UT/CSR geocenter estimates from SLR tracking to LAGEOS-1 and LAGEOS-2
Two arc lengths are used. The 60 day arcs span late 1992 to the present. The monthly arcs,
which span 2000 to present are equal in length but remain close to the calendar months.
The time tags assigned are the mid-point of each arc. The 60-day estimates tend to be less
noisy and require little smoothing.
ITRF/SLRF2014 station coordinates are now being used. Note that two 'flavors' of solutions are provided.
The first, denoted by CN-CM, reflect the correction required to best center the SLR network on
the geocenter in the absence of modeling any local site loading. This is the traditional
geocenter motion typically realized by SLR and consistent with other estimates (see reference
given below for additional references). While this represents the best model for centering
the SLR network on the geocenter, it is well known that local site loading (degree-2 and
above) can exaggerate the amplitude of the motion so that it does not necessarily reflect
the true degree-1 load signal.
The second, denoted by CF-CM, is intended to reflect the true degree-1 mass variations
without being affected by the higher degree site loading effects (particularly at the
annual frequency). In order to accommodate higher-degree site loading, a fairly tightly
constrained bias has been estimated for each station to absorb the higher degree loading
deformation, which is dominantly in the vertical and thus highly correlated with the bias.
These estimates are in much better agreement with results from, for example, GPS global
inversion. See https://cddis.nasa.gov/lw20/docs/2016/papers/14-Ries_paper.pdf for more
information and please cite [Ries, J. C. Reconciling estimates of annual geocenter motion
from space geodesy, 20th International Workshop on Laser Ranging, 10-14 October 2016,
Potsdam, Germany] if using one of these products.
Note that the AOD1B values for degree-1 have not been removed. The mean values of the AOD1B
model for degree-1 (GAC) are provided by most GRACE processing centers and can be used
to remove the atmosphere-ocean contribution to degree-1. As shown in the above paper, after
removing the AOD1b for degree-1, the results for X and Y are in good agreement with Swenson,
Chambers and Wahr (2008). There remains a significant discrepancy in Z. Because of the strong
agreement between the SLR results and those from the GPS global inversions, we believe the
annual variations in Z from this analysis should be more reliable.
The geocenter vector is consistent with IERS conventions, being the vector
from the origin of the TRF to the instantaneous mass center of the Earth.
The relationship between degree-1 and geocenter (CF-CM) as defined here is
C11 = X/Ae
S11 = Y/Ae
C10 = Z/Ae
where Ae = 6378136360 mm. For normalized coefficients, divide by sqrt(3). In the above
expression, only the CF-CM values should be used.
This research sponsored by NASA Contract NNX16AF20G and NASA/JPL Contract 1616713 (MEaSUREs).
For more information, contact J. C. Ries (ries@csr.utexas.edu).
~