DETERMINATION OF LONG-TERM CHANGES IN THE EARTHS GRAVITY-FIELD FROM SATELLITE LASER RANGING OBSERVATIONS

Temporal changes in the Earth's gravity field have been determined by analyzing satellite laser ranging (SLR) observations of eight geodetic satellites using data spanning an interval of over 20 years. The sate llites used in the analysis include Starlette, LAGEOS 1 and 2, Ajisai, Etalon 1 and 2, Stella, and BE-C. Geophysical parameters, related to both secular and long-period variations in the Earth's gravity field, including the geopotential zonal rates ((J) over dot(2), (J) over dot( 3), (J) over dot(4), (J) over dot(5), and (J) over dot(6)) and the 18. 6-year tide parameter, were estimated. The estimated values for these parameters are (J) over dot(2) = -2.7+/-0.4(10(-11)/yr); (J) over dot( 3) = 1.3+/-0.5(10(-11)/yr); (J) over dot(4) = -1.4+/-0(10(-11)/yr); (J ) over dot(5) = 2.1+/-0.6(10(-11)/yr); (J) over dot(6) = 0.3+/-0.7(10( -11)/yr); C-18.6(+20) = 1.56 +/-0.2 (centimeters) and S-18.6(+20) = -0 .1+/-0.2 (centimeters). The amplitude and phase for the 18.6-year tide are in general agreement with the effects predicted by the Earth's ma ntle anelasticity. The solution accuracy was evaluated by considering the effects of errors in various non-estimated dynamical model paramet ers and by varying the data span and data sets used in the solution. E stimates for (J) over dot(3) from individual LAGEOS 1 and Starlette SL R data sets are in good agreement. The lumped sum values for (J) over dot(3) and (J) over dot(5) are very different for LAGEOS 1 and Starlet te. The zonal rate determination is limited to degree 6 with the curre nt SLR data sets. Analysis of the sensitivity of the solution for the zonal rates to the satellite tracking data span suggests that the temp oral extension of the current SLR data sets will enhance the solution of zonal rates beyond degree 6.