Passive microwave remote sensing of snow constrained byhydrological simulations

This paper describes a snow parameter retrieval algorithm from passive microwave remote sensing measurements. The three components of the retrieval algorithm include a dense media radiative transfer (DMRT) model, which is based on the quasicrystalline approximation (QCA) with the sticky particle assumption, a physically-based snow hydrology model (SHM) that incorporates meteorological and topographical data, and a neural network (NN) for computational efficient inversions. The DMRT model relates physical snow parameters to brightness temperatures. The SHM simulates the mass and heat balance and provides initial guesses for the neural network. The NN is used to speed up the inversion of parameters. The retrieval algorithm can provide speedy parameter retrievals for desired temporal and spatial resolutions, Four channels of brightness temperature measurements: 19V, 19H, 37V, and 37H are used. The algorithm was applied to stations in the northern hemisphere. Two sets of results are shown. For these cases, the authors use ground-truth precipitation data, and estimates of snow water equivalent (SWE) from SHM give good results. For the second set, a weather forecast model is used to provide precipitation inputs for SHM. Additional constraints in grain size and density are used. They show that inversion results compare favorably with ground truth observations     

微波无源遥感隐身目标的机理与方法研讨

对于优化设计和涂有吸波材料的隐身目标一般用常规雷达无法发现。提出了一种用微波辐射计来进行遥感隐身目标的探测方法,它采用测量大气热噪声方法对目标进行探测,不仅能够探测隐身目标,而且本身也具有隐身功能。通过机理和方法分析表明:用微波辐射计探测隐身目标的方法是可行的,关键是要提高微波辐射计的空间分辨率和温度分辨率。     

Galactic noise and passive microwave remote sensing from space at L-band

The spectral window at L-band (1.413 GHz) is important for passive remote sensing of soil moisture and ocean salinity from space, parameters that are needed to understand the hydrological cycle and ocean circulation. At this frequency, radiation from celestial (mostly Galactic) sources is strong and, unlike the constant cosmic background, this radiation is spatially variable. This paper presents a modern radiometric map of the celestial sky at L-band and a solution for the problem of determining what portion of the sky is seen by a down-looking radiometer in orbit. The data for the radiometric map are derived from recent radio astronomy surveys and are presented as equivalent brightness temperature suitable for remote sensing applications. Examples using orbits and antennas representative of those contemplated for remote sensing of soil moisture and sea surface salinity from space are presented to illustrate the signal levels to be expected. Near the Galactic plane, the contribution can exceed several kelvin.     

Interference from 24-GHz automotive radars to passive microwave earth remote sensing satellites

The automotive industry is currently considering the introduction of short-range radars (SRR) operating near 24 GHz for improving road traffic safety. SSRs are intended to observe the full azimuthal space cover around a vehicle using up to eight sensors. The sensors would operate in an ultrawideband (UWB) mode, occupying 3-5 GHz of bandwidth. Interference from SRR transmitters with passive microwave remote sensing satellites used for weather and climate monitoring could occur as the result of several coupling mechanisms, including direct coupling via the transmit antenna beam and scattering and diffraction of the transmitted signals from leading vehicles, buildings, and other nearby objects. In this study, we estimate the amount of coupling anticipated to occur from SRRs, including the direct and scattered contributions. The calculations are based on bistatic scattering measurements of a typical automobile and ray optical simulations of reflection and propagation in an urban environment. Using these calculations, the maximum allowable SRR transmitted power for interference levels acceptable for meteorological and climatological remote sensing applications are quantified. The study provides criteria for SRR operation with the Earth Exploration Satellite Service on a noninterference basis.     

Ground-based passive microwave remote sensing observations of soilmoisture at S-band and L-band with insight into measurementaccuracy

A ground-based experiment in passive microwave remote sensing of soil moisture was conducted in Huntsville, AL, from July 1-14, 1996. The goal of the experiment was to evaluate the overall performance of an empirically-based retrieval algorithm at S-band and L-band under a different set of conditions and to characterize the site-specific accuracy inherent within the technique. With high temporal frequency observations at S-band and L-band, the authors were able to observe large scale moisture changes following irrigation and rainfall events, as well as diurnal behavior of surface moisture among three plots, one bare, one covered with short grass and another covered with alfalfa. The L-band emitting depth was determined to be on the order of 0-3 or 0-5 cm below 0.30 cm³/cm³ with an indication that it is less at higher moisture values. The S-band emitting depth was not readily distinguishable from L-band. The uncertainty in remotely sensed soil moisture observations due to surface heterogeneity and temporal variability in variables and parameters was characterized by imposing random errors on the most sensitive variables and parameters and computing the confidence limits on the observations. Discrepancies between remotely sensed and gravimetric soil moisture estimates appear to be larger than those expected from errors in variable and parameter estimation. This would suggest that a vegetation correction procedure based on more dynamic modeling may be required to improve the accuracy of remotely sensed soil moisture     

