Onboard calibration of the ASTER instrument

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is a high spatial resolution optical sensor for observing the Earth carried on the National Aeronautics and Space Administration Terra satellite. ASTER consists of three radiometers covering the following regions: visible and near-infrared (VNIR), shortwave infrared (SWIR), and thermal infrared (TIR). The preflight calibration of VNIR and SWIR utilized standard large integrating spheres whose radiance levels were traceable to primary standard fixed-point blackbodies. The onboard calibration devices for the VNIR and SWIR consist of two halogen lamps with photodiode monitors. In orbit, all three bands of the VNIR showed rapid decreases in the output signal while all SWIR bands remained stable. The TIR onboard blackbody was calibrated against a standard blackbody from 100-400 K in a vacuum chamber before launch. The TIR is unable to see the dark space. The temperature of the TIR onboard blackbody remains at 270 K for a short-term calibration to determine any offset and is varied from 270-340 K for a long-term calibration of both the offset and gain. The long-term calibration just after launch was consistent with the prelaunch calibration but then showed a steady decrease of the TIR response over the five years of operation to date.     

Design and preflight performance of ASTER instrument protoflightmodel

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is an advanced multispectral imager with high spatial, spectral, and radiometric resolution, built to fly on the EOS-AM1 spacecraft along with four other instruments, which will be launched in 1998. The ASTER instrument covers a wide spectral region, from visible to thermal infrared with 14 spectral bands. To meet the wide spectral coverage, optical sensing units of ASTER are separated into three subsystems: visible and near-infrared (VNIR) subsystem, shortwave infrared (SWIR) subsystem, and thermal infrared (TIR) subsystem. ASTER also has an along-track stereoscopic viewing capability using one of the near-infrared bands. To acquire the stereo data, the VNIR subsystem has two telescopes, one for nadir and another for backward viewing. Several new technologies are adopted as design challenges to realize high performance. Excellent observational performances are obtained by a pushbroom VNIR radiometer with a high spatial resolution of 15 m, a pushbroom SWIR radiometer with high spectral resolution, and a whiskbroom-type TIR radiometer with high spatial, spectral, and radiometric resolutions. The preflight performance is evaluated through a protoflight model (PFM)     

ASTER Level-1 data processing algorithm

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is an advanced multispectral imager with high spatial, spectral, and radiometric performance built for the EOS-AM1 polar orbiting spacecraft. ASTER covers a wide spectral region from visible to thermal infrared with 14 spectral bands. To meet this wide spectral coverage, ASTER has three optical-sensing subsystems: visible and near-infrared (VNIR), shortwave infrared (SWIR), and thermal infrared (TIR). In addition, the VNIR subsystem has two telescopes (nadir and backward telescopes) for stereo data acquisition. This ASTER instrument configuration with multitelescopes requires highly refined ground processing for the generation of Level-1 data products that are radiometrically calibrated and geometrically corrected. The algorithm developed for the ASTER Level-1 data processing is described     

Overview of Advanced Spaceborne Thermal Emission and ReflectionRadiometer (ASTER)

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is a research facility instrument provided by the Ministry of International Trade and Industry (MITI), Tokyo, Japan to be launched on NASA's Earth Observing System morning (EOS-AM1) platform in 1998. ASTER has three spectral hands in the visible near-infrared (VNIR), six bands in the shortwave infrared (SWIR), and five bands in the thermal infrared (TIR) regions, with 15-, 30-, and 90-m ground resolution, respectively. The VNIR subsystem has one backward-viewing band for stereoscopic observation in the along-track direction. Because the data will have wide spectral coverage and relatively high spatial resolution, it will be possible to discriminate a variety of surface materials and reduce problems in some lower resolution data resulting from mixed pixels. ASTER will, for the first time, provide high-spatial resolution multispectral thermal infrared data from orbit and the highest spatial resolution surface spectral reflectance temperature and emissivity data of all of the EOS-AM1 instruments. The primary science objective of the ASTER mission is to improve understanding of the local- and regional-scale processes occurring on or near the Earth's surface and lower atmosphere, including surface-atmosphere interactions. Specific areas of the science investigation include the following: (1) land surface climatology; (2) vegetation and ecosystem dynamics; (3) volcano monitoring; (4) hazard monitoring; (5) aerosols and clouds; (6) carbon cycling in the marine ecosystem; (7) hydrology; (8) geology and soil; and (9) land surface and land cover change. There are three categories of ASTER data: a global map, regional monitoring data sets, and local data sets to be obtained for requests from individual investigators     

