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Satellite altimetry has proved successful as a global tool for monitoring sea surface height, significant wave height and wind speed. Nevertheless, a global archive of 17 years of raw data from a series of missions is presently unexploited around the world coastline. This huge amount of unused data can be re-analyzed, improved and more intelligently exploited, possibly promoting coastal altimetry to the rank of operational service. Operational users interested in monitoring sea level change and wave conditions in the coastal zone (e.g. for coastal erosion, sediment/pollutant transport applications) still rely on sparse (and expensive) in situ monitoring stations or poor models. In this work we present a new approach in the exploitation of altimeter data in the coastal zone (currently impeded by unsuitable waveform retracking scheme and coarse along-track spatial sampling in the coastal zone, among others). The objective of this paper is to show how a new, robust, retracking algorithm is able to retrieve with high accuracy physical ocean parameters from altimeter waveforms in the coastal zone. The main focus lies on retrieving sea surface height in the coastal zone with the same precision as is achieved in the open ocean. In addition, the retrieval of more accurate altimeter-derived wave products in the coastal zone is also important as waves are more directly relevant to many operational applications in the coastal zone.

Fifteen years of global altimetry data over the coastal ocean lie, largely unexploited, in the data archives, simply because intrinsic difficulties in the corrections and issues of land contamination in the footprint. These data would be invaluable for studies of coastal circulation, sea level change and impact on the coastline. Amongst some initiatives, we describe here the COASTALT Project, funded by ESA. The main objective of the COASTALT Project is to contribute towards making the status of pulse-limited coastal altimetry operational. In this paper we will first illustrate the first project phase, based on the assessment of user requirements, and summarize those requirements. Then we will describe the COASTALT methodology and objectives. Finally, we will illustrate and discuss the various options for coastal waveform retracking, and present a plan for the validation of the retracked data. The first results in the radar altimeter waveform analysis show the complexity of the coastal signals due to land contamination and calm/rough waters.

We have used the first three years of ERS-2 altimeter data (May 1995 - June 1998) in order to analyse the space-temporal ocean response to atmospheric pressure variations in the three main ocean world basins: Atlantic, Indian and Pacific. We have also quantified the magnitude of the departures of this response from the hypothetical barometric factor (-0.995 cm/mbar), commonly applied to altimeter data records to eliminate the effect of the atmospheric pressure variations in the ocean (Inverse Barometer Correction). From the results obtained, we have found a different behaviour in the Atlantic Ocean with respect to the other two basins, as far as the magnitude of the barometric factor is concerned. Considering that we have estimated the meridional response to atmospheric pressure variations by applying the collinear track and the crossover track methods, the Atlantic Ocean response is quite similar to the one deduced from the isostatic assumption at all latitudinal bands. Nonetheless, Indian and Pacific Oceans show important departures from the hypothetical value at low latitudes. In order to understand why the Atlantic Ocean response is different from the one obtained in the others two, we can infer some explanations but it seems that the different climatology in the basins could be explaining the results obtained, especially the effect of winds.

The ocean response to pressure variations is subtracted from altimeter records using the standard Inverse Barometer Correction (IBC), based on the hypothetical isostatic assumption. Previous analyses have demonstrated that this assumption has to be applied with care when the high frequency pressure variations are considered, as is the case of using the crossover track method. Using ERS-2 radar altimeter data, we study the response of the Atlantic Sea Level (ASL) to pressure forcings at different ranges of frequency, in order to determine the validity of the isostatic assumption. We have also determined this response when using the outputs of a Global Ocean Model (GOM) forced by pressure and wind fields. From the comparison between both results we have observed that data errors could be underestimating our estimations of the response of the ocean to pressure variations; this underestimation could represent more than 20 percent of the values obtained in equatorial and tropical zones, being insignificant out of the latitudinal band.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.