Remote sensing of ocean colour from space began in 1978 with the successful launch of NASA’s Coastal Zone Color Scanner (CZCS). Despite the fact that CZCS was an experimental mission intended to last only one year, the sensor continued to generate a valuable time-series of data over selected test sites until early 1986. Ten years passed before other sources of ocean-colour data became available with the launch of MOS, OCTS and POLDER in 1996, and SeaWiFS in 1997. Several new ocean-colour sensors have recently been launched and still more are planned for the near future by various space agencies. You can consult the CEOS Missions, Instruments, and Measurements (MIM) database to see an updated list of all the Earth observation programmes and plans of the world’s civil space agencies.
There are two types of orbits for Earth observation satellites, polar orbiting and geostationary. Polar-orbiting satellites typically operate at an altitude of around 800 km, with a revisit time of 2-3 days, whereas geostationary satellites operate in time scales of hours, which could theoretically provide data on the diurnal variation in phytoplankton abundance and productivity.
Polar-orbiting sensors can be divided into two groups: dedicated polar orbiting sensors with a resolution greater than 500m (medium resolution), and hyperspectral sensors with a much finer resolution.
Polar orbiting satellites follow an overhead path around the Earth so that they pass close to the North and South Poles. They orbit the Earth as it turns beneath them. By so doing, a different part of the Earth’s surface is viewed on each successive orbit. In this way the entire Earth is covered, with a small amount of overlap at low latitudes, and a greater overlap at higher latitudes. Satellite orbits can be controlled so that they view the same latitude on the Earth’s surface at the same local sun time (referred to as sun-synchronous).
The altitude of the satellite affects the detail. At higher altitudes the satellite visits the same place more frequently, but the information obtained is less detailed. At lower altitudes the satellite visits less frequently, but more detailed information is acquired. Most ocean-colour sensors are on satellites at an altitude of around 700-800 km.
Hyperspectral sensors offer an improvement over existing sensors in that they have hundreds of wavebands and can detect many different wavelengths, including visible, reflected near infrared, short wave infrared and sometimes thermal infrared. Hyperspectral Imaging (HSI) is a passive technique (i.e., it depends upon the sun or some other independent illumination source). HSI creates a larger number of images from contiguous, rather than disjoint, regions of the spectrum, and usually with much finer resolution. The increased sampling of the spectrum yields greater information. Many remote sensing tasks which are currently impractical or impossible with broad-band remote-sensing systems will be accomplished with HSI. New algorithms will allow more precise identification of sub-pixel components, leading to more accurate data.
Geostationarysatellites which are placed in orbit 36,000 km above the equator, moving in the same direction that the Earth is turning, give the impression that they are stationary and remain over its sub-satellite point, the point where an imaginary line joining the satellite and the centre of the Earth intersects with the Earth’s surface. Satellites in a geostationary orbit continuously view the same portion of the surface of the earth. A large portion of the earth’s surface can be viewed in this manner, and images can be obtained more frequently. Such an instrument could provide high quality observations for a single study area with an image frequency of approximately 30 minutes.
The principal applications would be to:
- determine the effects of storms and tidal mixing on phytoplankton populations,
- monitor biotic and abiotic material in river plumes and tidal fronts,
- track hazardous materials (e.g. oil spills and noxious algal blooms).
This type of instrument would not provide routine global coverage as is possible from polar-orbiting, sun-synchronous satellites. However, a single imager on a geostationary satellite could provide multiple views during a single day of many locations.
South Korea’s (GOCI) instrument on board the COMS-1 satellite, which was launched on 26 June 2010, is the first ocean-colour sensor in a geostationary orbit.