CONCLUSION
Measuring the temporal variations of the gravity field, in particular of the zonal
harmonics, provide important constraints on global changes of the Earth's inertia tensor.
Precise orbit analyses of geodetic laser satellites on a long term basis provide now such
an opportunity. During the past decade, several studies have estimated the secular rate of
change of J 2 but it is only recently that attempts to solve also for j 3, j 4 and j 5 have
been carried out. As evident in Table 3, there is now a good consistency among the
recent solutions for j 2, in the range of -(2.7 - 3.0) x 10- 11 . This is far to be the case for
j 3, j 4 and j 5 for which differences between solutions greatly exceed the reported
uncertainties. Moreover solutions for j 3 with no j 5 must be considered as effective rates
that vary with the satellites considered. Thus, except perhaps for j 2, proposed solutions
for j 3, j 4 and j 5 are still quite preliminary but there is hope to improve them in the
future by adding new observations and combining the data of several laser satellites.
Reliable values for these j n will provide strong constraints on mantle viscosity models
and present-day mass balance of ice sheets.
At subdecadal time-scales, measuring the temporal variations of the zonal harmonics
will serve to study global changes in the Earth' hydrosphere, in particular will provide
upper limits on the redistribution of matter in the oceans and continental water storage.
Acknowledgment. This study was supported by the Centre National d'Etudes Spatiales
andCNRS.
REFERENCES
Boubli Y., A. Cazenave, IF. Minster and R Abarca, Effects of oceans inferred from the
Topex-Poseidon satellite on Earth's rotation and gravitational field at annual
frequency, in preparation, 1995.
Chao, B.F., and A. Y. Au, Temporal variations of the Earth's low degree zonal
gravitational field caused by atmospheric mass redistribution, 1980-88, Geophys. Res.
Lett., 96, 6569-6575, 1991.
Cheng, M. K., R J. Eanes, C. K. Shum, B. E. Schutz, and B. D. Tapley, Temporal
variations in low degree zonal harmonics from Starlette orbit analysis, Geophys. Res.
Lett., 16,393-396, 1989.
Cheng M.F., C.K. Shum and B.D. Tapley, Global constraint on the modeling of mass
transport within the Earth system using long term SLR observations, IUGG General
Assembly, Boulder, Co, 1995
DeMets, D.C., R G. Gordon, D. F. Argus, and S. Stein, Current plate motions, Geophys.
1. Int., 101,425-478, 1990.
150
Measuring the temporal variations of the gravity field, in particular of the zonal
harmonics, provide important constraints on global changes of the Earth's inertia tensor.
Precise orbit analyses of geodetic laser satellites on a long term basis provide now such
an opportunity. During the past decade, several studies have estimated the secular rate of
change of J 2 but it is only recently that attempts to solve also for j 3, j 4 and j 5 have
been carried out. As evident in Table 3, there is now a good consistency among the
recent solutions for j 2, in the range of -(2.7 - 3.0) x 10- 11 . This is far to be the case for
j 3, j 4 and j 5 for which differences between solutions greatly exceed the reported
uncertainties. Moreover solutions for j 3 with no j 5 must be considered as effective rates
that vary with the satellites considered. Thus, except perhaps for j 2, proposed solutions
for j 3, j 4 and j 5 are still quite preliminary but there is hope to improve them in the
future by adding new observations and combining the data of several laser satellites.
Reliable values for these j n will provide strong constraints on mantle viscosity models
and present-day mass balance of ice sheets.
At subdecadal time-scales, measuring the temporal variations of the zonal harmonics
will serve to study global changes in the Earth' hydrosphere, in particular will provide
upper limits on the redistribution of matter in the oceans and continental water storage.
Acknowledgment. This study was supported by the Centre National d'Etudes Spatiales
andCNRS.
REFERENCES
Boubli Y., A. Cazenave, IF. Minster and R Abarca, Effects of oceans inferred from the
Topex-Poseidon satellite on Earth's rotation and gravitational field at annual
frequency, in preparation, 1995.
Chao, B.F., and A. Y. Au, Temporal variations of the Earth's low degree zonal
gravitational field caused by atmospheric mass redistribution, 1980-88, Geophys. Res.
Lett., 96, 6569-6575, 1991.
Cheng, M. K., R J. Eanes, C. K. Shum, B. E. Schutz, and B. D. Tapley, Temporal
variations in low degree zonal harmonics from Starlette orbit analysis, Geophys. Res.
Lett., 16,393-396, 1989.
Cheng M.F., C.K. Shum and B.D. Tapley, Global constraint on the modeling of mass
transport within the Earth system using long term SLR observations, IUGG General
Assembly, Boulder, Co, 1995
DeMets, D.C., R G. Gordon, D. F. Argus, and S. Stein, Current plate motions, Geophys.
1. Int., 101,425-478, 1990.
150