Origin of vector parasites in numerical Maxwell solutions

Dispersion relations are derived for conventional finite-element (FE) and finite-difference (FD) approximations for four versions of the Maxwell equations in the plane: the double-curl equation; the vector Helmholtz equation; the penalty equation; and the primitive, coupled Maxwell curl equations. Comparison with their analytic counterparts reveals the presence and origin of vector parasites. For the double-curl and penalty methods, the dispersion relations are double-valued, admitting an extra, spurious dispersion surface of real-valued wavenumbers. As a result, low wavenumbers support well-resolved and poorly resolved vector parasites. The Helmholtz schemes have monotonic, single-valued dispersion relations for divergence-free physical modes. Specification of divergence-free boundary conditions is sufficient to guarantee the absence of parasites. The primitive schemes have single-valued but folded (nonmonotonic) dispersion relations, supporting poorly resolved vector parasites at low wavenumbers. Use of a staggered finite-difference grid eliminates these parasites and results in a dispersion relation identical to that for the Helmholtz scheme. In cases where vector parasites arise, the same essential weakness in the discretized form of either the first or cross-derivative is responsible     

Hybrid numerical method for harmonic 3-D Maxwell equations:scattering by a mixed conducting and inhomogeneous anisotropicdielectric medium

This article deals with a hybrid numerical method for solving harmonic Maxwell equations in the classical electrodynamic context. This formulation can be used with any body of arbitrary three-dimensional geometry, of perfectly conducting material or dielectric, with locally inhomogeneous and anisotropic behavior laws, and with or without dielectric losses. The mathematical formulation is presented along with applications validating it. The exterior problem is treated by the integral equation method while local equations are used for the dielectric parts of the body. A global variational formulation of the coupled problem is developed for use in discretization by the finite element method. Boundary finite elements are used for integral operators connected with the exterior problem. Localized finite elements are used for the interior problem. Difficulties of irregular frequencies, also called resonant frequencies in the perfectly conducting case, arising from the integral formulation are analyzed in detail and an efficient solution is developed     

Aperture synthesis concepts in microwave remote sensing of theEarth

The application of aperture synthesis concepts, used for many years in radioastronomy to achieve high image resolution at a reasonable cost, to remote sensing technology is discussed. The electronically scanned thinned array radiometer (ESTAR) is put forward as a viable alternative to improve spatial resolution by an order of magnitude over what is presently achieved by microwave imaging systems that are collecting data from Earth orbit. Future developments in airborne sensor technology and potential spacecraft application are described     

Comments on "Interference from 24-GHz automotive Radars to passive microwave Earth remote sensing Satellites"

In a recent paper, Younis et al. propose an apparently interesting methodology for the computation of the interference received by a spaceborne passive sensor from terrestrial interferences. However, the paper seems to require some clarifications, as some of the conclusions are questionable. This comments paper aims at identifying the most outstanding issues of the publication.     

热红外联合被动微波重构裸土区地表温度时间序列数据算法

在进行一定区域地表温度时间序列的研究中,常遇到由于天气原因造成的温度数据缺失问题,针对这一突出问题,在分析土壤发射率数据库的基础上,提出了一种裸露地表晴空时段热红外、微波亮温数据与非晴空时段微波亮温数据相结合来获取时间序列中缺失的地表温度数据的算法.此算法不仅解决了被动微波进行地表温度反演过程中下垫面发射率难以确定的问...     

Efficient analysis of microwave passive structures using 3-D envelope-finite element (EVFE)

A three-dimensional envelope-finite element (EVFE) technique is proposed to solve the transient responses of general microwave passive structures. EVFE simulates the signal envelope rather than the original signal waveform by de-embedding the carrier from the time-domain wave equation. The sampling rate of the time-domain waveform is only governed by the Nyquist rate of the envelope, rather than that of the carrier in traditional time-domain simulators. Compared to traditional finite-element time-domain (FETD) methods, the computational cost can be dramatically reduced when the signal envelope-to-carrier ratio is very small. It also provides much higher computational efficiency than frequency-domain finite-element methods for simulating frequency responses over certain bandwidth. This technique is applied to solve a waveguide structure with a dielectric post discontinuity and a microstrip patch antenna. The accuracy and efficiency is demonstrated and compared with traditional unconditionally stable FETD methods.     

