CHAPTER 2 IMAGING SPECTROMETRY: BASIC ANALYTICAL TECHNIQUES Freek VAN DER MEER α ,β , Steven DE JONG χ & Wim BAKKER α α International Institute for Aerospace Survey and Earth Sciences (ITC), Division of Geological Survey, Enschede, The Netherlands. β Delft University of Technology, Department of Applied Earth Sciences, Delft, The Netherlands χ Wageningen University and Research Center, Center for Geo-information, Wageningen, the Netherlands 1 Introduction Remote sensing (e.g., the observation of a target by a device separated from it by some distance thus without physical contact) of the surface of the Earth from aircraft and from spacecraft provides information not easily acquired by surface observations. Until recently, the main limitation of remote sensing was that surface information lacked detail due to the broad bandwidth of sensors available. Work on high-spectral resolution radiometry has shown that earth surface mineralogy can be identified using spectral information from sensor data (Goetz 1991). Conventional sensors (e.g., Landsat MSS and TM, and SPOT) acquire information in a few separate spectral bands of various widths (typically in the order of 0.1-0.2 µm), thus smoothing to a large extent the reflectance characteristics of the surface (Goetz & Rowan 1981). Most terrestrial materials are characterized by spectral absorption features typically 0.02-0.04 µm in width (Hunt 1980). High-spectral resolution remotely sensed images are acquired to produce reflectance or radiance spectra for each pixel in the scene. Based upon the molecular absorptions and constituent scattering characteristics expressed in the spectrum we seek to: • Detect and identify the surface and atmospheric constituents present • Assess and measure the expressed constituent concentrations • Assign proportions to constituents in mixed spatial elements • Delineate spatial distribution of the constituents • Monitor changes in constituents through periodic data acquisitions • Simulate, calibrate and intercompare sensors • Validate, constrain and improve models New analytical processing techniques have been developed to analyze such high spectral dimensional data sets. These methods are the scope of this chapter. The pre- 2 F.D. VAN DER MEER, S.M. DE JONG & W. BAKKER processing of imaging spectrometer data and the calibration of the instruments is briefly addressed. The chapter focuses on the processing of the data and new analytical approaches developed for the specific use with imaging spectrometer data. First we present a review of existing systems and design philosophy. 2 Imaging spectrometry: airborne systems Imaging spectrometers have been used for many years in military applications such as the detection of camouflage from real vegetation. Due to the classified nature of the data and sensors not much can be said about the origin and applications being served. The first scanning imaging spectrometer was the Scanning Imaging Spectroradiometer (SIS) constructed in the early 1970s for NASA's Johnson Space Center. After that, civilian airborne spectrometer data were collected in 1981 using a one-dimensional profile spectrometer developed by the Geophysical Environmental Research Company which acquired data in 576 channels covering the 0.4-2.5 µm wavelength range (Chiu & Collins, 1978) followed by the Shuttle Multispectral Infrared Radiometer (SMIRR) in 1981. The first imaging device was the Fluorescence Line Imager (FLI; also known as the Programmable Line Imager, PMI) developed Canada’s Department of Fisheries and Oceans (in 1981) followed by the Airborne Imaging Spectrometer (AIS), developed at the NASA Jet Propulsion Laboratory which was operational from 1983 onward acquiring 128 spectral bands in the range of 1.2-2.4 µm. The field-of-view of 3.7 degrees resulted in 32 pixels across-track. A later version of the instrument, AIS-2 (LaBaw, 1987), covered the 0.8-2.4 µm region acquiring images 64 pixels wide. Since 1987, NASA is operating the successor of the AIS systems, AVIRIS, the Airborne Visible/Infrared Imaging Spectrometer (Vane et al., 1993). Since that time many private companies also started to take part in the rapid development in imaging spectrometry. Initiatives are described in a later paragraph on airborne systems. Currently many space agencies and private companies in developed and developing countries operate there own instruments. It is impossible to describe all currently operational airborne imaging spectrometer systems in detail. Some systems will be highlighted to serve as examples rather than to provided an all-inclusive overview. The AISA Airborne Imaging Spectrometer is a commercial hyperspectral pushbroom type imaging spectrometer system developed by SPECIM based in Finland. The spectral range in standard mode is 430 to 900 nm and a spectral sampling interval of 1.63 nm a a total of 288 channels. Spectral channel bandwidth are programmable from 1.63 to 9.8 nm. The