IMAGING SPECTROMETRY: BASIC ANALYTICAL · PDF file IMAGING SPECTROMETRY: BASIC...
Embed Size (px)
Transcript of IMAGING SPECTROMETRY: BASIC ANALYTICAL · PDF file IMAGING SPECTROMETRY: BASIC...
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
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. The IFOV is 2 mrad and the FOV is 71 degrees.
ROSIS (Reflective Optics System Imaging Spectrometer) is a compact airborne imaging spectrometer, which developed by Dornier Satellite Systems, GKSS Research Centre (Institute of Hydrophysics) and the German Aerospace Center (DLR, Institute of Optoelectronics). ROSIS is a pushbroom imaging spectrometer acquiring 32 or 84 bands in the 430 to 850nm range with a spectral halfwidth of 7.6nm. The FOV is 16 degrees and the IFOV is 0.56 mrad. For a flight altitude of 10 km the pixel size is about 5.6 m. with a swath width of 2.8 km.
TRW currently performs airborne data collection with the TRWIS III with image spatial resolutions spanning from less than 1 meter to more than 11 meters, with spectral coverage from 380 to 2450 nm. Spectral resolution is 5.25 nm in the visible/near infrared (380 - 1000 nm) and 6.25 in the short wave infrared (1000 - 2450 nm).
2.1 AIRBORNE SIMULATORS
Several examples can be listed of airborne simulators for spaceborne instruments. NASA has a strong airborne programme to support future and planned spaceborne imaging spectrometer missions.
The MODIS Airborne Simulator (MAS) is a multispectral scanner configured to approximate the Moderate-Resolution Imaging Spectrometer (MODIS; Justice et al., 1998), an instrument orbiting on the NASA TERRA platform. MODIS is designed to measure terrestrial and atmospheric processes. The MAS was a joint project of Daedalus Enterprises, Berkeley Camera Engineering, and Ames Research Center. Sensor/Aircraft Parameters include 50 channels acquired at 16-bit resolution, a IFOV of 2.5 mrad yielding a ground Resolution of 50 meters at 65,000 feet and a swath width of 36 km.
The MODIS/ASTER Airborne Simulator (MASTER) is similar to the MAS, with the thermal bands modified to more closely match the NASA EOS ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer; Kahle et al., 1991; Table 1) satellite instrument. It is used to study geologic and other Earth surface properties. The MASTER sensor/aircraft parameters include 50 channels acquired at 16-bit resolution, a IFOV of 2.5 mrad yielding a ground Resolution of 12-50 meters.
The Airborne Ocean Color Imager (AOCI) is a high altitude multispectral scanner built by Daedalus Enterprises, designed for oceanographic remote sensing. It provides
ANALYTICAL TECHNIQUES IN SPECTROMETRY 5
10-bit digitization of eight bands in the visible/near-infrared region of the sp