Zhensen Wu’s research while affiliated with Xidian University and other places

The parabolic equation (PE) model is effective for predicting signal propagation over irregular terrains. The shift map method of the PE model is highly accurate and widely used for terrain propagation predictions. The maximum propagation angle is a crucial parameter of the shift map model. Here, unreasonable values may result in incorrect results or further computations. However, the prediction formula used in existing literature is limited, thereby leading to inaccurate results. In this study, a maximum propagation angle prediction method was developed. The maximum propagation angle with terrain should be calculated as the sum of the maximum propagation angle without terrain and twice the maximum terrain slope angle. Hence, the selection criteria for the different models in the hybrid terrain model were reanalyzed. An accurate maximum propagation angle is key for the PE model over irregular terrains, as it ensures prediction accuracy and improves calculation speed.

This paper proposes a hyperspectral imaging simulation method based on a ray-tracing algorithm. The algorithm combines calculations based on solar and atmospheric visible light radiation as well as the spectral bidirectional reflection distribution function (BRDF) of the target surface material and can create its own scenarios for simulation calculations on demand. Considering the presence of multiple scattering between the target and background, using the ray-tracing algorithm enables the precise computation of results involving multiple scattering. To validate the accuracy of the algorithm, we compared the simulated results with the theoretical values of the visible light scattering intensity from a Lambertian sphere. The relative error obtained was 0.8%. Subsequently, a complex scene of engineering vehicles and grass was established. The results of different observation angles and different coating materials were calculated and analyzed. In summary, the algorithm presented in this paper has the following advantages. Firstly, it is applicable to geometric models composed of any triangular mesh elements and accurately computes the effects of multiple scattering. Secondly, the algorithm combines the spectral BRDF information of materials and improves the efficiency of multiple scattering calculations using nonuniform sampling. The computed hyperspectral scattering data can be applied to simulate airborne or space-borne remote sensing data.

The climatic fluctuations in northern China exhibit remarkable variability, making it imperative to harness the power of MODIS data for conducting comprehensive investigations into the influences of desertification, desert expansion, and snow and ice melting phenomena. Consequently, the rigorous evaluation of MODIS land surface temperature (LST) and land surface emissivity (LSE) products takes on a momentous role, as this provides an essential means to ensure data accuracy, thereby instilling confidence in the robustness of scientific analyses. In this study, a high-resolution hyperspectral imaging instrument was utilized to measure mid-wave hyperspectral images of grasslands and deserts in the northwest plateau region of Qinghai, China. The measured data were processed in order to remove the effects of sensor noise, atmospheric radiation, transmission attenuation, and scattering caused by sunlight and atmospheric radiation. Inversion of the temperature field and spectral emissivity was performed on the measured data. The inverted data were compared and validated against MODIS land surface temperature and emissivity products. The validation results showed that the absolute errors of emissivity of grassland backgrounds provided by MCD11C1 in the three mid-wave infrared bands (3.66–3.840 μm, 3.929–3.989 μm, and 4.010–4.080 μm) were 0.0376, 0.0191, and 0.0429, with relative errors of 3.9%, 2.1%, and 4.8%, respectively. For desert backgrounds, the absolute errors of emissivity were 0.0057, 0.0458, and 0.0412, with relative errors of 0.4%, 4.9%, and 3.9%, respectively. The relative errors for each channel were all within 5%. Regarding the temperature data products, compared to the inverted temperatures of the deserts and grasslands, the remote sensing temperatures provided by MOD11L2 had absolute errors of ±2.3 K and ±4.1 K, with relative errors of 1.4% and 0.7%, respectively. The relative errors for the temperature products were all within 2%.

The scintillation of the orbital angular momentum (OAM) of a Bessel Gaussian beam was derived based on the Rytov method to characterize the performance of the OAM communication. Moreover, a multi-parameter demultiplexing method was also proposed which could decode the OAM state, the amplitude and two additional beam width information dimensions. The advantages of the OAM states as the communication carrier over the beam intensity were that the minimum scintillation of the fundamental mode was smaller, and its corresponding radius also diverged slower. The coefficient of variation of the decoding amplitude was approximated to the square root of the radial minimum scintillation, and it provided an estimated decoding precision for the input sample selection. This study not only provided theoretical basis for communication link design, but also had a promising application on the large capacity beam multiplexing in free-space laser communication.

