Radiometric Calibration and Validationof TG-1 Hyper-spectral Imager

TG-1,the first Chinese space laboratory module launched on 29 September 2011,has accumulated large numbers of high-resolution image data by the hyper-spectral imager.However,these images still can’t be applied to quantitative analysis because of the huge spectral and radiance difference between ground and onboard conditions.So a radiometric calibration is very necessary to correct these laboratory and on-board radiance calibration parameters for better quantitative application.This paper aims to calibrate TG-1 hyper-spectral imager using reflectance-based calibration method through performing ground calibration experiments on 3 February and 6 March 2011.Firstly,this study used the ground experiment data on 6 March to obtain each channel’s calibration correction coefficients,which were different a lot from laboratory and on-board ones,relative error in all channels are bigger than 10 percent.Then,the calibration experiment data on 3 February 2011 were used to validate the result.Results show that calibration correction coefficients improved the image accuracy,the calibration and validation experiments’ results are in good agreement.In most channels,the relative errors are less than 10 percent,except that bigger error appears in absorption channels.Therefore,this calibration experiment renewed the calibration coefficients,and improved the quantitative level of the TG radiance products.     

Atmospheric correction of hyperspectral airborne GCAS measurements over the Louisiana Shelf using a cloud shadow approach

As an image-driven method to correct for atmospheric effects, the cloud shadow (CS) approach does not require accurate radiometric calibration of the sensor, making it feasible to process remotely sensed data when radiometric calibration may contain non-negligible uncertainties. Using measurements from the Geostationary Coastal and Air Pollution Events Airborne Simulator and from the Moderate Resolution Imaging Spectroradiometer over the Louisiana Shelf, we evaluate the CS approach to airplane measurements in turbid-water environments. The original CS approach somehow produced remote-sensing reflectance (R_rs, sr^?1) with an abnormal spectral shape, likely a result of the assumption of identical path radiance for the pair of pixels in and out of the shadow, which is not exactly valid for measurements made from a low-altitude airplane. To overcome this limitation, an empirical scheme using an effective wavelength-dependent radiance reflectance for the cloud (γ, sr^?1) was developed and reasonable GCAS R_rs retrievals are then generated, which were further validated against in situ R_rs. Issues and challenges in applying CS to measurements of low-altitude airplanes are discussed.     

HJ-1B卫星热红外通道在轨绝对辐射定标

利用青海湖水面辐射校正场对HJ-1B卫星热红外通道进行绝对辐射定标,由CE312热红外辐射计测量水面辐亮度,结合辐射传输模型MODTRAN4.0计算大气透过率和上行程辐射,同时进行CE312通道与HJ-1B卫星热红外通道光谱匹配,计算传感器入瞳处等效辐亮度值。通过2010年8月1日和5日两组传感器入瞳等效辐亮度值和卫星通道计数值回归得到该卫星通道绝对定标系数,并使用内蒙达里湖水面辐射校正场2010年6月29日实测数据对定标结果进行检验。     

Using multiple radiometric correction images to estimate leaf area index

Ecological applications of remote-sensing techniques are generally limited to images after atmospheric correction, though other radiometric correction data are potentially valuable. In this article, six spectral vegetation indices (VIs) were derived from a SPOT 5 image at four radiometric correction levels: digital number (DN), at-sensor radiance (SR), top of atmosphere reflectance (TOA) and post-atmospheric correction reflectance (PAC). These VIs include the normalized difference vegetation index (NDVI), ratio vegetation index (RVI), slope ratio of radiation curve (K), general radiance level (L), visible-infrared radiation balance (B) and band radiance variation (V). They were then related to the leaf area index (LAI), acquired from in situ measurement in Hetian town, Fujian Province, China. The VI–LAI correlation coefficients varied greatly across vegetation types, VIs as well as image radiometric correction levels, and were not surely increased by image radiometric corrections. Among all 330 VI–LAI models established, the R ² of multi-variable models were generally higher than those of the single-variable ones. The independent variables of the best VI–LAI models contained all VIs from all radiometric correction levels, showing the potentials of multi-radiometric correction images in LAI estimating. The results indicated that the use of VIs from multiple radiometric correction images can better exploit the capabilities of remote-sensing information, thus improving the accuracy of LAI estimating.     

