Satellite measurements of tropospheric aerosols

The ability to measure tropospheric aerosols over ocean surfaces has been demonstrated using several different satellite sensors. Landsat data originally showed that a linear relationship exists between the upwelling visible radiance and the aerosol optical thickness (about 90% of this thickness is generally in the lowest 3 km of the atmosphere). Similar relationships have also been found for sensors on GOES, NOAA-5 and NOAA-6 satellites. The linear relationship has been shown theoretically to vary with the aerosol properties, such as size distribution and refractive index, although the Landsat data obtained at San Diego showed little variability in the relationship. To investigate the general applicability of the technique to different locations, a global-scale ground-truth experiment was conducted in 1980 with the AVHRR sensor on NOAA-6 to determine the relationship at ten ocean sites around the globe. The data for four sites have been analyzed, and show excellent agreement between the aerosol content measured by the AVHRR and by sunphotometers at San Diego, Sable Island and San Juan, but at Barbados, the AVHRR appears to overestimate the aerosol content. The reason for the different relationship at the Barbados site has not been definitely established, but is most likely related to problems in interpreting the sunphotometer data rather than to a real overestimation by the AVHRR. A preliminary analysis of AVHRR Channel 1 (0.65 μm) and Channel 2 (0.85 μm) radiances suggest that useful information on the aerosol size distribution may also be obtained from satellite observations.     

Satellite measurements of atmospheric composition

Since 1978 a number of satellite borne sensors have been used to measure the composition of the earth's atmosphere. These include the LIMS and SAMS instruments on the Nimbus 7 satellite (launched in October 1978), the SAGE instrument on the AEM2 satellite (launched in february 1979) and various instruments on the SME spacecraft (launched October 1981). For many species, these have provided the first abundance measurements with high spatial and temporal resolution and with global coverage. In this paper the composition measurements that have become available from these programs will be reviewed. The paper will then describe some recent studies that have made use of the new data. As it is the exclusive subject of another invited paper, ozone will not be discussed in in any detail.     

Satellite altimeter measurements of the geoid in sea ice zones

The Seasat radar altimeter provided surface height measurements to a precision better than 10cm over the open ocean, The data have been used to produce maps of the ocean geoid which reveal details of sub-surface topography such as sea mounts, ocean trenches and mid-ocean ridges /1/. In areas of the ocean covered by sea ice, however, the quasi-specular ice returns which occurred were incorrectly handled by the on-board processor. This resulted in a significant decrease in the precision of the surface height estimates. Consequently, researchers have generally eliminated data from regions where sea ice is suspected to have been present, including large areas of the Antarctic ocean. We have developed a technique for significantly improving the height measurements over such areas permitting the mapping of the geoid in such regions. The short wavelength RMS deviation of elevation measurements from collinear passes over such areas has been reduced from 1.032m to 0.632m. The application of the technique to ERS-1 altimeter data will be particularly important, since coverage of a substantial area of the Arctic ocean (up to 82° latitude) will be possible for the first time     

Determination of aerosol optical depth from ground measurements

The methods used to determine the aerosol optical depth as a function of wavelength are briefly described and discussed. Some results from the operational network of the World Meteorological Organization and other, more research oriented studies, are reviewed and critically analysed to assess the reliability and accuracy of such determinations and their value as ground truth measurements for space applications.     

Satellite derived aerosol optical depth climatology over Bangalore,India

Climatological aerosol optical depths (AOD) over Bangalore, India have been examined to bring out the temporal heterogeneity in columnar aerosol characteristics. AOD values at 550 nm derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor onboard NASA’s Terra and Aqua satellites, for the period of 2002–2011 have been analyzed (independently) for the purpose. Frequency distributions of the AOD values are examined to infer the monthly mean values. Monthly and seasonal variations of AOD are investigated in the light of regional synoptic meteorology. Climatological monthly and seasonal mean Terra and Aqua AOD values exhibited similar temporal variation patterns. Monthly mean AOD values increased from January, peaks during May and thereafter (except for a secondary peak during July) fall off to reach a minimum during December. Monsoon season recorded the highest climatological seasonal mean AOD, while winter season recorded the lowest. AOD values show an overall increasing trend on a yearly basis, which was found mainly due to sustained increase in the seasonal averaged AOD during summer. The results obtained in the present study are compared with that of the earlier studies over the same location and also with AOD over various other Indian locations. Finally, the radiative and climatic impacts are discussed.     

