Mesoscale mapping capabilities of multiple-satellite altimeter missions

The purpose of this paper is to quantify the contribution of merging multip le-satellite altimeter missions to the mesoscale mapping of sea level anoma ly (H), and zonal (U) and meridional (V) geostrophic velocities. A space/ti me suboptimal interpolation method is used to estimate the mean and standar d deviation of the H, U, and V mapping errors (as a percentage of signal va riance) for different orbit configurations. Only existing or planned orbits [TOPEX/Poseidon (T/P), Jason-1, ERS-1/2-ENVISAT, Geosat-GFO] are analyzed. Jason-1 and TIP orbits are assumed to be interleaved. A large number of si mulations are performed, including studies of sensitivity to a priori space scales and timescales, noise, and latitude. In all simulations, the Geosat orbit provides the best sea level and velocity mapping for the single-sate llite case. In most simulations, the Jason-1-T/P orbit provides the best tw o-satellite mapping. However, the gain from an optimized two-satellite conf iguration (Jason-1 + T/P) compared to a nonoptimized configuration (T/P + E RS or T/P + Geosat) is small. There is a large improvement when going from one satellite to two satellites. Compared to T/P, the combination of T/P an d ERS, for example, reduces the H mean mapping error by a factor of 4 and t he standard deviation by a factor of 5. Compared to ERS or even Geosat, the reduction is smaller but still by a factor of more than 2. The H mapping i mprovement is not as significant when going from two to three or three to f our satellites. Compared to the Geosat, ERS, and T/P mean mapping errors, t he Jason-1 + T/P mean mapping error is, respectively, reduced by 5%, 9%, an d 17% of the signal variance. The reduction in mean mapping error by going from two to three and from three to four satellites is, however, only 1.5% and 0.7% of the signal variance, respectively. These results differ from Gr eenslade et al, mainly because of the definition of resolution adopted in t heir study. The velocity field mapping is also more demanding in terms of s ampling. The U and V mean mapping errors are two to four times larger than the H mapping error. Only a combination of three satellites can actually pr ovide a velocity field mean mapping error below 10% of the signal variance. The mapping of V is also less accurate than the mapping of U but by only 1 0%-20%, even at low latitudes. These results are confirmed using model data from the Parallel Ocean Climate Model (POCM). POCM H, U, and V are thus ve ry well reconstructed from along-track altimeter data when at least two sat ellites are used. The study also shows that the Jason-1-T/P orbit tandem sc enario has to be optimized taking into account the other satellites (GFO an d ENVISAT). It also confirms the usually agreed upon main requirement for f uture altimeter missions: at least two (and preferably three) missions (wit h one very precise long-term altimeter system to provide a reference for th e other missions) are needed.