Núria Tous Ramon

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.

Independent System Calibration of Sentinel-1B

Sentinel-1B is the second of two C-Band Synthetic Aperture Radar (SAR) satellites of the Sentinel-1 mission, launched in April 2016—two years after the launch of the first satellite, Sentinel-1A. In addition to the commissioning of Sentinel-1B executed by the European Space Agency (ESA), an independent system calibration was performed by the German Aerospace Center (DLR) on behalf of ESA. Based on an efficient calibration strategy and the different calibration procedures already developed and applied for Sentinel-1A, extensive measurement campaigns were executed by initializing and aligning DLR’s reference targets deployed on the ground. This paper describes the different activities performed by DLR during the Sentinel-1B commissioning phase and presents the results derived from the analysis and the evaluation of a multitude of data takes and measurements.

In-orbit calibration of the TanDEM-X system

In addition to the first satellite TSX already in-flight since 2007 [1], the second satellite TDX of the TanDEM-X system could be successfully launched in 2010 [2]. The primary object of the TanDEM-X mission is to generate a highly accurate digital elevation model (DEM) with never achieved accuracy on global scale. But in addition to this DEM acquisition based on a bistatic satellite constellation, nominal TerraSAR-X operation shall be available anymore, i.e. the bistatic TanDEM-X mission and the monostatic TerraSAR-X mission have to be operated in parallel with both satellites. Consequently the second satellite TDX had to achieve the same accuracy and performance as those of the first satellite TSX. Based on a short overview of the different calibration procedures the paper discusses the calibration results achieved for the whole TanDEM-X system, successfully in-flight since June 2010.

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.

The DLR SAR calibration center

A SAR system has to be calibrated for quantitative measurement. During this process all essential parameters of a SAR image are linked to their geophysical quantity. This includes the location of the image pixel, its backscattering characteristics (in amplitude and in phase) and polarimetric information. The Microwave and Radar Institute of the DLR has gained a lot of experience in these calibration procedures during the last decades and has developed special methods and dedicated reference targets for spaceborne SAR system calibration. In example of calibration results obtained for different spaceborne SAR systems the DLR SAR Calibration Center is presented.

Radiometric Performance of the TerraSAR-X Mission over More Than Ten Years of Operation

The TerraSAR-X mission, based on two satellites, has produced SAR data products of high quality for a number of scientific and commercial applications for more than ten years. To guarantee the stability and the reliability of these highly accurate SAR data products, both systems were first accurately calibrated during their respective commissioning phases and have been permanently monitored since then. Based on a short description of the methods applied, this paper focuses on the radiometric performance including the gain and phase properties of the transmit/receiver modules, the antenna pattern checked by evaluating scenes acquired over uniformly distributed targets and the radiometric stability derived from permanently deployed point targets. The outcome demonstrates the remarkable performance of both systems since their respective launch.

Geometric Accuracy of Sentinel-1A and 1B Derived from SAR Raw Data with GPS Surveyed Corner Reflector Positions

The geometric accuracy of synthetic aperture radar (SAR) data is usually derived from level-1 products using accurately surveyed corner reflector positions. This paper introduces a novel approach that derives the range delay and azimuth shift from acquired SAR raw data (level-0 products). Therefore, the propagation path is completely retrieved from SAR pulse transmission up to the reception of the point target’s backscatter. The procedure includes simple pulse compression in range and azimuth instead of full SAR data processing. By applying this method, the geometric accuracy of ESA’s Sentinel-1 SAR satellites (Sentinel-1A and Sentinel-1B) is derived for each satellite overpass by using corner reflectors with precisely surveyed GPS positions. The results show that the azimuth bias of about 2 m found in level-1 products for Stripmap acquisitions is reduced to about 15 cm. This indicates an artificial bias arising from operational Sentinel-1 SAR data processing. The remaining range bias of about 1.0 m, observed in L0-products, is interpreted as the offset between the SAR antenna phase center and the spacecraft’s center of gravity. The relative pixel localization accuracy derived with the proposed method is about 12 cm for the evaluated acquisitions. Compared to the full processed level-1 SAR data products, this accuracy is similar in the range direction, but, for the azimuth direction, it is improved by about 50% with the proposed method.