Accurate Transponder Calibrations with the Novel Three-Transponder Method

Transponders are, besides trihedral corner reflectors, the most commonly used measurement standards in radiometric SAR calibration. They allow signal recording for the reconstruction of the azimuth pattern of the SAR system, adjustments of the backscattering matrix for polarimetric applications, and radar cross sections (RCSs) which are potentially much larger than those of passive point targets. These advantages led DLR to develop, manufacture, and install three new, accurate C-band “Kalibri” transponders in South Germany, which are now being used for the calibration and monitoring of the Copernicus Sentinel-1A satellite. Before the transponders could be used as radiometric measurement standards, they needed to be calibrated themselves. In an effort to find the most accurate RCS calibration approach for the given transponder design, several existing methods were compared [1], and a new, potentially highly accurate method, devised which exploits the specific design of the Kalibri transponders [2]. The new “three-transponder method” is similar in principle to the known “three-antenna method”, but is based on the radar equation instead of the Friis transmission formula. The approach exploits the fact that modern transponders like the “Kalibri” device can also be operated as radars because of the integrated digital sub-system (which is needed to implement a digital delay line and incorporates an AD and DA converter). To conduct a complete measurement, three transponders and three measurements (with one transponder pair each) are required; refined measurement schemas are also possible. In comparison to existing methods, no additional radiometric measurement standard is needed, which so far has been one of the limiting factors in accomplishing lower calibration uncertainties. Measurement traceability is achieved by tracing a comparatively simple length measurement back to a national realization of the meter. Such a length measurements can be performed with high accuracy. The presentation will include the setup and the measurement results of a first demonstration measurement campaign. Despite remaining challenges in the practical implementation, the uncertainty analysis shows that the method is a good candidate for highly accurate transponder RCS calibrations in the future

Comparison of conventional and shifted excitation Raman difference spectroscopy for bacterial identification

Raman spectroscopy is an emerging tool for fast bacterial identification. However, Raman spectroscopy is depending on suitable preprocessing of the spectra, thereby background removal is a decisive step for conventional Raman spectroscopy. The background has to be estimated, which is challenging especially for high fluorescence backgrounds. Shifted excitation Raman difference spectroscopy (SERDS) eliminates the background through the experimental procedure and holds as promising approach for highly fluorescent samples. Bacterial Raman spectra might be especially complex because these spectra consist of a multitude of overlapping Raman bands from a large multiplicity of biomolecules and only subtitle differences between the species Raman spectra enable the bacterial identification. Here, we investigate the benefits of SERDS compared with conventional Raman spectroscopy specific for the study and identification of bacteria. The comparison utilizes spectra sets of four bacterial species measured with conventional Raman spectroscopy and SERDS and covers three processing approaches for SERDS spectra, for example, the reconstruction with a non‐negative least square algorithm

Acute aortic dissection type A discloses Corpus alienum

We report an unusual case of an aortic type A dissection with a corpus alienum which compresses the right ventricle. The patient successfully underwent an aortic root replacement in deep hypothermia with re-implantation of the coronary arteries using a modified Bentall procedure and the resection of the corpus alienum. Intraoperative finding reveals 3 greatly adhered gauze compresses, which were most likely forgotten in the operation 34 years ago

Excitation energies for transition metal atoms - A comparison between coupled cluster methods and second-order perturbation theory

Coupled cluster methods are compared to multiconfigurational second-order perturbation theory in a study of the high spin d(x)s(2) -> d(x+1)s(1) excitations in first, second, and third row transition metals. Large basis sets of the atomic natural orbital type are used. Scalar relativistic effects are included using the Douglas-Kroll-Hess Hamiltonian. The effect of spin-orbit coupling is demonstrated for third row atoms. The results show that the two methods give results of the same accuracy, with the exception of first row atoms with less than five 3d-electrons. The CASPT2 method here overestimates the effect of 3p correlation with about 0.1 eV

Analysis of an Improved Temperature Management Concept for SAR System Calibration Transponders

For the radiometric calibration of spaceborne synthetic aperture radar systems, active targets with known backscatter, so called transponders, are used. These serve as an external absolute reference, and the quality of the derived calibration parameters depends on the quality of the deployed transponders. Due to the temperature dependent behavior of the implemented active radio frequency components the transponder radar cross section is sensitive to the temperature of the components. This effect requires the implementation of a reliable and precise temperature management System to stabilize the temperature of the radio frequency components to a constant value under all relevant environmental conditions. In order to fulfill the increasing requirements for future transponders towards radiometric accuracy new temperature control concepts must be developed. For this purpose, existing temperature management systems are analyzed and a promising concept is investigated. In the final version of this paper the results of measurements in a climatic chamber are presented in order to analyze the performance of the new control concept

Absolute Radiometric Calibration of the Novel DLR “Kalibri” Transponder

The technological advancement of the synthetic aperture radar (SAR) principle leads to an innovative challenge for the calibration as well. In order to provide an active reference target for an accurate absolute radiometric calibration the knowledge of the target’s backscattering characteristics is essential. For the recently developed DLR C-band transponder named “Kalibri” several strategies for an accurate determination of the radar cross section (RCS) have been analyzed. Based on a comparison with respect to accuracy and feasibility, several recommendations for the best transponder calibration strategy were established. The resulting RCS of the transponders retrieved from the most suitable measurement method is presented as well as a cross-validation to prove the plausibility of these results

Absolute Radiometric Calibration of C-Band Transponders with Proven Plausibility

The progressive development of upcoming satellite missions using synthetic aperture radar (SAR) lead to novel challenges also for the reference targets. In order to provide a reference target for an accurate absolute radiometric calibration the knowledge of the backscattering characteristics is essential. Whereas usually one single measurement is used for the estimation of the radar cross section (RCS), this paper presents two conducted, independent methods to derive the RCS of three C-band transponder leading to cross-validated results based on a compatibility analysis