An empirical (target-) BRDF normalization method has been implemented for Imaging Spectrometry data processing,
following the approach of Kennedy, published in 1997. It is a simple, empirical method with the purpose of a rapid
technique, based on a least-squares quadratic curve fitting process. The algorithm is calculating correction factors in
either multiplicative or additive manner for each of the identified land cover classes, per spectral band and view angle
unit. Image pre-classification is essential for successful anisotropy normalization. This anisotropy normalization method
is a candidate to be used as baseline correction for future data products of APEX, a new airborne Imaging Spectrometer
suitable for simulation and inter-calibration of data from various other sensors.
A classification algorithm, being able to provide anisotropy class indexing that is optimized for the purpose of BRDF
normalization has to be used. In this study, the performance of the standard Spectral Angle Mapper (SAM) approach
with RSL's spectral database SPECCHIO attached is investigated. Due to its robustness regarding directional effects,
SAM classification is estimated to be the most efficient. Results of both the classification and the normalization process
are validated using two airborne image datasets from the HyMAP sensor, taken in 2004 over the "Vordemwald" test site
in northern Switzerland.
Historically the atmospheric and meteorological communities are separate worlds with their own data formats and tools
for data handling making sharing of data difficult and cumbersome. On the other hand, these information sources are
becoming increasingly of interest outside these communities because of the continuously improving spatial and temporal
resolution of e.g. model and satellite data and the interest in historical datasets. New user communities that use
geographically based datasets in a cross-domain manner are emerging. This development is supported by the progress
made in Geographical Information System (GIS) software. The current GIS software is not yet ready for the wealth of
atmospheric data, although the faint outlines of new generation software are already visible: support of HDF, NetCDF
and an increasing understanding of temporal issues are only a few of the hints.
The Medium Resolution Imaging Spectrometer, MERIS, on board of ENVISAT-1 fulfils the information gap between the current high and low spatial resolution sensors. In this respect, the use of MERIS full resolution data (300 m pixel size) has a great potential for regional and global land cover mapping. However, the spectral and temporal resolutions of MERIS (15 narrow bands and a revisit time of 2-3 days, respectively) might be further exploited in order to get land cover information at a more detailed scale. The performance of MERIS for extracting sub-pixel land cover information was evaluated in this study. An iterative linear spectral unmixing method designed to optimize the number of endmembers per pixel was used to classify 2 MERIS full resolution images acquired over The Netherlands. The latest version of the Dutch land use database, the LGN5, was used as a reference dataset both for the validation and for the selection of the endmembers. This dataset was first thematically aggregated to the main 9 land cover types and then spatially aggregated from its original 25m to 300m. Because the fractions of the different land cover types present in each MERIS pixel were computed during the aggregation, a sub-pixel accuracy assessment could be done (in addition to the traditional assessment based on a hard classification). Results pointed out that MERIS has a great potential for providing sub-pixel land cover information because the classification accuracies were up to 60%. The correct number of endmembers to unmix every pixel was adequately identified by the iterative linear spectral unmixing. Future research efforts should be put in making use of the high revisit time of the MERIS sensor (temporal unmixing).
Since the launch of MERIS on ENVISAT long term activities using vicarious calibration approaches are set in place to monitor potential drifts in calibration in the radiance products of MERIS. We are using a stable, well monitored reference calibration site (Railroad Valley, Nevada, USA) to derive calibration uncertainties of MERIS over time. We are using interpolation of uncertainties to derive a second set of uncertainties for a national data validation in the Netherlands. A satellite image derived land use map of the Netherlands (LGN4) is used to determine the largest homogeneous land use classes using a standard purity index (SPI). Potential adjacency effects are minimized using moving window filters on the pixels of the aggregated map. Multiple error propagation is being used to assess the impact of calibration accuracy on land use classification. A classification in 9 land use classes is finally performed on MERIS FR images of the Netherlands using image based spectral unmixing and matched filtering with endmembers derived from the LGN. We conclude that the classification performance may significantly be increased, when taking into account long-term vicarious calibration results.
