In this letter, the recent progress in optimizing the microwave calibration target is reported. Based on the former findings on the wideband reflectivity of the target and the analysis model for the directionally radiated brightness temperature, the authors further investigate the comprehensive optimization in both the aspects of electromagnetic and thermal properties. Specifically, a design instance with a thinner coating at the tip and thicker coating at the bottom is concluded and optimized for stable wideband performance, without introducing further difficulties in manufacturing. The performance of the new design is compared to that of traditional uniform coating, in terms of total reflectivity, temperature gradient, and, finally, the brightness temperature toward the normal direction. Results show the comprehensively improved performance of the new design, which can be a good reference for practical applications.

This communication investigates the wideband reflectivity performance of calibration targets for spaceborne radiometers, which are fabricated as coated pyramids/cones. Specifically, the effects of the coating material properties on the wideband reflectivity performance are studied. Numerical studies based on finite-difference time-domain (FDTD) methods are carried out considering two coating materials with distinct characteristics. The mechanisms of structural absorption and reflection in both the low- and the high-frequency regions are discussed, and the relations between the coating material properties and the total reflectivity are determined. The results show that the absorption within the coating layer is not the only factor that determines the structural reflection in the high-frequency region; the impedance matching between the coating and free space, especially in the case of coated pyramids, is also a determinant. Different geometry optimization strategies are presented for the two materials based on their distinct characteristics. The wideband reflectivity performance is found to be well improved.

In recent years, bistatic synthetic aperture radar (SAR) technique has attracted considerable and increasing attention. Compared to monostatic SAR for which only the backscattering is measured, bistatic SAR expands the scattering measurements in aspects of angular region and polarization, and greatly enhances the capability of remote sensing over terrain and sea. It has been pointed out in recent theoretical researches that bistatic scattering measured in the forward region is preferable to that measured in the backward region in lines of surface parameters retrieval. In the forward region, both dynamic range and signal sensitivity increase to a great extent. For these reasons, bistatic SAR imaging is desirable. However, because of the separated positions of the transmitter and receiver, the degrees of freedom in the parameter space is increased and the forward bistatic imaging is more complicated than the backward bistatic SAR in the aspects of bistatic range history, Doppler parameter estimation and motion compensation, et, al. In this study, we analyze bistatic SAR in terms of ground range resolution, azimuth resolution, bistatic range history and signal to noise ratio (SNR) in different bistatic configurations. Effects of system motion parameters on bistatic SAR imaging are investigated through analytical modeling and numerical simulations. The results indicate that the range resolution is extremely degraded in some cases in forward bistatic SAR imaging. In addition, due to the different imaging projection rules between backward and forward bistatic SAR, the ghost point is produced in the forward imaging. To avoid the above problems, the forward bistatic imaging geometry must be carefully considered. For a given application requirement with the desired imaging performances, the design of the motion parameters can be considered as a question of solving the nonlinear equation system (NES). Then the improved chaos particle swarm optimization (CPSO) is introduced to solve the NES and obtain the optimal solutions. And the simulated imaging results are used to test and verify the effectiveness of CPSO. The results help to deepen understanding of the constraints and properties of bistatic SAR imaging and provide the reference to the optimal design of the motion parameters for a specific requirement, especially in forward bistatic configurations.

This work reports numerical investigations on the scattering from microwave blackbody, for developing a national standard facility on the emissivity determination in China. Based on the Kirchhoff’s law of thermal equilibrium, the standard facility follows a bistatic scattering measurement configuration and covers a wide frequency range. In general, the reflectivity is to be extracted based on partially collected scattering in the near-field region. The whole electromagnetic process, from the quasi-optical antenna illumination, to target scattering, and finally the aperture coupling reception, is modeled to be analyzed. It is shown in the simulation results that the Floquet scattering properties of the array-shaped blackbody can be obtained without fulfilling the far-field condition. Then, evaluations on the reflectivity extraction are performed at 54, 89, 118, and 183 GHz, addressing typical blackbody structures of coated cones and coated pyramids. The factors of referencing targets, curve integration, and the oblique incidence are discussed, as well as the case of geometry flawed blackbody. This work offers important reference and specific suggestions for achieving accurate and stable reflectivity determination in the bistatic near-field configuration.

Electromagnetic wave scattering suppression has applications in many scenes, including radio communication and electromagnetic compatibility. A large amount of research focuses on absorber structures with reflection suppression at the −10 dB level, while scattering suppression beyond −10 dB level is rather rare. In order to study the formation of higher suppression, metamaterial with absorption and scattering composite function is presented based on the coding sequence of absorptive electromagnetic genetic components. The quantized experimental measurement result of scattering suppression reaches −22.4 dB in consecutive 7.152–16.184 GHz band (relative bandwidth 77.4%), which shows a consistent trend with simulation and validates the design process. Optimization with a small gene coding variation rate (4.7%) makes the scattering suppression effect surpass −30 dB line in 9.45–13.8 GHz in simulation. Besides camouflage function, this metamaterial could alter the scattering information in spatial distribution with a high suppression effect, which has a good chance to serve as a transmitter in a local area network with good privacy.

Before practical usage in microwave radiometers, the calibration targets must be tested for its emissivity/total reflectivity. In this work, we address the issue that remains in the total reflectivity determination, which is estimating the total reflectivity based on measured backscattering in the monostatic configuration, specifically at high frequencies. Based on the efficient modeling on periodic unit of the array-shaped calibration target, we quantitatively evaluate the relationship between backscattering and the total reflectivity, on the common structure of coated cones and coated pyramids. It is found that the total reflectivity estimation becomes target-specific for the coated cones; meanwhile, stable compensation can be concluded for the pyramidal units, which is important for practical testing in submillimeter applications. Then, the disturbing effects of the geometric defects on the total reflectivity estimation are discussed, and it is inferred that wideband measurement is vital for diagnosing those defects. Furthermore, actual test results are analyzed, showing the complex scenario in estimating the total reflectivity of a prototype with shape defects. Also, the numerical concluded $C_{g}$ compensation is verified in the clear case of a fine prototype. The findings in this work offer a direct reference for the application of microwave radiometer calibration, especially in the region of submillimeter waves.