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TOPEX
The
TOPEX data are from the
Merged
Geophysical Data Records (MGDRs) and
have been modified to include the Wallops internal calibration, new
tide models, TMR path
delay corrections, and updated sea state bias models.
To
make comparisons with
Jason more compatible, we replaced the (M)GDR ocean and load tides with
those from GOT99.2 model (Ray, 1999) and the equilibrium long-period
ocean tides with those used for the Jason GDRs (P. Callahan, personal
communication).
Two
modifications to the
wet troposphere path delay are necessary. Keihm et al. (2000)
discovered a drift in one channel of the TOPEX Microwave Radiometer
(TMR), which caused a noticeable drift in comparisons between T/P SSH
measurements and the global tide gauge network (Mitchum, 1998). We have
applied a correction for this drift based on revised calibrations of
the 18GHz brightness temperature of the TMR.
We also include a TMR yaw correction to account for a slightly greater
path delay when the satellite is in fixed rather than sinusoidal yaw.
This TMR yaw correction accounts for the 15-hour thermal transition
between yaw
states (Zlotnicki and Callahan, 2002). The brightness temperature and
yaw corrections were made from the GDR Correction Product
Version C.
Chambers et
al., 2003 estimated
new models (here called T-CSR) for each of the
two different TOPEX altimeters (TOPEX-A and TOPEX-B) that have operated
during TOPEX/POSEDION mission.
The T-CSR SSB model for TOPEX-B is significantly different than the
model provided on the (M)GDRs, which was estimated using only TOPEX-A
data. A 1.5 mm bias between TOPEX-A and TOPEX-B has been applied. We
have not applied any
retracking corrections or corrections to the ionosphere path delay that
result from changes to the sea state bias model.
The
inverted barometer used is NOT the value from
the TOPEX MGDR-B, but is calculated using the 6-hour pressure values
from the European Centre for Medium-Range Weather Forecasts (ECMWF).
Inverted Barometer = -9.948*(atmospheric_pressure -
global_average_pressure)
The global_average_pressure is interpolated from values determined at 6
hour intervals from CNES. The file containing the average values is
obtained from CNES/CLS.
The inverted barometer does not have much apparent effect on the global
mean sea level, because the ocean as a whole is not compressible.
Jason
The
Jason-1 data are
from the Geophysical Data Records (GDRs). We replaced the GDR
orbits with JPL GPS
reduced-dynamic precise orbits (Desai and Haines, 2003). The reduced
dynamic orbits are thought to be accurate to better than 10 mm rms, in
part because the reduced dynamic technique can compensate for residual
gravity errors.
Global
mean sea level
We
chose to reference TOPEX
and Jason SSH anomalies on the common ground-track to an along-track
mean computed using data from the original TOPEX/POSEDION mission. A
cross-track gradient correction was estimated using the JPL along-track
mean sea
surface model. This model was derived by computing adjustments to the
GSFC00.1 global mean sea surface model using sea surface height data
from the TOPEX altimeter. The cross-track gradient component was not
adjusted and was therefore effectively a fit to the cross-track
gradients defined by the GSFC00.1 model along the T/P ground track. (S.
Desai, personal communication).
Spurious
data have
been removed with a restriction of SSH anomalies to be < 1 meter.
Global mean sea level was computed from the corrected SSH anomalies
using a simple equal-area weighted average (Wang and Rapp, 1994).
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