Introduction - Hyperspectral Image Fusion - page 7

Table 1.1 Specifications of AVIRIS system
Year of availability 1987

Number of bands 224

Spectral range 0 . 4-2 . 5 µ m

Bandwidth at FWHM 9.4-16 nm

Spatial resolution 4-20 m

Scan line width 1.9-11 km

Source http://aviris.jpl.nasa.gov

resolution where the width of the HS image ( X -axis) is determined by the sensor, and
the length ( Y -axis) depends on the distance covered by the aircraft. Although airborne
hyperspectral systems have been operated since 80s, the significant milestone was
the development of the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS)
designed by the Jet Propulsion Laboratory (JPL) of NASA [10]. The objective was
to measure the upwelling spectral radiance from those areas of the earth where the
solar reflected energy was dominant. The AVIRIS produced the first set of images
in 1987 becoming the first earth-looking spectrometer covering the entire spectral
range from 0

m. The spectral resolution of hyperspectral systems is often
reported in terms of full width at half maximum (FWHM) which is related to the
width of an approximated spectral channel. For the AVIRIS sensor, this measure
varies between 9.4 and 16 nm. This airborne sensor acquires the data in the form of
224 contiguous bands at 10 nm intervals. With 614 pixels per line and a spatial res-
olution of 20 m, it covers an 11 km wide region. These specifications of the AVIRIS
have been summarized in Table 1.1 . The AVIRIS applications range from geology,
hydrology, environmental monitoring, as well as land and ocean ecology.
Some other airborne hyperspectral imaging systems include Compact Airborne
Spectrographic Imager (CASI) by the Canadian company ITRES. The CASI has a
programmable spectral band filters where one can obtain up to 288 spectral bands.
CASI has been used for the GIS applications, and also for the vegetation analysis.
Naval Research Laboratory developed the HYDICE for civic applications such as
agriculture, geology, and ocean studies. The HYDICE, an acronym for the Hyper-
spectral Digital Imagery Collection Experiment, was one of the early airborne instru-
ments with a high spatial resolution achieved by flying at a relatively low altitude.
The HyVista corporation of Australia designed the HyMAP imaging spectrometer
which covers the spectral range from 0

. 4to2 . 5 µ

m in 126 contiguous bands. The
bandwidth of each channel is between 15 and 20 nm. The HyMAP is designed for
the purpose of mineral exploration.
While airborne sensors have been in demand, the spaceborne sensors provide
several advantages over the sensors from the former category, including:

. 4to2 . 5 µ

￿ Coverage of wider areas on the earth.

￿ Repeated (multi-temporal) data collection.

￿ Reduction in distortion due to sensor motion.