Techniques for the retrieval of aerosol properties over land andocean using multiangle imaging

Aerosols are believed to play a direct role in the radiation budget of Earth, but their net radiative effect is not well established, particularly on regional scales. Whether aerosols heat or cool a given location depends on their composition and column amount and on the surface albedo, information that is not routinely available, especially over land. Obtaining global information on aerosol and surface radiative characteristics, over both ocean and land, is a task of the Multi-angle Imaging SpectroRadiometer (MISR), an instrument to be launched in 1998 on the Earth Observing System EOS-AM1 platform. Three algorithms are described that will be implemented to retrieve aerosol properties globally using MISR data. Because of the large volume of data to be processed on a daily basis, these algorithms rely on lookup tables of atmospheric radiative parameters and predetermined aerosol mixture models to expedite the radiative transfer (RT) calculations. Over oceans, the “dark water” algorithm is used, taking full advantage of the nature of the MISR data. Over land, a choice of algorithms is made, depending on the surface types within a scene-dark water bodies, heavily vegetated areas, or high-contrast terrain. The retrieval algorithms are tested on simulated MISR data, computed using realistic aerosol and surface reflectance models. The results indicate that aerosol optical depth can be retrieved with an accuracy of 0.05 or 10%, whichever is greater, and some information can be obtained about the aerosol chemical and physical properties     

自由曲面反射式投影成像系统的设计

为了使投影设备具有超短投影距离,同时具有大尺寸高清显示画面,本文设计了一款采用自由曲面全反射式成像光路小投射比的超短焦距投影物镜.系统使用16.5 mm(0.65 in)DMD微透镜显示芯片,采用远心光路提高像面照度均匀性;利用光学设计软件CodeV进行镜片设计与优化,最终得到由4片非球面反射面构成的投影物镜.物镜投射...     

反射光栅的色散特性研究

本文分析了反射光栅的各种色散特性,提出光栅时间色散的概念,推导了光栅对光脉冲一级衍射的数学表达式。     

Directional ocean wave measurements in a coastal setting using afocused array imaging radar

A unique focused array imaging Doppler radar was used to measure directional spectra of ocean surface waves in a nearshore experiment performed on the North Carolina Outer Banks. Radar images of the ocean surface's Doppler velocity were used to generate two dimensional spectra of the radial component of the ocean surface velocity field. These are compared to simultaneous in-situ measurements made by a nearby array of submerged pressure sensors. Analysis of the resulting two-dimensional spectra include comparisons of dominant wave lengths, wave directions, and wave energy accounting for relative differences in water depth at the measurement locations. Limited estimates of the two-dimensional surface displacement spectrum are derived from the radar data. The radar measurements are analagous to those of interferometric synthetic aperture radars (INSAR), and the equivalent INSAR parameters are shown. The agreement between the remote and in-situ measurements suggests that an imaging Doppler radar is effective for these wave measurements at near grazing incidence angles     

Impact of horizontal and vertical heterogeneities on retrievals using multiangle microwave brightness temperature data

This paper investigates the impact of heterogeneity at the land surface on geophysical parameters retrieved from multiangle microwave brightness temperature data, such as would be obtained from the Soil Moisture and Ocean Salinity (SMOS) mission. Synthetic brightness temperature data were created using the Common Land (land surface) Model, coupled with a microwave emission model and set within the framework of the North American Land Data Assimilation System (NLDAS). Soil moisture, vegetation optical depth, and effective physical temperature were retrieved using a multiobjective calibration routine similar to the proposed SMOS retrieval algorithm for a typical on-axis range of look angles. The impact of heterogeneity both in the near-surface profiles of soil moisture and temperature and in the land cover on the accuracy of the retrievals was examined. There are significant errors in the retrieved parameters over regions with steep gradients in the near-surface soil moisture profile. These errors are approximately proportional to the difference in the soil water content between the top (at 0.7 cm) and second layer (at 2.7 cm) of the land surface model. The errors resulting from heterogeneity in the land cover are smaller and increase nonlinearly with increasing land-surface heterogeneity (represented by the standard deviation of the optical depth within the pixel). The most likely use of retrieved soil moisture is through assimilation into an LDAS for improved initiation of weather and climate models. Given that information on the soil moisture profile is already available within the LDAS, the error in the retrieved soil moisture as a result of the near-surface profile can be corrected for. The potential errors as a result of land-surface heterogeneity can also be assessed for use in the assimilation process.     

