Contributions to the improvement of the wet tropospheric correction for SARAL/AltiKa

Clara Lazaro (Universidade do Porto, Faculdade de Ciências; CIIMAR, Portugal)


M. Joana Fernandes (Universidade do Porto, Faculdade de Ciências; CIIMAR, Portugal); Alexandra L. Nunes (Instituto Politécnico do Porto - ISEP; CIIMAR, Portugal); Pires Nelson (Universidade do Porto, Faculdade de Ciências; CIIMAR, Portugal)

Event: 2014 SARAL/AltiKa workshop

Session: Instrument processing

Presentation type: Type Poster

The SARAL satellite, a joint mission by the Indian Space Research Organization and the Centre National d'Études Spatiales (CNES) carries the AltiKa instrument, built by CNES. Launched in February 2013 to the same orbit of Envisat, it is a mission of great relevance, allowing the continuity of altimeter measurements over the 35-day tracks of the ESA altimeter missions.

The AltiKa instrument includes a Ka band altimeter and a two-band (23.8 and 36.8 GHz) microwave radiometer (MWR) similar to that of Envisat, sharing the same antenna and allowing a smaller footprint.

The wet tropospheric correction derived from the MWR on board SARAL is being estimated by CNES using a retrieval algorithm similar to the one developed for Envisat, adapted to the technical characteristics of this new instrument and under improvement.

This study aims to give a contribute to the assessment of the AltiKa MWR-derived WTC and to suggest alternative corrections.

For this purpose, the WTC from the Data Combination (DComb) and GNSS-derived path delay (GPD) algorithms, developed by University of Porto, were computed for several SARAL cycles.

The DComb WTC, developed for CryoSat-2 In the framework of the CP4O project, is based on the data combination through objective analysis (OA) of three wet path delay data sources: scanning imaging MWR (SI MWR) on board remote sensing satellites; Global Navigation Satellite Systems (GNSS) measurements at coastal stations and ECMWF operational model fields, being independent of the on-board MWR measurements. The GPD algorithm, developed in the scope of the COASTALT and Sea Level CCI projects, is also based on the data combination through OA of three WTC data sources, GNSS and ECMWF model fields, the SI-MWR measurements used in the DComb estimation being replaced by valid on-board MWR measurements.

The three radiometer based WTC (original correction derived from the on-board MWR, DComb and GPD) and the WTC from ERA Interim and ECMWF operational models are inter-compared using various statistical analyses: direct comparison of the various WTC, Sea level (SLA) anomalies variance at crossovers, SLA variance difference function of distance from the coast and function of latitude.

The GPD correction removes outliers and the strong contamination by land and ice present in the original MWR WTC, providing a continuous along-track correction. Similar to previous results obtained for all ESA altimeter missions, the GPD correction shows a significant improvement, particularly in the coastal and polar regions. The DComb WTC enables an independent evaluation of the original MWR WTC, particularly in open-ocean where the SI-MWR measurements are free of land or ice contamination and may give insight to the quality of the original MWR WTC e.g. in rain conditions.

Clara Lazaro
Universidade do Porto, Faculdade de Ciências; CIIMAR