APEX is a dispersive pushbroom imaging spectrometer operating in the spectral range between 380 - 2500 nm. The spectral resolution will be better than 10 nm in the SWIR and < 5 nm in the VNIR range of the solar reflected range of the spectrum. The total FOV will be ± 14 deg, recording 1000 pixels across track with about 300 spectral bands simultaneously. A large variety of characterization measurements will be performed in the scope of the APEX project, e.g., on-board characterization, frequent laboratory characterization, and vicarious calibration. The retrieved calibration parameters will allow a data calibration in the APEX Processing and Archiving Facility (PAF). The data calibration includes the calculation of the required, time-dependent calibration coefficients from the calibration parameters and, subsequently, the radiometric, spectral and geometric calibration of the raw data. Because of the heterogeneity of the characterization measurements, the optimal calibration for each data set is achieved using a special assimilation algorithm. In the paper the different facilities allowing characterization measurements, the PAF and the new data assimilation scheme are outlined.
Spectro-directional surface measurements can either be performed in the field or within a laboratory setup. Laboratory measurements have the advantage of constant illumination and neglectable atmospheric disturbances. On the other hand, artificial light sources are usually less parallel and less homogeneous than the clear sky solar illumination. To account for these differences and for determining for which targets a replacement of field by laboratory experiments is indeed feasible, a quantitative comparison is a prerequisite. Currently, there exists no systematic comparison of field and laboratory measurements using the same targets.
In this study we concentrate on the difference in spectro-directional field and laboratory data of the same target due to diffuse illumination. The field data were corrected for diffuse illumination following the proposed procedure by Martonchik . Spectro-directional data were obtained with a GER3700 spectroradiometer. In the field, a MFR sun photometer directly observed the total incoming diffuse irradiance. In the laboratory, a 1000W brightness-stabilized quartz tungsten halogen lamp was used. For the first direct comparison of field and laboratory measurements, we used an artificial and inert target with high angular anisotropy. Analysis shows that the diffuse illumination in the field is leading to a higher total reflectance and less pronounced angular anisotropy.
Recently, a joint Swiss/Belgian initiative started a project to build a new generation airborne imaging spectrometer, namely APEX (Airborne Prism Experiment) under the ESA funding scheme named PRODEX. APEX is a dispersive pushbroom imaging spectrometer operating in the spectral range between 380 - 2500 nm. The spectral resolution will be better then 10 nm in the SWIR and < 5 nm in the VNIR range of the solar reflected range of the spectrum. The total FOV will be ± 14 deg, recording 1000 pixels across track with max. 300 spectral bands simultaneously. APEX is subdivided into an industrial team responsible for the optical instrument, the calibration homebase, and the detectors, and a science and operational team, responsible for the processing and archiving of the imaging spectrometer data, as well as for its operation. APEX is in its design phase and the instrument will be operationally available to the user community in the year 2006.
The handling of satellite or airborne earth observation data for scientific applications minimally requires pre-processing to convert
raw digital numbers into scientific units. However depending on sensor characteristics and architecture, additional work may be
needed to achieve spatial and/or spectral uniformity. Standard
higher level processing also typically involves providing orthorectification and atmospheric correction. Fortunately some of the computational tasks required to perform radiometric and geometric calibration can be decomposed into highly independent
subtasks making this processing highly parallelizable. Such
"embarrassingly parallel" problems provide the luxury of being
able to choose between cluster or grid based solutions to perform
these functions. Perhaps the most convenient solutions are grid-based, since most research groups making these kinds of measurements are likely to have access to a LAN whose spare computing resources could be non-obtrusively employed in a grid. However, since many higher level scientific applications of earth observation data might be composed of more highly interdependent subtasks, the parallel
computing resources allocated for these tasks might also be made
available for low level pre-processing as well. We look at two
modules developed for our prototype data calibration processor for
APEX, an airborne imaging spectrometer, which have been implemented
on both a cluster and a grid leading us to be able to make observations and comparisons of the two approaches.
