A crossover Adjustment Scheme for Reducing the Large-scale
Time-dependent Errors in Satellite Altimetry While Preserving the Ocean
Signal
Chang-Kou Tai and John M. Kuhn
J.Geophys. Res., (submitted), 1994.
National Ocean Service, NOAA, Silver Spring MD 20910
ABSTRACT
A crossover adjustment scheme for reducing the orbit, tide and
any other large-scale time-dependent errors in satellite altimetry
while preserving the ocean signal is devised and tested with 130
days of TOPEX/POSEIDON data. It employs a Fourier series truncated
at 3.33 cycle per revolution (i.e., 3.33 cpr, about 12,000 km in
along-track wavelength) to accurately describe these large-scale
errors. The adjustment is limited to each 10-day repeat cycle of
TOPEX/POSEIDON to ensure the preservation of the large-scale ocean
signal, which can be shown analytically (along with all large-scale
errors) to undergo transformations to well-sampled 10-day averages
after the adjustment by the ways the singularities are handled in
this scheme. The solution of the adjustment is comprised of large-
scale time-dependent orbit error, tide model error (mainly M2
tide), environmental correction error, and ocean signal relative to
their respective 10-day means. The solution reduces the crossover
difference from 9.5 cm (rms) to 6.8 cm, while accounting for 6.4
cm. The rms value of the solution is 4.9 cm (4.3 cm if counting
solution values at crossovers only). Eliminating influences from M2
and S2 tide errors (which are aliased along track into 6.26 and
5.92 cycles respectively) by forming 6-cycle differences of the
solution, the non-systematic orbit error is estimated to have a rms
value of 4.1 cm; while averaging of the solution over 12 cycles
produces an estimate of the systematic orbit error (which produces
no collinear difference) at 1.8 cm (rms), projecting an upper bound
for the gravity-error-induced orbit error at 2.5 cm, and pointing
to a simple way of using space-time averaging to reduce the orbit
and tide errors for TOPEX/POSEIDON if the signal can withstand the
averaging, such as in the tropics. The adjustment corrects for over
half of the tide model error, even though the scheme is not
designed for this purpose and there are more effective ways to
remove the tide error.