Daniel Schläpfer

Geo-atmospheric processing of airborne imaging spectrometry data. Part 1: Parametric orthorectification

An operational orthorectification solution in support of the combined geometric and radiometric processing of currently available imaging spectrometry data is presented. The described parametric geocoding procedure (PARGE) strictly considers the aircraft and terrain geometry parameters and uses a forward transformation algorithm to create orthorectified imaging spectrometry cubes. The implementation principles, the auxiliary data calibration strategies, and the workflow of the currently applied processor are discussed. The major error sources of the approach are identified, and possibilities are shown how to make the most out of the available auxiliary data Inertial Navigation System/Global Positioning System (INS/GPS) parameters. Results on HyMap and AVIRIS imaging spectrometry data show an absolute accuracy in the range of 1-3 pixels for this kind of imagery. The combination of PARGE with an atmospheric correction procedure is shown in part 2 to this paper. The depicted geo-atmospheric workflow is proposed as a standard processing approach for available and future imaging spectrometry data.

Atmospheric Precorrected Differential Absorption Technique to Retrieve Columnar Water Vapor

Abstract Differential absorption techniques are suitable to retrieve the total column water vapor contents from imaging spectroscopy data. A technique called Atmospheric Precorrected Differential Absorption (APDA) is derived directly from simplified radiative transfer equations. It combines a partial atmospheric correction with a differential absorption technique. The atmospheric path radiance term is iteratively corrected during the retrieval of water vapor. This improves the results especially over low background albedos. The error of the method for various ground reflectance spectra is below 7% for most of the spectra. The channel combinations for two test cases are then defined, using a quantitative procedure, which is based on MODTRAN simulations and the image itself. An error analysis indicates that the influence of aerosols and channel calibration is minimal. The APDA technique is then applied to two AVIRIS images acquired in 1991 and 1995. The accuracy of the measured water vapor columns is within a range of ±5% compared to ground truth radiosonde data.

SENSOR: a tool for the simulation of hyperspectral remote sensing systems

Abstract The consistent end-to-end simulation of airborne and spaceborne earth remote sensing systems is an important task, and sometimes the only way for the adaptation and optimisation of a sensor and its observation conditions, the choice and test of algorithms for data processing, error estimation and the evaluation of the capabilities of the whole sensor system. The presented software simulator SENSOR (Software Environment for the Simulation of Optical Remote sensing systems) includes a full model of the sensor hardware, the observed scene, and the atmosphere in between. The simulator consists of three parts. The first part describes the geometrical relations between scene, sun, and the remote sensing system 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 taking the atmospheric influence on the radiation into account. The third part consists of an optical and an electronic sensor model for the generation of digital images. Using SENSOR for an optimisation requires the additional application of task-specific data processing algorithms. The principle of the end-to-end-simulation approach is explained, all relevant concepts of SENSOR are discussed, and first examples of its use are given. The verification of SENSOR is demonstrated. This work is closely related to the Airborne PRISM Experiment (APEX), an airborne imaging spectrometer funded by the European Space Agency.

An automatic atmospheric correction algorithm for visible/NIR imagery

The automatic correction of atmospheric effects currently requires visible to short‐wave spectral bands (400–2500 nm) to derive high accuracy surface reflectance data. Common techniques employ spectral correlations of dark targets in the short‐wave infrared (SWIR, around 2.2 µm), blue (480 nm) and red (660 nm) regions to derive the aerosol optical depth. A large number of current Earth‐observing satellite sensors have only three or four spectral channels in the visible and near‐infrared (VNIR) region (400–1000 nm), making an automatic image‐based atmospheric correction very difficult. This contribution presents a new algorithm and first results with VNIR imagery. The method starts with the assumption of average clear atmospheric conditions (aerosol optical depth AOD = 0.27, corresponding to a visibility of 23 km) and calculates the surface reflectance in the red and near‐infrared (NIR) bands. The second step derives a mask of dark vegetation pixels. It is calculated using multiple thresholds of vegetation index combined with red and NIR surface reflectance values. Then the red band surface reflectance for the dark pixels is estimated from the NIR reflectance as ρred = 0.1 ρnir, from which the aerosol optical depth (or visibility) can be calculated. The core of the VNIR algorithm consists of two subsequent iteration loops (visibility and ρred) to improve the visibility estimate. Results of the VNIR method are presented for Landsat‐5 Thematic Mapper (TM) and Landsat‐7 Enhanced Thematic Mapper Plus (ETM+) imagery using only the first four bands. The performance of the method is compared to the established dark pixel technique where the SWIR bands are included. Results show that the deviation between both methods is usually less than 0.005 reflectance units if measured in terms of the scene‐average reflectance, indicating a useful potential for this approach.