The launch of ESA’s ENVISAT in March 2002 was followed by a commissioning phase for all ENVISAT instruments to verify the performance of ENVISAT instruments and recommend possible adjustments of the calibration or the product algorithms before the data was widely distributed. The focus of this paper is on the vicarious calibration of the Medium Resolution Imaging Spectrometer (MERIS) radiance product (Level 1b) over land. From August to October 2002, several vicarious calibration (VC) experiments for MERIS were performed by the Optical Sciences Center, University of Arizona, and the Remote Sensing Laboratories, University of Zurich. The purpose of these activities was the acquisition of in-situ measurements of surface and atmospheric conditions over a bright, uniform land target, preferably during the time of MERIS data acquisition. The experiment was performed on a dedicated desert site (Railroad Valley Playa, Nevada, USA), which has previously been used to calibrate most relevant satellite instruments (e.g., MODIS, ETM+, etc.). In-situ data were then used to compute top-of-atmosphere (TOA) radiances which were compared to the MERIS TOA radiances (Level 1b full resolution product) to determine the in-flight radiometric response of the on-orbit sensor. The absolute uncertainties of the vicarious calibration experiment are found between 3.36-7.15%, depending on the accuracies of the available ground truth data. Based on the uncertainties of the vicarious calibration method and the calibration accuracies of MERIS, no recommendation to update the MERIS calibration is given.
The underlying algorithmic architecture of the level 0 to 1 processing
of the APEX spectrometer is presented. This processing step calculates
the observed radiances in physical units from the recorded raw digital
numbers. APEX will operate airborne and record radiance in the solar reflected wavelength range. The system is optimized for land applciations including limnology, snow, soil, amongst others. The instrument will be calibrated with a flexible setup in a laboratory as well as on-board. A concept for the dynamic update of the radiance calibration coefficients for the APEX spectrometer is presented. The time evolution of the coefficients is calculated from the heterogeneous calibration measurements with a data assimilation technique. We propose a Kalman filter for the initial version of the processor. Additionally, the structure of the instrument model suitable for the analysis of APEX data is developed. We show that this model can be used for the processing of observations as well as for the calculation of calibration coefficients. Both processes can be understood as inverse problems with the same forward model, i.e. the instrument model.
The high resolution airborne imaging spectrometer APEX (Airborne Prism Experiment) is currently being built. In parallel, its data processing calibration chain is being designed. The complex design of this high resolution pushbroom instrument bears the risk of optical aberrations in the registered spatio-spectral frames. Such aberrations consist of so-called frown and smile effects, as well as ghost image, smear, and stray light contributions. A concept is presented which shall operationally improve image calibration by inversion of the sensor model.
The monitoring of the carbon stock in terrestrial environments, as well as the improved understanding of the surface-atmosphere interactions controlling the exchange of matter, energy and momentum, is of immediate interest for an improved assessment of the various components of the global carbon cycle. Studies of the Earth System processes at the global scale rely on models that require an advanced understanding and proper characterization of processes at smaller scales. The prime objective of the Surface Processes and Ecosystem Changes Through Response Analysis (SPECTRA) Mission is to determine the amount, assess the conditions and understand the response of terrestrial vegetation to climate variability and its role in the coupled cycles of energy, water and carbon. The amount and state of vegetation will be determined by the combination of observed vegetation properties and data assimilation. Many vegetation properties are related to features of reflectance spectra in the region 400 nm - 2500 nm. Detailed observations of spectral reflectance reveal subtle features related to biochemical components of leaves such as chlorophyll and water. The architecture of vegetation canopies determines complex changes of observed reflectance spectra with view and illumination angle. Quantitative analysis of reflectance spectra requires, therefore, an accurate characterization of the anisotropy of reflected radiance. This can be achieved with nearly - simultaneous observations at different view angles. Exchange of energy between the biosphere and the atmosphere is an important mechanism determining the response of vegetation to climate variability. This requires measurements of the component temperature of foliage and soil. The latter are closely related to the angular variation in thermal infrared emittance. Scientific preparations for SPECTRA are pursued along two avenues: a) the nature of the expected data and candidate algorithms are evaluated by generating and using synthetic hyper - spectral multi - angular\radiometric data; algorithms are evaluated with actual hyper -spectral data collected with a variety of airborne systems and concurrent ground measurements;
Over the past few years, a joint Swiss/Belgium ESA initiative resulted in a project to build a precursor mission of future spaceborne imaging spectrometers, namely APEX (Airborne Prism Experiment). APEX is designed to be an airborne dispersive pushbroom imaging spectrometer operating in the solar reflected wavelength range between 4000 and 2500 nm. The system is optimized for land applications including limnology, snow, and soil, amongst others. The instrument is optimized with various steps taken to allow for absolute calibrated radiance measurements. This includes the use of a pre- and post-data acquisition internal calibration facility as well as a laboratory calibration and a performance model serving as a stable reference. The instrument is currently in its breadboarding phase, including some new results with respect to detector development and design optimization for imaging spectrometers. In the same APEX framework, a complete processing and archiving facility (PAF) is developed. The PAF not only includes imaging spectrometer data processing up to physical units, but also geometric and atmospheric correction for each scene, as well as calibration data input. The PAF software includes an Internet based web-server and provides interfaces to data users as well as instrument operators and programmers. The software design, the tools and its life cycle are discussed as well.
