Optical Remote Sensing of the Hydrosphere: From the Open Ocean to Inland Waters

The History of Satellite Ocean Colour

The first spaceborne ocean colour images were obtained through hand-held cameras on the manned space missions in the 1960s and they clearly demonstrated the potential of satellites for monitoring SPM along the coasts, and the colour variation due to phytoplankton (e.g., Badgley and Childs 1969). The first quantitative work was undertaken by Clarke et al. (1970) who used an airborne spec-troradiometer to record the spectral variation of water-leaving radiance above waters with different phytoplankton concentrations. The observations demonstrated a change in the water-leaving radiance spectra with increasing chlorophyll-a concentrations (chl), and that the atmosphere had a notable effect, namely that scattering increased as measurements were recorded at increasing altitude.

Satellite monitoring of the coastal zone was made possible by the Earth Resources Technology Satellite (later renamed Landsat, with Landsat-1 launched in July 1972; Goward et al., in this volume), but the first dedicated marine sensor was the Coastal Zone Color Scanner (CZCS) launched in October 1978 onboard the Nimbus-7 satellite. Ocean colour satellites have generally been launched into polar orbits (following a path that passes close to the North and South Poles) so that almost the entire Earth is covered, with a small amount of overlap at low latitudes and a greater overlap at higher altitudes, as different parts of the Earth's surface are viewed during successive orbits.

CZCS was a proof of concept sensor with a number of design specifications and objectives that included defining requirements for future ocean colour instruments. The sensor recorded the upwelling visible radiance in four wavebands (Table 27.1) with a broad NIR waveband to differentiate sea from land and cloud, and a thermal infra-red band (10.5–12.5 μm) to detect Sea Surface Temperature (SST) variability. CZCS also possessed a tilting mechanism, which pointed the scan plane up to 20 degrees forward or aft of nadir along track, which helped the sensor avoid sunglint (sunlight reflected from the sea-surface without interacting with the components within the water). Finally, the detectors had four gain settings to allow observations at a range of radiance levels and hence sun angles. However, CZCS was operated on an intermittent schedule (due to power demands), the sensor lost the thermal infrared waveband within the first year, and the other wavebands began degrading from 1981 onwards, although they remained useable to some degree until 1984.

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