The exploration of the Moon has never ceased. One of the most significant challenges is determining the thickness of the lunar regolith. This paper employs the relative microwave brightness temperature (TB) to invert the thickness of the lunar regolith. A multi-layer parallel stratified model serves as the forward model. In the inversion process, the simulated microwave TB is derived by calculating the sum of the TB contributions from each layer. Based on the forward model, areas where the simulated TB is sensitive to lunar regolith thickness can be identified. Subsequently, the simulated TB is compared with the measured TB by the Chang'E-2 Microwave Radiometer (MRM) at 3 GHz at midnight (24:00) of the lunar local time. The discrepancy between the observed and modeled TB at a specified location such as the Apollo 12 landing site (A12) is regarded as a correction for the simulated TBs at other locations with the same latitude. Ultimately, the thickness of the regolith is inverted according to the corrected simulated TB. This paper compares the inverted result with the regolith thickness obtained by DEM data. It found that regions where the model inverted results closely align with the DEM data tend to have higher FeO/TiO2content. The uncertainty of the inversion is also discussed, which indicates that the method presented in this paper is feasible.

In recent years, the application of passive millimeter-wave imaging (PMWI) in target detection, reconnaissance, and surveillance has drawn our attention again, especially passive interferometric millimeter-wave imaging (PIMI). However, PIMI suffers with high hardware and signal processing complexity, and passive interferometric millimeter-wave sensors (PIMSs) are very expensive. In this letter, a W/D-band millimeter-wave radiometer on unmanned aerial vehicles (UAVs) is first developed for target detection, reconnaissance, and surveillance, especially for ship detection. The W/D-band millimeter-wave radiometer has the advantages of low hardware and signal processing complexity and low cost. The W/D-band millimeter-wave radiometer is introduced, and a proof prototype of the W/D-band millimeter-wave radiometer is manufactured. Numerical simulations are performed to analyze the brightness temperature (TB) characteristic of the metallic ships. Outdoor experiments are performed to assess the feasibility of the metallic target detected by the W/D-band millimeter-wave radiometer on a UAV. Initial experimental results have demonstrated that metallic targets can be detected by the W/D-band millimeter-wave radiometer from UAVs, as expected.

Fire accidents in forest or urban areas can result in serious casualties and financial losses. Due to the penetrating ability of low-frequency microwave signals, the microwave radiometer is able to detect a potential fire hazard before the fire breaks out. Considering the flexibility and convenience in practical applications, the fire monitoring system based on microwave radiometers faces problems. To solve the problems, a novel reconfigurable intelligent surface-assisted (RIS-assisted) radiometer is proposed in this article. The trigger model (also called the system model) of RIS-assisted radiometers for early fire warning is derived. The system models of single and distributed RIS-assisted radiometers are derived and analyzed. The derivations of the system models also expound the operating principle of RIS-assisted radiometers for potential fire monitoring in practical applications. The numerical simulations and experiments are conducted, and the results validate the feasibility of the RIS-assisted radiometer for potential fire monitoring. Finally, to understand the RIS-assisted radiometer more comprehensively in practical applications, the imperfections of the experiment are discussed. By utilizing the technique of the RIS, the microwave radiometric system may have the opportunity to be applied to potential fire monitoring with low hardware complexity and low cost.

The vertical temperature distribution in the permanently shaded region (PSR) has a significant impact on the temporal and spatial distribution of the cold trap. To obtain the vertical temperature profile of the PSR, an inversion method that fuses microwave and infrared brightness temperature (TB) data is proposed. In the inversion process, the infrared data were initially employed to derive the optimal value of the H-parameter that controls the density profile. Subsequently, high-frequency (37 GHz and 19.35 GHz) microwaveTBdata were employed to ascertain the range of surface density, whereas low-frequency (3 GHz) microwaveTBdata were utilized to determine the range of bottom density. A fixed correction was applied to the 3 GHz brightness temperature data to account for the calibration error. Due to the inherent uncertainties associated with the thermal model, both the Hayne and Woods models were employed in the inversion process, yielding disparate results. The PSR in the Haworth impact crater was selected as a case study for the inversion. The Woods model was found to provide a superior explanation for the microwave observation. The optimal surface density of the PSR of the Haworth crater was determined to be within the range of 1200~1300 kg m-3, while the bottom density was within the range of 2100~2200 kg m-3. The inverted vertical temperature distribution in the PSR of Haworth crater indicates that the depth of the cold trap can reach approximately 8.5 m. Additionally, the impact of heat flow on microwaveTBis discussed.

