Gabriel Castellanos Alfonzo

Independent Verification of the Sentinel-1A System Calibration

In the frame of the COPERNICUS program, the main objective of the Sentinel-1 mission is to ensure the continuity of C-band SAR data for global earth monitoring. Sentinel-1A is the first of two C-band satellites launched in April 2014. In addition to the commissioning of Sentinel-1A executed by the European Space Agency (ESA), an independent verification of the system calibration has been performed by DLR under an ESA contract. For this purpose, the complete calibration chain was developed and established, starting with a calibration concept, a detailed in-orbit calibration plan and the software tools for analyzing and evaluating all the measurements up to the calibration targets serving as accurate reference. Based on an efficient calibration strategy, this paper describes the different activities performed by DLR and presents the results obtained during the commissioning phase (CP) of Sentinel-1A.

The DLR Spaceborne SAR Calibration Center

Abstract A necessary activity for any SAR system is its calibration to establish the relation between radar measurements and geophysical parameters. During this process, all essential parameters of a SAR image are linked to their geophysical quantities. This includes the geolocation of the SAR image, its backscattering characteristics (in amplitude and in phase) and polarimetric information. The Microwaves and Radar Institute of the DLR has gained extensive experience in these calibration procedures during the last decades and has developed special methods and dedicated reference targets for spaceborne SAR system calibration. Through examples of calibration results obtained for different spaceborne SAR mission, the capabilities of the DLR SAR Calibration Center are presented.

First TerraSAR-X TOPS Mode Antenna Pattern Measurements Using Ground Receivers

The antenna model used for correcting the influence of the antenna pattern on synthetic aperture radar (SAR) images requires on-ground validation and in-flight verification. A methodology for the in-flight verification that is based upon the measurement of azimuth antenna patterns using ground receivers has been successfully demonstrated for the operational SAR modes of the TerraSAR-X (TSX) and TanDEM-X (TDX) missions. Recently, the novel (terrain observation by progressive scans) TOPS mode was for the first time implemented as an experimental mode on TerraSAR-X to demonstrate its feasibility in support of its implementation on ESAs Sentinel-1 mission. In this mode, besides scanning in elevation, the antenna beam is steered in flight direction from aft to the front at a constant rate to achieve an enhanced radiometric image performance. This paper discusses the methodology and presents results of the first in-flight antenna characterization of a SAR instrument operating in TOPS mode, in this case TerraSAR-X, using ground receivers. The results demonstrate that the TOPS one-way azimuth antenna pattern can be accurately modeled by the TSX antenna model indicating the general suitability of this approach for the in-flight antenna model verification during TOPS mode operations.

Phase Pattern Calibration for Interferometric Applications in Spaceborne SAR Systems

SAR is a widely used technique to acquire images for geoscience and earth observation applications. Active phased array antennas are commonly used in spaceborne SAR systems. For certain modes and applications, it is necessary to know the phase behavior of these phased array antennas. For applications utilizing the different polarization channels for interferometry, the phase difference between the polarizations needs to be calibrated very accurately as it is the main evaluation parameter. Also for single-pass interferometric missions, the difference between the two antennas in terms of phase gradients is of major importance. This paper demonstrates for the first time the usage of phase patterns in an operational interferometric SAR mission. It describes why these phase patterns are required and how they are used to fulfill the different goals of the missions. Then, the mathematical model to derive the phase of the antenna patterns is shown. Finally, the paper explains how the antenna patterns are calibrated in order to minimize their residual errors and describes in detail the measurements performed for this calibration and verification.