HF ground wave propagation over sea ice for a spherical Earth model

Calculations of the field strength at 10 MHz are reported for a ground wave signal where the path is sea water covered by a uniform layer of sea ice. It is shown that a surface wave type mode may dominate the conventional ground wave modes at the shorter ranges (i.e., < 10 km). At larger ranges the field strength due to the ice layer may be seriously degraded.     

HF Radio Wave Transmission over Sea Ice And Remote Sensing Possibilities

Ground wave propagation is analyzed for a path where sea water is covered by a uniform layer of sea ice. The source is taken to be a vertical electric dipole on or above the ice layer. The solution indicates that a trapped surface wave is significant at short ranges while, at longer ranges, the usual ground wave modes are dominant. The resulting interference pattern may produce rapid variations of the field at intermediate ranges. These characteristics, as well as the height dependence of the observed field strength, are strongly dependent on the thickness of the ice layer.     

粗糙海面建模及其在蒸发波导传播中的应用研究

海上蒸发波导微波超视距电波传播对舰载雷达的运行具有重要的影响. 本文基于粗糙海面电波散射特征和海面高度分布概率密度函数,提出了一种考虑海面遮蔽影响的海上电波传播计算方法,并与两种不包含遮蔽影响的粗糙海面建模的计算结果进行了模拟和试验对比. 计算结果为:1）随着计算频率和风速的增大,不同粗糙海面处理方法的计算结果差异增大,对应位置的路径损耗差异可达10 dB;2）基于试验数据的对比初步显示采用考虑遮蔽影响改进的抛物方程模型预测的路径损耗精度相对较高,粗糙海面对电波的遮蔽效应是蒸发波导传播损耗计算中的一个重要因素;3）试验结果为不稳定大气时采用NPS蒸发波导模型、稳定大气时采用PJ蒸发波导模型预测大气折射率剖面,所预测的路径损耗要优于采用单一蒸发波导模型的结果. 本文所得结果对海上电波传播计算和大气波导的反演等具有参考价值.     

区域蒸发波导及微波路径损耗预报与验证

蒸发波导对近海面电磁波传播有重要影响,区域蒸发波导及微波路径损耗预报对评估海上雷达、通信、侦察等电磁系统的性能至关重要. 本文基于美国国家环境预报中心的GFS预报产品和NAVSLaM蒸发波导预测模型,对我国南海海域进行了大面积蒸发波导预报,利用欧洲中期天气预报中心发布的ERA5再分析数据进行了对比,并结合电磁波传播抛物方程模型开展了微波在蒸发波导信道中的路径损耗预报;然后利用在南海北部的路径损耗测试链路,对预报结果的准确性及时效性进行了验证. 再分析数据对比及海上试验验证结果表明,基于GFS数据开展的蒸发波导层中路径损耗预报,除个别异常点外,预报的路径损耗与实测值趋势相同,在前24 h结果与实测值较为吻合,误差在5~10 dB,可以用于大面积蒸发波导层中路径损耗的预报. 另外,试验发现,降雨会导致预报结果与实测数据有很大的误差,在进行电磁波传播计算时,应充分考虑雨衰、海面粗糙等海洋环境.     

A numerical model for electromagnetic scattering from sea ice

A numerical model for scattering from sea ice based on the finite difference time domain (FDTD) technique is presented. The sea ice medium is modeled as consisting of randomly located spherical brine scatterers with a specified fractional volume, and the medium is modeled both with and without a randomly rough boundary to study the relative effects of volume and surface scattering. A Monte Carlo simulation is used to obtain numerical results for incoherent υυ backscattered normalized radar cross sections (RCSs) in the frequency range from 3 to 9 GHz and for incidence angles from 10° to 50° from normal incidence. The computational intensity of the study necessitates an effective permittivity approach to modeling brine pocket effects and a nonuniform grid for small scale surface roughness. However, comparisons with analytical models show that these approximations should introduce errors no larger than approximately 3 dB. Incoherent υυ cross sections backscattered from sea ice models with a smooth surface show only a small dependence on incidence angle, while results for sea ice models with slightly rough surfaces are found to be dominated by surface scattering at incidence angles less than 30° and by scattering from brine pockets at angles greater than 30°. As the surface roughness increases, surface scattering tends to dominate at all incidence angles. Initial comparisons with measurements taken with artificially grown sea ice are made, and even the simplified sea ice model used in the FDTD simulation is found to provide reasonable agreement with measured data trends. The numerical model developed ran be useful in interpreting measurements when parameters such as surface roughness and scatterer distributions lie outside ranges where analytical models are valid     