ASTER geometric performance

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) system acquires multispectral images ranging from the visible to thermal infrared region. The ASTER system consists of three subsystems: visible and near-infrared (VNIR), short-wave infrared (SWIR) and thermal infrared (TIR) radiometers. The VNIR subsystem has a backward-viewing telescope as well as a nadir one. To deliver data products of high quality from the viewpoint of geolocation and band-to-band registration performance, a fundamental program, called Level-1 data processing, has been developed for images obtained using four telescopes with a cross-track pointing function. In this work, the methodology of the geometric validation is first described. Next, the image quality of ASTER data products is evaluated in view of the geometric performance over a period of four years. The band-to-band registration accuracy in the subsystem is better than 0.1 pixels and that between subsystems is better than 0.2 pixels. This means that the geometric database is determined accurately and the image matching method based on a cross-correlation function is effective in the operational usage.     

Accurate atmospheric correction of ASTER thermal infrared imagery using the WVS method

The water vapor scaling (WVS) method involves an atmospheric correction algorithm for thermal infrared (TIR) multispectral data, designed mainly for the five TIR spectral bands of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on the Terra satellite. First, this method is improved for better applicability to ASTER/TIR imagery. The major improvement is the determination of a water vapor scaling factor on a band-by-band basis, which can reduce most of the errors induced by various factors such as algorithm assumptions. Next, the WVS method is validated by assessing the surface temperature and emissivity retrieved for a global-based simulation model (416 448 conditions), 183 ASTER scenes selected globally, and ASTER scenes from two test sites, Hawaii Island and Tokyo Bay. In situ lake surface temperatures measured in 13 vicarious calibration experiments, Moderate Resolution Imaging Spectroradiometer sea surface temperature products, and a climatic lake temperature are also used in validation. All the results indicate that although the ASTER/TIR standard atmospheric correction algorithm performs less well in humid conditions, the WVS method will provide more accurate retrieval of surface temperature and emissivity in most conditions including notably humid conditions. The expected root mean square error is about 0.6 K in temperature. Since the WVS method will be degraded by errors in gray pixel selection and cloud detection, these processing steps should be applied accurately.     

Inflight straylight analysis for ASTER thermal infrared bands

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument was launched into Earth orbit on the Terra platform in late 1999. ASTER produces images of the Earth in 14 spectral bands including five bands in the thermal infrared (TIR) part of the electromagnetic spectrum (8-12 /spl mu/m). On one occasion ASTER was used to image the Moon as part of the long-term calibration strategy for instruments on the Terra platform. Analysis of the imagery revealed that the TIR band had noticeable straylight effects (ghosting), and an algorithm was developed to correct for these effects. The algorithm was applied to ASTER/TIR images acquired over a vicarious calibration (VC) site at Cold Springs Reservoir (CSR), NV. Data from CSR had been evaluated in three previous VC experiments and showed large unexplained differences between the ASTER image radiance and vicarious predicted radiance not observed in other larger, more laterally homogenous sites. After straylight correction the vicarious and image radiances were in good agreement. A further comparison with nearly simultaneous airborne TIR data acquired with the MODIS/ASTER (MASTER) sensor indicated that the ASTER straylight corrected data also agreed with the airborne data. Finally, the algorithm was applied to artificially created models. The results indicated that a radiance change caused by straylight reached 6% to 8% of a radiance contrast for a smaller square target than 10/spl times/10 pixels or a narrower line target than five pixels. Straylight in ASTER/TIR imagery may not be very large for most targets, but may become an error factor for high-radiance-contrast targets.     

Correction of stray light and filter scratch blurring for ASTER imagery

Stray light components in images obtained by the shortwave infrared (SWIR) and visible near-infrared (VNIR) radiometers of the Advanced Spaceborne Thermal Emission Reflection Radiometer (ASTER) were investigated. A simple method, which is equivalent to the van Cittert method of deconvolution, was used for correction. The stray light components were estimated using the image obtained during lunar observation, and the improvement in image quality was examined after stray light correction. The calculation is performed in the space domain, and application to the filter scratch problem of the ASTER/SWIR sensor, which has a scratch on the interference filter resulting in partially degenerated images, is also demonstrated.     