FY-3气象卫星MERSI数据快速预处理的IDL实现

作为新型卫星数据源,FY\|3/MERSI(风云三号中分辨率光谱成像仪)影像的快速预处理方法与模块目前较少见。采用基于三角网的几何校正算法,根据研究区Shapefile文件和FY\|3/MERSI自带定位数据提取处理区域,实现对应区域全部20个通道几何校正,并利用后向映射重采样输出各通道图像纠正后像元点DN值,随后进行辐射定标、太阳高度角订正等预处理。该系列过程用IDL编写相关程序实现了用户界面化交互操作。运行结果表明:设计的算法流程占用内存小,计算速度快。对于一个面积约45万km²的区域,所有通道处理耗时仅450 s,且校正后影像质量和精度均较好。相较于相同区域借助ENVI软件下地理位置查找表法(GLT)仅实现单通道几何校正耗时超30 min,本文开发的模块大大提高了FY\|3/MERSI数据处理效率,为其在城市热环境、积雪等领域的应用做好准备工作。     

Point or pointless – quality of ground data

A method for measuring continuously the inherent optical properties of water together with the salinity and the temperature values was developed and tested at the Division of Geophysics, University of Helsinki. The flow‐through system operates from a moving boat and has been used to collect parameters for optical modelling, coastal zone and inland water studies and to track the path of the fresh water spreading into sea areas. The system was also used in Lake Vänern, Sweden, to calculate the radiance reflectance, R _r, and total back‐scattering efficiency, b _{b eff}, along a transect. An example from the Gulf of Finland is presented, which shows how the spectral behaviour of light changes when moving away from a fresh water source. Data collected from the River Kymijoki estuary showed that the details in a rapidly changing environment were strongly smoothed in a Moderate Resolution Imaging Spectroradiometer (MODIS) data image. The correlation for continuously measured scattering values and MODIS channel 1 data varied between 0.70 and 0.85, n?=?126. Flow‐through measurement datasets can be used to obtain representative sites for vertical profiles or calibration measurements.     

A dynamic scaling algorithm for the optimized digital display of VIIRS Day/Night Band imagery

The VIIRS Day/Night Band (DNB) is a visible and near-infrared sensor that is sensitive to a broad range of light intensities ranging from daylight down to airglow at night. The on-board calibration of the DNB allows for the quantification of radiance values over the full range of the instrument’s sensitivity, unlike the heritage Operational Linescan System (OLS). For scenes that span the day/night terminator, observed DNB radiance values may vary by up to eight orders of magnitude. Consequently, it is impractical to display the full range of radiance values in a single digital image. In this work, an algorithm is presented that scales the observed radiance values between expected maximum and minimum values that are a function of solar and lunar zenith angles as well as the fraction of the lunar disc that is illuminated by the Sun. This dynamic scaling algorithm preserves scene contrast over the full range of solar and lunar illumination conditions, similar to the Near Constant Contrast (NCC) imagery product. Unlike the NCC algorithm, however, the ‘erf-dynamic scaling’ algorithm (so-called for its likeness to the Gauss error function) presented here requires no ancillary information outside of what is included in the DNB data distributed according to the Joint Polar Satellite System (JPSS) program file standards. Results indicate that this algorithm has improved performance over simple methods for displaying DNB imagery and, in some instances, may exceed the performance of the NCC product itself. This algorithm is expected to replace many ad hoc methods of displaying DNB imagery and may serve as a substitute for operational users that do not have access to the NCC product.     