Determination of HgCdTe elasto-plastic properties using nanoindentation

Depth sensing indentation has been used to investigate the elasto-plastic behavior of Hg_0.7Cd_0.3Te prepared by molecular beam epitaxy, liquid phase epitaxy, as well as of bulk Hg_0.7Cd_0.3Te prepared by the modified Bridgman method. It was found that Hg_0.7Cd_0.3Te was characterized by a modulus of elasticity of E _avg ∼ 50 GPa and hardness of H _avg ∼ 0.66 GPa, independent of growth technology. The measured hardness was observed to increase with decreasing size of indentation owing to the nucleation of dislocations within the plastic zone. The elasto-plastic response of the samples to nanoindentation was observed to be purely elastic at low indentation depths and developed into ∼10% elastic and ∼90% plastic response, with an increase in penetration contact depth to above 100 nm exhibiting significant amounts of creep. The transition from purely elastic to elasto-plastic behavior has been observed to be marked by discontinuities, or “pop-in” events in the indenter load-penetration curves, with the indentation zone maximum sheer stress varying with HgCdTe growth method in the range 1.1–1.8 GPa. This onset and subsequent flow of plasticity is postulated to be associated with the spontaneous nucleation and propagation of dislocations.     

WindSat radio-frequency interference signature and its identification over land and ocean

Radio-frequency interference (RFI) in the spaceborne multichannel radiometer data of WindSat and the Advanced Microwave Scanning Radiometer-EOS is currently being detected using a spectral difference technique. Such a technique does not explicitly utilize multichannel correlations of radiometer data, which are key information in separating RFI from natural radiations. Furthermore, it is not optimal for radiometer data observed over ocean regions due to the inherent large natural variability of spectral difference over ocean. In this paper, we first analyzed multivariate WindSat and Scanning Multichannel Microwave Radiometer (SMMR) data in terms of channel correlation, information content, and principal components of WindSat and SMMR data. Then two methods based on channel correlation were developed for RFI detection over land and ocean. Over land, we extended the spectral difference technique using principal component analysis (PCA) of RFI indices, which integrates statistics of target emission/scattering characteristics (through RFI indices) and multivariate correlation of radiometer data into a single statistical framework of PCA. Over ocean, channel regression of X-band can account for nearly all of the natural variations in the WindSat data. Therefore, we use a channel regression-based model difference technique to directly predict RFI-free brightness temperature, and therefore RFI intensity. Although model difference technique is most desirable, it is more difficult to apply over land due to heterogeneity of land surfaces. Both methods improve our knowledge of RFI signatures in terms of channel correlations and explore potential RFI mitigation, and thus provide risk reductions for future satellite passive microwave missions such as the NPOESS Conical Scanning Microwave Imager/Sounder. The new RFI algorithms are effective in detecting RFI in the C- and X-band Windsat radiometer channels over land and ocean.     

Diagnosing refraction properties of troposphere over land using global navigation systems

Possibility of using global positioning systems to determine the refraction coefficient of troposphere is considered. Connection between errors of altitude measurements and condition of troposphere is revealed.     

Correlation properties of ocean altimeter returns

The correlation properties of altimeter returns are useful in the optimal design of altimeter systems. They are also necessary for implementing maximum likelihood estimation algorithms for altimeter waveform parameter estimation. The authors derive a simple expression for the bin-to-bin and pulse-to-pulse correlation of altimeter waveforms. This expression is a triple convolution of the specular point probability density function, a product of altimeter point target responses, and a generalized surface impulse response. This result is analogous to the usual expression for the mean altimeter waveform. The authors compare these theoretical predictions against real data, using measurements from the Skylab and Geosat altimeters. Finally, they present a simple application of the results to altimeter system design     

激光反射层析成像相位恢复算法研究

针对激光反射层析成像中目标随机抖动与径向运动造成的投影中心失配问题,提出采用相位恢复算法对重构图像进行相位恢复重建,通过反复光强迭代消除目标随机运动造成的相位误差,达到减少重构误差、恢复目标图像的目的。为改进G-S算法收敛速度与恢复精度,提出频域模值加权方法进行投影数据相位迭代恢复,仿真实验表明,将算法收敛速度与收敛精度提高了1.2倍以上。通过对三组仿真实验处理,重构图像的平均相对均方误差由2.487 5下降到0.792 7,有效地恢复了目标图像轮廓。外场实验表明,该算法能够有效消除重构图像伪迹,改善激光反射层析成像系统成像质量。     

On the theory of SAR ocean wave imaging

Ocean wave imaging with a high-resolution synthetic aperture radar (SAR) is considered. It is noted that the imaging is mainly due to a variable number of the subimages of different ocean surface elements superimposed at each image plane point. This mechanism is nonlinear, in principle, acting on the ocean surface represented as a random process. SAR image spectra are calculated for representative values of ocean state and imaging geometry parameters     

Synthetic aperture radar imaging of moving ocean waves

A theory for the radar imaging of ocean waves is presented under the assumptions that a swell propagates through an ensemble of Bragg scatterers and that the integration time of the synthetic aperture radar (SAR) is small compared to the angular velocity of the swell. Results are prsented which show image development and distortions caused by the radial velocities and accelerations of the swell. Neglecting small wave bunching and tilts due to the longer underlying waves, and considering only one-dimensional geometries, the mechanism of wave motions are considered and their efforts on the production of the usual intensity Pattern representing the wave image are studied. The analysis shows that in certain situations a processed image can appear which has twice the spatial period of the actual long wave on the ocean, which can confuse the interpretation of ocean wave analysis.     