The Traceable Radiometry Underpinning Terrestrial- and Helio-Studies (TRUTHS) mission offers a novel approach to the provision of key scientific data wtih unprecedented radiometric accuracy for Earth Observation (EO) and solar studies, which will also establish well-calibrated reference targets/standards to support other SI missions. This paper will present the TRUTHS mission and its objectives. TRUTHS will be the first satellite mission to calibrate its instrumentation directly to SI in orbit, overcoming the usual uncertainties associated with drifts of sensor gain and spectral shape by using an electrical rather than an optical standard as the basis of its calibration. The range of instruments flown as part of the payload will also proivde accurate input data to improve atmospheric radiative transfer codes by anchoring boundary conditions, through simultaneous measurements of aerosols, particulates and radiances at various heights. Therefore, TRUTHS will significantly improve the performance and accuracy of Earth observation misison with broad global or operational aims, as well as more dedicated missions. The providision of reference standards will also improve synergy between missions by reducing errors due to different calibration biases and offer cost reductions for future missions by reducing the demands for on-board calibration systems. Such improvements are important for the future success of strategies such as Global Monitoring for Environment and Security and the implementation and monitoring of international treaties such as the Kyoto Protocol. TRUTHS will achieve these aims by measuring the geophysical variables of solar and lunar irradiance, together with both polarized and un-polarized spectral radiance of the Moon, and the Earth and its atmosphere.
In the framework of the APEX (Airborne Prism Experiment) pushbroom imaging spectrometer, a complete processing and archiving facility (PAF) is developed. The PAF not only includes imaging spectrometer data processing up to physical units, but also geometric and atmospheric correction for each scene, as well as calibration data input. The PAF software includes an Internet based web-server and provides interfaces to data users as well as instrument operators and programmers. The software design, the tools and its life cycle is discussed as well. Further we will discuss particular instrument requirements (resampling, bad pixel treatment, etc.) in view of the operation of the PAF as well as their consequences on the product quality. Finally we will discuss a combined approach for geometric and atmospheric correction including BRDF (or view angle) related effects.
A method for the determination of aerosol optical properties from imaging spectrometer data on a local scale is investigated, making use of the continuous spectral coverage, high spatial resolution, and the well-calibrated radiometry of such data. The method (correlated spectral unmixing) is based on the decomposition of the sensor signal in the short-wave infrared using spectrum database ground spectra, the reconstruction of image ground spectra in the visible, and forward modelling with a radiative transfer code. The sensitivity of the imaging spectrometer signal to different atmospheric condititions is explored, as well as the correlation of spectral reflectances in the visible and short-wave infrared for a variety of surfaces. The potential of the presented method is demonstrated for a scene from the airborne visible and infrared imaging spectrometer AVIRIS over rugged heterogeneous coastal terrain in California, and comparisons to multispectral methods are made.
