Other Free Encyclopedias » Online Encyclopedia » Encyclopedia - Featured Articles » Contributed Topics from P-T

Remote Sensing - Introduction, History, The Electromagnetic Spectrum, Recent Advances

wavelengths images radiation satellites

NICHOLAS M. SHORT, PH.D.
NASA

DAVID MALIN
Anglo-Australian Observatory, RMIT University

DONALD L. LIGHT
Airborne and Space Systems for Mapping and Remote Sensing

Introduction

Remote sensing may be broadly defined as the collection of data which leads to the observing of objects, area, or phenomenon, without human physical contact. In this section data that can be viewed as images will be emphasized, but the term remote sensing may also embrace non-image data. The image information is usually derived from capturing electromagnetic radiation that is emitted, reflected, absorbed, or scattered in a way that reveals something about the object or scene of interest. This energy may be of natural origin, such as sunlight, or it may be artificial, such as radar or laser scanning, leading to the concepts of active and passive remote sensing. Energy can be detected and recorded by photographic film and digital detectors, while more sophisticated radio frequency receivers, radar systems, thermal sensors, radiometers, and other sensing instruments are required for other wavelengths and imaging regimes.

Remote sensing is used in a wide range of applications, including agriculture, forestry, geology, cartography and land cover mapping, meteorology, water resources control, hydrology, oceanography, and environmental management, and of course, the military, just to mention a few. Today, aerial and ground photography are considered subsets of remote sensing . Remote sensing is essentially earthward-looking. Apart from atmospheric research, upward-looking remote sensing is astronomy, and that is also discussed elsewhere in this volume .

History

The remote sensing of images began with the invention of photography. Close-up photography (proximal remote sensing) began in 1839 with the images of Daguerre. Distal remote sensing from above the ground started in the 1860s from balloons. With the invention of the airplane, aerial photography showed its value in the World War I. Before the World War II more sophisticated photographic methods had evolved, a few simple electronic sensors were used, and radar was being developed. At the end of WW II, radar had come into civilian use, thermal sensors were being developed, and the concept of multispectral photography was finding applications. It was these developments that led to the term “remote sensing,” which was first used in the 1950s by Evelyn Pruitt, a geographer/oceanographer at the U.S. Office of Naval Research. However, at that time, most remote sensing was still carried out photographically from the ground and airborne platforms.

As rockets became more reliable, remote sensing could cover large areas of the earth and look outward toward the planets and the cosmos. Early efforts involved installing automated sensors and cameras on captured German V-2 rockets. Though they never attained orbit, they took the first high-altitude aerial images, beginning in 1946. This was followed by the development of meteorological satellites in the 1960s, the first of which was the Television Infrared Observation Satellite (TIROS-1). Further refinements in imaging sensors revealed the surface of the earth (literally) in a new light and the modern era of multispectral imaging had begun. The value of the multispectral aspects of imagery from space was tested by using multi-band imagery from manned aircraft, where it is still widely used.

This early phase led to the birth of the Earth Resources Technology Satellite (ERTS, later renamed Landsat) that was launched in 1972. Six other Landsat satellites followed, each of which had several visible-light and infrared bands. The success of such missions led to a remote sensing industry and to more satellites (e.g., SPOT, JERS, IRS) each with improved capabilities. The first geostationary weather satellites were launched in the mid 1970s and also carried instruments to monitor solar activity, X-radiation from space, and the earth’s magnetic field. Most earth-observing sensors operate in the scanner (pushbroom) mode, some using mirrors to sweep across the scene as the satellite platform moves in orbit. Multispectral data are produced by using bandpass filters to isolate various parts of the reflected spectrum.

The early satellites were passive observers, but in 1978, Seasat was launched and used radar to produce images of ocean currents and surface detail that in turn revealed the presence of unsuspected features on the sea floor. Imaging radar has also been used from the Space Shuttle, where it produces remarkable images of landforms below the vegetation cover and other unexpected features such as ancient riverbeds in the Sahara desert. The military uses satellites equipped to image in the visible and infrared wavelengths for high resolution surveillance. These satellites use powerful telescopes and images from them are not widely available.

The Electromagnetic Spectrum

The visible region of the spectrum is small, with wavelengths between 0.4 and 0.7µm (micrometers, microns, millionths of a meter). Part of the near infrared (NIR) is made up mostly of reflected radiation, but radiation emitted from thermal processes (heat) is detectable at about 3detectable at about 3m. Much of the radiation in this part of the spectrum is absorbed and/or scattered by the atmosphere; however, there are some gaps shown in the irregular dark band in Figure 91, and the atmosphere is essentially transparent at visible wavelengths. At shorter wavelengths, the atmosphere is effectively opaque, so remote sensing detectors above a source will receive weak or no signals. The attenuation properties of the atmosphere are thus an important consideration in remote sensing. On the other hand, telescopes in earth orbit looking out into space are above the atmosphere and can sense radiation at
all these wavelengths, including radio radiation, originating from cosmic sources.

Recent Advances

Since the early days of multispectral remote sensing, the major advances include completely new types of detectors. A small number of individual detectors in a row (silicon for visible light, lead sulfide for longer wavelengths) were replaced by the CCD chip, usually consisting of many smaller detectors in a two-dimensional X-Y pattern. Pixilated arrays can be made to be
sensitive to a very wide range of wavelengths. This in turn has permitted greatly improved spatial and spectral resolution. The latter is now associated with hyperspectral sensing in which the bandwidths are reduced to 0.01-0.02 µm. A micrometer is a millionth of a meter; another term is nanometer which is 1000 µm, thus 0.55 µm is 550nm. With hyperspectral sensing the 200 or more channels cover a continuous range of wavelengths can extend from less than 0.4 to over 2.5 µm. Also much extended is the ability to record data at longer wavelengths, including thermal regions at 5-6 and 8-14 µm and into the passive microwave spectral region (millimeter wavelengths) and the active, where scene-illuminating radiation is produced by the sensor system (e.g., radar). Finally, much better computer-based image processing is available.

As software and imaging systems continue to improve, remote sensing is focused on developing methods to quantify the data extracted from imagery. As a result, more and more work is done to directly measure environmental factors and (for example) help to increase agricultural yields. This involves correcting for atmospheric degradation of these images and removing their effects on the final image brightness. In addition to the collection and correction of remote sensing images and the display and application of the data, a broad range of scientific disciplines are benefiting from these improvements. Among the disciplines developing or using remote sensing techniques are optical, imaging, and photographic scientists,
as well as geographers, geologists, agronomists, foresters, environmental scientists, and archaeologists.

Besides meteorological, oceanographic, and land-observing remote sensing from satellites (and manned spacecraft), remote sensing is also a key part of the exploration of the planets (e.g., Mars Orbiters) and the cosmic exploration of stars, galaxies, and intergalactic regions using sensors tied to such exceptional space platforms as the Hubble Space Telescope and the Chandra X-Ray Telescope. Almost their entire functions are remote-sensing based.

User Comments

Your email address will be altered so spam harvesting bots can't read it easily.
Hide my email completely instead?

Cancel or

Vote down Vote up

about 5 years ago

Explain the role of aerial photography and remote sensing in Kenya.

Vote down Vote up

about 5 years ago

Explain the role of aerial photography and remote sensing in Kenya.

Vote down Vote up

almost 6 years ago

make it more advanced by supporting images and photos with recent change on it!

Vote down Vote up

over 6 years ago

what is the remote sensing? and explaine the electromagnatic spectume? and which is the uses electromagnatic spectrume in remote sensing & gis, explain?

























Vote down Vote up

over 6 years ago

i have need the research of atmospheric remote sounding from the ground