In-flight calibration of large field of view sensors at short wavelengths using Rayleigh scattering

Abstract

Satellite observations over the ocean in the backscatter direction are dominated by Rayleigh scattering. We use this predictable radiance for in-flight calibration of the visible SPOT channels. Two methods are evaluated. The first method directly relates the measured numerical signal in a short wavelength channel to the predicted reflectance. In the second method, we use a second channel centred at a longer wavelength, to correct the short wavelength channel for the effect of the atmospheric aerosol contribution. These two methods are examined for channel B₀ , centred at 045 μm planned for launch on SPOT-4 VEGETATION, and for channel B₁ centred at 0-55 mu;mm, currently on-board SPOT-1 HRV. In both cases, the channel B₃ , centred at 0-85 mu;mm is used for aerosol correction. Error analysis shows that accuracies of 3 and 5 per cent respectively can be achieved for B₀ and B₁. The last section of the paper is devoted to a validation of the error analysis using SPOT-1 HRV data.     

Radiometric characterization of the ROCSAT Ocean Color Imager

The Ocean Color Imager (OCI), a multispectral optical imager for observing pigment distributions in low-latitude oceans, has been successfully built and parameterized. To recover at-sensor radiance data, a mathematical model was developed to characterize its radiometric response; the corresponding parameters were identified initially by pre-flight calibration and were adapted in-flight by a method of cross-platform calibration. The cross-platform in-flight calibration was carried out to compare radiance data with data measured by the vicarious orbital sensor at the cross points to determine possible changes in the parameters and performance of the OCI. The radiometric model, the pre-flight parameters and the method of cross-platform in-flight calibration are reported here.     

Evaluation of the Lakshadweep Sea as a site for vicarious calibration of the Ocean Colour Monitor

The uncertainty in the top-of-the-atmosphere (TOA) radiance is a result of uncertainties in aerosol components, water-leaving radiance (due to seawater constitutions) and whitecap radiance. This paper investigates the variability of these individual terms over the Arabian Sea and particularly in Lakshadweep region, to establish a site for vicarious calibration of the Ocean Colour Monitor (OCM). We found that fractional coverage of whitecap radiance is less than 0.5% for winds lower than 8 m s^?1 and its radiance contribution can be assigned to a constant value. For higher winds, the contribution from whitecap radiance to TOA radiance has to be considered along with the atmospheric stability factor. The Lakshadweep Sea, for most of the time, is characterized by a low concentration of chlorophyll-a, an oligotrophic water body and maritime aerosol.     

Non-isotropy of the upward radiance field in typical coastal (Case 2) waters

The in-water radiance field has been computed in typical Case 2 waters by using radiative transfer models and appropriate inherent optical properties (IOPs) combined with realistic boundary conditions. In particular, the bi-directional structure of the subsurface upward flux has been investigated in view of remote sensing applications related to ocean colour. In Case 2 waters, the IOPs are not controlled by the phytoplankton (or chlorophyll) concentration; rather they are essentially determined by the abundance of terrigenous optically active materials, either particulate or dissolved. Based on field data and related IOPs, two extreme situations were selected as representative instances of sedimentdominated and yellow-substance-dominated Case 2 waters. This study shows that even in very turbid natural waters, the upward radiance field is not isotropic and remains Sun-angle dependent. More than 100 successive events are needed to reach a quasi-isotropic, illumination independent, upward radiance field. In contrast, with a high yellow substance content resulting in high absorption (compared to scattering), single scattering prevails in such waters and this leads to strongly featured radiance fields that are heavily dependent on the Sun's position. It is necessary to account for these effects when interpreting waterleaving radiances as detected from space, and, perhaps more importantly, when carrying out at-sea radiometric measurements in support of calibration of remote ocean colour sensors. For this purpose, a practical approach and mean values of relevant coefficients are proposed to describe the bi-directional structure of the upward radiance field in the two extreme situations of strongly scattering or strongly absorbing waters.     

