A new side-lobe correction for Sentinel-3A Microwave Radiometer: definition and assessment

Mathilde Siméon (CLS, France)

CoAuthors

Marie-laure Frery (Cls, France); Franck Borde (ESA, Netherlands); Christophe Goldstein (CNES, France)

Event: 2019 Ocean Surface Topography Science Team Meeting

Session: Instrument Processing: Propagation, Wind Speed and Sea State Bias

Presentation type: Type Poster

The Sentinel-3A Surface Topography Mission has been launched on February 2016 and is now in its third year of operation. Its objectives are to serve primarily the marine operational users but also allow the monitoring of sea ice and land ice, as well as inland water surfaces.
A two-channels microwave radiometer (23.8 and 36.5 GHz) similar to the Envisat and ERS MWR sensors is combined to the altimeter in order to correct the altimeter range for the excess path delay (WTC for wet tropospheric correction) resulting from the presence of water vapor in the troposphere.
Brightness temperatures for each band are computed from the MWR raw measurements using an instrument model to consider calibration, losses and thermal behaviour. Finally, any contamination of the antenna measurement outside the main beam, such as sky or on-earth sidelobes is removed. Sidelobes aiming on Earth represent the main contributor. But it is complex to estimate as it depends on the observed scene. It is of great importance to estimate the most precise side-lobe correction, as it can contaminate the brightness temperatures of a few Kelvins near coastlines.
The current Sentinel-3 sidelobe correction is inherited from Envisat and uses both its antenna power pattern and measured temperatures. Even though Sentinel-3 and Envisat MWR are close (same frequencies, similar observation geometry), the footprints are significantly different, and the antenna power pattern is very specific to each instrument. It is obvious that using Envisat side-lobe correction leads to an overestimation of the sidelobe correction in coastal areas, therefore it is not suited to Sentinel3.
We will present here the thorough analysis of Sentinel-3A antenna pattern that has been conducted to estimate the importance of sidelobes and its asymmetry. By performing simulations of the side-lobe contribution to the antenna measurement from a given high-resolution scene, we pinpointed several methodological sources of errors that can impact the sidelobe-correction of a few Kelvins. Following these results, we will introduce the improvements brought to the Envisat sidelobe correction method to generate side-lobe correction maps for Sentinel-3A.
Then we will talk about the assessment of the MWR performances over ocean as well as coastal areas using the new side-lobe correction. The assessment will be performed on brightness temperatures by statistical analysis. The impact on the wet tropospheric correction quality is assessed using global metrics (differences of SSH variance at cross-overs) and comparison to in-situ measurements (GPS and radiosondes).

 

Poster show times:

Room Start Date End Date
The Gallery Tue, Oct 22 2019,16:15 Tue, Oct 22 2019,18:00
The Gallery Thu, Oct 24 2019,14:00 Thu, Oct 24 2019,15:45
Mathilde Siméon
CLS
France
msimeon@groupcls.com