Page 93 - Digital Analysis of Remotely Sensed Imagery

64 Cha pte r T w o

Hyperion data are processed to various levels before they are
released to the public. Level 0 processing includes removal of
transmission artifacts and reordering of data formats. VNIR and
SWIR data are merged to form a single raw image file, together
with the flight information and ancillary data. Level 1R data have
been radiometrically calibrated based on coefficients derived from
both laboratory and on-orbit calibration, but not corrected for
geometric distortions. Level 1Gst data have been terrain corrected
and are available in 16-bits. Standard datasets, which include image
as well as metadata and ancillary information, may be purchased
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in units of 20 7.5 km in the HDF. Hyperion data are priced from
$250 per scene for archived level 1R data, rising to $500 for level
1Gst data. An addition cost of $750 is charged for data acquired
upon the user’s request.
Hyperion data are ideally applied in fields in which the subtle
spectral variations among ground targets at some wavelengths need
to be identified and differentiated. Usually, such identification and
differentiation are almost impossible with standard multispectral
data, such as mapping of soil salinity, accurate mineral exploration,
better predictions and assessments of crop yield, and better
containment mapping.
2.5.2 AVIRIS
AVIRIS is a further development from the prototype, the Airborne
Imaging Spectrometer (AIS). It was designed and constructed by the
Jet Propulsion Laboratory in Pasadena, California, under contract to
NASA. This unique optical sensor captures solar radiant energy over
the wavelength range of 0.4 to 2.5 μm in 224 contiguous spectral
bands. The wavelength range of each spectral band is programmable.
Quantized to 12 bits, AVIRIS data are radiometrically calibrated to
<10 percent absolute value.
AVIRIS uses “whiskbroom” scanning to sweep the ground back
and forth, producing 614 pixels for the 224 detectors per scan. The
FOV and IFOV of AVIRIS imagery are fixed at 30° and 1 milliradian,
respectively (JPL, 2007). These two parameters translate into a varying
swath width and pixel size, depending upon the flying height of the
aircraft in which the sensor is aboard. If flown at approximately 20 km
above sea level, each AVIRIS pixel corresponds to a ground area of
approximately 20 20 m (with some overlap between pixels), thus
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yielding a ground swath width of about 11 km (Table 2.26). At a flight
height of 4 km above the ground, each pixel covers a ground area of
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4 4 m at a swath width of 2 km (Vane and Goetz, 1993). Since
AVIRIS data are airborne, not all areas of the Earth’s surface have
been sensed yet. The areas flown so far were decided by NASA on the
basis of their scientific merits. Of the flown areas, their archived data
are not routinely available to the public, which is the same as with
other commercial satellite data.