We present an approach to translate scientific requirements into instrument specifications by using a forward model for generic airborne imaging spectrometers in earth remote sensing. Based on scientific requirements, for each relevant variable detectable using imaging spectroscopy, ground reflectance spectra have been provided by specialists in their field of expertise. Relevant changes to be detected in the observed variable are used to derive critical delta reflectances. Realistic mission scenarios are subsequently combined with theses delta reflectances and a radiative transfer code to determine spectral NedL values at the sensor level. The combination of various fields of application in terms of detectable variables and the use of realistic mission scenarios leads to the determination of various NedL levels that are determined at given at sensor radiances. Using this concept, manufacturable specifications can be derived from scientific requirements.
Wolf Schweitzer, Michael Schaepman, Michael Ith, Karin Bruegger, Michael Thali, Tanya Doernhofer, Kathrin Tiefenthaler, Eva Scheurer, Peter Vock, Chris Boesch, Richard Dirnhofer
We currently evaluate MRI as add-on to dissection. Cases can only build on high evidential values of morphological findings as estimated using Bayesian likelihood-ratios. These values may vary among different cases depending on the quality of the morphology and the discrete hypotheses to be discerned. After scanning 20 bodies using MRI admitted to our institute for autopsy, we reconstructed selected imaging findings from a couple of illustrative cases according to a geometrical model ('Pink Box') designed as an object oriented bridging protocol to enable comparison of autopsy and MRI data. Although it appears obvious that 'three-dimensional imaging yields relevant diagnoses,' comparison of selected findings suggests, that the real evidential value of a postmortem scan depends on basic geometrical features of tissue structures examined. (1) Tissue surfaces are difficult to examine in MRI, including surface features of contact wounds in firearm injuries, lacerations of the pleura, or skin needle marks. (2) Specificity and sensitivity of solid tissue block data depend on contrast and resolution. (3) Tunnels or tubes, such as coronary arteries, linear wound tracks or the aorta offer more degrees of freedom for reconstruction, including spatial reconstruction or cross sectioning in different directions. (4) Three-dimensional rendering of complex objects results in spectacular images. Their evidential value is dependent on the way thresholding of 2D slices is validated. We present illustrative examples which suggest that a possible integration of non-invasive imaging methods into Forensic Pathology in fact need to take basic geometry into consideration when discussing evidential value.
The consistent simulation of airborne and spaceborne hyperspectral data is an important task and sometimes the only way for the adaptation and optimization of a sensor and its observing conditions, the choice and test of algorithms for data processing, error estimations and the evaluation of the capabilities of the whole sensor system. The integration of three approaches is suggested for the data simulation of APEX (Airborne Prism Experiment): (1) a spectrally consistent approach (e.g. using AVIRIS data), (2) a geometrically consistent approach (e.g. using CASI data), and (3) an end-to- end simulation of the sensor system. In this paper, the last approach is discussed in detail. Such a technique should be used if there is no simple deterministic relation between input and output parameters. The simulation environment SENSOR (Software Environment for the Simulation of Optical Remote Sensing Systems) presented here includes a full model of the sensor system, the observed object and the atmosphere. The simulator consists of three parts. The first part describes the geometrical relations between object, sun, and sensor using a ray tracing algorithm. The second part of the simulation environment considers the radiometry. It calculates the at-sensor-radiance using a pre-calculated multidimensional lookup-table for the atmospheric boundary conditions and bi- directional reflectances. Part three consists of an optical and an electronic sensor model for the generation of digital images. Application-specific algorithms for data processing must be considered additionally. The benefit of using an end- to-end simulation approach is demonstrated, an example of a simulated APEX data cube is given, and preliminary steps of evaluation of SENSOR are carried out.
Utilization of sub-pixel targets for radiometric calibration of airborne and space-borne imaging sensors involves the uncertainty of their contribution to the pixel-integrated radiance. This contribution depends not only on the target area but also on an unknown location of the sub-pixel target within a sensor pixel. A technique is proposed to retrieve both the target radiance and its sub-pixel location from the target image, taking into account the effects of the sensor point spread function. The technique was used for in-flight calibration of the thermal channels of the airborne imaging spectrometer DAIS-7915.