Vicarious calibration of ASTER thermal infrared bands

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on the Terra satellite has five bands in the thermal infrared (TIR) spectral region between 8-12 /spl mu/m. The TIR bands have been regularly validated in-flight using ground validation targets. Validation results are presented from 79 experiments conducted under clear sky conditions. Validation involved predicting the at-sensor radiance for each band using a radiative transfer model, driven by surface and atmospheric measurements from each experiment, and then comparing the predicted radiance with the ASTER measured radiance. The results indicate the average difference between the predicted and the ASTER measured radiances was no more than 0.5% or 0.4 K in any TIR band, demonstrating that the TIR bands have exceeded the preflight design accuracy of <1 K for an at-sensor brightness temperature range of 270-340 K. The predicted and the ASTER measured radiances were then used to assess how well the onboard calibration accounted for any changes in both the instrument gain and offset over time. The results indicate that the gain and offset were correctly determined using the onboard blackbody, and indicate a responsivity decline over the first 1400 days of the Terra mission.     

Land Surface Emissivity Retrieval From Different VNIR and TIR Sensors

This paper discusses the application and adaptation of two existing operational algorithms for land surface emissivity (epsiv) retrieval from different operational satellite/airborne sensors with bands in the visible and near-infrared (VNIR) and thermal IR (TIR) regions: (1) the temperature and emissivity separation algorithm, which retrieves epsiv only from TIR data and (2) the normalized-difference vegetation index thresholds method, in which epsiv is retrieved from VNIR data.     

运动补偿下双通道星载高光谱成像仪图像配准

最新一代可见近红外（VNIR）和短波红外（SWIR）双通道星载高光谱成像仪,多采用视场分离器将VNIR和SWIR通道分离为多个子视场,同一时刻各子视场对地成像区域不同,在采用运动补偿技术提高图像信噪比时,各子视场对同一地物的观测角不同,导致图像间失配关系复杂,无法获取同一地物的VNIR-SWIR连续光谱。通过建立运动补偿下的严格成像几何模型,定量分析了双通道图像的畸变和失配规律,进而提出了各子视场分别几何定位再相位相关法配准的方案,并利用东天山区域运动补偿下星载双通道高光谱仿真数据进行验证。结果表明,传统的基于图像的配准方案精度为3.9像元,仍无法得到同一地物像元的VNIR-SWIR光谱曲线,文中方案配准精度提高到0.3像元,VNIR和SWIR重叠波段的反射率光谱重合度误差由41.5%降低到1.2%。     

ASTER preflight and inflight calibration and the validation ofLevel 2 products

Describes the preflight and inflight calibration approaches used for the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). The system is a multispectral, high-spatial resolution sensor on the Earth Observing System's EOS-AM1 platform. Preflight calibration of ASTER uses well-characterized sources to provide calibration and preflight round-robin exercises to understand biases between the calibration sources of ASTER and other EOS sensors. These round-robins rely on well-characterized, ultra-stable radiometers. An experiment field in Yokohama, Japan, showed that the output from the source used for the visible and near-infrared (VNIR) subsystem of ASTER may be underestimated by 1.5%, but this is still within the 4% specification for the absolute, radiometric calibration of these bands. Inflight calibration will rely on vicarious techniques and onboard blackbodies and lamps. Vicarious techniques include ground-reference methods using desert and water sites. A recent joint field campaign gives confidence that these methods currently provide absolute calibration to better than 5%, and indications are that uncertainties less than the required 4% should be achievable at launch. The EOS-AM1 platform will also provide a spacecraft maneuver that will allow ASTER to see the Moon, allowing further characterization of the sensor. A method for combining the results of these independent calibration results is presented. The paper also describes the plans for validating the Level 2 data products from ASTER. These plans rely heavily upon field campaigns using methods similar to those used for the ground-reference, vicarious calibration methods     

低温红外镜头设计仿真方法及试验验证

在低温环境下镜头结构会产生热变形,对镜头光学传递函数（MTF）及离焦量均会产生影响,从而影响光学成像质量。在此基于某红外遥感器,针对210 K低温工作环境,设计了一套具备热卸载功能的透射式低温镜头。对其建立有限元模型,并加载模拟在轨工作环境温度场,得到热变形数据,最终计算出镜头MTF及离焦量变化,并通过该仿真分析手段对低温镜头结构进行优化设计。低温镜头装调完成后,将低温镜头及其他配合测试设备置于真空罐内,在常温与低温环境条件下,对光学系统MTF及最佳焦面位置进行测试标定。测试结果表明,各项偏差在可接受范围之内,MTF仅变化0.2%,说明使用的低温镜头多场耦合仿真方法是可靠的,能够对红外遥感器低温镜头设计进行指导。     

