, tracking data from satellites at lower altitudes are also required in order to
measure shorter wavelength variations in the gravity field. A robust "satellite-only" gravity
model (one based solely on satellite tracking and not altimetry or surface gravity data)
should contain tracking data from satellites covering as wide a range of orbit inclinations
and altitudes as possible.
One of the recent successful gravity models using satellite tracking data as its foundation,
JGM-2 (Nerem et al., 1994b), contains tracking data from more than 30 different satellites
spanning nearly as many years in time. Such a large collection of historical tracking data
by definition contains data sets with widely varying accuracies and spatial coverage.
Relatively crude optical tracking data are employed because these early satellites occupied
low orbital inclinations that have not been re-observed by newer tracking systems/satellites.
Early Doppler tracking data are also included in current gravity models for similar reasons.
Precise satellite laser ranging (SLR) data form the foundation of the "satellite-only" gravity
models, with satellites such as Lageos 1, Starlette and Ajisai, providing the most strength
to the solution. Tracking of SPOT-2 (Nerem et al., 1994a) and TIP (Nerem et al., 1994b)
95
Existing models of the Earth's gravitational potential have clearly been useful for many of
the applications previously discussed. However, current models have weaknesses caused
by several factors including the lack of surface gravity data in some regions, the lack of
uniform coverage of the oceans with high resolution satellite altimetry, and the lack of
satellite tracking data at low « 35°) orbital inclinations. In addition, there is a clear lack of
precise tracking data for satellites at altitudes less than 700 km.
Developments in each of these areas in the past few years indicated that the determination
of a new potential model combining satellite observations, altimeter information, new
terrestrial gravity data, and improved elevation data sets, was a very desirable goal. To this
end, GSFC and DMA signed a Memorandum of Understanding for a Joint Gravity Field
and Geoid Improvement Project in April 1994 with the following objective: "The primary
goal (of the project) is to improve the Earth Gravity Model (EGM) and its associated global
geoid, to support terrestrial and extra-terrestrial scientific endeavors, as well as to meet the
mapping, charting and navigation requirements of both the civil and military sections." The
primary responsibilities for developing the new model reside at GSFC and DMA. This
work is supported by activities at the Naval Surface Warfare Center (NSWC) and The Ohio
State University.
The general direction for the new model development are defined by representative
persons from the organizations involved. More specific activities are carried out by the
following working groups: Working Group I: Combination Methods and High Degree
Expansions; Working Group II: Surface Gravity Data Preparations; Working Group III:
Evaluation of Altimeter Implied Gravity Anomalies; Working Group N: Satellite Gravity
Model Development. In the following sections, a brief description of the procedures and
data being used by each working group in the development of the 360 x 360 model is
given.
SATELLITE TRACKING DATA AND MODELING PROCEDURES
Over the last 30 years, a variety of different types of tracking data have been collected for
satellites in a number of different orbits. The gravity field is best determined by having
tracking data from satellite orbits with a wide range of inclinations and altitudes. Different
satellite inclinations are principally required in order to define the unique individual
coefficients of the model. Data from different altitudes are desirable since both the forces
of gravity and atmospheric drag vary inversely with altitude. Satellites at altitudes of 1
Earth radius, such as Lageos 1 and 2, are optimal for determining the long wavelength
gravity field since the effects of non-conservative forces, such as atmospheric drag, are
small. However, tracking data from satellites at lower altitudes are also required in order to
measure shorter wavelength variations in the gravity field. A robust "satellite-only" gravity
model (one based solely on satellite tracking and not altimetry or surface gravity data)
should contain tracking data from satellites covering as wide a range of orbit inclinations
and altitudes as possible.
One of the recent successful gravity models using satellite tracking data as its foundation,
JGM-2 (Nerem et al., 1994b), contains tracking data from more than 30 different satellites
spanning nearly as many years in time. Such a large collection of historical tracking data
by definition contains data sets with widely varying accuracies and spatial coverage.
Relatively crude optical tracking data are employed because these early satellites occupied
low orbital inclinations that have not been re-observed by newer tracking systems/satellites.
Early Doppler tracking data are also included in current gravity models for similar reasons.
Precise satellite laser ranging (SLR) data form the foundation of the "satellite-only" gravity
models, with satellites such as Lageos 1, Starlette and Ajisai, providing the most strength
to the solution. Tracking of SPOT-2 (Nerem et al., 1994a) and TIP (Nerem et al., 1994b)
95