Measurements made from large distances, as from high-flying aircraft or Earth satellites. Especially refers to measurements of either natural radiation (passive methods) or radiation from a source in the sensor (active methods) that has been reflected back from the earth. Determination of characteristics without direct physical contact. Often implies detecting, imaging, and interpretation by means of reflected or emitted electromagnetic radiation. Data covering all parts of the world are available from a number of remote sensing satellites operated by the US or other countries. Sensors record different wavelengths and have different spatial resolutions, and some spacecraft carry active systems such as radar altimeters. These data form the basis of NASA’s Earth Observing System (EOS). Data can be purchased publicly in either digital or graphical form. Passive data yield information only about the surface of the Earth and are useful in the study of topography and geomorphologic features, in identifying rock type, in identifying land use and environmental changes, vegetated and agricultural usage, the extent of urbanization, roads and coastal features, etc.
The most common data are AVHRR, Landsat, and SPOT data, each having different orbital characteristics and each has several versions of data (see Figure L-1). AVHRR (Advanced Very High Resolution Ratiometer) instruments on NOAA’s polar orbiting weather satellites cover a huge swath width, nearly 2700 km, but with the poorest spatial resolution. A full Landsat scene is 185×185 km represented by 3240 pixels east-west and 2340 scans north-south. Landsat bands for multispectral scanners are band 4: 05 to 0.6 μm (green); band 5: 0.6 to 0.7 μm (red); band 6: 0.7 to 0.8 μm (infrared); band 7: 0.8 to 1.1 μm (infrared). A number of Landsat instruments have been placed in orbit, including Landsat 7, launched in April, 1999. The more recent Landsat instruments have better resolution, record more components of the electromagnetic spectrum, and are better calibrated. SPOT (Satellite Pour l’Observation de la Terre, operated by CNES of France) gives the greatest resolution of the three data types. In general, as the resolution increases, the size of the scene decreases, and the price per square kilometer greatly increases. Displays sometimes use imaging (to produce a picture of the radiation, as in a photograph), sometimes are nonimaging (to produce a profile of the variation of radiation along the flight path). May also include measurements of the magnetic field or other nonradiation measurements.