Validation of ASTER/TIR standard atmospheric correction using water surfaces

The standard atmospheric correction algorithm for the five thermal infrared (TIR) bands of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is based on radiative transfer calculation using the MODTRAN code. Atmospheric profiles input to MODTRAN are extracted from either the Global Data Assimilation System (GDAS) product or the Naval Research Laboratory (NRL) climatology model. The present study provides validation results of this algorithm. First, in situ lake surface temperatures measured in 13 vicarious calibration (VC) experiments were compared with surface temperatures retrieved from ASTER data. As the results, the mean bias was 0.8 and 1.8 K for GDAS and NRL, respectively. The NRL model performed worse than GDAS for four experiments at Salton Sea, CA, probably because the model was not suitable for this site, which has typically higher surface temperature and humidity than other VC sites. Next, the algorithm was validated based on the max-min difference (MMD) of water surface emissivity retrieved from each of 163 scenes acquired globally. As a result, the algorithm error increased quadratically with the precipitable water vapor (PWV) content of the atmosphere, and the expected MMD error was 0.049 and 0.067 for GDAS and NRL, respectively, with a PWV of 3 cm, where 0.05 on MMD is roughly corresponding to -0.8 or +2.3 K on the retrieved surface temperature error. The algorithm performance degraded markedly when the surface temperature exceeded about 25/spl deg/C, particularly for NRL. Consequently, GDAS performs better than NRL as expected, while both will perform less well for humid conditions.     

FY-3B MERSI短波红外波段温度响应与在轨定标分析

风云三号B星(FY-3B)中分辨率光谱成像仪(Medium resolution spectral imager, MERSI)的两个短波红外波段(1.64和2.13 $\mu$m) 采用光伏碲铬汞探测器,由于辐冷问题导致短波红外波段探测器的实际工作温度远高于设计值,影响了其在轨辐射特性。 对处于高温工作状态的FY-3B MERSI短波红外波段的长时间序列在轨辐射特性进行了较为系统的研究分析。采用冷空观测和 遥测温度时间序列数据开展了工作温度对遥感器响应的影响分析,发现短波红外波段的冷空值与探测器温度之间存在正 相关关系。采用线性模型描述仪器响应的温度依赖性,获得了温度校正因子;温度每变化1度, 1.64和2.13 $\mu$m波段冷空观 测值分别变化约0.7\%和5\%。进行温度校正后,冷空观测时间序列的波动显著降低。采用全球多目标定标方法获得了 短波红外波段的在轨辐射响应变化,在参考温度下,2011年11月至2016年12月,1.64和2.13 $\mu$m波段的总衰减分别 约为6\%和11\%。通过与Aqua中分辨率成像光谱仪(Moderate-resolution imaging spectroradiometer, MODIS)的 多年交叉比对分析,发现不管是否在定标过程中进行温度校正,采用基于长时间序列趋势建模的日定标更新后MERSI 与MODIS的辐射偏差较为稳定,可以满足7\%的定标指标要求。     

FY-2卫星热红外波段MTF在轨评价及其在改进图像质量中的应用

提出了基于理想斜坡模型及目标延拓空间卷积变换的采样成像系统热红外波段MTF在轨评价方法.仿真结果表明,该方法对性能良好的成像系统而言,系统特征MTF值的绝对评价误差约0.05.同时,利用2006～2008年间的观测数据,定量评价了FY-2C卫星热红外通道的系统MTF特性,与卫星发射前地面测试结果基本一致.采用在轨评价得到的系统点源扩展函数(PSF),对FY-2C卫星热红外通道图像进行了复原处理,得到了清晰度更高的图像产品,并给出了对典型台风监测结果的改进.传统在轨MTF评价方法中采用的理想阶跃模型,可视为理想斜坡模型中参数Nslope为0时的特例.因此,该方法同样适用于可见光、近红外波段的在轨MTF评价.     

