You are here: Home:Departments:Satellite SAR-Systems:Calibration

Calibration

The calibration of imaging radar systems is the main focus of this group and has been an important research field in the Institute for over 20 years. The essential task of calibrating a SAR system is to transform the image information into geophysical parameters like radar cross section maps or digital elevation models. The different error contributions of a SAR system have to be identified, estimated and finally corrected. Increasing complexity of new SAR systems combined with the demand for high accuracy and short duration of the commissioning phase require new calibration concepts. Only such concepts allow to calibrate multiple mode high resolution SAR systems like TerraSAR-X with reasonable effort.

The Calibration Group has been developing and operating the Calibration Facility Oberpfaffenhofen (CALIF), comprising various calibration targets like corner reflectors, transponders and ground receivers in L-, C- and X-band, and the required infrastructure for the preparation and execution of calibration campaigns over large test sites. Furthermore, various S/W tools for the analysis and modelling of a SAR system and for deriving calibration parameters have been developed and implemented and can be modified for future SAR systems. Successful preparation and execution of calibration campaigns has been demonstrated for a number of SAR missions: ERS-1/2 1991-1996, SIR-C/XSAR 1994, SRTM 2000, ENVISAT/ASAR 2002. Currently the group is responsible for the calibrating TerraSAR-X and started developing the interferometric height calibration concept for TanDEM-X.

Tasks / Methods

Antenna Model Approach

SAR sensors based on active phase array antennas feature several operational modes with a multitude of different antenna beams (~10000 beams in case of TerraSAR-X). Conventional approaches to calibrate all these antenna beams individually are not feasible within reasonable time and cost constraints. A precise antenna model is the key element of a novel method, which allows reducing the in-flight measurements at the expense of additional characterisation on ground. In combination with optimisation algorithms, the antenna model can be also used for defining the excitation coefficients of an active antenna even with failed and/or unstable transmit/receive modules. Antenna optimisation has already been performed for the ASAR an ENVISAT and for TerraSAR-X.

Full article

TRM Characterisation PN-Gating

The Pseudo Noise (PN) gating method offers a technique for characterizing individual transmit/receive modules under realistic operational (power supply load) conditions. For this purpose the instrument is operated in a special transmit/receive module characterization mode (PN-gating). This method has been developed by the Institute and already verified on ground with TerraSAR-X, where it will be used for the first time.

Full article

Calibration Facility Oberpfaffenhofen (CALIF) A functional calibration facility is the basic requirement for an accurate adjustment of the complete radar system and thus for the success of the SAR mission. The Institute is well equipped with three different types of calibration targets and maintains a laboratory for the precise adjustment of electronic devices.
Full article

Algorithms and Tools

The implementation of calibration and verification concepts requires algorithms and tools to perform specific measurements and analyses. Early developments date back to the SIR-C/X-SAR missions in 1994 and have been subsequently continuously improved and extended. In 1999 a contract from ESA/ESRIN was awarded for the development of the SAR product control software (SARCON) for ERS and ASAR products. With BAE Systems Ltd. as software engineering partner, SARCON has been extended and improved over the last years and is currently the standard software in ESRIN’s product control service. With the growing complexity of the sensors, more sophisticated algorithms, e.g. for PN-Gating, distributed target ambiguity analysis or antenna beam pattern optimisation, have been added. The core for our calibration and verification algorithms is the CALIX software, which is currently being upgraded for TerraSAR-X and ALOS.

Full article

Projects

The Calibration Concept of TerraSAR-X

The German national TerraSAR-X satellite based on a public-private partnership agreement between DLR and EADS Astrium GmbH and planned for launch in October 2006, will be a SAR system with StripMap, ScanSAR and Spotlight mode, as well as several experimental modes like wide bandwidth operation, along track interferometry or right/left looking radar. The calibration group is responsible for the complete calibration of TerraSAR-X. In addition to the development and the integration of the calibration functions for TerraSAR-X, all calibration activities including the planning, the coordination and the execution of the campaigns during the commissioning phase will be performed by the group.

Full article

TanDEM-X TerraSAR-X will be operated in a tandem orbit configuration with a second X-band SAR satellite, in order to generate a global Digital Elevation Model. A height calibration concept is under development.
Full article

Sentinel-1 Sentinel-1 is C-band SAR system as part of the ESA/EU GMES (Global Monitoring for Environment and Security) programme. As a member of an industrial consortium, the group is contributing with the calibration and verification concept to the Phase B1 study.
Full article

TerraSAR-L Under a contract to EADS Astrium Ltd. the group was responsible for the System Calibration and Verification Plan and the specification of the Instrument Calibration Segment for ESA’s TerraSAR-L phase B study.
Full article


Compute TerraSAR-X or TanDEM-X reference target alignment angles

Calibration

Antenna Model Approach

TRM Characterisation PN-Gating

Calibration Facility Oberpfaffenhofen (CALIF)

Algorithms and Tools

The Calibration Concept of TerraSAR-X

TanDEM-X

Sentinel-1

TerraSAR-L