Transponders (also known as polarimetric active radar calibrators or PARCs) are commonly used for radiometric calibration of synthetic aperture radars (SARs). Currently three methods for the determination of a transponder's frequency-dependent radar cross section (RCS) are used in practice. These require either to measure disassembled transponder components, or a separate radiometric measurement standard (like a flat, metallic plate or a corner reflector), leading to additional uncertainty contributions for the calibration result. In this paper, a novel method is introduced which neither requires disassembly nor an additional radiometric reference. Instead, the measurement results can be directly traced back to a realization of the meter, lowering total measurement uncertainties. The method is similar in approach to the well known three-antenna method, but is based on the radar equation instead of Friis transmission formula. The suitability of the method is demonstrated by a measurement campaign for DLR's three new Kalibri C-band transponders, completed by an uncertainty analysis. The method is not universally applicable for all transponder calibrations because (a) three devices are necessary (instead of only one for the known methods), and (b) the transponders must provide certain additional features. Nevertheless, these features have become standard in modern SAR calibration transponder designs. The novel, potentially more accurate three transponder method is thus a viable alternative for transponder RCS calibration, ultimately contributing to synthetic aperture radars with a reduced radiometric measurement uncertainty.
2. Curlander, J. C., Synthetic Aperture Radar: Systems and Signal Processing, John Wiley & Sons, Inc., 1991.
3. Döring, B. J. and M. Schwerdt, "The radiometric measurement quantity for SAR images," IEEE Transactions on Geoscience and Remote Sensing, Vol. 51, No. 12, 5307-5314, Feb. 2013.
4. Döring, B. J., et al., "Absolute radiometric calibration of TerraSAR-X - Approach and ground," Microwave Conference (GeMIC), 55-58, VDE, 2008.
5. Döring, B. J., et al., "Hierarchical Bayesian data analysis in radiometric SAR system calibration: A case study on transponder calibration with RADARSAT-2 data," Remote Sensing, Vol. 12, No. 5, 6667-6690, 2013.
6. Döring, B. J., et al., "Highly accurate calibration target for multiple mode SAR systems," 8th European Conference on Synthetic Aperture Radar, 1-4, Jun. 2010.
7. Döring, B. J., et al., "Reference target correction based on point target SAR simulation," IEEE Transactions on Geoscience and Remote Sensing, Vol. 50, No. 3, 951-959, Mar. 2012.
8. Foged, L. J., M. Sierra-Castaner, and L. Scialacqua, "Facility comparison campaigns within EurAAP," EUCAP Conference, IEEE, 2011.
9. Freeman, A., "SAR calibration: An overview," IEEE Transactions on Geoscience and Remote Sensing, Vol. 30, No. 6, 1107-1121, Nov. 1992.
10. Gelman, A., et al., Bayesian Data Analysis, 3rd Ed., Chapman & Hall/CRC, Boca Raton, 2013.
11., "Evaluation of measurement data - Guide to the expression of uncertainty in measurement," ISO/IEC Guide 98-3:2008, Nor. 2008.
12. Jackson, H. D. and A. Woode, "Development of the ERS-1 active radar calibration unit," IEEE Transactions on Microwave Theory and Techniques, Vol. 40, No. 6, 1063-1069, Jun. 1992.
13. Jirousek, M., et al., "Development of the highly accurate DLR Kalibri transponder," Proceedings of 10th European Conference on Sythetic Aperture Radar, 1-4, 2014.
14. Knott, E. F., J. F. Shaeffer, and M. T. Tuley, Radar Cross Section, 2nd Ed., SciTech Publishing, Raleigh, NC, 2004.
15. Kummer, W. H. and E. S. Gillespie, "Antenna measurements - 1978," Proceedings of the IEEE, Vol. 66, No. 4, 483-507, Apr. 1978.
16. Raab, S., Planung und Durchf¨uhrung einer Freifeld-RCS-Messreihe zur genauen Kalibrierung von Referenzzielen, MA Thesis, Hochschule für angewandte Wissenschaften Würzburg-Schweinfurt, 2013.
17. Raab, S., Transponderausrichtung bei der Drei-Transponder-Methode: Anleitung und Protokoll zur Kampagne im Dezember 2013, Tech. Rep., Issue 1.0, DLR (German Aerospace Center), Jan. 2014.
18. Raab, S., et al., "Comparison of absolute radiometric transponder calibration strategies," Proceedings of the European Conference on Synthetic Aperture Radar, 1-4, 2014.
19. Ross, S. M., Statistik für Ingenieure und Naturwissenschaftler, 3rd Ed., Spektrum Akademischer Verlag, 2006.
20. Rudolf, D., et al., "Absolute radiometric calibration of C-band transponders with proven plausibility," Proceedings of the European Conference on Sythetic Aperture Radar, 2014.
21. Schwerdt, M., et al., "Final TerraSAR-X calibration results based on novel efficient methods," IEEE Transactions on Geoscience and Remote Sensing, Vol. 48, No. 2, 677-689, Feb. 2010.
22. Schwerdt, M., et al., "Independent Verification of the Sentinel-1A system calibration: First results," Proceedings of the European Conference on Synthetic Aperture Radar, 1259-1262, VDE Verlag GmbH, Berlin, 2014.
23. Warren, L., Stutzmann and Gary A. Thiele. Antenna Theory and Design, 2nd Ed., John Wiley & Sons, Inc., 1998.