Normalized Differential Spectral Attenuation (NDSA) Measurements Between Two LEO Satellites: Performance Analysis in the Ku/K-Bands

Normalized differential spectral attenuation (NDSA) is a novel differential measurement method to estimate the total content of water vapor [integrated water vapor (IWV)] along a tropospheric propagation path between two low Earth orbit (LEO) satellites. A transmitter onboard the first LEO satellite and a receiver onboard the second one are required. The NDSA approach is based on the simultaneous estimates of the total attenuation at two relatively close frequencies in the Ku/K-bands and of a “spectral sensitivity parameter” that can be directly converted into IWV. The spectral sensitivity has the potential to determine the water vapor contribution, to cancel out all spectrally flat unwanted contributions, and to limit the impairments due to tropospheric scintillation. In this paper, we focus on the measurement accuracy of the spectral sensitivity parameter. Specifically, we examine this accuracy at three different frequencies and for two models of atmospheric structure. We first provide an approximate expression of the accuracy and then validate this expression through Monte Carlo simulations based on microwave propagation models.      

Normalized differential spectral attenuation (NDSA): a novel approach to estimate atmospheric water vapor along a LEO-LEO satellite link in the ku/K bands

We introduce here the normalized differential spectral attenuation (NDSA) approach, which is a novel differential measurement way for estimating the total content of water vapor integrated water vapor (IWV) along a tropospheric propagation path between two Low Earth Orbit (LEO) satellites. The NDSA approach requires a transmitter onboard the first LEO satellite and a receiver onboard the second one. It is based on the simultaneous measurement of the total attenuation at two relatively close frequencies in the Ku/K bands, and on the estimate of a "spectral sensitivity parameter" that can be directly converted into IWV. NDSA is potentially able to emphasize the water vapor contribution, to cancel out all spectrally flat unwanted contributions and to limit the impairments due to tropospheric scintillation. The objective of the paper is to analyze the level of correlation between the spectral sensitivity parameter and the IWV at a given altitude from ground of the LEO-LEO link (tangent altitude), in order to single out the best performing frequencies. Simulation results are based on microwave propagation models and on radiosonde data. The results shows the potential of the NDSA approach to provide direct estimates of IWV along LEO-LEO tropospheric propagation paths in the 15-25 GHz frequency range, under different atmospheric conditions.     

机载大气红外高分辨率光谱的信息量分析

机载红外高分辨率干涉光谱仪（HIS）是遥感大气信息的大气红外背景辐射测量装置。基于HIS红外高光谱大气数据,对大气温度和主要大气吸收气体进行了敏感性分析;并通过引入信息容量的概念,利用一维变分方法,对HIS测量装置遥感探测大气参数的能力进行评估,定量描述了HIS测量的红外光谱反演大气温度和水汽的信息容量、自由度和垂直分辨率等信息,其中温度、水汽的信息容量分别为49.5、25.2;自由度分别为10.5、5.6,温度的平均垂直分辨率为2.2 km,水汽的垂直分辨率为2 km;讨论了HIS的反演精度与仪器测量误差之间的关系,获取了大气温湿廓线的最小可探测精度。     

MIR measurements of atmospheric water vapor profiles

Three subjects related to atmospheric water vapor profiling using the 183.3 GHz absorption line are discussed. First, data acquired by an airborne millimeter-wave imaging radiometer (MIR) over ocean surface in the western Pacific are used to estimate three-dimensional (3D) distribution of atmospheric water vapor. The instrument's radiometric measurements with mixed vertical and horizontal polarizations require modifications to the retrieval algorithm used in the past. It is demonstrated that, after the modifications, the new algorithm can provide adequate retrieval of water vapor profiles, even though the measured data are of mixed polarizations. Next, the retrieved profiles, in terms of water vapor mixing ratio ρ (g/kg), are compared with those measured in near concurrence by dropsondes from a research aircraft in the western Pacific and by a ground-based Raman lidar at Wallops Island, Virginia. The ratio of the standard deviation to the mean ρ is found to be 0.12 at 0.25 km altitude and gradually degraded to 0.67 at the highest altitude of the retrieval of 10.25 km. Finally, the effect of the “initial guess” relative humidity profile on the final retrieved product is analyzed with respect to the condition for the convergent retrieval. It is found that the effect is minimal if the initial profile is not unrealistically different from the true one. If the initial profile is very different from the true one, the final retrieved product could be subject to a substantial error. Tightening of the convergent condition in the retrieval helped reduce magnitude of the error, but not remove it totally. It is concluded that an initial profile based on climatology is likely to provide most reliable retrieval results     

红外高光谱探测CO_2柱总量的敏感性分析

利用辐射传输模型MODTRAN5模拟计算了红外高光谱仪在光谱分辨率、光谱定标精度、地面反射率、大气廓线以及辐射分辨率对CO_2气体含量探测敏感性的影响,同时定义探测辐亮度随气体含量改变的变化量指标S值,并对探测灵敏度、光谱分辨率和信噪比SNR的关系进行了综合分析。为我国红外高光谱大气探测仪的设计以及高光谱大气成分反演研究的开展提供重要的科学依据。对CO_2柱总量反演通道的选择和实现CO_2气体探测敏感参数分析具有重要的参考价值。     

Spectral and nonlinear analyses of MEG background activity in patients with Alzheimer's disease.

