SFSI-2: a New Source of Airborne Hyperspectral SWIR Imagery

This paper was presented at the 22nd Annual Canadian Remote Sensing Symposium
Victoria, British Columbia, 21-25 August 2000.

ABSTRACT

Borstad Associates Ltd. has been flying the SFSI-2 (SWIR Full Spectrum Imager) operationally since August 1998 to provide a new source of remotely sensed Short Wave Infra Red imagery. SFSI-2 is a highly modified and updated instrument originally designed and constructed by the Canada Centre for Remote Sensing (CCRS). SFSI-2 operates in the Short Wave Infrared range, from 1230 nm to 2380 nm and after the Borstad modifications, it is smaller, requires less power, and has the ability to simultaneously acquire 230 spectral bands with 5 nm spacing. The instrument utilizes a two-dimensional detector array, refractive optics and a transmission grating with an angular field of view of 34 degrees. Continuous recording routinely produces image files of more than 20 km length. The instrument is small enough to fly in a single engine aircraft, and has often been flown with the Borstad Associates Ltd. casi (Compact Airborne Spectrographic Imager) to provide both VNIR and SWIR data. This paper presents examples of mapped data from the SFSI-2 over a hydrothermally altered target near Cuprite, Nevada in the western USA. It compares spectral signatures produced at altitude by the SFSI-2 with reflectance spectra acquired with ground instruments, and image maps from SFSI-2 with those produced from AVIRIS data.

INTRODUCTION

The use of the Short Wave Infrared part of the spectrum for the remote sensing of minerals dates back to even before the launch of the Landsat Thematic Mapper and before (Goetz and Rowan, 1981) . One of the first imaging spectrometers for remote sensing, the Airborne Imaging Spectrometer (AIS) (Vane and Goetz, 1988), was developed specifically for mineral exploration; it operated in the Short Wave Infrared (SWIR) with 128 contiguous bands. This development was followed by the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) (Vane et al., 1993) which collects data in 224 bands distributed across the Visible and Near Infrared (VNIR) and SWIR ranges. This latter instrument, which is still in operation, has produced extensive data sets, spurring interest in research into imagery of high spectral resolution and high spectral dimensionality. The SWIR Full Spectrum Imager (SFSI) was designed and developed at the Canada Centre for Remote Sensing (Neville et al., 1995) to provide remote sensing researchers with both high spectral and high spatial resolution SWIR imagery for use in developing the methodology and promoting applications in this spectral region. In the design of the instrument, the spectral range 1200 to 2400 nm was selected to include the region 2100 nm to 2400 nm that is of specific interest for mineral identification. SFSI was designed to give a nominal band width of 10.3 nm but we are over sampling this at 5 nm band centers. In the following sections we describe the sensor design, calibration and operation, and some imagery from an airborne mission in Nevada in 1999.

Sensor description

SFSI-2 is a pushbroom line imager and as such uses no moving parts in the imaging process. All spectral bands for all pixels are imaged simultaneously. This is accomplished by using a two-dimensional array in which the spectra are dispersed over the 'vertical' dimension of the array and the across-track line of pixels is imaged onto the 'horizontal' dimension. SFSI-2 employs a two-dimensional platinum silicide Schottky barrier CCD array with 488 rows of 512 detector elements. In operation, a region of 480 lines by 496 columns is used; two adjacent lines are summed together to yield an effective array of 240 by 496 detector elements. This gives 240 spectral bands for each of 496 pixels in the across-track dimension for each integration period, which is variable between 40 and 66 ms. Not all bands are used and the full useful range is 1230 to 2380 nm across 230 bands. Data are digitised to 13 bits and recorded as 16 bit. The optical train is f/1.8 and entirely refractive; the spectral dispersive component is a blazed transmission grating. The across-track pixel instantaneous-field-of-view (IFOV) is 1.133 mrad giving an across-track field-of-view (FOV) of 32.8° The along track IFOV is currently 3.5mrad. This is determined by the slit width, which in the interest of maximizing signal strength has been left at the original setting which gives a nominal spectral band width of 10.3nm. The current system records data at 33.55 MB/sec directly to removable hard disk, and image segments of up to 20 km or 1 Gigabyte are routinely taken. The number of flight lines acquired is only limited by the on-board hard disk space, which currently allows 72 Gigabytes or in excess of 4 hours of recording. Before commencing the acquisition of image data, a shutter is closed and a series of 100 dark reference data frames consisting of the dark signal (minus an electronically introduced residual offset) from each detector in the array is acquired. A dark reference is also acquired at the end of each image file. In the post-flight processing, this dark data is averaged and the resultant frame of offset signals is subtracted from the image data. The net sensor signals are those resulting from the radiance collected from the target.

Sensor calibration

The spectral calibration includes the measurements of the centre wavelengths and the band widths for all the detector array elements. This has been performed using a calibrated black body source and a series of well defined spectral filters. Ideally, each row of detector elements in the array would have the identical band centre wavelength. In reality, there is a variation in centre wavelength with across-track pixel number due to the combination of expected slit curvature (which is present in any spectrograph), small optical distortions, and any mis-orientation of the detector array relative to the spectrograph slit. For SFSI-2 the deviations of the band centre wavelengths from the means for the corresponding detector rows vary, at the extremes, from -8 nm to +9.5nm. The radiometric calibration gives both the relative and absolute responsivity of the 240 by 496 detector elements. A calibrated radiance source was used to present the sensor with a uniform, accurately variable radiance target. The signal for each detector element, corrected for dark offset, divided by the known blackbody radiance at the element's centre wavelength provides the responsivity for each of the elements in the array. The geometric calibration, performed on a precision indexing table, provides the view angle for all pixels and all bands of the sensor. It was found that there is a minor mis-registration of the pixels from band to band caused by a small rotation of the detector array about the optic axis of the instrument (1.5 pxl maximum deviation relative to the centre of the array) combined with approximately 1 pixel deviation caused by the keystone effect. The calibration results are applied to the sensor image data in the first stage of processing, which consists of the removal of sensor artifacts from the data and the conversion from sensor signal to radiance. Following the subtraction of the 'dark' signal, the image data are converted to radiance by dividing by the responsivity matrix. Next, the 2-dimensional spectral-spatial frames of radiance data are remapped to give frames in which every pixel in a given row has the same wavelength. Each frame is then re-sampled to correct for the band mis-registration measured by the geometric calibration.

