AGORA - CLS publications
Journal of Atmospheric and Oceanic Technology (in press)
April 1997
ABSTRACT: Objective analysis of altimetric data (Sea Level Anomaly) usually assumes that measurement errors are well represented by a white noise, though there are long-wavelength errors which are correlated over thousands of kilometers along the satellite tracks. These errors are typically 3 cm rms for TOPEX/POSEIDON (T/P) which is not negligible in low energy regions. Analyzing maps produced by conventional objective analysis thus reveals residual long wavelength errors in the form of tracks on the maps. These errors induce sea level gradients perpendicular to the track, and therefore high geostrophic velocities which can obscure ocean features. To overcome this problem, an improved objective analysis method which takes into account along-track correlated errors is developed. A specific data selection is used to allow an efficient correction of long wavelength errors while estimating the oceanic signal. The influence of data selection is analyzed and the method is first tested with simulated data. The method is then applied to real T/P and ERS-1 data in the Canary basin (a region typical of low eddy energy regions) and the results are compared to those of conventional objective analysis method. The correction for the along-track long wavelength error has a very significant effect. For T/P and ERS-1 separately, the mapping difference between the two methods is about 2 cm rms (30% of the signal variance). The variance of the difference in zonal and meridional velocities is roughly 30% and 60% respectively of the velocity signal variance. The effect is larger when T/P and ERS-1 are combined. Correcting the long wavelength error also considerably improves the consistency between the T/P and ERS-1 data sets. The variance of the difference (T/P - ERS-1) is reduced by a factor of 1.7 for the sea level, 1.6 for zonal velocities and 2.3 for meridional velocities. The method is finally applied globally to T/P data. It is shown that it is tractable at the global scale and that it provides an improved mapping.
P.-Y. Le Traon and F. Ogor - CLS Space Oceanography Division
Journal of Geophysical Research (in press)
June 1997
ABSTRACT: The ERS-1 orbit error reduction method using TOPEX/POSEIDON data as a reference (Le Traon et al., 1995a) has been applied to ERS-1 data from the first 35-day repeat mission and from the 168-day geodetic mission. The method has been refined and formal error on the estimation is now calculated. With the new T/P and ERS-1 data sets which include in particular JGM-3 orbits and CSR3.0 tidal model, the estimated accuracy of the ERS-1 orbit error estimation is now about 2 cm rms only. The adjustment has been systematically performed for the ERS-1 JGM-3 and D-PAF orbits. The E-E crossover differences are reduced from 17 cm (using the D-PAF orbit) or 11 cm (using the JGM-3 orbit) to only 7 cm rms for all processed cycles. Similarly the TP-E crossover differences are reduced from 13 cm (using the D-PAF orbit) or 10 cm (using the JGM-3 orbit) to only 7 cm rms. The adjusted D-PAF and JGM-3 orbit errors have a mean rms of 10 and 7 cm respectively. The corrected Sea Surface Heights for the D-PAF and JGM-3 orbits have an rms difference of about 1 cm rms only while it is about 11 cm before T/P orbit error correction. This shows that the adjustment is not sensitive to the initial ERS-1 orbit used. It also confirms the 2 cm accuracy of the method. Repeat-track analysis for the 35-day repeat cycles (cycles 6 to 18) is finally performed. Mean difference of sea level variance before and after orbit error correction is 34 cm2 (D-PAF orbit) and 17 cm2 (JGM-3 orbit). Without the correction, even with the recent ERS-1 JGM-3 orbits, the signal is thus still too much corrupted by orbit error to allow an analysis of the large scale oceanic signal and to be merged with T/P data. It is shown, on the other hand, that the corrected ERS-1 and T/P sea level variabilities are in excellent agreement.