Enforcing causality in numerical solutions of Maxwell's equations

Maxwell's equations in differential form do not distinguish between advanced and retarded solutions. Unless special precautions are taken, a point-by-point numerical integration, based on a finite-difference analog of Maxwell's equations, will lead to a mixture of advanced and retarded fields, inadmissible on physical grounds. The causality requirement can be satisfied if the Maxwell theory is expressed in integral-equation form, with retardation incorporated in all the integrands. A solution using numerical integration will then be physically acceptable.     

Passive microwave remote sensing of forests: a model investigation

A model, based on the radiative transfer theory and the matrix doubling algorithm, is described and used to compute the emissivity e of forests. According to model simulations, the L-band emissivity trend versus forest biomass is more gradual than that of the backscatter coefficient. This gradual behavior is observed, in absence of leaves, also at C- and X-bands, while leaves anticipate saturation and make e higher in coniferous forests and lower in deciduous forests. Model results are successfully validated by some available experimental data. Operational aspects, concerning the potential of airborne and spaceborne radiometers in identifying forest type and estimating biomass, are discussed     

Reply to comments on "Interference from 24-GHz automotive radars to passive microwave Earth remote sensing satellites"

We appreciate that authors Kerr et al. concur that our paper [ibid., vol.42, no.7, p.1387-98 (2004)] provides the right approach to the analysis of potential interference from anthropogenic sources to remote sensing satellites. The potential for such interference is likely to grow as new active systems are developed, necessitating acceptable procedures for interference analysis based on accepted scientific knowledge and engineering principles. While some simple clarifications are in order to improve the acceptability of our procedure, we suggest, however, that Kerr et al. have not studied our paper in detail and misinterpret several points.     

卫星遥感资料在蒸发波导数值预报中的应用

基于美国国家环境预报中心(NCEP)再分析资料和卫星遥感资料,利用天气预报模式(WRF),并耦合Babin蒸发波导模式,对发生在福建省平潭岛地区的一次蒸发波导过程进行了三维变分同化数值模拟预报对比试验。结果表明:24h之内模式预报结果与平潭岛观测结果变化规律基本一致(相关系数都在0.92以上);卫星遥感资料同化可以使模式初始场优化,有助于改善预报结果,"同时同化海表风和海面温度"情况下,经过偏差订正后的预报效果最好(无偏均方根误差最小为0.680m);单独同化海面风或海面温度资料都会改善预报结果。     

Quantification of 3-D field effects during 2-D microwave imaging

Two-dimensional (2-D) approaches to microwave imaging have dominated the research landscape primarily due to the moderate levels of measurement data, data-acquisition time, and computational costs required. Three-dimensional (3-D) approaches have been investigated in simulation, phantom, and animal experiments. While 3-D approaches are certainly important in terms of the potential to improve image quality, their associated costs are significant at this time. In addition, benchmarks are needed to evaluate these new generation systems as more 3-D methods begin to appear. In this paper, we present a systematic series of experiments which assess the capability of our 2-D system to image classical 3-D geometries. We demonstrate where current methods suffer from 3-D effects but also identify situations where they remain quite useful. Comparisons between reconstructions utilizing phantom measurements and simulated 3-D data are also shown to validate the results. These findings suggest that for certain biomedical applications, 2-D approaches remain quite attractive.     

微波遥感在岩土工程中应用的基础实验研究

由电磁辐射理论知道,物质的微波辐射是热能激发组成物质分子的转动态能级跃迁辐射出的电磁波。在等温过程加载的条件下,对岩石和混凝土进行了２ｃｍ、３ｃｍ、１０ｃｍ３个波段以及水平和垂直两种极化状态的实验,得出岩石和混凝土的微波辐射能量随压力变化面变化的实验结果,实验了不同岩性岩石和不同强度混凝土共计４０个试件,无一例外。变化规律是：在２ｃｍ和１０ｃｍ波段微波辐射能量随压力增加而增加,在３ｃｍ波段微波辐射     

Moving boundaries in 2-D and 3-D TLM simulations realized byrecursive formulas

A novel technique for arbitrary boundary positioning in transmission line matrix (TLM) networks is described. This capability removes the restriction that dimensions of TLM models can only be integer multiples of the mesh parameter and allows superior boundary resolution. Since the position of boundaries can be continuously varied even during a simulation, this feature can model moving boundaries for time-domain optimization and phenomena such as the Doppler effect