Imaging spectroscopy is the acquisition of images wherein for each spatial-resolution element of an image, a continuous spectrum of the energy arriving at the sensor is measured. These spectra are used to derive information based on the signature of the interaction of matter and energy expressed in the spectrum. Spectroscopy has been used in the laboratory and astronomical observatory for more than a hundred years. The AVIRIS sensor is the prototype imaging spectrometer for remote sensing.
AVIRIS
AVIRIS is an acronym for airborne visible-infrared-imaging spectrometer. AVIRIS is a world-class instrument in the realm of Earth remote sensing, a unique optical sensor that delivers calibrated images of the upwelling spectral radiance in 224 contiguous spectral channels (also called bands), with wavelengths from 380 to 2500 nm. The instrument was designed to fly aboard a NASA ER-2 plane (a U2 modified for increased performance) at approximately 20 km above sea level, at about 730 km/hr.
The AVIRIS instrument contains 224 detectors, each with a wavelength-sensitive range (also known as spectral bandwidth) of approximately 10 nm, allowing it to cover the range between 380 nm and 2500 nm. Plotted on a graph, the data from each detector yields a spectrum that, compared with the spectra of known substances, reveals the composition of the area under surveillance.
Figure 5.15 AVIRIS line diagram. AVIRIS uses silicon (Si) detectors for the visible range and indium-antimonide (InSb) for the near infrared, cooled by liquid nitrogen. The sensor has a 30° total field of view (full 614 samples) and one milliradian instantaneous field of view(IFOV, one sample), calibrated to within 0.1 mrad. Dynamic range has varied; 10-bit data encoding was used through 1994, 12-bit data have been read out since 1995.
AVIRIS uses a scanning mirror to sweep whiskbroom fashion, producing 614 pixels for the 224 detectors on each scan. An individual pixel produced by the instrument flown at an altitude of 20 km covers an approximately 20-m square area on the ground (with some overlap between pixels), yielding a ground swath about 11 km wide.16
Hyperion
The first major VNIR/SWIR hyper-spectral sensor to fly in space was the Hyperion sensor, on the NASA EO-1 platform. EO-1 is a test bed for Earth-resources instruments, launched in conjunction with Landsat 7 and designed to test follow-on technology for NASA systems. EO-1/SAC-C was launched November 21, 2000, from VAFB in a 705-km orbit, trailing just after Landsat 7. The Hyperion sensor is the TRW-built cousin to the payload of NASA’s ill-fated Lewis satellite effort.
Hyperion offers 30-m spatial resolution covering a 7.5-km swath. The 0.42.5 ^m spectral range is analyzed at 10-nm spectral resolution (220 bands). Figure 5.17 contains a nice illustration of the spectral nature of the sensor and an unusual look at the beginnings of a blackbody curve for the hot lava of Mount Etna, glowing at some 800-1000 K.
Figure 5.16 This image from AVIRIS shows elements of a scene acquired on August 20, 1992. The mission was flown on a NASA ER-2 plane at an altitude of 20,000 m (65,000 ft) over Moffett Field, California, at the southern end of the San Francisco Bay. The image is roughly true color. Blue: 458.46 nm; green: 557.07 nm; red: 655.84 nm (bands 10,20, and 30). The red on the east side of the bay is correct, due to the presence of one-cm-long red brine shrimp in the evaporation pond.
Figure 5.17 Mount Etna. Hyperion offers 12-bit dynamic range.
The lava curve (brown in the line plot) shows a spectrum that rises above the intensity of reflected sunlight beginning at about 1600 nm, and appears to be peak around 2.4 ^m. By contrast, a vegetation signature (the green curve), shows the IR ledge expected of healthy vegetation at 700 nm. The small arrows along the bottom of the curve, at 1234, 1639, and 2226 nm, indicate the spectral bands used to construct the image on the right, coded as blue, green, and red, respectively.
MightySat II—FTHSI18
The first hyperspectral sensor to fly on a satellite was the Fourier transform spectrometer flown by AFRL on the USAF MightySat-II. It was launched from Vandenberg Air Force Base into a 550-km polar orbit by a Minotaur rocket on 19 July 2000. The Fourier transform hyper-spectral imager (FTHSI) can produce 150 narrowband images in the 0.45-1.05-^m band. Image quality was modest—but it was the first. Air Force security games prevented anyone from seeing much of the data.19
From a space-systems design effort, the most interesting aspect of the sensor was the use of commercial, off-the-shelf parts. No radiation-hardened parts were used, though some additional shielding was placed around critical components. The sensor operated nominally until the satellite was shut down.20
Figure 5.18 RGB of first scene, taken near Keenesburg, CO. The data are presented as a false-color IR image—red regions are areas of vegetation.
