As a member of the industrial consortium the Group is contributing the concept for the calibration and verification of the Sentinel-1 SAR system to ESA’s Phase B1 study. The concept identifies and describes all facilities and activities necessary to deliver verified and calibrated SAR products. It specifies the associated algorithms and tools and plans the relevant activities. The Sentinel-1 calibration concept is based on the methodology developed for TerraSAR-L (Link to ‘TerraSAR-L’), which is in turn based on the Group’s heritage from calibration work on ERS-1, SIR-C/X-SAR, SRTM, ENVISAT/ASAR, as well as TerraSAR-X. Consequently, Sentinel 1 will profit from new, innovative methods shortly to be validated on TerraSAR-X (Link to ‘ TerraSAR-X Cal’) and briefly described in the following.
Antenna Model Approach
Sentinel-1 is also an advanced multi-mode synthetic aperture radar system providing Stripmap operation with adjustable swath positions or Wide Swath modes (both in dual polarisation) and the Wave mode (single polarisation). This results in a multitude of beams with different antenna patterns.
Following the developments for TerraSAR-X and TerraSAR-L, the Sentinel-1 calibration concept is also built around an antenna model, which enables the antenna pattern to be accurately derived from the applied excitation coefficients.
The life cycle of this antenna model is shown in Figure 1. It will commence with a definition of requirements and will then be produced and validated prior to launch by using ground based measurements. This activity may involve some levels of iteration of the model once the validation has commenced. The validated model will be available prior to launch, will be verified in orbit in the commissioning phase and will be used from commissioning onwards. Input data from ground based characterisation and in-orbit internal characterisation will be used to provide the beam patterns which will be required during data analysis and image processing.
With the aid of the antenna model, system calibration can be achieved by a limited number of in-orbit verification measurements using suitable ground targets. In this way the commissioning phase can be significantly reduced compared to the traditional (e.g. ASAR) approach based on the calibration of individual beams.
Pulse Coded Calibration (PCC)
The pulse coded calibration is a means of characterising individual rows or modules of an active antenna while all are operating. The Sentinel-1 instrument is capable of calibrating during each imaging mode thanks to PCC. The measured excitation coefficients directly feed into the antenna model for dynamic re-calibration. The PCC or PN-Gating method (Link to ‘TRM Characterisation PN-Gating’) was developed in the Group and is to be implemented for the first time on TerraSAR-X (Link to ‘ TerraSAR-X Cal’).
Noise Characterisation
In an imaging radar system noise not only impacts on the image contrast (radiometric resolution) but can lead to radiometric errors and image impairment due to noise artefacts.
Although the resolution cannot be restored in the data processing, the radiometric bias errors due to noise can be corrected. A prerequisite is the characterisation of the system noise level, which is achieved by collecting data with the radar transmitter switched off, the so-called receive only mode. The noise measurements obtained are adjusted for the system gain and annotated to each product to allow the user to correct the radiometric bias if required.
Last December’s approval of the first phase of the GMES programme includes the continuation of the preparatory activities and in the case of Sentinel-1 the full implementation of the mission. With our current contribution to the Phase B1 study and our long-term experience in SAR calibration we are well prepared to take over responsibility for the detailed Sentinel-1 system calibration and verification plans, design and development of the required tools and facilities but also for the planning and execution of the campaigns in the commissioning phase. For the latter the Group is well equipped with calibration targets and operates and maintains a large calibration site in Southern Germany, see Figure 2.