GNSS Satellite Autonomous Integrity Monitoring (SAIM) using inter-satellite measurements

Integrity is the ability of Global Navigation Satellite Systems (GNSS) to detect faults in measurements and provide timely warnings to users and operators when the navigation system cannot meet the defined performance standards, which is of great importance for safety of life critical applications. Compared with both Receiver Autonomous Integrity Monitoring (RAIM) and ground based GNSS Integrity Channel (GIC) methods which are widely adopted nowadays, the Satellite Autonomous Integrity Monitoring (SAIM) method can be used to monitor orbit/ephemeris and clock errors, and has advantages in monitoring orbit and clock quality and providing instantaneous responses when faults happen.     

Infrared emission and optical scattering by the high galactic latitude dust cloud L1642

We present a comparison of optical surface brightness and extinction data with IRAS infrared maps at 60 and 100 μm for the high galactic latitude cloud L1642. We have derived the optical albedo of the grains and the ratio of 100 μm and optical opacities. The albedo does not depend strongly on wavelength; on the average the albedo is 0.5 between 3450 and 5500 Å. L1642 is detected also at 12 and 25 μm, where the morphology of the brightness distribution supports an explanation in terms of non-equilibrium emission from small grains and/or large aromatic molecules.     

Precision measurements of stratospheric ozone profiles by rocket-borne optical ozonesondes

Sunset observations of the upper stratospheric and mesospheric ozone were made at Uchinoura (31.25°N, 131.08°E) with rocket-borne optical ozonesondes, which consist of multi-color solar ultraviolet radiometers and sun tracking devices. Three ozone density profiles were obtained in this study. A comparison with the 30°N zonal and monthly average of the interim reference ozone model shows a variability that our present ozone mixing ratios below −50 km are larger in January and February and smaller in September than those of the model.     

Identification of a debris cloud from the nuclear powered SNAPSHOT satellite with haystack radar measurements

Data from the Massachusetts Institute of Technology Lincoln Laboratory Long Range Imaging Radar (known as the Haystack radar) have been used in the past to examine families of objects from individual satellite breakups or families of orbiting objects that can be isolated in altitude and inclination. This is possible because, for some time after a breakup, the debris cloud of particles can remain grouped together in similar orbit planes. This cloud will be visible to the radar, in fixed staring mode, for a short time twice each day, as the orbit plane moves through the field of view. There should be a unique three-dimensional pattern in observation time, range, and range rate which can identify the cloud. Eventually, through slightly differing precession rates of the right ascension of ascending node of the debris cloud, the observation time becomes distributed so that event identification becomes much more difficult.     

Mapping ocean dynamic structure using optical spectroscopic measurements from space

Ocean circulation is an important element in many of the large scale experiments planned for the World Climate Program. Satellite infrared data, particularly that from polar oribiting NOAA weather satellites, have demonstrated a capability for showing flow patterns in the ocean in areas where western boundary currents or upwelling provide sufficient thermal contrast. Many areas, however, have thermal contrasts too low to be mapped reliably and other tracers are needed.Images showing variations in the colour (spectral reflectance) of the surface layers of the ocean, from the Coastal Zone Color Scanner on Nimbus 7, have provided excellent examples of flow patterns traced by movements of water bodies having different phytoplankton content. Examples are presented here which show coastal flow patterns off the British Columbia coast, Gulf Stream flow between the New England sea mounts and the form of the Alaskan Stream.Such imagery has only recently become available, and could contribute greatly to a more detailed understanding of ocean circulation. The CZCS was launched in 1978 and is slowly degrading in operation. It appears that a six-year gap in supply of ocean colour imagery may now occur before a replacement can be launched. A large back-log of data remains to be analysed. The CZCS was the first to make this type of measurement and it seems certain that improved sensor designs could increase the value of the data. One such design being developed in Canada makes use of two dimensional arrays of optical detectors to provide greatly increased spectral resolution, and improved sensitivity.     

Cloud optical thickness feedbacks in the CO2 climate problem

A radiative-convective equilibrium model is developed and applied to study cloud optical thickness feedbacks in the CO₂ climate problem. The basic hypothesis is that in the warmer and moister CO₂-rich atmosphere, cloud liquid water content will generally be larger than at present, so that cloud optical thickness will be larger too. For clouds other than thin cirrus, the result is to increase the albedo more than to increase the greenhouse effect. Thus the sign of the feedback is negative: cloud optical properties alter in such a way as to reduce the surface and tropospheric warming caused by the addition of CO₂. This negative feedback can be substantial. When observational estimates of the temperature dependence of cloud liquid water content are employed in the model, the surface temperature change due to doubling CO₂ is reduced by about one half.     

