TEMPORAL VARIATIONS OF THE EARTHS GRAVITATIONAL-FIELD FROM SATELLITE LASER RANGING TO LAGEOS

We have estimated monthly values of the J2 and J3 Earth gravitational coefficients using LAGEOS satellite laser ranging (SLR) data collected between 1980 and 1989. For the same time period, we have also compute d corresponding estimates of the variations in these coefficients caus ed by atmospheric mass redistribution using surface atmospheric pressu re estimates from the European Center for Medium Range Weather Forecas ts (ECMWF). These data were processed both with and without a correcti on for the ''inverted barometer effect,'' the ocean's isostatic respon se to atmospheric loading. While the estimated zonal harmonics in the orbit analysis accommodate gravitational changes at a reduced level ar ising from all other higher degree zonal effects, the LAGEOS and atmos pheric time series for J2 compare quite well and it appears that the n on-secular variation in J2 can be largely attributed to the redistribu tion of the atmospheric mass. While the observed changes in the 'effec tive'' J3 parameters are not well predicted by the third degree zonal harmonic changes in the atmosphere, both odd zonal time series display strong seasonality. The LAGEOS J3 estimates are very sensitive to as yet unmodeled forces acting on the satellite and these effects must be better understood before determining the dominant geophysical signals contributing to the estimate of this time series.