First measurement results of a new highly-accurat SAR calibration target

The requirements on new spaceborne synthetic aperture radar (SAR) missions are always pushed towards better image quality with respect to signal-to-noise ratio, radiometric accuracy, and spatial resolution. An accurate radiometric calibration of the whole SAR system is crucial to cope with the demand on high image quality. Among other factors, the quality of the calibration depends on the utilized reference targets. Also permanent system monitoring, onboard and by reference targets, is required to guaranty the image quality over the whole mission time. In this paper a new highly-accurate active calibration target (transponder) is presented. The device is currently under development in the DLR-project “Kalibri” [1] and features remote control and alignment as well as an improved internal calibration for stable operation. Furthermore, first representative measurement results could be achieved by acquiring TerraSAR X images

Linearity Measurements of an Accurate Transponder for Calibrating Future Spaceborne SAR Systems

The requirements on new spaceborne synthetic aperture radar (SAR) missions are always pushed towards better image quality with respect to signal-to-noise ratio, radiometric accuracy, and spatial resolution. Therefore an accurate calibration of the SAR system and the final product is essential. The quality of the calibration depends on the utilized reference target. In this paper a new active calibration target (transponder), currently under development at DLR, and measurement results of the linearity of the high frequency section is presented

Variations of the Transponder’s RCS Due to Environmental Impacts on the Antennas

Transponders for synthetic aperture radar (SAR) need to be extremely precise in order to qualify as absolute calibration references for the increasingly demanding new SAR systems. To guarantee highest accuracy and stability even components which normally are considered ideal, have to be be taken into account. This paper shows the environmental influence on the antenna gain and thus on the overall transponder gain, compares two different housing designs and explains why this particular design has been chosen for the new transponder currently being developed at the DLR

Linking Reference Target Properties to Its Perceived RCS in SAR Images

Reference point targets like active transponders or passive corner reflectors provide a reference reflectivity against which the instrument offset can be determined. Up to now, no direct link between the reference point target properties (like its frequency response) and its perceived backscattering was available. In this paper it is proposed to close this gap using a point target SAR simulator, which links the target properties to the derived absolute calibration factor. This allows to derive correction factors which result in a normalization of calibration results over SAR mode settings and target properties

Highly Accurate Calibration Target for Multiple Mode SAR Systems

The product quality with respect to radiometric accuracy of current and future space-borne synthetic aperture radar (SAR) systems depends on the grade of the utilized external calibration targets. This paper presents the first steps taken in designing and building a highly accurate active target (transponder), which exhibits high radar cross section (RCS) stability (in the order of a tenth of a decibel) over its lifetime even for future multiple mode SAR systems. Furthermore, the transponder allows the recording of the azimuth pattern and fully sampled pulses

Reference Target Correction Based on Point Target SAR Simulation

The backscattering from man-made point targets like passive corner reflectors and active transponders is often used as a radiometric calibration standard for synthetic aperture radar (SAR) calibration. As new systems emerge and the demand for more accurate systems increases, it becomes necessary to better understand the effects of real or imperfect targets on the radiometric calibration results. Therefore, a point target SAR simulator is presented which models the complete external radiometric calibration process. It incorporates a number of target properties like frequency response, transponder internal calibration strategies, noise, and interference signals, and it takes the instrument SAR mode settings into consideration. Thereby, the relevant target backscatter variation as observed in the processed SAR image with respect to an ideal or any other target can be determined. The simulation results are relevant during the design process of a new target as well as during the actual calibration of a SAR system. Based on these point target simulations, correction coefficients can be stated for each target and SAR mode, therefore decreasing the remaining radiometric point target uncertainties. The quantitative examples in this paper show that these corrections can influence the absolute radiometric calibration by more than 1 dB