Field of view is 21 degrees across-track and 0.055 degrees along-track resulting in typical spatial Resolutions of 360 pixels per swath, 1 m across- track resolution at an aircraft altitude of 1000 m. The Advanced Solid-state Array Spectroradiometer (ASAS) is a hyper-spectral, multi-angle, airborne remote sensing instrument maintained and operated by the Laboratory for Terrestrial Physics at the NASA Goddard Space Flight Center. The system acquires 62 spectral channels in visible to near-infrared (404 to 1020 nm) with a spectral bandwidth of approximately 10 nm. The across-track resolution is 3.3 m (at nadir) to 6.6 m (60 deg) at 5000 m altitude, the along-track resolution is 3 m (at nadir) at 5000 m altitude. ANALYTICAL TECHNIQUES IN SPECTROMETRY 3 In 1987 NASA began operating the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS). AVIRIS was developed as a facility that would routinely supply well-calibrated data for many different purposes. The AVIRIS scanner collects 224 contiguous bands resulting in a complete reflectance spectrum for each 20*20 m. pixel in the 0.4 to 2.5 µm region with a sampling interval of <10 nm. The field-of-view of the AVIRIS scanner is 30 degrees resulting in a ground field-of-view of 10.5km. AVIRIS uses scanning optics and four spectrometers to image a 614 pixel swath simultaneously in 224 contiguous spectral bands over the 400 to 2500nm. wavelength range. The Thermal Infrared Multispectral Scanner (TIMS) is a multispectral scanner that collects data in six channels. The six-element HgCdTe detector array provides six discrete channels: • Channel 1: 8.2 - 8.6 micrometers • Channel 2: 8.6 - 9.0 micrometers • Channel 3: 9.0 - 9.4 micrometers • Channel 4: 9.4 - 10.2 micrometers • Channel 5: 10.2 - 11.2 micrometers • Channel 6: 11.2 - 12.2 micrometers Flown aboard NASA C-130B, NASA ER-2, and NASA Learjet aircraft, the TIMS sensor has a nominal Instantaneous Field of View of 2.5 milliradians with a ground resolution of 25 feet (7.6 meters at 10,000 feet). The sensor has a selectable scan rate (7.3, 8.7, 12, or 25 scans per second) with 698 pixels per scan. Swath width is 2.6 nautical miles (4.8 kilometers at 10,000 feet) while the scanner's Field of View equals 76.56 degrees. In Canada, ITRES developed the Compact Airborne Spectrographic Imager (CASI) that became operational in 1989. Recently, a revised version of the instrument has been put on the market. This pushbroom sensor has 288 spectral narrow (1.9nm.) bands with 512 pixels across track in the 400-870nm. range . The Geophysical Environmental Research Corporation (GER) based in Millbrook, U.S. develops a line of imaging spectrometers that included the GERIS (a 63 channel instrument no longer in production), the Digital Airborne Imaging Spectrometer (DAIS), and the Environmental Protection Systems (EPS) systems. The 79-channel Digital Airborne Imaging Spectrometer built by the Geophysical Environmental Research corp. (GER) is the successor of the 63-channel imaging spectrometer GERIS. This 15-bit instrument covers the spectral range from the visible to the thermal infrared wavelengths at variable spatial resolution from 3 to 20 m. depending on the carrier aircraft flight altitude. Six spectral channels in the 8 - 12 µm. region are used for temperature and emissivity of land surface objects. These and 72 narrow band channels in the atmospheric windows (e.g. those wavebands that pass relatively undiminished through the atmosphere) between 0.450 and 2.45 µm. are sensed using four spectrometers. The FOV is 32-39 degrees (depending on aircraft) and the IFOV is3.3 mrad. (0.189 degrees) yielding a GIFOV depending on aircraft altitude of between 5 - 20 m. The Hyperspectral Mapper (HyMAP), in the U.S. known as Probe-1, was build by Integrated Spectronics in Australia and operated by HyVISTA. The 126 channel Probe 1 (operated by ESSI in the US) is a "whiskbroom style" instrument that collects data in a cross-track direction by mechanical scanning and in an along-track direction by movement of the airborne platform. The instrument acts as an imaging spectrometer in 4 F.D. VAN DER MEER, S.M. DE JONG & W. BAKKER the reflected solar region of the electromagnetic spectrum (0.4 to 2.5 µm) and collects broadband information in the MIR (3 - 5 µm) and TIR (8 - 10 µm) spectral regions. In the VNIR and SWIR, the at-sensor radiance is dispersed by four spectrographs onto four detector arrays. Spectral coverage is nearly continuous in these regions with small gaps in the middle of the 1.4 and 1.9 µm atmospheric water bands. The spatial configuration of the Probe-1 is: • IFOV - 2.5 mrad along track, 2.0 mrad across track • FOV - 61.3 degrees (512 pixels) • GIFOV - 3 – 10 m (typical operational range) The Multispectral Infrared and Visible Imaging Spectrometer (MIVIS) is a 102 channel imaging spectrometer developed by SensyTech. It has 92 channels covering the 400-2500nm region and 10 thermal channels.