Posidonia oceanica genetic and biometry mapping through high-resolution satellite spectral vegetation indices and sea-truth calibration

In the framework of Posidonia oceanica (PO) preservation activities, a small-scale restoration pilot project was implemented in 2005 at a Santa Marinella site to replace the loss of this important species of seagrass in this zone of the central Tyrrhenian coast via an innovative transplantation approach. In this context, taking into account the recent advances in the fields of high-resolution (HR) satellite/airborne remote-sensing and genetics laboratory analysis techniques, we propose this integrated methodology for monitoring changes in transplanted meadows in regard to perspective to provide support in the assessment of the entire local PO and seagrass population dynamic. According to specific information requirements in terms of radiometric and spectral/spatial resolution, the multispectral data currently available from the QuickBird polar satellite’s four-band (red, green, blue visible and near-infrared) HR sensor were exploited for methodology implementation using a practical ‘image-based’ approach to account for atmospheric and water column turbidity typical of this mid-coastal Mediterranean region. First, the extents and types of seagrass cover were suitably mapped, and then also the distributions of specific vegetation parameters related to PO dynamics and health were assessed by exploiting the remotely sensed satellite-derived radiance signals and point sea-truth calibration measurements of the bio-genetic parameters. In particular, we implemented maps of leaf area index, genetic similarity, and density Giraud indices corresponding to distributions of PO patches using multivariate and data-mining models (artificial neural network) based on appropriately preprocessed radiometric and auxiliary (bathymetry) input variables.     

Atmospheric effects on radiometry from zenith of a plane with dark vertical protrusions

Effects of an optically thin plane-parallel scattering atmosphere on radiometric imaging from the zenith of a specific surface type are analysed. The surface model was previously developed to describe arid steppe, where the sparse vegetation forms dark vertical protrusions from the bright soil plane. The analysis is in terms of the surface reflectivity to the zenith r_p for the direct beam, which is formulated as r_p = r_i exp(?s tan θ₀), where r_i is the Lambert-law reflectivity of the soil, the protrusion parameter 5 is the projection on a vertical plane of protrusions per unit area and θ⁰ is the zenith angle. The surface reflectivity r_P is approximately equal to that for the global irradiance (which is directly measured in the field) only for a narrow range of the solar zenith angles. The effects of the atmosphere when imaging large uniform areas of this type are comparable to those in imaging a Lambert surface with a reflectivity r_P . Thus, the effects can be approximated by those in the case of a dark Lambert surface (analysed previously), inasmuch as r_P is smalleSr than the soil reflectivity r_i for any off-zenith illumination. The surface becomes effectively darker with increasing solar zenith angle.

Adjacency effects of a reflection from one area and scattering in the instantaneous field of view (object pixel) are analysed as cross radiance and cross irradiance, The analysis is only for the case of a small object pixel embedded in a different terrain, extending to infinity as a uniform area. The effects of the cross radiance (which are dominant) are found to be smaller than those over a Lambert plane for the same surroundings-to-object-pixel contrast and atmospheric conditions. However, the adjacency effects are highly variable, because the effective contrast for our plane with dark protrusions is a function of not only the surface parameters but also of the solar zenith angle and the atmospheric conditions.     

Radiometric normalization for change detection in peatlands: a modified temporal invariant cluster approach