Digital Airborne Photogrammetry—A New Tool for Quantitative Remote Sensing?—A State-of-the-Art Review On Radiometric Aspects of Digital Photogrammetric Images

The transition from film imaging to digital imaging in photogrammetric data capture is opening interesting possibilities for photogrammetric processes. A great advantage of digital sensors is their radiometric potential. This article presents a state-of-the-art review on the radiometric aspects of digital photogrammetric images. The analysis is based on a literature research and a questionnaire submitted to various interest groups related to the photogrammetric process. An important contribution to this paper is a characterization of the photogrammetric image acquisition and image product generation systems. The questionnaire revealed many weaknesses in current processes, but the future prospects of radiometrically quantitative photogrammetry are promising.

APEX - the hyperspectral ESA Airborne Prism Experiment

The airborne ESA-APEX (Airborne Prism Experiment) hyperspectral mission simulator is described with its distinct specifications to provide high quality remote sensing data. The concept of an automatic calibration, performed in the Calibration Home Base (CHB) by using the Control Test Master (CTM), the In-Flight Calibration facility (IFC), quality flagging (QF) and specific processing in a dedicated Processing and Archiving Facility (PAF), and vicarious calibration experiments are presented. A preview on major applications and the corresponding development efforts to provide scientific data products up to level 2/3 to the user is presented for limnology, vegetation, aerosols, general classification routines and rapid mapping tasks. BRDF (Bidirectional Reflectance Distribution Function) issues are discussed and the spectral database SPECCHIO (Spectral Input/Output) introduced. The optical performance as well as the dedicated software utilities make APEX a state-of-the-art hyperspectral sensor, capable of (a) satisfying the needs of several research communities and (b) helping the understanding of the Earths complex mechanisms.

Spatial PSF Nonuniformity Effects in Airborne Pushbroom Imaging Spectrometry Data

Efficient and accurate imaging spectroscopy data processing asks for perfectly consistent (i.e., ideally uniform) data in both the spectral and spatial dimensions. However, real pushbroom-type imaging spectrometers are affected by various point spread function (PSF) nonuniformity artifacts. First, individual pixels or lines may be missing in the raw data due to bad pixels originating from the detector, readout errors, or even electronic failures. Second, so-called smile and keystone optical aberrations are inherent to imaging spectrometers. Appropriate resampling strategies are required for the preprocessing of such data if emphasis is put on spatial PSF uniformity. So far, nearest neighbor interpolations have been often recommended and used for resampling. This paper shall analyze the radiometric effects if linear interpolation is used to optimize the spatial PSF uniformity. For modeling interpolation effects, an extensive library of measured surface reflectance spectra as well as real imaging spectroscopy data over various land cover types are used. The real measurements are systematically replaced by interpolated values, and the deviation between original and resampled spectra is taken as a quality measure. The effects of nearest neighbor resampling and linear interpolation methods are compared. It is found that linear interpolation methods lead to average radiometric errors below 2% for the correction of spatial PSF nonuniformity in the subpixel domain, whereas the replacement of missing pixels leads to average errors in the range of 10%-20%

SPECCHIO: a spectrum database for remote sensing applications

Representative and comprehensive information on the spectral properties of natural and artificial materials on the Earths surface is highly relevant in aircraft or satellite remote sensing, such as geological mapping, vegetation analysis, or water quality estimation. For this reason, the spectrum database SPECCHIO (Spectral Input/Output) has been developed, offering ready access to spectral campaign data, modelled data, and existing spectral libraries. Web-based and command line interfaces allow for the input of spectral data of heterogeneous formats and descriptions, as well as interactive queries, previews, and downloads. ASCII and ENVI spectral library data formats are currently supported. SPECCHIO is used as a reference database for the retrieval of geophysical and biophysical parameters from remotely sensed data, accounting for the frequent lack of surface spectra. The database is also used for the general management of spectral data, including detailed ancillary data.

Structure, Components, and Interfaces of the Airborne Prism Experiment (APEX) Processing and Archiving Facility

The product generation from hyperspectral sensor data has high requirements on the processing infrastructure, both hardware and software. The Airborne Prism Experiment (APEX) processing and archiving facility has been set up to provide for the automated generation of level-1 calibrated data and user-configurable on-demand product generation for higher processing levels. The system offers full reproducibility of user orders and processing parameters by employing a relational database. The flexible workflow software allows for the quick integration of novel algorithms or the definition of new processing sequences. Reprocessing of data is supported by the archiving approach. Configuration management based on the database enables the control over different versions of processing modules to be applied. The system is described with a focus on the APEX instrument; however, its generic design allows adaptation to other sensor systems.

APEX: current status of the airborne dispersive pushbroom imaging spectrometer

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.