Satellites, Environmental Sensing

Aerial photography was the precursor of satellite imagery for recording earth-surface characteristics using instrumentation that is located at a distance from (i.e., remotely), rather than in contact with, the subject. Established in the late 1800s, the first aerial photographs were obtained from cameras aboard balloons and kites until the advent of aircraft in the early 1900s.

These early aircraft made a substantial impact on the collection of land- use and land-cover information, resource inventory, and archaeological data as well as being an important tool in wartime. In the late 1940s the first attempts at space exploration provided new opportunities for remote sensing of the earth. The first satellite for remote sensing was TIROS-1 launched in 1960; it was designed to collect meteorological data such as cloud cover.

As improvements in sensors occurred, the range and quality of the data collected by meteorological satellites also improved. In particular the physics of the sensors became sufficiently sophisticated to compensate for distortions due to turbulence and refraction of light in regions of varying densities and temperatures in the atmosphere.

Further developments occurred as space flights increased. In the 1960s astronauts used hand-held cameras to capture images of the earth. Experiments involving color-reversal film to determine geological features were conducted successfully as part of the Gemini program. Subsequently, other aspects of the earth’s surface became the focus of photographic record; for example, the oceans and the world’s vegetation cover.

New frontiers were breached with the Apollo space program, especially the use of mechanically operated Hasselblad cameras in conjunction with multispectral photography. The latter involves different types of film with various filters; for example, panchromatic (multicolored) film, infrared monochrome (black and white) film, and infrared color film.

Piloted and robotic spacecraft with radar on board began as part of Russia’s military intelligence program, but by the late 1970s weather radar (looking for radar echoes of precipitation) and radar altimeters (measuring heights of the earth’s surface from the time delay of a signal) were deployed from satellites.

Radar sensor systems are ‘‘active’’ in that they emit the same microwave radiation that is used for the remote sensing. ‘‘Passive’’ sensors, on the other hand, are dependent upon receiving reflected sunlight or thermal infrared emission. All passive sensors record electromagnetic radiation (i.e., radiant energy) which is emitted from the earth’s surface or atmosphere and which is classified according to wavelength.

These types of radiant energy, listed in order of increasing wavelength, are gamma radiation, x-rays, ultraviolet radiation, visible radiation, microwaves, and radiowaves. As instrumentation have become increasingly sophisticated, sensors that can record a variety of radiant-energy wavelengths, (i.e., multispectral) have been developed. Skylab, launched in 1973 and carrying the earth resources experiment package (EREP) was an early example of space-based multispectral remote sensing involving photographic and electronic measurements.

From this the National Aeronautics and Space Administration (NASA) began to develop a program of recording earth resources from satellites. The first satellite in what later became the Landsat programme was launched in 1972. Landsat has become one of the most enduring earth observation missions, the latest satellite being Landsat 7 launched in 1999. The instruments on the Landsat satellites record data in the electromagnetic range between visible light and thermal radiation and with resolutions between 80 and 15 meters.

The detail revealed by these missions confirmed the value of satellite remote sensing for monitoring the status of earth-surface features and resources. The Landsat program was the start of true satellite remote sensing. It shed light on a range of features, contributed to the explanation of their structure and formation and highlighted the magnitude of environmental change in many regions, including the rapidly declining extent of tropical forests.

Subsequent satellites for remote sensing include ‘‘Seasat,’’ launched in 1978. Carrying radar sensors, it focused on the oceans, their circulation and sea-ice cover. The French Centre National d’Etudes (CNES) in combination with Belgium and Sweden launched the first non-U.S. satellite in 1986. This was one of a series known as Systeme Pour l’Observation de la Terre (i.e., SPOT), which subsequently developed into an international project involving five satellites launched between 1986 and 2002.

Soviet satellite monitoring facilities included the Cosmos-1870 and ALMAZ-1 in 1987 and 1991 respectively. Both collected radar images and ALMAZ-1 was the first satellite to operate commercially. The European Space Agency (ESA) also launched a satellite, ERS-1, in 1991, and ERS-2 in 1995. The primary sensors record microwave, radar and radiometric data. Other nations that have developed satellite monitoring systems include Japan whose space agency launched JERS-1 in 1992, Canada launched its first Radarsat in 1995 and China launched the satellites YZ-1 and YZ-2 in 1999 and 2000. There were at least 31 satellites in orbit in 2000.

These satellites transmit images to earth and sophisticated computer programs translate data from various wavelengths into information on geology, water resources, vegetation, soils, minerals, tectonics, agriculture, forestry and urban environments. The end products are images from which maps of particular characteristics can be constructed. The continuous recording of earth-surface features provides detailed information on global environmental change at various scales. Such data can facilitate management, the prediction of future change and the assessment of the quality and quantity of biological and mineral resources.

The varied applications of satellite imagery have turned what began as government-funded endeavor into a commercial activity. However, satellites have also been developed for other purposes, two of which include weather and climate, and military applications. The recording of weather and climatic data involves monitoring of the atmosphere; today’s efforts have advanced considerably on those of TIROS-1 (see above), and now weather and climate prediction, including advance warning of extreme conditions, is a primary objective.

Satellites so designed are known as metsats; examples include those of the U.S. National Oceanic and Atmospheric Administration (NOAA), the Geostationary Operational Environmental Satellite (GOES), and the U.S. Air Force Defense Meteorological Satellite Program (DMSP). Military satellites differ from nonmilitary satellites insofar as they are always government funded, provide encrypted data that requires a special and secure receiver, and operate at a higher resolution.

Recent developments include a new family of agile satellites, such as IKONOS (1-meter resolution and Quickbird (0.61-meter resolution), which exploit emerging optical technologies to compete in the market for high-resolution images normally provided by aerial photography. The rapid and continuous collection of varied environmental data, the increasing resolution possible by sensors and increasing sophistication of analytical techniques mean that satellite monitoring has a bright future.