Radiometric normalization is a vital stage in any change detection study due to the complex interactions of radiance and irradiance between the Earth's surface and atmosphere. Compensation for variables such as sun's angle, surface profile, atmospheric conditions, and sensor calibration coefficients are essential in achieving a radiometrically stable data base of multi-temporal, multi-spectral imagery for a change detection study. In this study, five Landsat Enhanced Thematic Mapper Plus (ETM+) images taken over the east coast of Ireland in 2001 were geometrically corrected and topographically normalized for further processing and analysis. Assessment of various vegetation indices showed that the enhanced vegetation index 2 (EVI2) gave the highest accuracy in identifying the various vegetation types and habitats in the Wicklow Mountains National Park. The initial analysis of radiometric normalization with temporal invariant clusters (TICs) gave poor results due to the spectral heterogeneity of urban pixels within each image. A revised TIC subset normalized method was developed using regional growth parameters in urban environments to limit the spatial and spectral extent of pixels used in the TIC scene normalization process. Correlation analysis between the TIC-subset-normalized ETM+ data and Landsat Ecosystem Disturbance Adaptive Processing System (LEDAPS) absolute corrected data produced coefficient of determination (R²) values between 0.88 and 0.98. Such results demonstrated the robustness of the TIC subset normalization procedure when correcting for atmospheric variability between images while maintaining spectral integrity. Statistical analysis on master slave and TIC-subset-normalized slave data using cumulative distribution curves derived from image histograms showed an 86.93% reduction in the maximum difference between master and slave data due to the TIC subset normalization process. This procedure of radiometric normalization is suitable in landscapes with a low density of spectrally stable targets.     

Relation between power and endurance for treadmill running of short duration

Abstract

An exercise test was devised to investigate the relationship between power and endurance for treadmill running. The subjects were 19 males aged 21-25 yr (11 distance runners and 4 sprinters of provincial grade, and 4 non-competitive runners). Each subject ran to exhaustion on a treadmill at 15kmhr^?1at five different inclinations (31%-9%), giving maximum performance times in the range 10s to 3 min. An iterative least-squares procedure was used to fit the following exponential model to each subject's data: I₁= I_∞+ (I₀?I_∞)exp(?t/τ) where I₁, I₀and 1_∞are inclinations at time t = t, t = 0 and t → ∞, and τ is a time constant. The fit was excellent (r ²= 0.96? 1.00). I₀and 1_∞are interpreted as measures of maximum anaerobic (instantaneous) and maximum aerobic (continuous) power respectively. Inclinations corresponding to performance times of 10-180s (I₁₀?I₁₈₀) were calculated from these parameters. Test-retest reliability was highest for I₀-I₃₀(intraclass r= 0.97?0.94), lower for I₆₀-I_∞(r= 0.89?0.84), and lowest for τ (r= 0.78). Good correlations were observed between I₀-I₃₀and peak power in a 30s all-out test on a cycle ergometer (r= 0.73?0.81), and between I₁₈₀, I_∞and maximum oxygen consumption (r= 0.87, 0.81). The test may be useful for ranking or monitoring running performance for events of up to 1 min duration.     

Radiometric Transfer: Example‐based Radiometric Linearization of Photographs

We present an example‐based approach for radiometrically linearizing photographs that takes as input a radiometrically linear exemplar image and a target regular uncalibrated image of the same scene, possibly from a different viewpoint and/or under different lighting. The output of our method is a radiometrically linearized version of the target image. Modeling the change in appearance of a small image patch seen from a different viewpoint and/or under different lighting as a linear 1D subspace, allows us to recast radiometric transfer in a form similar to classic radiometric calibration from exposure stacks. The resulting radiometric transfer method is lightweight and easy to implement. We demonstrate the accuracy and validity of our method on a variety of scenes.     

Validation of MERIS ocean-color products in the Bohai Sea: A case study for turbid coastal waters