Estimates of convective heat fluxes and gravity wave amplitudes in the Venus middle cloud layer from VEGA balloon measurements

Atmospheric temperatures and vertical velocities obtained from the VEGA balloon measurements indicate that the dynamical heat flux is upward and has an amplitude that ranges from 0 to 360 W m⁻² in the middle cloud region. The static stability is positive and ranges from 0 to 2.0 K km⁻¹. Time series analysis of these results indicates that convection is the principal mechanism for generating the large vertical motions. Gravity waves were also detected at these levels and account for about 15% of the covariance between temperature and vertical velocity.     

A surface reflectance correction model to improve the retrieval of MISR aerosol optical depth supported by MODIS data

Combined use of different satellite sensors are known to improve retrievals of aerosol optical depth (AOD). In this study, we propose a new method for retrieving Multi-angle Imaging SpectroRadiometer (MISR) AOD data supported by Moderate Resolution Imaging Spectroradiometer (MODIS) data in Jiangsu Province, China, over the period of 2016–2018 using MODIS L1B, bidirectional reflectance distribution function (BRDF), MISR 1B2T, and ground-measured AOD data. This method is based on the surface reflectance determined by the MODIS V5.2 algorithm. Through the observation angle and spectral conversion between different sensors, the MISR AOD can be obtained. The correlation coefficient (R) and root-mean-square error (RMSE) between the retrieved MISR and ground-measured AOD data varied between different seasons. The accuracy of the MISR AOD retrieval was notably improved after correcting the MISR surface reflectance. Therefore, the method proposed in this study is feasible for the retrieval of MISR AOD supported by MODIS data, and will be applicable to atmospheric environmental monitoring over large areas in the future.     

Retrieval of vegetation equivalent water thickness from reflectance using genetic algorithm (GA)-partial least squares (PLS) regression

This study aimed to investigate the performance of genetic algorithms coupled with partial least squares (GA-PLS) modeling of spectral reflectance in retrieving equivalent water thickness (EWT) at leaf and canopy level. A genetic algorithm was used to identify a subset of spectral bands sensitive to the variation in EWT, and PLS was then applied to relate the identified bands to EWT values. GA-PLS was applied to leaf level reflectance available from LOPEX dataset, and canopy data includes reflectance simulated by a leaf radiative transfer model PROSPECT and a canopy radiative transfer model SAILH and acquired by airborne visible/infrared imaging spectrometer (AVIRIS). The results indicate that GA-PLS has the capability of retrieving EWT from leaf and canopy reflectance, and achieved good estimation accuracy, i.e. low root mean square errors (RMSE) and high squared correlation coefficients (R²). For the retrieval at leaf level, the estimation accuracy can be as good as RMSE = 0.0019 g/cm² and R² = 0.939 or better. For the retrieval at canopy level, the model accuracy is RMSE = 0.0061 g/cm² and R² = 0.966 or better when PROSPECT-SAILH simulated reflectance was used; when AVIRIS image spectra were used, the model accuracy is RMSE = 0.0094 g/cm² and R² = 0.8734 for the calibration, and RMSE = 0.0132 g/cm² and R² = 0.7756 for the validation. These results from GA-PLS modeling support the conclusion that GA-PLS has the potential to be applied to AVIRIS, Hyperion and HyMap imagery for retrieving EWT. The selected bands for the AVIRIS datasets differ from those for the LOPEX and PROSPECT-SAILH simulated datasets, and this inconsistence of the selected bands for different datasets indicates that the GA-PLS method has the advantage of tuning the optimum bands for PLS regression and accommodating the effects of confounding factors.     

基于三维点云模型的空间目标光学图像生成技术

空间探测任务中大量先验图像数据的缺乏,使得基于光学图像的态势感知和导航算法无法被有效定量测试和评估。针对此问题,提出了一种基于三维点云模型和射影变换基本理论的空间目标光学图像生成方法。在完成对空间目标三维点云模型和仿真摄像机模型构建基础之上,利用射影变换基本理论依次计算像平面所有像素点与空间目标三维点云模型空间点的对应关系,并基于Lambertian漫反射模型和相对应空间目标三维点云模型空间点的光照方向,得到所有像素点的灰度值,从而生成给定空间目标的光学图像。大量仿真实验表明:与传统的基于解析模型的仿真图像生成方法相比,所提的空间目标光学图像生成技术能够以更快的速度生成更加真实的仿真图像,且生成的仿真图像可以广泛应用于椭圆拟合、陨石坑检测、着陆器视觉导航、航天器交会对接、空间目标跟踪等典型空间应用算法的定性与定量评估。      

