SPECTRAL SIGNATURE ANALYSIS USING A FIELD GONIOMETER MOUNTED SPECTRORADIOMETER

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1 SPECTRAL SIGNATURE ANALYSIS USING A FIELD GONIOMETER MOUNTED SPECTRORADIOMETER Michael E. Schaepman, Stefan Sandmeier, Klaus I. Itten, Peter Meyer and Tobias Kellenberger Remote Sensing Laboratories Department of Geography Winterthurerstrasse 190 CH 8057 Zurich, Switzerland ABSTRACT A ground spectroradiometric measurement system is introduced to measure bidirectional reflectance using a transportable field goniometer. The calibration site and the spectroradiometer used are discussed, followed by the concept of the goniometer. Finally, an approach to an expert system to handle spectral signatures is presented. 1.0 INTRODUCTION Hyperspectral remote sensing data are widely available since the introduction of various airborne imaging spectrometers. Simultaneous ground data collection using spectroradiometric instruments must be carried out to provide for the necessary reference data. The demand to better understand the information content of the measurements leads to the following steps (modified after Sturm (Sturm, 1992)), which have to be carried out to compare the remotely sensed data with the ground reference: Accurate calibration in terms of radiance of the received signals (the calibration problem) A sufficiently precise assessment of all alterations of the object inherent spectral radiance in duced by atmospheric effects (the atmospheric correction problem) Better understanding of the object and illumination geometry and the objects non Lamber tian behaviour (the Bidirectional Reflectance Distribution Function (BRDF) problem) Full understanding and quantification of the relations between nature and status of the object and its electromagnetic emittance and/or reflectance spectrum (the signature problem) In this paper, we will first address the ground data acquisition system and its calibration. Then a new concept of a transportable field goniometer is presented. The detection of the variation in reflectance with view and illumination direction on natural targets is a major goal in modelling and understanding the BRDF. A discussion of the atmospheric correction itself and the use of a physical model is not part of this paper and can be found in Sandmeier (Sandmeier, 1994a). Collecting signatures and managing them in a knowledge base is a possible approach to the signature problem. Classification and comparison methods for signatures are an essential enhancement to a knowledge base. Thus MATSS (Management and Analysing Tools for Spectral Signatures) will be presented as an approach of an expert system for spectral signature analysis. 2.0 TECHNICAL SETUP This section is to describe the instruments which perform the measurements, and the calibration, followed by the concept of the transportable field goniometer. 2.1 THE SPECTRORADIOMETER The spectroradiometer used is a GER 3700 ground spectroradiometer. This instrument has been chosen because of its very fast scan time (50 ms and up) and the relatively small weight of the

2 optics head (approx. 10 kg). It will be operated in a helicopter as well as mounted on the goniometer described below or on a normal tripod in the field. Table 1 lists some of the technical features of the GER Table 1. Technical Specifications of the GER 3700 Spectral Range nm Bandwidth Sampling 1.5 nm ( nm) 6.2 nm ( nm) 8.6 nm ( nm) Detectors Si, PbS, PbS Channels 704 Spectrometers 3 Dispersing Gratings Scan Time 50 ms and up The control unit including the signal processor (a lap-top computer) can operate the optics head remotely and the user can perform all the operations from the keyboard. This setup is essential for the use with the goniometer, as no access to the sled mounted radiometer is necessary during a hemispherical measurement. 2.2 CALIBRATION The accurate measurement of optical radiation involves not only the use of a stable and well characterised spectroradiometer, but also requires a reflection standard. A commercially available standard, which is traceable to NIST (National Institute of Standards and Technology) will be used. Also a panel with a known reflectance will be used in the field and in the laboratory Laboratory Setup Calibration will be performed in a laboratory using an integrating sphere calibration standard. The integrating sphere, having a diameter of 8 inches and a radiation port diameter of 2 inches, has a silicon detector filter combination with a photopic response mounted in the sphere wall to monitor the sphere luminance. The 4:1 ratio in sphere diameter to the radiating port diameter typically exhibits a ± 0.5 % nonuniformity in near normal radiance (Schneider, 1993). The source module has a reflectorized 150 Watt tungsten halogen lamp which is located external to the sphere. The controller part consists of a stabilised power supply with ramp up power features and the photometer amplifier. The calibration for spectral radiance ranges from nm. The spectroradiometer is mounted directly to the exit port of the integrating sphere. Using the goniometer in the laboratory, the angular dependencies of the reflectance standard will be measured. The reflectance standard used is a Spectralon panel made from PTFE (polytetrafluoroethylene). It is a white reflecting material and has been chosen due to its qualities in spatial and spectral uniformity and stability with time. Bruegge et al. (Bruegge, 1991) have shown that measuring the BRDF of this kind of panels fulfills this substitution. The precalculated sun position for the experiment outside is simulated in the laboratory using the tungsten halogen lamp. Because the lamp is mounted outside to the sphere, it can easily be taken off the sphere and used as a directional irradiance source.

