Imaging spectrometers

Two recent datasets produced by imaging spectrometers will particularly drive the theoretical, numerical and software developments of our project.

The first data set under consideration (mostly hyperspectral images with vertical pointing) comes from the Mars-Express Mission operated by the European Space Agency. The second data set (multi-angular hyperspectral images - HMA) is generated by the CRISM instrument of the Mars Reconnaissance Orbiter (NASA) that started its scientific activities in November 2006 for a nominal duration of two years.

The OMEGA imaging spectrometer [Bibring 2004] has been mapping the Martian surface with a spatial resolution varying between 300 to 3000 meters depending on spacecraft altitude. It acquires for each resolved pixel the spectrum from 0.36 to 5.2 µm in 352 contiguous spectral channels (spectels). Combining imagery and spectrometry, OMEGA is designed to provide the mineralogical and molecular composition of the surface and atmosphere of Mars through the spectral analysis of the re-diffused solar light and surface thermal emission. OMEGA provides a global coverage at medium resolution (1 to 5 km) of the entire surface of Mars as well as snapshots of selected areas with a resolution of a few hundreds meters. OMEGA allows to:

  • characterize the composition of surface materials, discriminating between various classes of silicates, hydrated minerals, oxides and carbonates, organic frosts and ices.
  • study the time and space distribution of atmospheric CO2 and H2O both in the atmosphere as gases and at the surface as icy deposits.
  • identify the aerosols and dust particles in the atmosphere, and observe their time and space distributions.

In March 2007 the OMEGA collection was worth 500 Gbytes of raw images not accounting for auxiliary data and derived products.

The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) [Murchie 2004] on the MRO (Mars Reconnaissance Orbiter) spacecraft is the first HMA imager to operate from deep space. After a successful orbit insertion that occurred mid-March 2006, MRO started its main observational campaign in November 2006. CRISM spectral range spans the ultraviolet (UV) to the mid-wave infrared (MWIR), 383 nm to 3960 nm, with a 6.6 nm spectral spacing. CRISM acquires three major types of observations of the Martian surface and atmosphere, one being HMA 10 km wide images at 18 meters per pixel of specifically targeted sites. CRISM/MRO spectrally scanning the martian surface and atmosphere (Courtesy of the CRISM team JHU/APL)
The objectives of CRISM are to:

  • search for evidence of aqueous and/or hydrothermal activity, and to map and characterize the composition, geology, and stratigraphy of surface deposits.
  • characterize seasonal variations in dust and ice aerosols and water content of surface materials.

How do imaging spectrometers take an image ?

(Courtesy of the CRISM team JHU/APL) One two-dimensional frame of hyperspectral data, outlined in white in the upper left panel, is a single line of a spatial image. Indeed imaging spectrometers take an image one line (CRISM) or even one pixel (OMEGA NIR) at a time. But each pixel along the line has a spectrum (the decomposition of solar light reflected or emitted by the atmosphere and/or surface of Mars) that fills out the second dimension of the frame. A two-dimensional spatial image of a target is built up by taking successive data frames as the spectrometer field of view is swept across a target, either by scanning an instrument gimbal or by an orbiter along-track motion over the Martian surface. The stack of resulting data frames (shown in the upper left panel) is a multiband image, or “image cube.” The lower right panel shows the spectrum for a single spatial pixel from the center panel. The absorptions present in the spectrum are distinctive of different materials (minerals, ices, etc.) and also bear the imprints of their physical and structural properties.

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CRISM's Targeted Observations

(Courtesy of the CRISM team JHU/APL) To take high-resolution observations, CRISM operates in targeted mode. The instrument's gimbal - basically, a scan platform - is scanned to compensate for motion of the spacecraft and to cover a region approximately 10 km x 10 km at about 18 meters per pixel, in 544 channels covering 0.36-3.92 microns. Ten additional abbreviated, spatially binned images are taken before and after the main image. In time sequence, as a target is approached, five short scans across it are performed during which hyperspectral data are taken spatially binned to conserve data volume (purple). Then, centered on the time of target overflight, a slow scan across the target is performed with minimal or no spatial binning (green). Finally, five additional short scans are performed as the target is departed (red). The result is 11 images, the 6th of which is at high spatial resolution. Projected onto a map, the footprints of the 11 images overlap.

This sequence of multiple measurements of the same target at different geometries - over a short time while the illumination is constant - is called an emission phase function. It provides measurements to study principally the atmosphere and to correct surface spectra of the central high resolution observation for atmospheric effects. Once corrected for the atmospheric effects the EPF phase function, which can be built at low spatial resolution (~ 180 meters) for a network of points spread across the scene, may provide with good precision the variation of surface reflectance according to geometry.

 LPG, after data acquired by CRISM/MRO JHU/APLLeft: Superimposed onto the high resolution control image, the network of points (colored crosses) for which an EPF curve can be built
 LPG, after data acquired by CRISM/MRO JHU/APL
Right: Variation of surface-atmosphere reflectance according to phase angle for a given point in the image and at a wavelength of 2.6 microns

A supplementary dimension coming in addition to the traditional spectral (one) and spatial (two) dimensions is thus provided. Data are hyperspectral and multi-angular, i.e. HMA.
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imaging_spectrometers.txt · Last modified: 2011/12/06 13:48 by leaumerc
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