Thematic Mapper Image Quality: Registration, Noise, And Resolution

This paper provides an assessment of Thematic Mapper data quality in terms of band-to-band registration, periodic noise, and spatial resolution. Based on the Thematic Mapper images analyzed so far, the band-to-band registration accuracy is very good. For bands within the same focal plane, the mean misregistrations are well within the specification, 0.2 pixel, except for the thermal band. The thermal band was misregistered by three pixels in each direction in early data products. The error in the across-scan direction was close to zero in later data products. For bands between the cooled and uncooled focal planes, there was a consistent mean misregistration of 0.5 pixels along-scan and 0.2-0.3 pixels across-scan, larger than the specified 0.3 pixel error for bands between focal planes. An analysis of the standard deviation of the misregistration indicated all band combinations would meet the registration specifications if the mean misregistrations were removed by the data processing software. Analysis of the periodic noise in one image indicated a noise component in bands 1-4 with a spatial frequency of 0.31 cycles/pixel. Other lower amplitude periodic components were also present. The periodic noise components obscured detail in areas of low contrast. Modulation transfer function (MTF) analysis in a comparative mode showed no difference in MTF between the forward and backward scans. The difference in MTF between radiometrically corrected data and geometrically corrrected data appeared to be attributable largely to the cubic convolution resampling used to derive the geometrically corrected data.     

宽光谱可见-短波红外成像光学系统设计

针对目前的红外成像光学系统在机器视觉工业检测领域难以同时实现成像质量好和结构紧凑设计的问题,提出了一种宽光谱可见-短波红外成像光学系统的设计方法。运用光学设计软件ZEMAX设计了一种适用于可见光和短波红外的红外成像光学系统。该系统由7组10片透镜组成,利用多组双胶合透镜来消色差,在第15个面使用非球面提高成像质量,最后对系统的成像质量进行研究。设计结果表明：该系统的的工作波长为0.4～1.7μm,全长为79.6 mm,F数为2.8,焦距为25.7 mm,畸变小于1.4%,调制传递函数值在奈奎斯特频率100 lp/mm处均大于0.4,接近衍射极限,成像质量良好。该系统可以对光滑表面的装配件进行缺陷检测,具有结构简单、易于加工装调的优点,有助于高效地完成机器视觉检测。     

基于误差分析的岩芯高光谱数据几何校正方法

南京地质调查中心研制的推扫式岩芯成像光谱仪由可见近红外成像光谱仪、短波红外成像光谱仪以及载有岩芯盘的导轨构成。导轨匀速运动的控制误差、两台独立成像光谱仪不同的空间分辨率不同以及不重合的视场范围,导致所获得的数据存在几何畸变,无法直接进行应用处理。针对上述问题,在分析了畸变产生机理的基础上,提出了基于三角形靶标的拉伸压缩畸变校正方法以及像元级与亚像元级联合配准方法。通过在岩芯盘一侧布设等腰直角三角形靶标,实现无位置姿态参数下的几何拉伸压缩畸变检测与校正;同时将尺度不便特征变化与扩展的相位相关方法相结合进行图像配准,提高图像配准的精度。实验结果表明,利用南京地质调查中心研制的岩芯成像光谱仪的实测高光谱数据进行方法性能验证,经过几何校正处理后的岩芯高光谱数据,拉伸压缩畸变校正精度为0.28个像元,配准精度优于0.1个像元。     

An atmospheric correction algorithm for thermal infraredmultispectral data over land-a water-vapor scaling method

Thermal infrared (TIR) multispectral data over land can be atmospherically corrected by radiative transfer calculations combined with global assimilated data from a weather forecast system. This approach is advantageous to operational processing but is not accurate. A new atmospheric correction algorithm with global assimilated data, a water vapor scaling (WVS) method, has improved results. In this algorithm, the accuracy of global assimilated data is markedly improved on a pixel-by-pixel basis as follows: (1) selecting gray pixels from an image; (2) estimating the scaling factors for the water-vapor profiles of gray pixels by an improved multichannel algorithm; (3) estimating the scaling factors for the water-vapor profiles of nongray pixels by horizontal interpolation; and (4) improving the water-vapor profiles of all pixels with the scaling factors. The proposed method can be applied if the image has one or more gray pixels. The simulation results for the advanced spaceborne thermal emission and reflection radiometer (ASTER) TIR subsystem show that the proposed method reduces errors on air temperature profiles as well as on water-vapor profiles and is as accurate as atmospheric correction with radiosonde measurements