A procedure has been developed to measure the spatial mis-registration of the bands of imaging spectrometers using data acquired by the sensor in flight. This is done for each across-track pixel and for all bands, thus allowing the measurement of the instrument's 'keystone' and related inter-band spatial shifts. The procedure uses spatial features present in the scene. The inter-band spatial relationship determinations are made by correlating these features as detected by the various bands. Measurements have been made for a number of instruments including the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS), Hyperion, Compact Airborne Spectrographic Imager (<i>casi</i>), SWIR (Short Wave Infra-Red) Full Spectrum Imager (SFSI), and Aurora. The measurements on AVIRIS data were performed as a test of the procedure; since AVIRIS is a whisk-broom scanner it is expected to be free of keystone. The airborne Aurora, <i>casi</i>, and SFSI and the satellite sensor Hyperion are all pushbroom instruments, exhibiting varying degrees of keystone. The potential impact of keystone upon spectral similarity measures is examined.
A procedure has been developed to measure the band-centers and bandwidths for imaging spectrometers using data acquired by the sensor in flight. This is done for each across-track pixel, thus allowing the measurement of the instrument's slit curvature or spectral 'smile'. The procedure uses spectral features present in the at-sensor radiance which are common to all pixels in the scene. These are principally atmospheric absorption lines. The band-center and bandwidth determinations are made by correlating the sensor measured radiance with a modelled radiance, the latter calculated using MODTRAN 4.2. Measurements have been made for a number of instruments including Airborne Visible and Infra-Red Imaging Spectrometer (AVIRIS), SWIR Full Spectrum Imager (SFSI), and Hyperion. The measurements on AVIRIS data were performed as a test of the procedure; since AVIRIS is a whisk-broom scanner it is expected to be free of spectral smile. SFSI is an airborne pushbroom instrument with considerable spectral smile. Hyperion is a satellite pushbroom sensor with a relatively small degree of smile. Measurements of Hyperion were made using three different data sets to check for temporal variations.
Hyperspectral image data sets acquired near Cuprite, Nevada in 1995 with the SWIR full spectrum imager (SFSI) and in 1996 with the Airborne Visible/IR Imaging Spectrometer (AVIRIS) are analyzed with a spectral unmixing procedure and the result compared. The SFSI image has pixels on 1 m by 1.5 m centers, the AVIRIS on 17 m centers; the region imaged by SFSI is a small portion of the full AVIRIS scene. Both have nominal spectral band center spacings of about 10 nm. The image data, converted to radiance units, are atmospherically corrected and converted to surface reflectance. Spectral end members are extracted automatically from the two data sets; those representing mineral species common to both are compared to each other and to reference spectra obtained with a portable IR mineral analyzer. The full sets of end members are used in a constrained linear unmixing of the respective hyperspectral image cubes. The resulting unmixing fraction images derived from the AVIRIS and the SFSI data sets for the minerals alunite, buddingtonite, and kaolinite exhibit strong similarities.
The SWIR full spectrum imager (SFSI), an imaging spectrometer covering the short-wave IR (SWIR) from 1220 to 2420 nm, has been developed at the Canada Centre for Remote Sensing for use on an airborne platform. The sensor has ben designed to acquire simultaneously a full spectrum resolution and a full image swath at high spatial resolution. The sensor was test flown in Nevada in June 1995. Data from this mission are analyzed on the Imaging Spectrometer Data Analysis System. A look-up-table driven atmospheric correction procedure is used to retrieve surface reflectances. An image cube of a site near Virginia City is processed via spectral unmixing using reflectance spectra extracted from the image at ground sample sites and spectra of mineral samples acquired by a ground-based field spectrometer. The resulting end member abundance maps indicate that SFSI data can be analyzed successfully in this way for mineral identification.
The SWIR full spectrum imager is an imaging spectrometer covering the short-wave infrared from 1220 to 2420 nm, which has been developed for remote sensing from an airborne platform. The sensor has been designed to acquire the full spectrum at high spectral resolution and the full image swath at high spatial resolution simultaneously. The instrument utilizes a 2D detector array, refractive optics and a transmission grating. The fore-optics and spectrograph are f/1.8, and the angular field-of-view is 9.4 degrees. The detector is a 488 line by 512 pixel PtSi Schottky barrier photodiode array. A VME bus computer communicates with the array controller, performs the data acquisition and provides the operator interface. The optical design and sensor system are described: calibration methods and results are presented. Post flight data processing procedures are described and the spectral signal-to-noise ratio is calculated from in-flight data. A sample single-band image from data collected on the JUne 1995 Nevada mission is displayed, spectra of minerals and trees are extracted, and a classification of this image is shown.
The SWIR full spectrum imager (SFSI) is a hyperspectral push-broom imager, acquiring imagery in 120 0.010 micrometers wide bands simultaneously covering the 1.20 micrometers to 2.47 micrometers spectral region. During the first flights of the instrument hyperspectral imagery was acquired over a calcite quarry and a dolomite quarry. Both these minerals show distinctive carbonate absorption features in the 1.7 to 2.5 micrometers region. These absorption features are centered at wavelengths approximately 0.1 micrometers shorter in dolomite than in calcite. The two minerals are clearly distinguished using a single Gaussian fit to the prominent carbonate absorption feature near approximately 2.3 micrometers in the quarry reflectance data from the SFSI test flights. The spectral resolution of the airborne spectra also allowed the 2.3 micrometers absorption feature to be resolved into tow closely spaced features. Both these absorption features show the mineralogical shift in band center. This has also been seen in laboratory spectra and a detailed comparison with this data is made.
The SWIR full spectrum imager (SFSI) is an imaging spectrometer, covering the short-wave infrared (SWIR) from 1200 to 2400 nm, which has been developed for remote sensing from an airborne platform. The sensor has been designed to acquire the full spectrum at high spectral resolution (10 nm) and the full image swath at high spatial resolution (50 cm) simultaneously. The instrument utilizes a platinum silicide (PtSi) detector array, refractive optics, and a transmission grating. A VME bus computer communicates with the array controller, performs the data acquisition, and provides the operator interface. The camera and data acquisition subsystems have been completed and test flown. The fore-optics, spectrograph, and sensor housing have been fabricated. Integration of the camera, spectrograph, and auxiliary components is scheduled for July 1994 followed by laboratory testing and calibration. Our goal is to obtain pilot project data by the end of autumn 1994. Here we describe the optical design, the sensor system, early test flight image data, and expected sensor performance based on laboratory testing. The objectives and procedures for the spectral, geometric, and radiometric calibration of this sensor are also discussed.