Another objective of the SeRAC project is to refurbish and upgrade the existing ESA sigma-0 transponder for calibrating now the backscatter coefficient of Sentinel-3 SRAL instruments. That old ESA sigma-0 transponder had been used in the past to calibrate the Envisat Radar Altimeter 2(RA-2) (https://doi.org/10.1016/j.asr.2012.12.014).

Fig.1: Left: The sigma0 transponder of ESA in June 2019. Right: The transponder’s modes diagram.

Refurbishment has been carried out by the Radio Analog Micro Electronics (RAME, https://ramesrl.it), Rome, Italy and completed in 2019 and included, among others:

• New transponder configuration for ensuring its operation in both Low-Resolution-Mode (with only one antenna) and Synthetic-Aperture-Radar (SAR) (two antennas) for Sentinel-3A and Sentinel-3B satellite altimetry missions;
• Assessment of new time domain of calibration sequences, both relative and absolute in test-bench environment with the developed board;
• Development of new software for on-site and remote operations and control of the refurbished transponder;
• Characterization of the receive and transmit antennas using outdoor test range facilities;
• Factory Acceptance Tests for the Radio-Frequency components (carried out in May 2019 in RAME, Rome, Italy).

Fig.2: Left: The ESA sigma-0 transponder consisted from five boards. Right: After its refurbishment, only one board is used to provide the advanced operating features.

In order to evaluate the performance of the refurbished transponder under real-life conditions for both Sentinel-3A and Sentinel-3B several field tests have been scheduled and carried out and results delivered. A crossover location of Sentinel-3A Pass No. 193 and Sentinel-3B Pass No. 214 had been identified in Tuscany, Italy (Fig. 3).

Fig.3: The location of the crossover of Sentinel-3A and Sentinel-3B for the field testing of the sigma0 transponder of ESA in Tuscany, Italy. Test sites are indicated by SP1A and SP1B.

A series of six Sentinel-3A (29/1/2020, 25/2/2020, 12/6/2020, 9/7/2020, 1/9/2020 and 28/9/2020) and two Sentinel-3B (23/6/2020 and 20/7/2020) follow-on field tests have been carried out in the period January – September 2020.

The Sentinel-3A signal as recorded by the sigma-0 transponder in September 2020 is shown in Fig. 4.

Fig.4: Left: The received power of Sentinel-3A as captured during the 28-Sept-2020 field testing (credit: RAME). Right: fully-focused radargram around the transponder location Radio-Frequency input power (credit: isardSAT).

The Sentinel-3 backscatter coefficient bias is determined employing two diverse processing methods: Delay Doppler and Fully Focused. First preliminary results are expected to be delivered and published in the next few months.