The European Space Agency (ESA) has identified the necessity to initiate a study that concentrates on the definition of an airborne imaging spectrometer which could represent a precursor to the spaceborne PRISM. The study included the definition of an Airborne PRISM Experiment (APEX) that will contribute to the preparation, calibration, validation, simulation, and application development for the PRISM mission. The APEX instrument is defined as an airborne pushbroom imager with 1000 pixels across track and 200 user selectable spectral bands over the wavelength range from 450-2500 nm. The complete APEX system will include an aircraft, navigation data, laboratory and in-flight calibration as well as a data archiving and processing facility. The definition of the specifications of the APEX instrument is based on a sensor model taking into account various parameters of the expected operation range of the instrument. The approach used defines the radiometric properties of expected scene radiances including SNR and NE(Delta) (rho) . The APEX system is presented and in compliance with the PRISM instrument, conclusions on the simulation possibilities are derived and discussed.
A multisensor airborne campaign is carried out in Switzerland in summer 1997. The campaign did not only involve a suite of different sensors but also extensive ground supporting measurements. Amongst the sensor that acquired data over a predefined set of three standard test sites were the hyperspectral imagers DAIS 7915 and CASI, a wide angle airborne camera (WAAC) and a SAR (E-SAR) system as well as an imaging laserscanner. On the ground, geolocation is performed with differential GPS systems and a number of georeferenced ground control points. An active navigation system for the aircraft is used for accurate flight path determination in order to support single- and multi-pass interferometric flights. The thermal ground references consist of a number of targets in the field to verify the thermal performance of the DAIS. Radiometric validation on the ground involves spectroradiometric measurements of a number of selected reference targets, measurements of global flux and radiant temperature, as well as sunphotometer measurements. Conventional field mapping completes the full documentation of the three test sites. The generation of digital surface models using the stereo approach of the WAAC camera and the laserscanner is a goal to support the georeferencing of the different acquired image data. Finally all data are projected onto a common reference system and can be used for further analysis.
Nearly all current imaging spectroscopy data are obtained by scanning airborne systems. The stability of such systems is always worse than that of spaceborne platforms. Thus, geometric distortions occur due to variations of the flightpath as well as of the attitude of the plane. These distortions cannot be corrected simply by ground control point based traditional georeferencing procedures since the movements cannot be approximated satisfactorily by polynomial transformations of the image. A pixel by pixel calculation has to be performed instead, to account for the position and attitude of the plane during the scanning process. A georeferencing procedure is described which is based on a parametric approach and theoretically allows sub- pixel accuracy even in steep terrain. The current work resulted in a new algorithm and application for parametric geocoding. A ground control point based procedure has been developed to recalibrate the offsets of the attitude data since they usually are given as relative angles. It exactly reconstructs the scanning geometry for each image pixel using position, attitude, and terrain elevation data. The procedure is tested on AVIRIS and on DAIS data and compared to digital topographic data. The geocoding results are of reliable accuracies of down to 1-2 pixels for both data sets.
In the past various authors pointed out, that the value of imaging spectrometer data is closely related to the accuracy with which the data are calibrated to represent physical parameters. the AVIRIS team at JPL gave good examples on how the calibration can be performed in the laboratory and how its accuracy can be evaluated independently by means of an in-flight calibration/validation experiment. The first part of this paper presents the laboratory instrumentation and measurements that were brought into place at the German Aerospace Research Establishment (DLR) to calibrate the DAIS 7915 sensor. Some estimates of the accuracy of these measurements are given to allow the derivation of an overall precision of the laboratory calibration. It is the purpose of an in-flight calibration and validation campaign to check the validity of the laboratory calibration for data acquired under in-flight conditions. In a joint experiment of DLR and the Remote Sensing Laboratories of the University of Zurich the DAIS instrument flew a standard test site in the center of Switzerland in summer 1996. In parallel to this overflight a number of ground reference measurements are acquired. The influence of the atmosphere is accounted for using the MODTRAN radiative transfer code. Sample spectra for different in-flight calibration targets are displayed.
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