The calibration target is a vital instrument for calibrating space-borne microwave radiometers, and its emissivity performance must be accurately determined before usage. Based on the Kirchhoff's law of thermal equilibrium, the emissivity of a calibration target can be determined from its electromagnetic reflectivity, which is defined as space integration of scattering. However, due to the general shape of periodic coated sharp pyramids, the scattering from calibration targets shows Floquet mode properties with scattering lobes in upper space. That phenomenon must be considered in the reflectivity measurement of calibration target, especially in the mono-static backscattering configuration. To support such backscattering-based reflectivity measurement, the Floquet mode and scattering patterns from periodic unit and finite-sized array are investigated by numerical simulations, more specifically, by the finite-difference time domain (FDTD) algorithm. The investigations include the scattering power distributions among scattering lobes from coated and bare pyramid arrays, and the ratio of total reflection to backscattering in cases of typical parameters. It is found in the millimeter wave region that the scattering power from bare pyramids is still concentrated in the backscattering lobe in the mono-static configuration, while for the coated pyramids the scattering power is distributed around Floquet modes. For the considered geometry and coating parameters, the power ratio of total scattering to backscattering can be more than 10 dB at the cared frequencies. After all, the numerical results provide referencing correction factor for actual measurement studies. It is also validated by numerical results and suggested in practice, to use periodic simulations of low computational burden to evaluate the compensation factor for the mono-static reflectivity measurement.
2. Wang, Z.-Z., J. Li, S. Zhang, and Y. Li, "Prelaunch calibration of microwave humidity sounder on China’s FY-3A," IEEE Geosci. Remote Sens. Lett., Vol. 8, No. 1, 29-33, 2011.
3. Nian, F., Y.-J. Yang, Y.-M. Chen, D.-Z. Xu, and W. Wang, "Recent progress on space-borne microwave sounder pre-launch calibration technologies in China," Journal System Engineering Electronics, Vol. 19, No. 4, 643-650, 2008.
4. Randa, J., A. Cox, and D. K. Walker, "Proposed development of a national standard of microwave brightness temperature," IEEE Proc. IGARSS, 3979-3982, Jul. 31–Aug. 4, 2006.
5. Nian, F., Y.-J. Yang, and W. Wang, "Research of optimizing the microwave wide band blackbody calibration target," Journal of Systems Engineering and Electronics, Vol. 20, No. 1, 6-12, 2009.
6. Wang, J.-H., J.-G. Miao, Y.-J. Yang, and Y.-M. Chen, "Scattering property and emissivity of a periodic pyramid array covered with absorbing material," IEEE Trans. Antennas Propagat., Vol. 56, No. 8, 2656-2663, 2008.
7. Sandeep, S. and A. J. Gasiewski, "Electromagnetic analysis of radiometer calibration targets using dispersive 3D FDTD," 2012 IEEE Trans. Antennas Propagat., Vol. 60, No. 6, 2821-2828, 2012.
8. Jin, M., M. Bai, and J.-G. Miao, "Emissivity study of the array shaped blackbody in the microwave band," Acta Phys. Sin., Vol. 61, No. 16, 164211, 2012 (in Chinese).
9. Wang, J.-H., Y.-J. Yang, J.-G. Miao, and Y.-M. Chen, "Emissivity calculation for a finite circular array of pyramidal absorbers based on Kirchhoff’s law of thermal radiation," 2010 IEEE Trans. Antennas Propagat., Vol. 58, No. 4, 1173-1180, 2010.
10. Bai, M., M. Jin, N.-M. Ou, and J.-G. Miao, "On scattering from an array of absorptive material coated cones by the PWS approach," 2013 IEEE Trans. Antennas Propagat., Vol. 61, No. 6, 3216-3224, 2013.
11. Pan, G., M. Jin, L.-S. Zhang, M. Bai, and J.-G. Miao, "An efficient scattering algorithm for smooth and sharp surfaces: Coiflet-based scalar MFIE," IEEE Trans. Antennas Propagat., Vol. 62, No. 8, 4241-4250, 2014.
12. Jin, M., M. Bai, L.-S. Zhang, G. Pan, and J.-G. Miao, "On the coiflet-TDS solution for scattering by sharp coated cones and its application to emissivity determination," IEEE Trans. Geosci. Remote Sensing, Vol. 54, No. 3, 1399-1409, 2016.
13. Sandeep, S. and A. J. Gasiewski, "Effect of geometry on the reflectivity spectrum of radiometer calibration targets," IEEE Geosci. Remote. Sensing. Lett., Vol. 11, No. 1, 84-88, 2014.
14. Chen, C.-Y., F. Li, Y.-J. Yang, and Y.-M. Chen, "Emissivity measurement study on wide aperture microwave radiator," IEEE Proc. ICMMT, 914-917, Apr. 21–24, 2008.
15. Gu, D.-Z., D. Houtz, J. Randa, and D. K. Walker, "Reflectivity study of microwave blackbody target," IEEE Trans. Geosci. Remote Sens., Vol. 49, No. 9, 3443-3451, 2011.
16. Houtz, D., D. K. Walker, and D.-Z. Gu, "Progress towards a NIST microwave brightness temperature standard for remote sensing," IEEE Proc. IGARSS, 3485-3488, Jul. 26–31, 2015.
17. Gu, D.-Z., J. Randa, and D. K. Walker, "A geometric error model for misaligned calibration target in passive microwave remote-sensing systems," IEEE Geosci. Remote. Sensing. Lett., Vol. 10, No. 6, 1597-1601, 2013.
18. Gu, D.-Z. and D. K. Walker, "Application of coherence theory to modeling of blackbody radiation at close range," IEEE Trans. Microwave Theory Tech., Vol. 63, No. 5, 1475-1488, 2015.
19. http://www.eccosorb.com/products-eccosorb-cr.htm; www.eccosorb.com/products-eccosorbmf.htm, .