The reconfigurable intelligent surface (RIS) stands as an emergent technique poised to revolutionize future vehicular communication. In order to facilitate the application of RIS in vehicular communication, this paper proposes two effective RIS-aided schemes tailored for polarization modulation (PM) aiming to achieve remarkable spectral efficiency. Specifically, Scheme I proposes a novel index modulation (IM) aided PM-RIS framework that innovatively encodes information across three distinct dimensions, the indices of dual-polarized antennas (DP-AEs), PM symbols and the reflecting surface indices of the RIS. Scheme II employs a deep neural network method to analyze the channel state information (CSI) for transmit antenna selection, that enhances the reliability and interference immunity in dynamic conditions while reducing computational complexity. Furthermore, a stepwise estimation (SE) method is introduced at the detection stage, which significantly reduces the complexity associated with information decoding. The simulation results highlight the superiority of the proposed system. Compared with traditional PM systems, it demonstrates enhanced performance in terms of spectral efficiency, channel stability, and bit error rate (BER).

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.

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.

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.

The mirrored aperture synthesis radiometer (MASR) has been proposed for high-resolution observation in recent years. Since the MASR is a kind of linear system, the matrix reconstruction method should be an alternative for image reconstruction of a practical MASR. However, the matrix reconstruction method is neither theoretically analyzed nor experimentally validated at present, and the performance of the matrix reconstruction method for a MASR in practical applications is still unknown. In this letter, the H-matrix reconstruction method including the truncated singular value decomposition (SVD) and Tikhonov regularization, not yet applied to a MASR, is introduced. Then, the numerical simulations and imaging experiments are performed to validate the presented method, and the performance of the presented method is also shown. Finally, the comparisons between the presented method and the cosine-transform-based iterative method are made in the view of imaging experiments. The H-matrix reconstruction method is more convenient in the situation of a small MASR with a small alias-free field of view (FOV).

The presence of unauthorized radio frequency interference (RFI) sources seriously affects the retrieval of brightness temperature of a synthetic aperture interferometric radiometer (SAIR). Postobservation RFI geolocation is important for improving the RFI situation hereafter and mitigating the impact of the identified RFIs. Previous works have demonstrated the potential of using high-/super-resolution direction-of-arrival (DOA) estimation techniques to achieve accurate localization of RFI sources in SAIR imagery. In this article, a multiple snapshot difference method is proposed to further reduce the RFI source geolocation bias. The covariance matrices of the consecutive snapshots with power changed RFIs are picked out and subtracted to cancel out the background scene. A MUSIC-based DOA algorithm is developed. Numerical simulations validate the theoretical correctness and the practical effectiveness of the proposed method.

The mirrored aperture synthesis radiometer (MASR) has been proposed as a promising technique for Earth observation. However, considering the system imperfections, the cosine visibility function and the brightness temperature distribution do not constitute a cosine transform pair. The method to improve the quality of reconstructed brightness temperature images for a practical MASR has not yet been developed. In this article, the complete formulation of cosine visibility functions in a practical MASR is derived to better understand the measurement of a MASR with non-negligible effects of the system imperfections. The array factor of a practical MASR is analyzed. Then, the alias-free field of view (FOV) for a MASR is presented, and an iterative method is proposed to reconstruct the brightness temperature image in the alias-free FOV to reduce the computational burden. The numerical simulation and experimental results demonstrate that the proposed method can be applied to practical MASRs and that the reconstruction quality is significantly improved by the proposed method. Finally, the H-matrix inversion method is discussed as an alternative reconstruction method for a practical MASR. Compared with the H-matrix inversion method, the proposed method is more convenient for practical applications.

Airborne passive remote sensing is an important method for Earth observation. In the airborne platform, it is difficult to mount an aperture synthesis radiometer (ASR) with a large co-plane antenna array due to the limitation of aerodynamic drag. In this situation, a conformal antenna array has the advantage of achieving large antenna array to improve the spatial resolution in low frequencies. In this paper, the basic principle of an ASR with a conformal array is analyzed. To reconstruct images with large field of view (FOV), the reconstruction method is introduced. The numerical simulation is conducted to validate the proposed method.

Aperture synthesis technique is able to provide high spatial resolution without requiring very large and massive real aperture. Recently, in addition to the well-known aperture synthesis radiometer, the concept of mirrored aperture synthesis has been proposed, and the experiments has been carried out to validate this technique. However, the field of view (FOV) of a mirrored aperture synthesis radiometer (MASR) has not been analyzed. In this paper, the FOV of an one-dimensional MASR is analyzed based on the symmetric convolution. A simulation and experiment are carried out to validate the derived FOV. The FOV of a two-dimensional MASR can be analyzed in the same way.

The spatial resolution is still a problem in passive microwave remote sensing, especially in low frequency. In recent years, the satellite formation flying has been proposed. Based on this technique, a large array is able to be synthesized in orbit to achieve higher spatial resolution. However, it is a big challenge for the control system to constrain all the satellites in a coplane in orbit. The 3-D array configuration is a good choice for a synthesized array based on satellite formation flying. In this paper, the complete formulation of visibility functions, including system imperfections, in a 3-D aperture synthesis radiometer (3-D ASR) is derived. The array factor of a 3-D ASR is defined. The reconstructed modified brightness temperature (BT) is a 3-D linear convolution of the modified BT and the array factor. Based on this relationship, the reconstruction method for a practical 3-D ASR is studied. The numerical results demonstrate that the reconstruction method is correct and stable. Finally, a discussion is given to analyze several existing methods that were proposed to reconstruct BT image for 3-D arrays in radio astronomy and earth observation. Compared with these existing methods, our imaging method is more suitable for earth observation based on the technique of satellites formation flying in low earth orbit. In addition, according to the derivations, some mature techniques that were developed for 2-D ASRs may be applied to 3-D ASRs.