Sea ice concentration, ice temperature, and snow depth using AMSR-E data

A summary of the theoretical basis and initial performance of the algorithms that are used to derive sea ice concentration, ice temperature, and snow depth on sea ice from newly acquired Earth Observing System-Aqua/Advanced Microwave Scanning Radiometer-EOS (AMSR-E) radiances is presented. The algorithms have been developed and tested using historical satellite passive microwave data and are expected to provide more accurate products, since they are designed to take advantage of the wider range of frequencies and higher spatial resolution of the AMSR-E microwave instrument. Validation programs involving coordinated satellite, aircraft, and surface measurements to determine the accuracies of these sea ice products and to improve further our capability to monitor global sea ice are currently underway.     

Surface roughness characterizations of sea ice and ice sheets: case studies with MISR data

This work is an examination of potential uses of multiangular remote sensing imagery for mapping and characterizing sea ice and ice sheet surfaces based on surface roughness properties. We use data from the Multi-angle Imaging SpectroRadiometer (MISR) to demonstrate that ice sheet and sea ice surfaces have characteristic angular signatures and that these angular signatures may be used in much the same way as spectral signatures are used in multispectral classification. Three case studies are examined: sea ice in the Beaufort Sea off the north coast of Alaska, the Jakobshavn Glacier on the western edge of the Greenland ice sheet, and a region in Antarctica south of McMurdo station containing glaciers and blue-ice areas. The MISR sea ice image appears to delineate different first-year ice types and, to some extent, the transition from first-year to multiyear ice. The MISR image shows good agreement with sea ice types that are evident in concurrent synthetic aperture radar (SAR) imagery and ice analysis charts from the National Ice Center. Over the Jakobshavn Glacier, surface roughness data from airborne laser altimeter transects correlate well with MISR-derived estimates of surface roughness. In Antarctica, ablation-related blue-ice areas, which are difficult to distinguish from bare ice exposed by crevasses, are easily detected using multiangular data.     

Evaluation of late summer passive microwave Arctic sea iceretrievals

The melt period of the Arctic sea ice cover is of particular interest in studies of climate change due to the albedo feedback mechanisms associated with meltponds and openings in the ice pack. The traditionally used satellite passive microwave sea ice concentration algorithms have deficiencies during the summer months due to the period's highly variable surface properties. A newly developed ice concentration algorithm overcomes some of these deficiencies. It corrects for low ice concentration biases caused by surface effects through the use of 85 GHz data in addition to the commonly used 19 and 37 GHz data and, thus, the definition of an additional ice type representing layering and inhomogeneities in the snow layer. This new algorithm will be the standard algorithm for Arctic sea ice concentration retrievals with the EOS Aqua advanced microwave scanning radiometer (AMSR-E) instrument. In this paper, we evaluate the performance of this algorithm for the summer period of 1996 using data from the special sensor microwave imager (SSM/I) which has frequencies similar to the AMSR instrument. The temporal evolution of summertime passive microwave sea ice signatures are investigated and sea ice concentration retrievals from the standard NASA team and the new algorithm are compared. The results show that the introduction of the additional sea ice type in the new algorithm leads to improved summertime sea ice concentrations. The SSM/I sea ice retrievals are validated using SAR-derived ice concentrations that have been convolved with the SSM/I antenna pattern to ensure an appropriate comparison. For the marginal ice zone, with ice concentrations ranging from 40% to 100%, the correlation coefficient of SAR and SSM/I retrievals is 0.66 with a bias of 5% toward higher SAR ice concentrations. For the central Arctic, where ice concentrations varied between 60% and 100%, the correlation coefficient is 0.87 with a negligible bias     