An extensive in situ data set in the Bohai Sea of China was collected to assess radiometric properties and concentrations of ocean constituents derived from Medium Resolution Imaging Spectrometer (MERIS). The data collected include spectral normalized water-leaving radiance L_wn(λ) and concentrations of suspended particulate matter (SPM) and chlorophyll a (Chl-a). A strict spatio-temporal match-up method was adopted in view of the complexity and variability of the turbid coastal area, resulting in 13, 48 and 18 match-ups for MERIS L_wn(λ), SPM and Chl-a estimates, respectively. For MERIS L_wn(λ), the match-ups showed mean absolute percentage differences (APD) of 17%-20% in the 412, 443, 620 and 665 nm bands, whereas L_wn(λ) at bands from 490 and 560 nm had better APD of 15-16%. The band ratio of L_wn(490) to L_wn(560) of the satellite data was in good agreement with in situ observations with an APD of 4%. MERIS SPM and Chl-a products overestimated the in situ values, with the APD of approximately 50% and 60%, respectively. When match-up criteria were relaxed, the assessment results degraded systematically. Hence, in turbid coastal areas where temporal variability and spatial heterogeneity of bio-optical properties may be pronounced as the result of terrestrial influences and local dynamics, the strict spatio-temporal match-up is recommended.     

Optical closure for remote-sensing reflectance based on accurate radiative transfer approximations: the case of the Changjiang (Yangtze) River Estuary and its adjacent coastal area,China

Optical closure exercises are pivotal for evaluating the accuracy of water quality remote-sensing techniques. The agreement between radiometrically derived and inherent optical property (IOP)-derived above-water spectral remote-sensing reflectance R_rs(λ) is necessary for resolving IOPs, the diffuse attenuation coefficient, and biogeochemical parameters from space. We combined spectral radiometric and IOP measurements to perform an optical closure exercise for two optically contrasting Chinese waters – the Changjiang (Yangtze) River Estuary and its adjacent coastal area in the East China Sea. The final aim of our investigation was to compare two derivations of R_rs(λ): R_rs(λ), derived from radiometric measurements; and R_rs(λ), derived from simultaneous IOP measurements. Five subsequent steps have been taken to achieve this goal, including (1) estimation of the R_rs(λ) from radiometric measurements; (2) scattering correction for the non-water spectral absorption coefficient a_pd(λ); (3) estimation of the below-water spectral remote-sensing reflectance r_rs(λ) from IOPs measurements; (4) the estimation of the R_rs(λ) from the r_rs(λ) values; and (5) the comparison between the R_rs(λ) derived from radiometric and IOP measurements. All steps were realized by using both direct measurements and different models based on radiative transfer theory. Results demonstrated that the impact of the errors caused by the scattering correction procedure and conversion of radiometric quantities into R_rs(λ) may be rather significant, especially in the long-wavelength spectrum range. Nevertheless, spectral features were similar between these R_rs(λ) sets for all waters – from relatively clear to very turbid. Exploiting this fact allows use of the spectral reflectance ratios for remote sensing of the estuarine and coastal Chinese waters.     

Estimation of soil clay and calcium carbonate using laboratory, field and airborne hyperspectral measurements

This paper examines how reflectance spectrometry used in the laboratory to estimate clay and calcium carbonate (CaCO₃) soil contents can be applied to field and airborne measurements for soil property mapping. A continuum removal (CR) technique quantifying specific absorption features of clay (2206 nm) and CaCO₃ (2341 nm) was applied to laboratory, field and airborne HYMAP reflectance measurements collected in 2003 (33 sites) and 2005 (19 sites) over bare soil sites of a few meters within the La Peyne Valley area, southern France. Nine intermediate stages from the laboratory up to HYMAP sensor measurements were considered for separately evaluating the possible degradation of estimation performances when going across scales and sensors, e.g. radiometric calibration, spectral resolution, spatial variability, illumination conditions, and surface status including roughness, soil moisture and presence and nature of pebbles.Significant relationships were observed between clay and CaCO₃ contents and CR values computed respectively at 2206 nm and 2341 nm from reflectance measurements at the laboratory level with an ASD spectrophotometer up to the HYMAP spectro-imaging sensor. Performances of clay and CaCO₃ estimations decreased from the laboratory to airborne scales. The main factors inducing uncertainties in the estimates were radiometric and wavelength calibration uncertainties of the HYMAP sensor as well as possible residual atmospheric effects.