Rotating mirrored aperture synthesis (RMAS) has been proposed as a new method for high spatial resolution earth observation from the geostationary orbit. However, in previous researches, the reflector sizes are assumed to be infinite so that all antennas in the antenna array can receive the reflection signals. This assumption is obviously unacceptable in practical applications. How to estimate the reflector sizes is still a problem. In this letter, the principle of RMAS is briefly reviewed. Then, the reflector sizes of an RMAS radiometer are analyzed based on solid geometry, and detailed calculation formulae are derived. A numerical example is given for calculating the reflector sizes of an RMAS radiometer applied in the geostationary orbit.

The mirrored aperture synthesis radiometer (MASR) has been proposed as a new technique for high-resolution observation. Compared with ASRs, MASRs have the advantage of lower system complexity. However, the relationship between MASRs and ASRs has not been studied. In order to thoroughly study MASRs, it is necessary to establish the relationship between MASRs and ASRs. In this letter, the array factor of 1-D MASRs in the discrete cosine transform domain (DCT-domain) is defined. The reconstructed image for a 1-D MASR is a symmetric convolution of the observed brightness temperature distribution and the defined array factor in the DCT domain. Since a symmetric convolution can be turned into a linear convolution, the relationship between 1-D MASRs and 1-D ASRs can be established. A 1-D MASR is equivalent to a 1-D ASR that has a mirrored window. Additionally, for the equivalent 1-D ASR, its observed scene is a symmetrically extended real scene. This established relationship is validated by the simulation results. In practical applications, a MASR can be understood as an ASR.

For one dimensional large aperture synthesis radiometers (ASRs), the low redundancy linear arrays (LRLAs) are usually the choice for the reason of the lowest system complexity. However, the number of LRLAs is very few, which makes the antenna arrangement of LRLAs inflexible. In this paper, the array configuration optimization for AFF-based nonuniform array is proposed. The AFF-based nonuniform array after the array configuration optimization is called the optimized nonuniform array (ONA). The method how to find out the ONAs is introduced. In numerical results, some examples of ONA are given. The numerical result demonstrates that the performance of ONAs is almost equivalent to LRLAs. Since the antenna positions of ONA are non-integers, in fact, the number of ONAs is infinite in theory, which can be a supplement to the LRLAs in practical applications.

Nonuniform aperture synthesis radiometers (NASRs) have the advantage of flexibility in practical applications. However, the main limitation of NASRs for Earth observation is the poor reconstruction quality. In this letter, the array factor of 1-D NASR is analyzed. Array factor forming (AFF) is introduced to improve the reconstruction quality of 1-D NASRs. The limitations of the AFF on array configurations are given. The NASR that meets the given limitations is called the optimizable NASR (ONASR). The numerical results and experimental results demonstrate that the AFF is able to improve the reconstruction quality of 1-D ONASRs. To assess the performance of the AFF thoroughly, the comparisons are made against the linear interpolation and the regularized G-matrix method. Meanwhile, the comparisons between the AFF-based 1-D ONASRs and the 1-D uniform ASRs are also made. The comparison results demonstrate that the AFF can obtain better reconstruction quality than the linear interpolation and the regularized G-matrix method. Additionally, the comparison results also demonstrate that the reconstruction quality of AFF-based 1-D ONASRs can approach that of 1-D uniform ASRs.

The calibration of aperture synthesis radiometers (ASRs) with large arrays by internal noise injection suffers from high increased mass and volume and is not capable to deal with the out-of-plane distortion of antenna arrays. In this letter, a hybrid calibration method is proposed, which combines the redundant baselines and reduced noise injection. The hybrid method needs only a small noise injection network and can deal with the out-of-plane distortion of antenna arrays. The performance of the hybrid method depends on the noise injection element distribution, which is analyzed and given based on minimizing the root mean square of all residual amplitude and phase errors. Simulation results demonstrate its performance. The proposed hybrid method is particularly helpful to the calibration of ASRs with large arrays in geostationary orbit.

Nonuniform aperture synthesis radiometers (NASRs) are emerging in Earth remote sensing. The main disadvantage of NASRs is the complexity of the image reconstruction. In this letter, the gridding method is introduced as an efficient method to reconstruct brightness temperature images for NASRs. In the gridding method, a Voronoi diagram is applied to dividing the sampling region in the spatial frequency domain into a number of small cells, and the area of each cell is regarded as the density compensation factor. Meanwhile, a method is proposed to produce the outermost Voronoi cells for the outermost visibility samples. The errors induced by using the gridding method are analyzed. The nonuniformity factor (NF) is defined to indicate the nonuniformity of a sampling pattern. The numerical results verify that the gridding method can be applied to image reconstruction for NASRs and that the accuracy of reconstructed images is dependent on NF. The comparison between the gridding method and the nonuniform fast Fourier transform algorithm shows that the gridding method has lower computational complexity.