SECULAR VARIATIONS OF THE ZONAL HARMONICS AND
POLAR MOTION AS GEOPHYSICAL CONSTRAINTS
R. S. Nerem
Space Geodesy Branch, NASNGSFC, Greenbelt MD 20771
S. M. Klosko
Hughes STX Corporation, Greenbelt MD 20770
INTRODUCTION
Monitoring temporal changes in geodetic constants remains an important challenge
for space geodetic investigations where the accurate determination of the effect
significantly lags geophysical modeling requirements and inferences. For example,
temporal changes in the Earth's gravity field are manifestations of complex
geophysical processes; an accurate determination of these changes can provide
important insights and constraints on their underlying causes. The temporal
variations of the external gravitational field represent a dynamic aspect of the mass
redistribution within the combined solid-Earth/ocean/atmosphere system.
Conservation of angular momentum for the system as a whole imposes constraints
on the variations of the angular momenta of its various components. Coincident
monitoring of the temporal variations of the geopotential, the Earth's pole position,
and its rotation rate are important objectives which can provide insight into key
geophysical processes and the unique structure of the Earth itself.
Temporal variations of the external gravitational field occur at various spatial and
temporal scales, ranging from episodic variations due to earthquakes to secular
variations dominated by the crust's post-glacial rebound and/or global mean sea level
rise from ice sheet ablation. Many geophysical studies have predicted a range of
secular gravitational changes which await definitive confirmation. Through the study
of the evolution of near-Earth satellite motion using highly precise satellite laser
ranging (SLR), the aggregate changes in the external gravitational field can be
determined. However, beyond resonant tidal effects (cf. Christodoulidis et aI., 1988),
primarily because of the limitation on available satellites having sufficiently long
tracking histories, only the time-dependent behavior of the lowest degree zonal terms
in the gravitational model can now be determined.
DETERMINING SECULAR CHANGES IN TIlE ZONAL HARMONICS
SLR-geodesy is based on the exploitation of the functional relationships between very
precise observations and the underlying model parameters. SLR measurements made
over the last 18 years to geodetic satellites provide an unprecedented observational
resource for the estimation of time variations of the longest wavelength components
of the Earth's gravitational field. Lageos-l (launched in 1976), and Starlette (1975)
offer the longest records of SLR data. However, SLR tracking systems achieved the
accuracy and global coverage necessary to support this study only since the early
1980s which coincides with the time period where VLBI provides independent
monitoring of the Earth's UTI variations.
The zonal harmonics of the gravity field produce both secular and long period orbit
perturbations. A consideration of the zonal disturbing potential as expressed in terms
of the orbital elements of the satellite (Kaula, 1966; eq. 3.70) shows:
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POLAR MOTION AS GEOPHYSICAL CONSTRAINTS
R. S. Nerem
Space Geodesy Branch, NASNGSFC, Greenbelt MD 20771
S. M. Klosko
Hughes STX Corporation, Greenbelt MD 20770
INTRODUCTION
Monitoring temporal changes in geodetic constants remains an important challenge
for space geodetic investigations where the accurate determination of the effect
significantly lags geophysical modeling requirements and inferences. For example,
temporal changes in the Earth's gravity field are manifestations of complex
geophysical processes; an accurate determination of these changes can provide
important insights and constraints on their underlying causes. The temporal
variations of the external gravitational field represent a dynamic aspect of the mass
redistribution within the combined solid-Earth/ocean/atmosphere system.
Conservation of angular momentum for the system as a whole imposes constraints
on the variations of the angular momenta of its various components. Coincident
monitoring of the temporal variations of the geopotential, the Earth's pole position,
and its rotation rate are important objectives which can provide insight into key
geophysical processes and the unique structure of the Earth itself.
Temporal variations of the external gravitational field occur at various spatial and
temporal scales, ranging from episodic variations due to earthquakes to secular
variations dominated by the crust's post-glacial rebound and/or global mean sea level
rise from ice sheet ablation. Many geophysical studies have predicted a range of
secular gravitational changes which await definitive confirmation. Through the study
of the evolution of near-Earth satellite motion using highly precise satellite laser
ranging (SLR), the aggregate changes in the external gravitational field can be
determined. However, beyond resonant tidal effects (cf. Christodoulidis et aI., 1988),
primarily because of the limitation on available satellites having sufficiently long
tracking histories, only the time-dependent behavior of the lowest degree zonal terms
in the gravitational model can now be determined.
DETERMINING SECULAR CHANGES IN TIlE ZONAL HARMONICS
SLR-geodesy is based on the exploitation of the functional relationships between very
precise observations and the underlying model parameters. SLR measurements made
over the last 18 years to geodetic satellites provide an unprecedented observational
resource for the estimation of time variations of the longest wavelength components
of the Earth's gravitational field. Lageos-l (launched in 1976), and Starlette (1975)
offer the longest records of SLR data. However, SLR tracking systems achieved the
accuracy and global coverage necessary to support this study only since the early
1980s which coincides with the time period where VLBI provides independent
monitoring of the Earth's UTI variations.
The zonal harmonics of the gravity field produce both secular and long period orbit
perturbations. A consideration of the zonal disturbing potential as expressed in terms
of the orbital elements of the satellite (Kaula, 1966; eq. 3.70) shows:
152