Geocenter - Degree 1
The spherical harmonic coefficients of degree 1 represent the distance between the center of mass of the Earth and its 'center of figure', which in practice is the center of a set of tracking stations on the surface of the Earth. Because of their physical meaning, time changes in degree 1 coefficients can be expressed in several equivalent forms (1) as distances in mm along the Z (along the axis of rotation), X and Y axes; (2) as (fully normalized) coefficients of the geopotential; (3) as the changes in mass (per unit area) that would give rise to the geopotential coefficients, expressed either in kg/m^2 or cm of equivalent water thickness.
The relation between these forms can be found in Swenson et al (2008), equations 5 and 4.
One way to obtain these coefficients uses Satellite Laser Ranging to geodetic satellites. Coefficients obtained in that manner since 1992 are available here
. These coefficients are expressed in the form (1) above. See Cheng, Tapley and Ries (2010)
Another way to obtain these coefficients combines GRACE data and the output of a numerical ocean model, and is applicable to land hydrology and cryospheric applications (Swenson, Chambers and Wahr, 2008). Coefficients computed in this manner are available here
. These coefficients are expressed in the form (2) above.
Cheng, M.K., B.D. Tapley, J.C. Ries (2010) Geocenter Variations from Analysis of SLR data, IAG Commission 1 Symposium 2010, Reference Frames for Applications in Geosciences (REFAG2010), Marne-La-Vallee, France, 4-8 October 2010.
Swenson, S.; Chambers, D. & Wahr, J. Estimating geocenter variations from a combination of GRACE and ocean model output J. Geophys. Res., 2008, 113, 8410
LAST UPDATE: 2012-05-01 V.Zlotnicki