Remote Sensing Instruments: Sensor Types Relevant to Wetlands
A wide range of sensors operate in the spectral region of the electromagnetic spectrum where they provide information that allows characterization, mapping, and monitoring of wetlands. Key among these are the Landsat sensors and Moderate Resolution Imaging Spectroradiometer (MODIS) . The Landsat series of sensors have been the Multispectral Scanner System (MSS), Thematic Mapper (TM), Enhanced Thematic Mapper (ETM+), and the Operational Land Imager (OLI). The Landsat MSS was launched on July 23, 1972 and provided the first moderate resolution global observations of the world’s surface in the visible and near infrared regions. Three more MSS sensors (Landsat 2-4) followed. While the radiometric range was low, these data provide the oldest and most detailed record of the extent of wetlands. First launched on 16 July 1982, the TM operated in six spectral bands and one thermal. The Landsat ETM was launched in 1999 and again observed in six spectral bands (visible to shortwave infrared) with an additional panchromatic band as well as a thermal band. Following the failure of the Landsat-6 at launch on October 5, 1993, users were dependent upon the Landsat-5 and 7 and observations were compromised by a partial failure of the latter. However, as part of the Landsat Continuity Mission (LCM), the Landsat 8 OLI was launched on 11 February 2013 and operated in the same wavelength regions as the ETM+. In addition to the bands used in the previous TM and ETM+ missions, the coastal blue channel was included as well as a cirrus channel (1.36–1.38 μm), with the latter facilitating better atmospheric correction because of sensitivity to atmospheric constituents. A particular advantage of the Landsat series is that a long historical record of the extent and characteristics of wetlands globally is provided, although observations have been compromised by the relatively low temporal frequency (every 16 days with this increasing when more than two sensors are operating) and cloud and cloud shadow in many regions.
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