Polar-Orbiting Satellites

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.




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