3 2.2.2 Field Setup The same Spectralon reflectance standard serves as a reference in field measurements. Comparison between the laboratory and the field setup must take into account that in the field a reflectance factor for hemispherical incidence will be measured as opposed to the laboratory's conical incidence. Due to the high variability of the atmosphere and the time necessary for a full hemispherical measurement, the reflectance properties of the reflectance standard should be invariant over the course of the measurements (later a sunphotometer will collect global incident radiation). Considering that a full hemispherical measurement (one measurement each 15 in zenith and azimuth) will take 132 sample points, the scan time of the spectroradiometer and the positioning time of the radiometer must be optimised. Otherwise the position of the sun and the atmosphere cannot be assumed constant. Measuring 132 single sample points over one hemisphere may not deliver good data quality. Depending on the desired SN ratio averaging over a multitude of measurements may be necessary. Preliminary tests show that at least three measurements for each sampling point have to be collected for a representative sample. Furthermore an adequate number of measurements must be made within one single field or landuse type. 2.3 THE FIELD GONIOMETER The scientific rationale behind a transportable field goniometer is the need of measuring the directional reflectance at different incident and viewing angles of naturally illuminated targets. Various requirements have to be met to make a goniometer transportable and stable for the field use. A complete description of the gonoimeter can be found in Sandmeier (Sandmeier, 1994b). Light source (Sun) 2 m Zenithal arc Sled with radiometer Target Azimuthal arc 4 m Figure 1. Concept of the Field Goniometer

4 The transportable field goniometer consists of four major parts. The azimuthal arc, the zenith arc, the motor driven sled and the remote control. Figure 1 sketches these major parts, except for the remote control. The azimuthal arc is based on 12 aluminium sockets. Interconnected, they draw a circle of 2 meter radius. A track on top of these sockets allows the zenith arc of 2 meter radius to be rotated by 360 degrees. The spectroradiometer itself is mounted on a sled which is driven by a precision motor to obtain any desired zenithal position on the bow. By moving the bow on the tracks manually, any azimuthal position can be acquired and the complete sphere over a selected target can be covered. In a later stage, azimuth drive motors may be installed. Figure 2. The Field Goniometer in the End Phase The concept of the goniometer requires two persons to operate the goniometer. One to operate the radiometer remotely by the computer and the other to move the sled on the zenithal arc.

5 3.0 MATSS 3.1 BACKGROUND Analysis of spectroscopic data from laboratory, from aircraft and from spacecraft requires a knowledge base (Clark, 1993). Several approaches to such knowledge bases have been realised since the broad availability of imaging spectroscopy data to the remote sensing community (Clark, 1993; Grove, 1992). Especially during the AVIRISwiss 91 campaign (Itten, 1992), the need for a spectral data management system arose, due to extensive spectra collection underwater, on the surface and from helicopter (Schaepman, 1992). On contrary to the mass-, NMR- and infrared spectroscopy, where algorithms of relations between the structure of a molecule and the spectral signature have been implemented (Walz, 1990), it is much more difficult to apply these techniques to the visible part of the spectrum. But in remote sensing at the biospheric level single species are not of interest but much larger groups that share common physiological mechanisms or pathways (Goetz, 1992a). Therefore it is more important to relate spectra to certain classes or subsets of classes than to structures of molecules. But the accurate definition of these classes referred above as larger groups is essential for the data retrieval. Therefore a spectral indexing system should not only allow to characterise (classification problem) and find (indexing problem) the desired spectra, but also allow for a mathematical comparison of the selected ones. Unfortunately comparison methods of spectral signatures suffer from a high level of uncertainty. These uncertainties are being considered 10 % or greater of the measured signal and are due to the distribution of radiant energy over wavelength, position, direction, time and polarisation (Grum, 1979). A radiometric measurement will usually measure the distribution of power or energy over one parameter while integrating over or holding the other parameters constant. Therefore different approaches to classify and identify types or subsets of spectral signatures are implemented in MATSS. 3.2 IMPLEMENTATION The Management and Analysing Tools for Spectral Signatures (MATSS) is an approach to provide the necessary methods to allow not only a fast and reliable clutch to structures of spectra but also to develop a new method for the classification of spectra within the library. A method to classify spectra whether this is a library of a certain type of endmembers, laboratory or field spectra or image data must be invariant to uncertainties of the quality of spectra in the visible region and must provide a fast access to their signatures. Basically three different approaches will allow such a classification: (1) a thematic classification of all the spectra in the database is necessary, (2) tools for finding and comparison of spectra and (3) a collection of prominent absorption lines can be accessed for easy comparison of absorption features. The thematic classification of spectras requires an accurate definition of the observed objects. As stated above, the level of the accuracy of the description depends on the object investigated. In any case the storage of the full acquired data is necessary and no data reduction will be applied. At the moment the bottleneck seems to be the knowledge representation. A strict hierarchical approach (Zupan, 1986) restricts the system to a too simplistic model. The knowledge data model must be easily modifiable and rather dynamic. An approach to a more fuzzy system is given from a link to a reference data base. The reference data base consists of a collection of prominent absorption lines extracted from recent publications and integrating as well existing spectroscopic reference data. The spectral indexing part of MATSS includes a variety of tools to find and identify absorption features. An excellent collection of such tools can be found in the specpr (Clark, 1993).

