Improving the Historical ERS and ENVISAT Fundamental Data Records by Means of a New Set of GPD+ Wet Tropospheric Corrections

For more than 30 years, satellite altimetry has been providing a unique data record, crucial for many applications such as ocean circulation and the monitoring of sea level rise at global and regional scale. The stringent requirements of such applications set the need for continuous improvement of satellite altimeter products, including those of past missions, in order to get consistent and homogeneous datasets.

The accurate retrieval of sea surface heights from satellite altimetry requires the measured ranges to be corrected for a number of atmospheric and sea state effects.  Amongst these effects, the path delay induced by the water vapor and cloud liquid water, the wet tropospheric correction (WTC), is still a major source of uncertainty, in particular in the long-term sea level trend.

Sponsored by the European Space Agency, the FDR4ALT project aims at generating improved, up-to-date altimetric records for the historical multidisciplinary 35-day repeat missions, ERS-1, ERS-2 and Envisat. To achieve this goal, a new set of wet tropospheric corrections based on the GNSS-derived Path Delay Plus (GPD+) algorithm, has been developed for these missions and is the focus of this study. GPD+ are wet path delays, obtained by data combination through objective analysis, using all available WTC sources, including onboard radiometers, GNSS, external imaging radiometers and an atmospheric model.

Since the GPD+ WTC are combined values, the intercalibration of the various datasets is a crucial step. To this end, a novel method to intercalibrate the various WTC datasets against one of the current best radiometric references, the Special Sensor Microwave Imager (SSM/I) and the Special Sensor Microwave Imager Sounder (SSMIS), hereafter designated SSMIS, has been developed.

The sensor calibration consists of the adjustment of each dataset to SSMIS using a time-varying set of two parameters (2P), offset and scale factor. For this purpose, 5°x5° grids of WTC mean values for the period of each altimeter cycle and each month, respectively, are computed for SSMIS. Similar grids of mean WTC values for the period of interest are computed for each non-SSMIS sensor, including the MWR onboard each altimeter mission. Using the set of corresponding grid cells for each pair (non-SSMIS, SSMIS) the 2P of the adjustment of each sensor to the SSMIS dataset has been computed. Finally, for the whole study period (1991-2012), ERA5 model and the GNSS dataset have also been adjusted to SSMIS by means of a similar 2P model, with yearly parameters.

This paper summarizes the results, focusing on the impact of the inter-calibration and the added value if these corrections for the FDR4ALT products.

The new calibration revealed to be robust, less sensitive to sampling issues and able to intercalibrate any set of sensors, even with different orbits and sampling patterns. Results show that the new GPD+ WTC are continuous, consistent and intercalibrated datasets, valid over all surface types. In addition to open ocean, they also cover conditions where the measurements from the onboard microwave radiometers are invalid, mostly coastal and polar regions, recovering, on average, 25-30% of the ocean points.