The aim of the present study is to analyze the magnetoencephalogram (MEG) background activity from patients with Alzheimer's disease (AD) and elderly control subjects. MEG recordings from 20 AD patients and 21 controls were analyzed by means of two spectral [median frequency (MF) and spectral entropy (SpecEn)] and two nonlinear parameters [approximate entropy (ApEn) and Lempel-Ziv complexity (LZC)]. In the AD diagnosis, the highest accuracy of 75.6% (80% sensitivity, 71.4% specificity) was obtained with the MF according to a linear discriminant analysis (LDA) with a leave-one-out cross-validation procedure. Moreover, we wanted to assess whether these spectral and nonlinear analyses could provide complementary information to improve the AD diagnosis. After a forward stepwise LDA with a leave-one-out cross-validation procedure, one spectral (MF) and one nonlinear parameter (ApEn) were automatically selected. In this model, an accuracy of 80.5% (80.0% sensitivity, 81.0% specificity) was achieved. We conclude that spectral and nonlinear analyses from spontaneous MEG activity could be complementary methods to help in AD detection.     

Spectral and Nonlinear Analyses of MEG Background Activity in Patients With Alzheimer's Disease

The aim of the present study is to analyze the magnetoencephalogram (MEG) background activity from patients with Alzheimer's disease (AD) and elderly control subjects. MEG recordings from 20 AD patients and 21 controls were analyzed by means of two spectral [median frequency (MF) and spectral entropy (SpecEn)] and two nonlinear parameters [approximate entropy (ApEn) and Lempel-Ziv complexity (LZC)]. In the AD diagnosis, the highest accuracy of 75.6% (80% sensitivity, 71.4% specificity) was obtained with the MF according to a linear discriminant analysis (LDA) with a leave-one-out cross-validation procedure. Moreover, we wanted to assess whether these spectral and nonlinear analyses could provide complementary information to improve the AD diagnosis. After a forward stepwise LDA with a leave-one-out cross-validation procedure, one spectral (MF) and one nonlinear parameter (ApEn) were automatically selected. In this model, an accuracy of 80.5% (80.0% sensitivity, 81.0% specificity) was achieved. We conclude that spectral and nonlinear analyses from spontaneous MEG activity could be complementary methods to help in AD detection.     

Remote sensing of water vapor in the near IR from EOS/MODIS

The LOWTRAN-7 code was used to simulate remote sensing of water vapor over 20 different surface covers. The simulation was used to optimize the water vapor channel selection and to test the accuracy of the remote sensing method. The channel selection minimizes the uncertainty in the derived water vapor due to variations in the spectral dependence of the surface reflectance. The selection also minimizes the sensitivity of the selected channels to possible drift in the channel position. The use of additional MODIS channels reduces the errors due to the effect of haze, subpixel clouds and uncertainties in the temperature profile. Remote sensing of the variation of water vapor from day to day will be more accurate, because the surface reflectances vary slowly with time. The method was applied to Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data     

单波长发射五通道接收激光雷达系统研制

后向散射激光雷达是探测大气气溶胶参数的有力工具,但它存在盲区和过渡区,且需要假设气溶胶的消光后向散射系数比来反演气溶胶的参数,这些限制了它的探测范围和精度。集侧向散射、后向散射和拉曼散射于一体的单波长发射五通道接收激光雷达系统,克服了上述困难。该激光雷达可以探测气溶胶的退偏比廓线、水汽混合比廓线、后向散射系数廓线和消光系数廓线等。气溶胶后向散射系数和消光系数可从地面到对流层顶进行探测,气溶胶退偏比廓线可以在对流层内进行探测,水汽混合比廓线可以在边界层内进行探测。在硬件条件的基础上,分析了各通道的信噪比和探测结果的随机相对误差。实例探测表明:该激光雷达系统数据可靠,探测范围较广。该系统的建立,为进一步深入研究气溶胶消光系数、水汽时空分布以及它们之间相互关系奠定了坚实的实验基础。     

Measurements of Atmospheric Water Vapor Above Mauna Kea Using an Infrared Radiometer

This paper presents the first results from a prototype infrared radiometer which has been developed to measure variations in atmospheric water vapor column abundance from high altitude sites. The performance of the infrared radiometer is compared and contrasted with that of a water vapor monitor operating at radio frequencies. Analysis shows that the infrared radiometer can measure variations at the level of ～ 1 μm precipitable water vapor (pwv) in an integration time of 1 s when the total column abundance is ～0.5 mm pwv. Since variations in atmospheric water vapor are the dominant source of phase noise in (sub)millimeter astronomical interferometry, an instrument capable of rapid and high sensitivity water vapor measurements has the potential to provide the necessary phase correction information for interferometric arrays.     

