Remote Sensing

I

Introduction

Remote Sensing, process of obtaining information about land, water, or an object, without any physical contact between the sensor and the subject of analysis. The term remote sensing most often refers to the collection of data by instruments carried aboard aircraft or satellites. Remote sensing systems are commonly used to survey, map, and monitor the resources and environment of Earth. They also have been used to explore other planets (see Space Exploration).

There are several different types of remote sensing devices. Many systems take photographs with cameras, recording reflected energy in the visible spectrum. Other systems record electromagnetic energy beyond the range of human sight, such as infrared radiation and microwaves (see Electromagnetic Radiation). Multispectral scanners produce images across both the visible and the infrared spectrum.

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Sensors

The most familiar form of electromagnetic energy is visible light, which is the portion of the electromagnetic spectrum to which human eyes are sensitive (see Color). When film in a camera is exposed to light, it records electromagnetic energy. Photographic images obtained from airborne cameras have been used in urban planning, forest management, topographic mapping, soil conservation, military surveillance, and many other applications (see Aerial Survey; Photogrammetry; Photography).

Infrared sensors and microwave sensors record invisible electromagnetic energy. The heat of an object, for example, can be measured by the infrared energy it radiates. Infrared sensors create images that show temperature variations in an area—a difficult or impossible task using conventional photography. Thermal infrared sensors can be used to survey the temperatures of bodies of water, locate damaged underground pipelines, and map geothermal and geologic structures.

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Microwave sensors, such as radar, transmit electromagnetic energy toward objects and record how these objects reflect the energy. Microwave sensors operate at very long electromagnetic wavelengths capable of penetrating clouds, a useful feature when cloud cover prohibits imaging with other sensors. By scanning an area with radar and processing the data in a computer, scientists can create radar maps. The surface of Venus, which is entirely shrouded by dense clouds, has been mapped in this way. Radar imagery is also used in geologic mapping, estimating soil moisture content, and determining sea-ice conditions to aid in ship navigation.

Multispectral scanners provide data electronically for multiple portions of the electromagnetic spectrum. Scientists often use computers to enhance the quality of these images or to assist in automated information-gathering and mapping. With computers, scientists can combine several images obtained by multispectral scanners operating at different frequencies.

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Satellites

Satellites have proved extremely useful in the development of remote sensing systems. In 1972 the United States launched Landsat 1, the first in a series of satellites designed specifically for remote sensing. Launched in 1999, Landsat 7 produced images of most of Earth’s surface every 16 days. Each Landsat image covers more than 31,000 sq km (11,970 sq mi). Objects as small as 230 sq m (2,500 sq ft) can be seen in the images produced by Landsat’s Enhanced Thematic Mapper Plus, a type of multispectral scanner. Landsat data have been used for applications such as mapping land use, managing forested land, estimating crop production, monitoring grazing conditions, assessing water quality, and protecting wildlife.

First launched in 1986, France's SPOT satellites have provided images showing objects as small as 100 sq m (1,076 sq ft) and have produced stereoscopic images useful for topographic mapping. Earth-observing satellites have also been launched by the European Space Agency and by Japan, Russia, India, and other nations.

Meteorological satellites, such as those operated by the U.S. National Oceanic and Atmospheric Administration, provide images for use in weather forecasting (see Meteorology), as well as in oceanic and terrestrial applications. Remote sensors on weather satellites can track the movement of clouds and record temperature changes in the atmosphere.

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Outlook for Remote Sensing

Remote sensing is changing rapidly. Some satellites carry instruments that can provide images of objects as small as an automobile and constantly improving technology promises even better resolution in the near future. Remote sensing satellites share some capabilities with intelligence-gathering satellites operated by military and intelligence agencies of the United States and other nations. In general, civilian satellites provide less detailed images than dedicated intelligence-gathering spacecraft, but have also raised privacy issues. Detailed satellite images of nearly every location on Earth have been made available on the World Wide Web.

Computer-assisted image-analysis techniques are leading to many new applications for remote sensing. In the late 1990s and the early 21st century, the U.S. National Aeronautics and Space Administration (NASA) began launching a series of satellites to study environmental changes on the planet as part of the Earth Observing System, a key program in NASA’s Mission to Planet Earth. A new Landsat satellite is planned, as well.