Electromagnetic analysis of radiometer calibration targets using dispersive Finite Difference Time Domain method

Abstract

MICROWAVE and millimeter wave remote sensing offer several advantages over infrared and visible observations. This is primarily due to the fact that microwave radiation is transparent to clouds and aerosols, thereby aiding in the accurate determination of temperature and water-vapor profiles. Radiometer calibration is the process by which a relationship is obtained between the output indicator of the radiometer usually voltage or current and the noise temperature at the radiometer input [1]. Microwave radiometer calibration targets are used in radiometer systems to provide a calibration source of known brightness temperature. Precise absolute calibration of radiometers for many Earth remote sensing applications require knowledge of the brightness temperature when viewing a calibration target to within 100 mK or better. The targets are typically periodic array of wedges or pyramids constructed of a thermally-conductive substrate coated with a thin layer of microwave absorbing material. Microwave absorbent materials are electrically dispersive in nature. The ideal radiometer calibration target should be a blackbody radiator with an emissivity of unity at the frequencies of interest. In order to efficiently design wideband calibration targets, an accurate electromagnetic and thermal analysis of these structures is essential [2]. Electromagnetic and thermal analysis of one-dimensional periodic wedge shaped structures has been presented in [3,4]. This work uses the coupled-wave approach for the electromagnetic analysis. However the work is not extended to two dimensionally periodic (doubly periodic) pyramidal structures. In this work, we have used the three dimensional Finite Difference Time Domain (FDTD) method to perform full wave electromagnetic analysis of doubly periodic pyramidal structures with a thin coating of electrically dispersive radar absorbent materials. Due to the higher accuracy required in microwave radiometry than in traditional electromagnetic or optical full wave simulations, we have developed a new precise 3D FDTD code rather than use commercial software packages. The code can be used to optimize presently used calibration targets as well as develop novel target designs.