小卫星星务管理技术

阐述了第三代星上测、控、管系统 (星务系统 )的基本设计思想 ,它是基于现场控制、内嵌式微控制器和星上现场网络等概念开发的一项卫星新技术。该项技术实现传统卫星上的系统功能集成 ,具有全数字化、全网络连接、全分散式和在线组态的特点。该文给出星务系统与过去星上测控系统相比的最大两个概念跳跃 :内嵌式 ,整星全网络化。星务系统构成了一种星载柔性服务系统 ,用现场网络来协调、控制星上智能设备的相互联系 ,完成包括信息流、动作流、能量流的动态作业 ,可以在线下载任务和在线整定参数 ,改变传统卫星测控系统接口层的封闭性和专用性 ,实现星上设备“即连即用”。这就是构成星上集成电子学系统一体化的关键 ,构成“平台化”的核心。按星务系统技术设计 ,可以提高整星级可靠性和运行功能有效性 ,并可以加快研制进度和降低开发成本。该文还指出了星务系统和数管系统的不同 ,CAN总线和 15 5 3B的不同     

Particle and optical measurements aboard the Aureol 3 spacecraft (ARCAD 3 project): A — The particle experiments B — Optical measurements

The CESR Toulouse - IKI Moscow particle instrument package aboard the AUREOL-3 satellite consists of a complete set of charged particle spectrometers which measure electron and ion fluxes from 15 eV to 25 keV in 128 steps and in 11 directions. In addition, 4 channel spectrometers (2 electron and 2 ion channels in parallel) allow high time resolution measurements (up to 10 msec) with onboard calculation of auto and cross correlation functions. For higher energies (40 – 280 keV), solid-state spectrometers are used to measure electron and proton fluxes in 4 channels in parallel. In addition, two Geiger counters are used for the determination of the trapping boundaries. Two mass-energy ion spectrometers (1 to 32 A.M.U., 0.02 – 15 keV) are placed with viewing angles which allow a distinction between nearly isotropic auroral proton precipitation and conical beams accelerated in the auroral ionosphere. Auroral and airglow photometry is performed aboard the AUREOL-3 satellite by a set of 3 parallel directed photometers with tiltable interference filters for 6300 Å, 4278 Å and Doppler shifte H_β emissions. Various modes of energy, angular and mass scanning, correlation function calculation and various Soviet and French telemetry regimes provide the possibility of choosing the sequences of measurements according to particular experimental programs along the orbit. Finally, examples of data from inflight measurements using the above instruments are presented and briefly discussed, showing several interesting features.     

Retrieval of aerosol optical characteristics from polarization measurements of reflected solar radiation above the oceans

The reflected near infrared solar radiation observed from space above the oceans is due mainly to the atmosphere scattering, as the ocean surface is nearly black. The molecular Rayleigh contribution is also minimized at infrared wavelengths and it can be evaluated. It is shown that the degree of polarization is much more sensitive to the aerosol properties than the radiance. Measurements of polarization at two wavelengths and with an angular scanning are simulated and an inversion algorithm is proposed. It aims at finding an “equivalent aerosol model,” which reproduces the optical thickness and the asymmetry factor of the actual aerosol at all wavelengths in the solar spectrum.     

Spatial and temporal monitoring of drought conditions using the satellite rainfall estimates and remote sensing optical and thermal measurements

Drought is an important natural disaster that causes devastating impacts on the ecosystem, livestock, environment, and society. So far, various remote-sensing methods have been developed to estimate drought conditions, each of which has advantages and restrictions. This study aims to monitor the real-time drought indices at the field scales via the integration of various earth observations. Our proposed method consists of two steps. In the first step, the relationships between long-term standardized precipitation indices (SPI) derived from PERSIANN-CDR rainfall data and two drought-dependent parameters derived from MODIS products, including normalized NDVI and soil-air temperature gradient, are obtained at the spatial resolution of PERSIANN-CDR grid (approximately 25 km). As the next step, the corresponding relationships are applied to estimate the drought index maps at the spatial resolution of MODIS products (1 km). Numerous analyses are carried out to evaluate the proposed method. The results revealed that, from various drought indices, including SPIs of different timescales (1, 3, 6, and 12-months), SPI-3 and SPI-6 are more appropriate to the proposed method in terms of correlation with temperature and vegetation parameters. The findings also demonstrate the competency of the proposed method in estimating SPI indices with average RMSE 0.67 and the average correlation coefficient of 0.74.