Visible sensors

The relevance of visible and infra-red imaging instruments is primarily for mapping the extent and, to a lesser degree, the characteristics of ice on the planet. Because of the relatively long time series provided by some of these instruments they are particularly valuable for determining variations in areal extent of glaciers, ice caps and sea ice. Surface velocities of land ice have also been obtained using 'feature tracking' methods (section 73.3.4). Given below are some brief details pertaining to the instruments that have been used most extensively for cryospheric monitoring applications.

The Landsat series of satellites has been providing visible and near infra-red (IR) imagery of the Earth's surface (up to a latitudinal limit of ca. ±82.5°) since 1972 and is, at present, the most ubiquitous satellite in Earth resource applications. It merits, therefore, some space here. The original instrument was known as the multispectral scanner (MSS) and had a resolution of 79 m and a repeat cycle of 18 days. The MSS was superseded by the thematic mapper (TM), which has a resolution of 30m, improved dynamic range in digitization (0-255) and seven channels in the visible, near and thermal infra-red (Table 73.1).

Landsat 7 was launched in April 1999 with the enhanced thematic mapper plus (ETM+) onboard. The ETM+ includes a number of new features that make it a more versatile and effective instrument compared with its predecessors. The primary new features on Landsat 7 are a panchromatic band with 15 m spatial resolution, on board, full aperture, 5% absolute radiometric calibration and a thermal IR channel with 60 m spatial resolution (Lillesand & Kiefer, 2000). A preliminary assessment of the glaciological utility of the ETM+ was undertaken shortly after the data became available (Bindschadler et al., 2001b). A summary of the principal characteristics of the ETM+ is given in Table 73.1.

Figure 73.1 A full 60 km ASTER image covering part of an icecap, Austfonna, on the island of Nordaustlandet in the Svalbard archipelago, Norwegian Arctic. The three visible channels were combined to produce a colour composite image. The scene was acquired in June 2001.

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