Applications of dense media radiative transfer theory for passivemicrowave remote sensing of foam covered ocean

The effect of the foam covered ocean surface on the passive microwave remote sensing measurements is studied based on the electromagnetic scattering theory. In formulating an electromagnetic scattering model, the authors treat the foam as densely packed sticky air bubbles coated with thin seawater coating. The layer of foam covers the ocean surface that has air bubbles. They then use dense media radiative transfer (DMRT) theory with quasi-crystalline approximation (QCA) for densely distributed sticky moderate size particles to calculate the brightness temperatures of the foam-covered ocean surface. Results are illustrated for 19 GHz and 37 GHz and for both vertical and horizontal polarizations as a function of foam microstructure properties and foam layer thickness. Comparisons are also made with experimental measurements     

Nd:YAG quantum efficiency and related radiative properties

Measured quantum efficiencies of ten different Nd:YAG crystals show a variation from 0.47 to 0.87. In phototropic crystals, the quantum efficiency can be degraded by color center formation or improved by bleaching. Color center bleaching occurs by oxidation, using a high-temperature reactive atmosphere process to control crystal defects related to hydroxyl impurities. Ion-ion and ion-lattice interaction effects are examined for further evidence of Nd³⁺ fluorescence quenching centers. Fluorescence rates, nonequivalent crystal field (NCF) effects, line broadening, Nd⁺³ lattice-defect interactions, and their temperature dependences are studied. Nd³⁺ site perturbations occur in all crystal samples. Five NCF Nd³⁺ sites show 6 cm^-1 site-to-site Stark level shifts at liquid helium temperatures. Smaller scale shifts are characteristic of particular samples. While the crystals examined show individual differences, only hydroxyl-related coloration provides specific evidence of quenching centers     

Models of radar imaging of the ocean surface waves

A number of models which would explain ocean wave imagery taken with a synthetic aperture imaging radar are analyzed analytically and numerically. Actual radar imagery is used to support some conclusions. The models considered correspond to three sources of radar backscatter cross section modulation:tilt modulation, roughness variation, and the wave orbital velocity. The effect of the temporal changes of the surface structure, parametric interactions, and the resulting distortions are discussed.     

Novel VOA using in-plane reflective micromirror and off-axis light attenuation

A new off-axis light attenuation configuration combining with a movable reflective micromirror is proposed for variable optical attenuator (VOA) application. Comprehensive study of new comb drive actuators of a two-step structure is done to achieve large displacement of an actuated micromirror for VOA. The measured optical and electrical performances have proven this new design promising for practical applications.     

Cooling of vapors using collisionally aided radiative excitation

Collisionally aided radiative excitation (CARE) is proposed as a mechanism for cooling an atomic vapor. With a CW laser power of 1.0 W/cm² and the resonant dipole-dipole interaction providing the collision mechanism, we estimate that temperature gradients of tens of degrees per cm can be achieved at vapor densities of order 10¹⁶ atoms/cm³.     

Wind direction over the ocean determined by an airborne, imaging,polarimetric radiometer system

The speed and direction of winds over the ocean can be determined by polarimetric radiometers. This has been established by theoretical work and demonstrated experimentally using airborne radiometers carrying out circle flights and thus measuring the full 360° azimuthal response from the sea surface. An airborne experiment, with the aim of measuring wind direction over the ocean using an imaging polarimetric radiometer, is described. A polarimetric radiometer system of the correlation type, measuring all four Stokes brightness parameters, is used. Imaging is achieved using a 1-m aperture conically scanning antenna. The polarimetric azimuthal signature of the ocean is known from modeling and circle flight experiments. Combining the signature with the measured brightness data from just a single flight track enables the wind direction to be determined on a pixel-by-pixel basis in the radiometer imagery     

Waveguiding and reflective properties of plane boundaries with an anisotropic chiral impedance

The properties of circularly polarized surface waves propagating along a plane boundary with an anisotropic chiral impedance are investigated. Boundaries with rectangular and radial-ring structures of anisotropy are considered. The problem of the normal incidence of plane circularly polarized waves on such boundaries is solved.