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The term remote sensing was first used in the United States in the 1960's to refer to studies using aerial photography. Since the launch of the first earth observation satellite in 1972, it is increasingly used to refer to satellite imagery. There are many different definitions of remote sensing, but perhaps the most useful for the applied biologist is that given by the United Nations. They define it as "the sensing of the Earth's surface from space by making use of the properties of electromagnetic waves emitted, reflected or diffracted by the sensed objects, for the purpose of improving natural resources management, land use and the protection of the
The principle of remote sensing is fairly simple. When electromagnetic radiation is incident on the earth's surface, various fractions of the energy are reflected, absorbed or transmitted. The proportions of each will vary for different earth features. Moreover for a given feature type the proportions will vary at different wavelengths so two features that look the same in one wavelength band may look very different in another band. The spectral reflectance of a feature at a particular wavelength is the percentage of the energy of that wavelength reflected of the total incident energy of that wavelength. A graph of the spectral reflectance of an object in relation to wavelength is called a spectral reflectance curve. Although vegetation is particularly reflective to 'near' (nonthermal) infrared radiation, different vegetation types each have their own specific spectral signatures. The condition of vegetation affects its spectral signature - for example vegetation under water stress will appear different from that in good condition. Other features such as water or bare rock also have their own signatures. Remote sensing systems based in space (known as platforms) have sensors which can detect these signatures. The data is transmitted to receiving stations on earth where the information is compiled and analysed.
Systems and sensors
Landsat-1 was the first earth observation satellite in the world, and since then several Landsat's have been launched. Sensors include a multispectral scanner (MS) and a thematic mapper (TM). MSS and TM data are composed in a scene of coverage 185 x 170 km. Each scene is coded with path number and row number, based on what is called the world reference system (WRS). There are 15 Landsat receiving stations in the world from which Landsat data are distributed to users for resource management and environmental monitoring.
SPOT was first launched in February, 1986 by the French Government. Since then a further four SPOT satellites have been launched. SPOT has two high resolution visible imaging system (HRV) sensors . Resolution is better than in the Landsat system - a scene of HRV has a nadir coverage of 60 x 60 km, but an oblique coverage of 81 km square Each scene is coded with column number and row number, termed the SPOT grid reference system (GRS). SPOT data are received at 14 ground receiving stations. The main purpose of data utilization is for resource management and topographic mapping.
The NOAA satellite series are the third generation of meteorological satellites operated by the National Oceanic and Atmospheric Administration (NOAA) in the USA. Two NOAA satellites are in operation at any one time. A NOAA satellite can observe the same area twice a day (day and night), so that the two satellite can cover the same area four times a day. The major sensors are the advanced very high resolution radiometer (model 2) (AVHRR/2), and the TIROS operational vertical sounder (TOVS). NOAA has edited global cloud free mosaics in the form of a global vegetation index (GVI) on a weekly basis since April 1982. The GVI data include information on the normalized difference vegetation index (NDVI) which is an indicator of the amount of biomass. The larger the NDVI, the denser the vegetation. Although the original resolution of is 1.1 km per pixel at the Equator, the GVI has a lower resolution of 16 km x 16 km per pixel. NDVI has been used extensively in epidemiological and ecological research.
Geostationary meteorological satellites are launched under the World Weather Watch project organized by the World Meteorological Organization. The earth is covered with five satellites, namely METEOSAT (ESA), INSAT (India), GMS (Japan), GOES-E (USA) and GOES-W (USA). The image data are transmitted to the ground station in both a high resolution and a low resolution mode. Data such as cloud volumes and sea surface temperatures are recorded in the archives.
In recent years a number of commercial earth observation satellites have been launched. DigitalGlobe, USA is a commercial company which gathers and sells remote sensing data; its main satellites are Quickbird (launched in 2001) WorldView1-1 launched in 2007 and WorldView-2 due for launch in 2009. Google Earth and Google Maps obtain much of their high resolution imagery from DigitalGlobe. GeoEye USA is the world's largest space imaging corporation and operates a number of satellites (Ikonos, Orbview and GeoEye) launched between 1999 and the present day. It provides satellite map images to Microsoft, Yahoo and Google.