How does satellite remote sensing work? Satellite image of tea growing area of Azerbaijan The data for the Azerbaijan project were collected by Landsat 5, a satellite launched by the US National Aeronautic and Space Administration (NASA) in March 1984. Landsat 5 was positioned around 700 km above the earth's surface in a near-polar, sun-synchronous orbit. This means that it circles the planet in an almost north-south direction, while the earth rotates from west to east. Landsat 5's speed is synchronized to the speed of the earth's rotation in such a way that the local sun time in the area beneath the satellite is always the same, about 9:30 a.m. - essential for data interpretation. Landsat 5 passes over the same spot every 16 days. On board Landsat 5 is a sensor called a thematic mapper. The earth's rotation below the satellite allows this sensor to scan a new area of the planet's surface with each consecutive pass. The width of the area scanned, called the swath, is 185 km. A full scene is defined as 185 km x 185 km, an area of approximately 34 000 km2, or roughly twice the size of Kuwait. The thematic mapper detects the solar energy reflected off the earth's features as well as the earth's own thermal energy on seven well-defined portions (bands) of the electromagnetic spectrum. For the bands sensing reflected light, the sensor can distinguish spatial features at a resolution of 30 m &endash; meaning features 30 m in size are identifiable. The thematic mapper's thermal channel, however, identifies features at the much lower resolution of 120 m. The energy detected by the sensor is recorded electronically, not on photographic film. The images produced by remote sensing are not photographs. Because various features on the earth's surface interact with and reflect solar energy differently in the various bands, the thematic mapper can be used to identify clearly a broad range of elements of land cover. For example, healthy green leaves are excellent reflectors of near-infrared wavelengths. The thematic mapper's fourth band detects reflected energy in this range, so it is particularly useful for identifying and mapping vegetation in a given area. Data from the thematic mapper are transmitted to stations on the ground. By international agreements, this information must be made available to anyone who requests it. It comes at a cost, however. The going rate for a full scene (34 000 km2) is US$4 400, which works out to approximately 13 cents per km2. Discounts are available for bulk purchases or for older inventory. For the Azerbaijan project, the satellite data were purchased in a digital format on a series of CD-Roms. ...and what are Geographic Information Systems? Information and maps derived from satellite remote sensing become even more powerful tools when they are integrated into a geographic information system (GIS). A GIS is a computerized storage, processing and retrieval system with hardware and software specifically designed to cope with geographically referenced spatial data and the corresponding attribute information - that is, maps that illustrate topography, water availability, population densities, administrative boundaries and transportation, energy and communication lines. A GIS allows information from these different maps to be combined and manipulated to meet a broad range of planning needs. This 'overlaying' function is one of the the most important tools of GIS, together with modelling and site selection for specific tasks. Back to main story  FAO Home page   Search our site  Comments?: Webmaster@fao.org © FAO, 1999