Retrieval of cloud base heights from passive microwave and cloudtop temperature data

Water vapor profiling algorithms that treat liquid clouds explicitly yield a cloud base height as a byproduct. A single case of a water vapor profile retrieval using a combination of the SSM/T-2 on the DMSP satellite and cloud parameters from the AVHRR on the NOAA satellite retrieved a reasonable cloud base. While hardly definitive, this case is suggestive. The authors examine the cloud base signal in a combination of the SSM/1 and SSM/T-2 on the DMSP satellite from a theoretical point of view. It is shown that the signal is strong enough for a useful retrieval only over the ocean. For low altitudes, a cloud top temperature (CTT) constraint, as could be provided from an infrared radiometer, is required. While difficult with the DMSP-NOAA satellite combination, this has become much easier with the recent launch of NOAA-K with the AMSU-B and AVHRR. It is shown that the signal is acceptable over the relevant range of cloud liquid water content values. To achieve useful results, some local tuning of the algorithm will be necessary. This tuning could take the form of water vapor profile covariance matrices, climatological estimates of the cloud liquid water density, or purely empirical methods. Broken and multilayer clouds provide additional complications to the problem     

Microwave attenuation measurements in satellite-ground links: thepotential of spectral analysis for water vapor profiles retrieval

The authors address the problem of estimating vertical profiles of atmospheric water vapor by means of attenuation measurements simultaneously made at different frequencies along a vertical satellite-ground link. The operating frequencies selected are those around the spectral absorption lines of water vapor at 22.235 GHz. A simulation is presented of multifrequency attenuation measurements, based on an atmospheric propagation model and on radiosonde data providing true profiles of temperature, pressure, and water vapor. The results indicate that such multifrequency measurements are correlated to variations of the vertical profiles of water vapor. It is therefore expected that vertical detail of such profiles depends on number and position of the frequencies utilized     

Absorption coefficients of marine waters: expanding multiband information to hyperspectral data

For many oceanographic studies and applications, it is desirable to know the spectrum of the attenuation coefficient. For water of the vast ocean, an effective way to get information about this property is through satellite measurements of ocean color. Past and present satellite sensors designed for ocean-color measurements, however, can only provide data in a few spectral bands. A tool is needed to expand these multiband measurements to hyperspectral information. The major contributors to the attenuation coefficient are absorption and backscattering coefficients. The spectral backscattering coefficient can generally be well described with a couple of parameters, but not so for the spectral absorption coefficient. In this paper, based on available hyperspectral absorption data, spectral-transfer coefficients are developed to expand multiband absorption coefficients to hyperspectral (400-700 nm with a 10-nm step) absorption spectrum. The derived transfer coefficients are further applied to data from field measurements to test their performance, and it is found that modeled absorption matches measured absorption very well (/spl sim/5% error). These results indicate that when absorption and backscattering coefficients are available at multiple bands, a hyperspectral attenuation-coefficient spectrum can now be well constructed.     

用于测量流层水汽的拉曼激光雷达

水汽在大气中含量很少,但变化很大,变化范围在0.1%~4%之间,水汽绝大部分集中在对流层。随着光电探测技术的不断发展,大气衰减对光电探测造成的影响也越来越显著,其中水汽是主要影响因子之一,也是最为不确定参数。光电探测中常用红外波段,但是水汽分子浓度较大,对辐射吸收造成很大的影响。拉曼激光雷达是测量大气水汽的主要技术手段之一。介绍了自行研制的水汽测量拉曼激光雷达的总体结构和主要技术参量。测量结果显示:该激光雷达可以对夜晚8 km高度范围内以及白天边界层内的水汽进行测量。实验数据与当地探空数据进行比对,取得了较好的一致性,充分验证了该拉曼激光雷达测量水汽的有效性和可靠性。     

A Future Millimeter/Sub–Millimeter Radiometer for Satellite Observation of Ice Clouds

The instrument concept of a future spaceborne millimeter/sub-millimeter radiometer is proposed in this paper for the remote sensing of ice clouds from satellite. The proposed radiometer is expected to operate at a series of frequencies within the millimeter and sub-millimeter wave range from 150 to about 900 GHz. Five frequencies are selected in the atmospheric windows, i.e., 150, 220, 463, 683, 874 GHz, and at each frequency there are two orthogonally polarized channels. Three water vapor channels located close to 183.31 GHz are also included in this instrument, since they can provide water vapor information, which is needed for ice cloud parameter retrieval. To simplify system design and test, a modular design philosophy is followed in the receiver frontend design and two antennas are used separately for the millimeter and sub-millimeter channels. Overall, the instrument requirements can be met with today's technology, except for the channels at the highest frequencies, where the radiometric sensitivity can be larger than the required 1.0 K for the 10 km spatial resolution (2.5 ms integration time). However, this situation can be improved by averaging neighboring pixels in data processing if certain compromise in the spatial resolution can be made at these frequencies.     

