Sentinel-1 C&D SAR instruments are designed to continue the observations beyond the lifetime of Sentinel-1 A & B. They are to a high degree rebuilds of S-lA&B and will have the same imaging modes and interfaces to ground. Changes have been implemented within the SAR antenna by using a new Tile Amplifier design, which will contribute to improved radiometric stability and accuracy. The new tile amplifiers simplify the internal calibration approach by reducing the number of internal measurements from five to three. Another improvement is the introduction of interleaved noise measurements during image acquisition in addition to noise acquisitions in preamble and postamble. This is important as the surface brightness temperature have a significant impact on the acquired noise and can change the measured levels by up to 1 dB. A first tile for S-lC with the new design has been successfully tested. Results confirm the updated design and indicate improved measurement accuracy.

Sentinel-1 C&D SAR instruments are designed to continue the observations beyond the lifetime of Sentinel-1 A & B. They are to a high degree rebuilds of S-1A&B and will have the same imaging modes and interfaces to ground. Changes have been implemented within the SAR antenna by using a new Tile Amplifier design, which will contribute to improved radiometric stability and accuracy. The new tile amplifiers simplify the internal calibration approach by reducing the number of internal measurements from five to three. Another improvement is the introduction of interleaved noise measurements during image acquisition in addition to noise acquisitions in preamble and postamble. This is important as the surface brightness temperatures have a significant impact on the acquired noise and can change the measured noise levels by up to 1 dB. A first tile for S-1C with the new design has been successful tested. Results confirm the updated design and indicate improved measurement accuracy.

ESA is developing the Sentinel-1 European Radar Observatory, a constellation of two polar orbiting satellites that provide C-band SAR products for operational applications. The Sentinel-1 mission has been designed to comply with stringent radiometric stability and accuracy requirements. To ensure these requirements are achieved, the mission relies on an efficient and robust strategy for in-flight calibration. This consists of (a) pulse-coded Internal Calibration pulses that achieve leakage cancellation and robust estimation and separation of different types of leakage signals and (b) an Antenna Model that estimates very accurately the antenna radiation patterns based on the instrument configuration and pre-launch measurements. Usage of calibration data and the Antenna Model supports maintaining the long-term absolute radiometric accuracy and enables a graceful degradation of performance in the event of unrecoverable failures of individual transmit/receive modules. During the commissioning phase, calibration also relies on precise external calibration transponders and measurements with notch patterns over the rainforest and transponders for accurate pointing determination.

The Sentinel-1 satellite carries on board a C-band Synthetic Aperture Radar built from 560 active radiating waveguide arrays. The 280 horizontally polarised waveguide arrays and the 280 vertically polarised waveguide arrays are arranged as dual-polarised pairs in suficient proximity to allow for grating lobe free electronic steering of the S-1 SAR antenna in the elevation plane. Each waveguide array is connected to a transmit-receive module (TRM) feeding or receiving signals via their common antenna port. The SAR instrument has an internal RF Characterisation mode exploiting the phase shifting capabilities of the TRMs, that allows monitoring of the individual excitations of each waveguide array in both Tx and Rx. The present paper reports on a novel Pulse Coded Calibration (PCC) scheme, that whilst maintaining some salient features of the Hadamard based PCC, also allows to optimise the antenna active return losses. A low active return loss is beneficial to minimise the RF stress of the TRM output circuitry and improves the accuracy of the RF Characterisation itself. (4 pages)

The Sentinel-1 C-band SAR instrument implements an internal calibration system with a minimum of dedicated calibration elements. The transmit power times receiver gain product, PG, used for correction of the image data, is obtained from five types of calibrations measurements. Leakage signals from TX to RX are the main error sources on the calibration signals. Their impact is minimised by the PCC2 (pulse coded calibration) technique. An RF Characterization mode is implemented to monitor individual elements of the antenna. This mode is based on the PCC512 pulse coded calibration technique. The expected 3sigma accuracy for internal calibration of PG is about 0.3 dB/ 3deg. The expected 3sigma accuracy of RF characterization mode is about 0.4 dB / 5deg.