6 MATSS is developed using is the Interactive Data Language (IDL). The extreme processing needs of MATSS require a final implementation in C or C++. The DBMS used at the moment is Blyth Omnis 7.2, a SQL capable database. 4.0 CONCLUSIONS It is expected, that the ability to determine directional reflectance at different incident and viewing angles of natural illuminated targets will improve the knowledge of remotely sensed signals. Nevertheless besides a good calibration, a stable spectroradiometer, a field goniometer, and an expert system to handle the data, we must exchange data with other goniometer facilities (laboratory and field) and cross calibrate the instruments used. 5.0 ACKNOWLEDGEMENTS The construction of the goniometer facility is supported by Fa. Lehner & Co., Gränichen / Aarau, Switzerland and was made possible by the dedication of W. Sandmeier. 6.0 REFERENCES C.J. Bruegge, A.E. Stiegman, D.R. Coulter, R.R. Hale, D.J. Diner, and A.W. Springsteen, "Reflectance Stability Analysis of Spectralon Diffuse Calibration Panels." In Calibration of Passive Remote Observing Optical and Microwave Instrumentation, ed. B.W. Guenther, Proceedings of the SPIE, Vol. 1493, Orlando, pp , R.N. Clark, G.A. Swayze, A.J. Gallagher, T.V.V. King, and W.M. Calvin, The U.S. Geological Survey, Digital Spectral Library: Version 1: 0.2 to 3.0 µm, U.S. Geological Survey, Open File Report , Denver, p. 1326, A.F.H. Goetz, Imaging spectrometry for earth remote sensing. In Imaging Spectroscopy: Fundamentals and Prospective Applications, eds. F. Toselli and J. Bodechtel, Kluwer Academic Publishers, Chap. 1, pp. 1-20, 1992a. A.F.H. Goetz, Principles of narrow band spectrometry in the visible and IR: Instruments and Data Analysis. In Imaging Spectroscopy: Fundamentals and Prospective Applications, eds. F. Toselli and J. Bodechtel, Kluwer Academic Publishers, Chap. 2, pp , 1992b. C.I. Grove, S.J. Hook, and E.D. Paylor II, Laboratory Reflectance Spectra of 160 Minerals, 0.4 to 2.5 Micrometers, NASA JPL Publication 92 2, Pasadena, p. 398, F. Grum and R.J. Becherer, Optical Radiation Measurements: Radiometry, Academic Press New York, Vol. 1, p. 6, K.I. Itten, P. Meyer, K. Staenz, T. Kellenberger, and M. Schaepman, Evaluation of AVIRISwiss 91 Campaign Data. In: Summaries of the Third Annual JPL Airborne Geoscience Workshop June 1 5, 1992, ed. R.O. Green, NASA JPL Publication 92 14, Vol. 1, pp , St. Sandmeier, P. Meyer, and K.I. Itten, A Shortwave Radiation Model for Radiometric Correction of Optical Satellite Data in Rugged Terrain. In International Geoscience and Remote Sensing Symposium IGARSS, Pasadena, in press, August 8 12, 1994a. St. Sandmeier and W. Sandmeier, A Transportable Field Goniometer for Remote Sensing Applications, in preparation, Remote Sensing Laboratories, Department of Geography, University of Zürich, 1994b.

7 M. Schaepman, Bildspektrometrie: Grundlagen und Datenerhebung im AVIRISwiss 91 Projekt, Masters Dissertation, Geographisches Institut, Universität Zürich, Zürich, p. 101, W.E. Schneider, Fundamentals of Measuring Optical Radiation, Optronic Laboratories Inc., Orlando, p. 4, B. Sturm, Atmospheric and radiometric corrections for imaging spectroscopy. In Imaging Spectroscopy: Fundamentals and Prospective Applications, eds. F. Toselli and J. Bodechtel, Kluwer Academic Publishers, Chap. 2, pp , R. Walz, Spektrensuche und Vergleich über spektrale Indizierung, Ph.D. Dissertation, Fakultät für Chemie und Pharmazie, Eberhard Karls Universität, Tübingen, p. 125, J. Zupan and M. Novic, Hierarchical ordering of spectral databases. In Computer Supported Spectroscopic Databases, ed. J. Zupan, Ellis Horwood Series in Analytical Chemistry, Chap. 2, pp , 1986.