基于DSP傅里叶变换红外光谱基线漂移快速校正实现

傅里叶变换红外光谱仪（FTIR）在长时间开放光程条件下进行测量时,FTIR光谱基线会发生漂移,影响光谱信息,改变光谱形状进而影响气体分析的准确度。因此对测量到的光谱信息进行基线校正是非常重要的预处理步骤,为下一步的气体定性定量分析提供良好的数据基础。本文针对基线漂移采用多项式迭代拟合逼近的方法,针对三种不同类型的基线漂移光谱进行校正,分别在MATLAB环境下及DSP中实现,校正后的的光谱数据采用均方根误差R作为评价标准,分别达到了4.21?0-6和5.38?0-6,校正时间分别消耗0.9350s和0.7783s, 结果表明该方法自动化程度高,校正效果好,速度快,可以为基于DSP实现系统小型化集成化的光谱处理系统提供理论基础。     

Multicomponent Physical Vapor Deposited Films with Homogeneous Molecular Material Distribution Featuring Improved Resist Sensitivity

Each film preparation technique affects the physical properties of the resulting coating and thus defines its applicability in modern device construction. In this context solvent based spin coated and solvent‐free physical vapor deposited molecular glass photoresist films are systematically investigated for their dissolution behavior, sensitivity, and overall lithographic performance. These investigations demonstrate that the solvent‐free physical vapor deposition leads to a marked increase in sensitivity. This could be explained by the individual molecule by molecule deposition step producing a more homogeneous distribution of the multicomponent resist system, especially the photoacid generator. In addition, this assumption is supported by former published simulations focusing on aggregate formation within thin films. This work demonstrates that the lithographic sensitivity of multicomponent resist system is an intrinsic parameter to investigate molecular material distribution and indicates that the applied film preparation technique is crucial for the corresponding performance and applicability.     

基于NCEP FNL资料的定标场地气象信息获取及应用

以美国国家环境预报中心(NCEP)提供的 FNL资料作为研究对象,对该资料的特点、包含内容、解码以及使用方法做出介绍。以Visual C++6.0为编程工具,开发针对FNL资料的数据分析程序,提取温度、气压、臭氧等气象和大气分子参数。结果表明FNL资料具有较高的可信度,可以为场地定标提供部分参数输入。基于数据分析结果,实现瑞利散射光学厚度、水汽含量和臭氧吸收光学厚度计算等方面的应用。提出下一步的研究方向。     

The Ground-Based Scanning Radiometer: A Powerful Tool for Study of the Arctic Atmosphere

Measurements of water vapor and clouds in the polar regions are difficult, because conventional instruments show little sensitivity (~1.3 K/mm) to low amounts. On the other hand, millimeter- and submillimeter-wavelength radiometry offers greatly enhanced sensitivity (up to 51.4 K/mm, depending upon frequency). For this reason, the National Oceanic and Atmospheric Administration's Physical Science Division designed a new instrument, the Ground-Based Scanning Radiometer (GSR), for continuous and unattended observations at millimeter and submillimeter wavelengths (50-380 GHz). The GSR was deployed for the first time during the Arctic winter radiometric experiment in March-April 2004. In this paper, we discuss the GSR calibration procedure, which allows for accurate measurements during clear and cloudy skies. Error-budget analysis and comparison with independent measurements show an absolute accuracy on the order of 1-2 K. Examples of multifrequency and multiangle GSR observations are illustrated, representing a valuable new data set for the study of water vapor, clouds, and atmospheric absorption models in the Arctic.     

Enhancement of the spectral sensitivity of photodiodes for the mid-IR spectral range

A new method is used to raise the spectral sensitivity of photodiodes based on GaSb/GaInAsSb/GaAlAsSb heterostructures for the spectral range 1.1–2.4 μm. It is shown that, with a profile formed as pits on the metal-free unilluminated rear surface area of the photodiode chip, it is possible to improve the spectral sensitivity of the photodiodes at wavelengths in the range 1.8–2.4 μm. The most pronounced increase of up to 53% at the sensitivity maximum, compared with the sensitivity of conventional photodiodes with a fully metallized rear surface of the chip, is observed for photodiodes with shallow pits 30 μm in radius on their rear surface. These devices can find wide application in systems measuring the amount of water in petroleum products and the moisture content of paper, soil and grain.