Airborne data acquisition

The data used in the analysis reported in this paper were collected on 28 September, 1999 over a west to east flight line located about 2 km north of Cuprite, Nevada. This area was chosen because it has been extensively studied and well mapped (Abrams et al, 1977, Clark et al, 1993, Swayze et al, 1998). Cuprite lies in southwest Nevada, along Highway 95, about 40 miles south of Tonapah and 10 miles south of the Goldfield Mining District. The area is named after a ghost town site. This is an arid to desert environment with low scrubby grass, sage and rabbit brush sparsely covering the terrain. The Stonewall Playa (a dry salt flat) lies to the south and east and is used as a reflectance calibration site. Cuprite is a steam-heated, hot-springs, alteration system that has been emplaced in volcanic host rocks. Although there is no known mineralization at Cuprite, the hydrothermal alteration mineral suite is extensive and exhibits excellent zoning, which can easily be detected with airborne sensors. The eastern side of the district contains a distinctive "Bulls Eye" pattern of silica, surrounded by alunite, surrounded by kaolinite. Cuprite has been used as a benchmark site for calibrating many new hyperspectral sensors (Swayze et al, 1998).

Figure 1. A SFSI-2 mean image of the Cuprite Mining District acquired in September 1999. The yellow box shows the area analyzed here.

Ground spectrometer data of the study area were acquired in 1998, and an independently classified mineral map done by the USGS (1998) was also conveniently available on line (speclab.cr.usgs.gov). The coordinates for the center of the area outlined in figure 1 are 37° 32 ' 38 " N and 117°11'2" W. For this mission the sensor was mounted in a Piper Navajo PA-31. The flying altitude was 3300 m above ground level (AGL) at an aircraft speed 75 to 80 m s-1. This gave an along track sampling interval of 4 m and an across-track pixel size of 4 m. Acquisition of the flight line began at 20.40.59 and ended at 20.42.38 UTC, during which time the solar zenith angle was approximately 52°. Motion effects due to aircraft attitude were corrected in post-processing using data from a mechanical gyro, while the orientation of the flight line was determined using an on-board Global Positioning System (GPS) receiver.

Figure 2. Side by side comparison of SFSI-2 and AVIRIS image (Average intensities of Red: 2.10 um, Green: 2.20 um, Blue: 2.35 um).
AVIRIS image taken from ENVI tutorial CD.

Data processing methodology

The SFSI-2 data were transformed to apparent reflectance via a flat field correction using the nearby Stonewall playa. The data set was then processed to obtain pure pixel end-members using standard hyperspectral mapping tools available in the commercial image processing package "Environment for Visualizing Imagery" (ENVI). For this paper, only three spectral end-members - Alunite, Buddingtonite and Kaolinite, were extracted from this process. Using the three spectra collected from 'pure pixel targets' within the image as end-members, Spectral Angle Mapping (SAM) in 2.0 um to 2.4 um range was then applied to the SFSI-2 image. The rule classes from the SAM classification were then thresholded subjectively to produce the final classification map (Figure 3). The final classified image was then compared to the AVIRIS 17-meter resolution mineral map of the area. The corresponding Regions of Interest for both SFSI and AVIRIS are shown in Figure 4. Collected end-members were also compared with the corresponding Jet Propulsion Laboratory library spectra available within ENVI.

Figure 3. SAM classification map derived from SFSI-2 image.  Collected end-member spectra with corresponding JPL library spectra are shown on the right.

Results

A comparison of the classified map produced from the fused SFSI-2 image with that of the AVIRIS mineral map available from the USGS website shows very good spatial correlation for Alunite and Buddingtonite (Figure 4). The Kaolinite regions delineated in the fused SFSI-2 image are observed to be larger than that of the AVIRIS map. This minor disagreement may be due to differences in the way the two images were processed or to differences in spatial resolution. The overall spectral shape and form of the 'flat field reflectance' spectral end-members collected from the SFSI-2 fused image display good correlation with JPL's spectral library (Figure 3), although it is apparent that the JPL library has less spectral resolution than SFSI.

Figure 4. Comparison of Alunite, Buddingtonite, and Kaolinite mineral maps derived from SFSI-2 image (at left and centre) with AVIRIS mineral map from the USGS website.

SUMMARY

Hyperspectral remote sensing data in the SWIR were acquired with the airborne imaging spectrometer SFSI over a site near Cuprite, Nevada in September 1999. This 8 km x 2 km image cube has been corrected for sensor artifacts and has been converted, first to at-sensor radiance, and then to surface reflectance using the Stonewall Playa as a 'flat field'. The SFSI image cube was analysed in the commercial remote sensing package ENVI. 'Pure pixel end-members' were first extracted, and then used in a Spectral Angle Mapper procedure to create abundance maps of these components. These end member spectra were matched to reference spectra [and PIMA ground spectra] also acquired from the image site. Three minerals, alunite, kaolinite and buddingtonite, were identified and mapped. The flat field corrected airborne SFSI-2 spectra closely match the ground-based reference spectra and the mineral maps produced are very similar to those produced independently using the AVIRIS instrument.

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