TEMPORAL VARIATIONS OF THE GRAVITY FIELD FROM
LAGEOS 1 AND LAGEOS 2 OBSERVATIONS
A. Cazenave, P. Gegout, G. Ferhat, and R Biancale
GRGS-CNES, 18 Avenue Edouard Belin, 31400 Toulouse, France
ABSTRACT
Precise orbit computations, based on a numerical approach, have been carried out for
the Lageos 1 and Lageos 2 satellites to derive temporal variations of the Earth's gravity
field. Eleven years (1984-1994) and two years (1993-1994) of laser data have been
analysed respectively for Lageos 1 and Lageos 2. The secular rates of 12, 13 and 14 have
been estimated, with the following solution:
j2 = -3.0 ± O.5x10- 11 ,i3 = -1. 7 ± O.lx10- 11 and j4 = -0.8 ± 1.5x10- 11
The seasonal variation of J 2 has also been computed over the eleven years period. As
reported earlier, the annual component of 12 is essentially due to air mass redistribution
in the atmosphere. Over the years 1993-1994, the annual ocean mass contribution has
been estimated using sea surface height data of the Topex-Poseidon altimeter satellite. It
is shown that the ocean effect is quite small, only a few % of the atmospheric effect at
the annual frequency.
INTRODUCTION
The zonal harmonics of the Earth's gravity field are expected to vary because of mass
redistribution inside the solid Earth and inside the hydrosphere. Several values of the
temporal changes of the first zonal harmonics In based on analyses of satellite laser
ranging data have been published (Yoder et al., 1983 ; Rubincam, 1984 ; Cheng et aI.,
1989, 1995 ; Gegout and Cazenave 1991, 1993 ; Nerem et al. 1993 ; Nerem and Klosko,
1994 ; Eanes and Bettadpur, 1995 ; Klosko and Nerem, 1995) Estimates of their
variations concern two main spectral bands : the secular and subdecadal time scale with
emphasis on the seasonal fluctuations. The first published secular change of 12 (e.g. ,
Yoder et al., 1983, Rubincam, 1984) has been generally interpreted as reflecting the post
glacial rebound phenomenon and has been used to constrain the mantle rheology, in
particular the viscosity contrast between upper and lower mantle (peltier, 1985). More
recently it has been argued that present-day retreat of small ice sheets and glaciers, as
well as present-day variations in the mass balance of the Antarctic and Greenland ice
sheets may give rise to pseudo secular changes of the zonal harmonics of the same order
141
LAGEOS 1 AND LAGEOS 2 OBSERVATIONS
A. Cazenave, P. Gegout, G. Ferhat, and R Biancale
GRGS-CNES, 18 Avenue Edouard Belin, 31400 Toulouse, France
ABSTRACT
Precise orbit computations, based on a numerical approach, have been carried out for
the Lageos 1 and Lageos 2 satellites to derive temporal variations of the Earth's gravity
field. Eleven years (1984-1994) and two years (1993-1994) of laser data have been
analysed respectively for Lageos 1 and Lageos 2. The secular rates of 12, 13 and 14 have
been estimated, with the following solution:
j2 = -3.0 ± O.5x10- 11 ,i3 = -1. 7 ± O.lx10- 11 and j4 = -0.8 ± 1.5x10- 11
The seasonal variation of J 2 has also been computed over the eleven years period. As
reported earlier, the annual component of 12 is essentially due to air mass redistribution
in the atmosphere. Over the years 1993-1994, the annual ocean mass contribution has
been estimated using sea surface height data of the Topex-Poseidon altimeter satellite. It
is shown that the ocean effect is quite small, only a few % of the atmospheric effect at
the annual frequency.
INTRODUCTION
The zonal harmonics of the Earth's gravity field are expected to vary because of mass
redistribution inside the solid Earth and inside the hydrosphere. Several values of the
temporal changes of the first zonal harmonics In based on analyses of satellite laser
ranging data have been published (Yoder et al., 1983 ; Rubincam, 1984 ; Cheng et aI.,
1989, 1995 ; Gegout and Cazenave 1991, 1993 ; Nerem et al. 1993 ; Nerem and Klosko,
1994 ; Eanes and Bettadpur, 1995 ; Klosko and Nerem, 1995) Estimates of their
variations concern two main spectral bands : the secular and subdecadal time scale with
emphasis on the seasonal fluctuations. The first published secular change of 12 (e.g. ,
Yoder et al., 1983, Rubincam, 1984) has been generally interpreted as reflecting the post
glacial rebound phenomenon and has been used to constrain the mantle rheology, in
particular the viscosity contrast between upper and lower mantle (peltier, 1985). More
recently it has been argued that present-day retreat of small ice sheets and glaciers, as
well as present-day variations in the mass balance of the Antarctic and Greenland ice
sheets may give rise to pseudo secular changes of the zonal harmonics of the same order
141