Passive interferometric microwave sensors (PIMSs) have been proposed for target detection, such as ships and aircrafts. However, the detection performance of the PIMSs has not been analyzed and discussed for target detection in detail before. In this letter, the detection performance of the PIMSs is analyzed in terms of detection probability at a constant false alarm rate (FAR). An end-to-end performance simulator is established to analyze the detection performance of the PIMSs in terms of detection probability at a constant FAR that 10-5in detail. Numerical simulations are performed to analyze the detection performance of the airborne PIMS for ship detection. The numerical results indicate that the detection performance depends on the height (distance), the targets, and the PIMSs, as expected.

Researches have demonstrated that passive interferometric microwave sensors (PIMSs) can be used in the imagery acquired and target detections, especially in low visibility and/or high sea clutter environments. Target detection by PIMSs is based on the high contrast against the background in the brightness temperature (TB) maps, and it is found that the targets still exhibit a high contrast within the alias field of view (FOV) of the background cancellation TB maps obtained by a background canceling method in the target detection algorithm. As a result, the targets can also be detected within the alias FOV of PIMSs, and PIMSs exhibit a wider FOV for target detection than that for Earth’s remote sensing. However, the effective FOV of PIMSs has never been studied for target detection before. In this article, the effective FOV of PIMSs is first studied for target detection. The effective FOV of PIMSs is analyzed and discussed for target detection in theory, which is based on the FOV of PIMSs for Earth’s remote sensing and the background canceling method. Numerical simulations are performed to analyze the effective FOV of PIMSs in terms of the metallic with a “cold” TB characteristic and the island with a “hot” TB characteristic. A series of experiments have been performed to verify the effective FOV of the PIMSs for target detection in ships, islands, and aircrafts. Both numerical and experimental results have demonstrated that the effective FOV of the PIMSs for target detection based on the background canceling method can be extended to the central hexagonal regions (${H}$), and it exhibits a wider FOV than alias-free FOV (AF-FOV) and/or extended AF-FOV (EAF-FOV) in Earth’s remote sensing, as expected.

A microwave radiation model with coherent surface scattering is developed for analyzing the topographic effects of the lunar permanently shadowed region (PSR) on microwave radiometric observations. The coherent surface scattering to the observer, which comes from the nearby surface due to the variation of the surface slope, is quantified by the ray-tracing method. In addition, the vertical distribution of temperatures of the PSR is estimated by a one-dimensional thermal model with three-dimensional shading and scattering effects. The impact of coherent scattering on microwave brightness temperatures by a nadir-look radiometer becomes more noticeable with high-resolution microwave brightness temperature data, such as 4 pix/deg by 4 pix/deg in this study. The rise in brightness temperature at certain locations in PSR may reach up to ~8 K at 37 GHz. However, when compared with the low resolution of the brightness temperature by Chang’E-2, the averaged contribution of coherent surface scattering is less than 1 K. Meanwhile, there is a discrepancy between the model-generated microwaveTBand the measuredTBof Chang’E-2, both in small and large craters. This discrepancy may be explained by a calibration issue or the uncertainty of model parameters. Nonetheless, the trend in the model-generated brightness temperature is consistent with the measurements, indicating that the proposed model has the potential to predict brightness temperature effectively within the permanently shadowed region.

Airborne passive interferometric microwave sensors (PIMSs) were proposed as a powerful complementary tool for ship detection, especially in low visibility and high sea clutter environments. At present, there is only one algorithm for ship detection by airborne PIMSs, and this algorithm is only based on the “cold” brightness temperature (TB) characteristic of metal ships with wakes in passive interferometric microwave images (PIMIs). Moreover, it has been found that ship wakes always exhibit a “hot” TB characteristic in PIMIs. In this article, an improved algorithm is innovatively proposed for ship detection by airborne PIMSs to improve detection performance, and it is simultaneously based on the “cold” and “hot” TB characteristics of metal ships with wakes. The microwave TB model of metal ships with wakes is established to analyze its TB characteristics. Given the TB model and its characteristics, an improved algorithm is introduced, which comprises five steps: searching targets, improving TB maps, clustering targets, estimating ship headings, and estimating ship trajectories. The improved algorithm can detect ships with a low false-alarm probability (FAR) and accurately estimate ship headings using only a single snapshot and the ship trajectory. The practicability and detection performance of the improved algorithm is verified by airborne experiments using an X-band PIMS. The experimental results demonstrate that the improved algorithm can accurately detect ships, estimate ship headings using only a single snapshot, and estimate ship trajectories. The impact of the algorithm parameters on the detection performance is analyzed and discussed. Moreover, the detection performance of the improved algorithm is analyzed and compared with the performance of the only existing algorithm, and the experimental results indicate that the improved algorithm has a better detection performance with a lower FAR value than the only existing algorithm, as expected.