In this paper, a statistical analysis of the diurnal, seasonal and solar cycle variation in the TEC longitudinal difference in midlatitudes of East Asia is presented using CODE GIMs data in 2015–2019. Moreover, the empirical neutral wind model HWM-14 and geomagnetic field model IGRF-2020 were employed to analyze the influence of geomagnetic configuration-neutral wind mechanism on the TEC longitudinal difference, and the F2 layer peak electron density (NmF2) data from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) were also used to study the role of local electron density in the TEC longitudinal difference. For the high solar activity year, the results show that east-west TEC longitudinal difference index Re/w is negative in the noon and positive at evening-night. Moreover, the longitudinal difference of daytime TEC is most evident in summer, less in autumn and least in spring and winter, while the nighttime difference is most obvious in equinox, followed by summer and winter during nighttime. The model simulation shows that the TEC longitude difference around noon is mainly caused by the zonal wind-declination mechanism, and a 4-h time delay seems to be an optimal result for the vertical drift velocity to cause the longitudinal TEC difference during pre-noon hours. At night, the uplifting electron flux, which is a product of local electron density and vertical drift velocity, shows a good correlation with Re/w, indicating that the local electron density is also an important factor affecting the TEC longitudinal difference during the nighttime. Moreover, there was about a 3-h time delay between the TEC longitudinal variations and the uplifting electron flux at night. For the low solar activity years, the western TEC is greater than eastern TEC during most of the year except in the summer nighttime. The TEC diurnal variation in the east and west suggested that the nighttime Re/w should be related to other physical process, such as the midlatitude summer nighttime anomaly (MSNA) in the east and the ionospheric nighttime enhancement (INE). The current study provides evidence for the longitudinal difference of NmF2 in East Asian midlatitudes and geomagnetic configuration-neutral wind mechanism proposed in previous studies and finds some new features which need further studying to improve our current understanding of ionospheric longitudinal difference in the low solar activity years. The results provide new insight into TEC longitudinal variations at midlatitudes, and they can contribute to understanding the ionosphere-thermosphere coupling system.

The ionospheric equivalent slab thickness (EST, also named τ) is defined as the ratio of the total electron content (TEC) to the F2-layer peak electron density (NmF2), and it is a significant parameter representative of the ionosphere. This paper presents a comprehensive statistical study of the ionospheric slab thickness at Yakutsk, located at the high latitude of East Asia, using the GPS-TEC and ionosonde NmF2 data for the years 2010–2017. The results show that the τ has different diurnal and seasonal variations in high- and low-solar-activity years, and the τ is greatest in the winter, followed by the equinox, and it is smallest in the summer in both high- and low-solar-activity years, except during the noontime of low-solar-activity years. Specifically, the τ in inter of high-solar-activity year shows an approximate single peak pattern with the peak around noon, while it displays a double-peak pattern with the pre-sunrise and sunset peaks in winter of the low-solar-activity years. Moreover, the τ in the summer and equinox have smaller diurnal variations, and there are peaks with different magnitudes during the sunrise and post-sunset periods. The mainly diurnal variation of τ in different seasons of high- and low-solar-activity years can be explained within the framework of relative variation of TEC and NmF2 during the corresponding period. By defining the disturbance index (DI), which can visually assess the relationship between instantaneous values and the median, we found that the geomagnetic storm would enhance the τ at Yakutsk. An example on 7 June 2013 is also presented to analyze the physical mechanism. It should be due to the intense particle precipitation and expanded plasma convection electric field during the storm at high-latitude Yakutsk station. The results would improve the current understanding of climatological and storm-time behavior of τ at high latitudes in East Asia.

The use of the low‐frequency scattering coefficient leads to inaccurate receiving‐power predictions in the millimeter‐wave band. The inaccurate signal will hinder optimization of the base station location. In the millimeter waveband, a building surface becomes rougher in an urban environment. The scattering becomes increasingly significant and a non‐negligible factor. In this article, the scattering characteristics of rough cement wall surfaces are studied to improve the scattering coefficients for millimeter waveband communications. The ray tracing method is adopted, and the lambertian and directive models are incorporated into this method, respectively. Moreover, the signal distributions of rough cement wall surfaces are acquired through an outdoor experiment at different incidence angles. The results reveal that the scatterings that are embedded into ray tracing are better than those which were not considered in the ray tracing, compared with the experiment. The directive model agrees more with the measurements than the lambertian model. The scattering coefficient S and exponent αR of the directive model are 0.19 and 1 in the millimeter waveband, respectively, which disagree with the results at low frequencies. The scattering properties can be applied to simulate signal propagation in urban environments to improve the reliability of mobile telecommunication systems. The use of the low‐frequency scattering coefficient leads to inaccurate receiving‐power predictions in the millimeter‐wave band. In the paper, the ray tracing method which considers the lambertian and directive models respectively is adopted, compared with the scattering coefficient of rough cement wall surfaces which are acquired through an outdoor experiment at different incidence angles. The results reveal that the scattering coefficient of the millimeter waveband greatly improves the accuracy of receiving power.

