Sigma-0 Estimation Improvements, using Jason-CS Altimetric SAR Mode: Results using Simulations and in Orbit CryoSat Data
Escolà, Roger1; Martin-Puig, Cristina2; Garcia-Mondéjar, Albert2; Roca, Mònica1
1isardSAT, POLAND; 2isardSAT, SPAIN
Measurement of the vertical-incidence backscatter coefficient, sigma-0, by radar altimeters has largely been used for the determination of wind-speed over the ocean, using empirical models initially formulated for GeoSat. Until 1991 (Witter and Chelton) it was accepted that sigma-0 differences between altimeters would require customised windspeed algorithms for each instrument. Cox and Munk (1954) established an empirical relationship between surface mean-square-slope of ocean surfaces and windspeed, using an optical sun-glint technique. In 1978 Brown showed that sigma-0 at normal incidence is inversely proportional to surface mean-square-slope. Then in 1979 Brown developed the first wind algorithm (with 2 branches) based on the Cox and Munk results. This was followed by an extension (Brown et al, 1981) to provide a 3-branch wind algorithm. Recent applications of the altimeter sigma-0, such as physically based models of sea-state bias and wave period, require an accurate measurement of Sigma-0.
In calculating sigma-0, three contributions shall be considered: the factors related to the (a) instrument (antenna gain, system gains and attenuations, etc.); (b) the geometry; and (c) the surface. These 3 factors are reconstructed at different levels of processing in the altimetric processing chain. In this paper we aim at investigating the three of them for the new altimetric mode: the SARM. Note that the Level 1b processor (calibrated pulse-width limited or multi-looked SAR data) shall provide the information of the two firsts factors (a and b), whereas the re-tracker used at Level 2, shall provide the last one (c). At isardSAT we have the capacity of such study since our team is composed of Level 1 and 2 SARM experts. Furthermore, isardSAT is the responsible for the development of the Poseidon 4 (P4) Ground Processor Prototype (GPP) of the Jason-CS altimeter and this work is of benefit for the GPP. Jason-CS is an operational oceanography program of two satellites that will ensure continuity to the Jason series of operational missions. It is being developed by a multi-Agency partnership consisting of ESA, EUMETSAT, NOAA, CNES and NASA-JPL. ESA is responsible for the Jason-CS space segment development along with Astrium GmbH as a prime contractor.
In conventional pulse limited altimeters the surface is always illuminated from the zenith. Therefore, the geometry of the scenario affects in the same manner all the echoes in a tracking cycle as the incidence angle remains almost the same and the altitude shall be considered constant. In turn, the geometry factor (b) can be assumed constant for all echoes of an averaged pulse-limited waveform, and therefore it can be applied to the incoherently integrated echo. On the contrary, in SAR altimeters each surface location is illuminated from a wide range of positions, thus the backscatter coefficient depends on the incidence angle of the illuminating beam. For this new observation scenario the geometry factor (b) can no longer be assumed constant for all beams illuminating a given surface. Furthermore, the antenna pattern shall also be taken into consideration, thus another angle dependency is added. This last implies that the instrument factor (a) cannot be assumed constant either. In addition, provided the contributing beams have different range values, the effective area of the surface per beam is not the same either.
The derivation of Sigma0 from SARM has never been attempted before . The objective of this study is to investigate the most adequate methodology to derive sigma0 from SARM at Level 1 and 2 for the Jason-CS mission, and also for the benefit of the Sentinel-3 mission.