遮蔽效应对潜艇海面尾迹后向散射影响研究

分析了合成孔径雷达(SAR)探潜对随机粗糙海面特性的应用需求和随机粗糙海面的生成机理,计算了满足Gauss形态相关函数的随机粗糙海面后向散射系数,提出了高海况下考虑海浪遮蔽效应的潜艇海面兴波尾迹的后向散射计算方法。仿真结果表明,海浪遮蔽效应会使海面后向散射降低从而更接近于海面散射的真实值,且其具体降低量与海况等因素有关。     

垂直电偶极子在涂有介质层球形导电基底上的场

利用解析方法导出了垂直电偶极子在涂履有介质层的球形导电基底上产生的电磁场的解析表达式。所得结果表明，当介质层达到一定的厚度时，与文献[1]中的平面导电基底情况相类似，垂直电偶极子同样能够在球形介质层的表面激励起吸附表面波（Trapped surface wave)。文中对具有特殊地貌情况下和有冰层的海面上的通信有新的指导意义。     

Analysis of surface roughness and morphology of first-year sea ice melt ponds: implications for microwave scattering

Variations in wind forcing over summer first-year sea ice (FYI) melt ponds occur at hourly to weekly scales and are a significant contributor to microwave backscatter (/spl sigma//spl deg/) variability observed from spaceborne synthetic aperture radar (SAR) platforms (e.g., ENVISAT-ASAR and RADARSAT-1). This variability impairs our ability to use SAR to derive information on summer sea ice thermodynamic state and energy balance parameters such as albedo and melt pond fraction. The surface roughness contribution of FYI melt ponds in the Canadian Arctic Archipelago to like-polarized, C-band /spl sigma//spl deg/ estimates is analyzed through a spectral and statistical analysis of surface wave height profiles for varying wind speeds, upwind fetch lengths, and melt pond depths. A unique derivation of melt pond surface wave height spectra is presented based on digital video of melt pond surface wave trains. Significant scale surface roughness was observed even at wind speeds of 3 m/spl middot/s/sup -1/, resulting in small perturbation model estimates of /spl sigma//spl deg/ (HH) ranging from -5 dB at 20/spl deg/ incidence to -22 dB at 50/spl deg/ incidence. Results from a multivariate linear regression analysis show that 53.5% of observed variance in /spl sigma//spl deg/ (HH or VV) can be explained by wind speed, upwind fetch from melt pond edges, and melt pond depth, with no appreciable difference in the relative contribution of explanatory variables. Modeled omnidirectional /spl sigma//spl deg/ as a function of wind speed and incidence angle for 100-m transects collected throughout the melt pond season act to elaborate the role of fetch and depth, as well as the modulating effect of hummocks, on /spl sigma//spl deg/.     

基于三角面元波面分割的海面红外成像仿真

从海面小波面法线出发,采用海浪谱模型模拟海面的形态,提出一种新的简便算法,将海面各小波面划分成三角面元,通过求三角面元的法向量来计算海面发射率,大大降低了计算的难度。其次,综合考虑海面自身热辐射及海面反射天空、太阳辐射和探测器与海面之间的大气路径光谱辐射,建立海面红外辐射模型。最后,生成海面红外辐射的灰度图像,并给出在不同探测器天顶角下的仿真结果。通过结果分析,该方法可以有效地模拟海面红外辐射特性,仿真的红外图像对于舰船红外仿真研究及红外探测具有重要意义。     

离散复镜像法在海水极低频电磁波传播计算中的应用

该文利用并矢格林函数法求解海水中水平电偶极子辐射场的矢量位和标量位格林函数。将在高频领域获得广泛应用的离散复镜像法引入海水里极低频电磁波传播计算中。高频和极低频情况下的波数公式不同导致积分路径发生变化,该文提出了一条新的适用于极低频情况的积分路径。使用广义笔函数束法将被积函数表示成指数和的形式,使用二级离散复镜像法得到空气-海水-海床3层水平分层模型的空间域闭式格林函数。基于混合位积分方程对电场进行了计算。与软件FEKO和WHAM的结果比较说明离散复镜像法在极低频电磁波传播计算中是有效的,并且在远区比WHAM能更好地体现出电场的衰减趋势。     

表面波飞机结冰传感器的研究

首先分析了覆盖冰层的各向同性半无限大弹性体中的声表面波(SAW),求得了声表面波波速与冰层厚度间的关系.结果表明,当冰层厚度为0时,求得的波速与没有冰层覆盖相对应的半无限大弹性体的声表面波波速相同;当冰层厚度不断增加时,求得的声表面波波速趋向于冰层中的波速.随后分析了基体是各向同性无限大弹性板的情况.计算结果表明,由于弹性板与冰层的相互作用,实际振动会出现多个声表面波模态和相应的波速.     

