REMOTE sensing is the science of studying an area or object without actually coming into contact with it. All objects on the earth emit electromagnetic energy. This energy includes not only visible light but also other radiations in the electromagnetic spectrum. The portion that is visible to the human eye is confined to wavelengths ranging from 0.4 micrometre (blue) to 0.7 micrometre (red). Just beyond the red end of the this band is the infrared region, which is subdivided into the near infrared reflected by the earth's surface and the thermal infrared emitted by objects.

Scientists have developed sensors and cameras which can detect and record this energy transmitted from the surface of the earth. These cameras and sensors are placed aboard earth-orbiting satellites that record and transmit the data to receiving stations on the earth in digital form. Technologists use computers and other devices to produce images that can be analysed and interpreted to obtain meaningful information on conditions on the earth.

For instance, water bodies turn up as dark blue or black in satellite imagery because of the low level of energy that they emit. Shallow water bodies are usually found in different shades of blue depending on the turbidity. Green vegetation shows up as red or deep purple because of the photosynthetic activities of plants.

The atmosphere -- the medium through which electromagnetic waves travel -- transmits, absorbs and scatters energy, but the effect of scattering is greater in the visible and infrared wavelengths. Those portions of electromagnetic spectrum where the atmosphere is transparent are called atmospheric windows or bands. The portions between atmospheric windows are called absorption bands. Water vapour and carbon dioxide absorb infrared from some portions of the spectrum. Shorter wavelengths of energy such as X-rays are absorbed by the ozone layer in the atmosphere.

Developments in film and lens technology have increased the satellites' Space satellites are equipped with sensors that measure both visible information as well as variations in the temperature of objects on earth. The spectral resolution of a camera or sensor refers to the range of spectral bands sensed.

Satellites are equipped with sensors that measure both visible information as well as variations in the temperature of objects on earth. The spectral resolution of a camera or sensor refers to the range of spectral bands sensed.

The LANDSAT-4 and LANDSAT-5 satellites have a Thematic Mapper (TM) that records data in seven bands of the spectrum of which three are in the visible range. Data from each of the seven TM bands can be viewed separately as black-and-white images, with varying tones of grey showing energy intensity. TM has higher resolution and accuracy in comparison with MSS.

Because clouds prevent clear imaging in visual and infrared bands, imaging radars have to be used. Radars that have high resolution are side looking, which means they direct a narrow beam of energy at right angles to the flight path of their antenna.

The resolution, or resolving power of a sensor or a camera, refers to the smallest meaningful element detectable in a scene and it is usually measured in metres. A scanner on board an aircraft can resolve or distinguish a one-metre object or feature on the ground from an altitude of 1,000 m, while a satellite sensor at about 750 km can resolve a 20-m object or feature on the ground. Over the years the spatial resolution of newer generation satellites has improved from 80 m to 10 m.

Remote sensing satellites are set in repetitive, circular, sun-synchronous, near-polar orbits at an altitude of 700-900 km. Satellites travel from the southern to the northern hemisphere in an east-west direction but because of the earth's equatorial bulge, their orbital plane always makes the same angle to the sun-earth line. This alignment ensures that a satellite passes over a point at always the same local mean solar time. This enables comparisons of images of a given location, acquired on different days.