Sentinel 3 Sigma0 Calibration with Transponder

Adrián Flores (isardSAT S.L., United Kingdom)

CoAuthors

Albert Garcia-Mondejar (isardSAT SL., Spain); Mònica Roca (isardSAT SL., Catalonia); Stefano Perticaroli (RAME , Italy); Antonio Mosca (RAME , Italy); Francesco Lombardi (RAME , Italy); Lacrouts Clément (ACRI, France); Stelios Mertikas (TUC, Crete); Achilles Tripolitsiotis (Space Geomatica, Crete); Frantzis Xenophon (TUC, Crete); Franck Borde (ESA , The Netherlands); Constantin Mavrocordatos (ESA, The Netherlands); Sophie Legac (CNES, France); Pierre Femenias (ESA, Italy)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Regional and Global CAL/VAL for Assembling a Climate Data Record

Presentation type: Type Forum

The backscatter from a radar altimeter σ0 is a measure of the surface roughness at scales of a few radar wavelengths; over the ocean, this is used to infer wind speed, which is an essential parameter used in weather forecasting. Over the ice, it gives information about the ice characteristics, which is crucial to understand the dynamics of the sea-ice and ice-sheets.

Some discrepancies have been observed between Envisat and Sentinel-3 for the open ocean backscatter σ0 coefficient. In addition, the Ice Community is requesting to get an accurate absolute knowledge of the backscatter coefficient to within 0.1 dB ideally. In order to cover both points, ESA/ESRIN decided to refurbish the Envisat σ0 transponder so that it can be used operationally for current and future altimetry missions.

The RF of the transponder is switched on upon the detection of the Radar pulses. In addition, it is thermally controlled and its gain is accurately measured/monitored. All these features make this transponder fully relevant to properly validate the Radar Altimeter link budget for a point target. However, it must be emphasised that the transponder has an internal optical fiber delay line of 16.5 kilometers (representing a delay of55 μs). As such, even very minor temperature change could impact the delay. This is why it is not advised to use this transponder as a range transponder but only as a σ0 transponder.

Before implementing operationally the transponder, a field test campaign has been carried out. A total of 6 acquisitions over a preliminary location have been performed. The final operation site will be selected at a later stage once the transponder commissioning is successful.

The σ0 results obtained during this first stage are shown in this poster.
 
Adrián Flores
isardSAT S.L.
United Kingdom
albert.garcia@isardsat.co.uk