激光致声技术在跨冰层介质的空中-水下通信应用方法

无线电波、声波和光波通信技术可有效应用于复杂的海洋通信环境中,然而在极寒地区和极地区域海面冰层覆盖的情况下,其在远端发送信号将无法穿透冰层介质,而难以实现空中与水下的通信联络,故提出一种应用于跨冰层介质传输的激光致声通信体制,通过空中发射激光与海面固体冰层的互作用诱导声波信号,再利用海面上覆盖冰层的声源辐射,向下方水体传播来建立空中到水下光波与声波的通信链路,系统将光学与声学技术的有机融合,充分利用固体冰层对声波的快速传导作用,探讨光声转换机理、调制编码及激光声传输特点与性能分析,有效解决海洋通信中受冰层覆盖影响和单一种类信号传输受限问题,为海洋跨空通信提供一种新的技术途径。     

Curves for ground wave propagation over mixed land and sea paths

Specific numerical results are presented for ground wave propagation over paths which are part sea and part land. The problem is idealized to the extent that the earth is a smooth spherical surface. The method is based on a previous formulation in terms of mutual impedance between two vertical electric dipoles on an inhomogeneous spherical earth. Amplitude and phase of the ground wave are given for various combinations of the following parameters: frequency 1000, 100, and 20 kc; land conductivities 100 and 10 mmhos/meter; and a sea conductivity of 4 mhos/meter. Most of the curves exhibit the well-known recovery effect which occurs beyond the coast line for propagation from land towards the sea.     

Sea ice identification using dual-polarized Ku-band scatterometerdata

This paper describes a classification algorithm using dual-polarized scatterometer measurements to identify the edge of the sea ice cover. The distinct polarization scattering signatures of sea ice and open water are discussed and illustrated with the dual-polarized radar measurements from the Seasat-A scatterometer (SASS). The analysis of SASS data suggests that the ratio of vertical and horizontal polarization backscatter, denoted as the copol ratio, is a useful discriminator of sea ice and open ocean. A simple classification algorithm using the thresholds of the copol ratio and backscatter levels is proposed. The feasibility of this algorithm is demonstrated using the SASS data from the single-sided, dual-polarization mode. The results indicate that the dual-polarized measurements from the NASA scatterometer (NSCAT) can be used to produce routine maps of sea ice edges     

On the Condition for Transformation of a Fast Surface Wave into a Slow Surface Wave

Surface plasmons (polaritons) along the boundary between vacuum and a dissipative half-space with given complex permittivity, including the case of placing an electrically thin dielectric layer (system of layers) on the boundary, are considered. Conditions for transformation of a fast gliding Zenneck surface wave (polariton) into a slow wave (slow surface plasmon) are considered. The behavior of the group velocity and the energy velocity of motion (transfer) in such structures are analyzed. The reflection coefficient of the Zenneck wave along the sea for the ice pack of a given thickness is determined. It has been shown that rigorous solution of the Sommerfeld problem by means of numerical calculation of integrals in the near zone allows obtaining the solution of the inverse problem of the structure of the surface region on the basis of redundant experimental data on components of the electromagnetic field on the surface at different ranges from the antenna.     

Characteristics of L-band multipath fading due to sea surface reflection

For lowG/Tship-based earth stations in future maritime satellite communications, the effect of multipath fading due to sea surface reflection will be very important at low elevation angles. However, a practical model of theL-band multipath fading for fade prediction has not been available so far. A practical fading model is presented based on the shadowed Kirchhoff approximation theory for the coherent and incoherent components of the scattered power as a function of sea surface conditions. Using this model, theoretical fading depths are presented as a function of elevation angle, wave height, and antenna gain. The results indicate that intense fading occurs for wave heights greater than 50 cm, and the dependence on wave height is small under these conditions atL-band frequencies. Theoretical results presented agree well with experimental results obtained by field experiments.