Lunar thermal conductivity is significantly important for understanding the geological processes of the Moon. The Apollo in situ heat flow experiments and diviner remote sensing data provide us with good constraints on the thermophysical properties of the lunar regolith fines layer. As the first farside in situ experiments, Chang’E-4 (CE-4) temperature sensors will further extend our understanding. In this study, a 1-D thermal model with updated bulk densities by the CE-4 lunar penetrating radar (LPR) data is used. Then, the nighttime surface temperatures from the sensors of the CE-4 lander are applied to retrieve the surface thermal conductivity at the CE-4 landing site. After minimizing the differences between the predicted temperature and CE-4 measurements, the range of surface layer contact conductivity ($k_{s}$) within the upper 1 cm is about (0.95–2.26)$\times 10^{-3}\,\,\text {Wm}^{-1}\cdot \text {K}^{-1}$for T1, (1.04–2.44)$\times 10^{-3}\,\,\text {Wm}^{-1}\cdot \text {K}^{-1}$for T2, (0.60–1.40)$\times 10^{-3}\,\,\text {Wm}^{-1}\cdot \text {K}^{-1}$for T3, and (0.60–1.39)$\times 10^{-3}\,\,\text {Wm}^{-1}\cdot \text {K}^{-1}$for T4, respectively. In addition, the factors that may affect the inversion or the measurements were discussed, such as the shadowing effect, the rock abundance (RA), the lateral conduction from the rover transfer mechanism, and the densification effect during the measurements.

Near-surface temperatures of permanently shadowed regions (PSRs) on the Moon provide fundamental information for water ice exploration. Seasonal temperature variations of PSRs are found in both Chang’E-2 microwave radiometer data and Diviner Lunar radiometer observations. Furthermore, unusual microwave brightness temperature variations between February 2011 and May 2011 of double-shaded PSRs are shown in the Chang’E-2 observational data, i.e., that the minimum microwave brightness temperature occurs before the time when the infrared brightness temperature reaches the minimum in double-shaded PSRs. To interpret this phenomenon, the 1-D thermal model and the microwave radiation transfer model are used. In the thermal model, the reradiation energy from the illuminated area is estimated by effective solar irradiance, which is an analytic solution for the radiative equilibrium temperature in the shadowed area of a spherical bowl-shaped crater. In the simulation, an assumed internal 0.4 W/m2heat flow beneath the lunar surface made a plausible fit to the unusual variations during some lunations. However, this is a huge value compared with the well-known heat flow value of about 0.018 W/m2. Furthermore, it is difficult to obtain this extra heat energy by lateral conduction below the surface in a large impact crater due to the small thermal conductivity of the lunar regolith. Finally, the unusual microwave brightness temperature (TB) changes are concluded to be caused by a calibration problem after excluding other possible reasons. In addition, a statistical correction method is applied to revise the problematic TB data to obtain the proper variation trend of the brightness temperature.

In aperture synthesis radiometers (ASRs), the mutual coupling effect between antennas is a key problem, which degrades the performance of ASRs and should be compensated. In this paper, an improved compensation method is proposed for ASRs, which is based on the receiving mutual impedance. The proposed method is verified by a one-dimension 5-channel L-band ASR. The experimental results show that the proposed method is superior over the existing method based on the transmitting mutual impedance, and is more appropriate to compensate the mutual coupling effect for ASRs.

A novel compact size dual-band dual-polarized antenna array for MIMO WLAN is reported. The array element comprises a periodic antenna with bowtie dipoles and a printed dipole antenna with a reflector and a director. The presented design is characterized by dual-band operation, good front-to-back ratios, average gains of 4 and 5 dBi over the 2.4 and 5.2 GHz bands respectively. A prototype 12-element array exhibits dual broad bandwidth and low mutual coupling among 2.25GHz-2.6GHz and 4.95GHz-6GHz. The measured results indicate the array is suitable for 802.11a/b/g/n systems employing MIMO and beam selection techniques.