The ionospheric equivalent slab thickness (τ) is defined as the ratio of the total electron content (TEC) to the F2-layer peak electron density (NmF2), and it is a significant parameter representative of the ionosphere. In this paper, a comprehensive statistical analysis of the diurnal, seasonal, solar, and magnetic activity variations in the τ at Guam (144.86°E, 13.62°N, 5.54°N dip lat), which is located near the magnetic equator, is presented using the GPS-TEC and ionosonde NmF2 data during the years 2012–2017. It is found that, for geomagnetically quiet days, the τ reaches its maximum value in the noontime, and the peak value in winter and at the equinox are larger than that in summer. Moreover, there is a post-sunset peak observed in the winter and equinox, and the τ during the post-midnight period is smallest in equinox. The mainly diurnal and seasonal variation of τ can be explained within the framework of relative variation of TEC and NmF2 during different seasonal local time. The dependence of τ on the solar activity shows positive correlation during the daytime, and the opposite situation applies for the nighttime. Specifically, the disturbance index (DI), which can visually assess the relationship between instantaneous τ values and the median, is introduced in the paper to quantitatively describe the overall pattern of the geomagnetic storm effect on the τ variation. The results show that the geomagnetic storm seems to have positive effect on the τ during most of the storm-time period at Guam. An example, on the 1 June 2013, is also presented to analyze the physical mechanism. During the positive storms, the penetration electric field, along with storm time equator-ward neutral wind, tends to increase upward drift and uplift F region, causing the large increase in TEC, accompanied by a relatively small increase in NmF2. On the other hand, an enhanced equatorward wind tends to push more plasma, at low latitudes, into the topside ionosphere in the equatorial region, resulting in the TEC not undergoing severe depletion, as with NmF2, during the negative storms. The results would complement the analysis of τ behavior during quiet and disturbed conditions at equatorial latitudes in East Asia.

The probability property of the orbital angular momentum (OAM) distortion of the Bessel Gaussian beam propagating through the turbulence is investigated in this study. The mean and variance of the beam harmonic intensity are derived from the Rytov theory with a bias of less than 6% when compared with the data calculated by the phase-screen method. Based on these statistics, the probability density function (PDF) of the harmonic intensity fluctuation is proposed to characterize the randomness property of the beam OAM distortion, which agrees well with the result obtained from the phase-screen method. The PDF of the intensity difference between the fundamental and its adjacent crosstalk modes is derived. Furthermore, the probability of the OAM decoding error is also provided. This study not only facilitates beam OAM crosstalk characterization, but also provides the applicable condition of beam multiplexing for the beam parameter selection and the communication link design.

This paper studied the Laser Radar Cross Section (LRCS) of the target scale model by using the Bidirectional Reflection Distribution Function (BRDF) of materials and the Monte Carlo method for reverse ray tracing. The results of the research are shown through data from the satellite scale model. On the basis of the classification of the roughness of different materials and in conjunction with probability density distribution, we adopted a non-uniform sampling method to improve the split-ray collection method in the spatial distribution of scattering energy in the reverse ray tracing algorithm. By conducting the typical Monte Carlo simulation of a simple dihedron and comparing the theoretical calculation results, we tested and verified the correctness of the proposed method. We analyzed the difference between the primary and secondary scattering results of the target LRCS with different conditions. The calculation results show that for rough materials, if only primary scattering is considered in a plurality of detection directions, a large calculation error will occur, so the influence of the secondary scattering must be considered; and the contribution of the diffuse reflection component to the secondary scattering is more significant. Finally, we compared the secondary scattering of different rough materials to describe the circumstances of secondary scattering in which diffuse reflection conditions need to be considered. In addition, the inverse Monte Carlo method used in this paper to calculate the multiple scattering of the LRCS of the target can effectively improve the calculation speed of the acquisition processing. This improves the reliability of the calculation results while taking into account the estimated speed.