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Mean Sea Level
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O Calibration
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The method of
producing the tide gauge estimates of
altimeter drift that we report here is described in detail by Mitchum
(2000), and will not be discussed in full here. Briefly, the method
works by creating an altimetric time series at a tide gauge location,
and then differencing this time series with the tide gauge sea level
time series. In this difference series, ocean signals common to both
series largely cancel, leaving a time series that is dominated by the
sum of the altimetric drift and the land motion at the tide gauge site.
Making separate estimates of the land motion rates and combining the
difference series from a large number of gauges globally results in a
times series that is dominated by the altimeter drift. Since the
difference series at separate time gauge locations have been shown to
be nearly statistically independent (Mitchum,
1998), the final drift
series has a variance much smaller than any of the individual series
that go into it. Because of the relatively large number of degrees of
freedom, this method outperforms calibrations from dedicated
calibration sites, although it is only a relative calibration, meaning
that it cannot determine any absolute bias. It can, however, detect
change in a bias, either a drift or a step change. For the tide gauge
calibrations, global mean sea level during the period of TOPEX-A
operation is used as zero level for TOPEX-B, POSEIDON, and Jason.
Ideally,
one would want to include all of the available tide gauges in
the calibration. A number of gauges, however, have a significant lag in
reporting of records and are not available for the Jason calibration.
On the other hand, some others do not extend backward through most of
the T/P mission. We have restricted the ~100 available gauges to a set
of 64 near real-time stations that span the majority of both the T/P
and Jason missions, and will therefore provide a relatively consistent
calibration for both.
TOPEX calibration: cal_cu2004_rel1_topex
Jason calibration: cal_cu2004_rel1_jason
TOPEX
calibration
Jason
calibration
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