EM Wave Scanners

EM Wave Scanners

In order to observe phenomena that cannot be glimpsed through direct contact, for example, the activities of an isolated weapons-testing site in a hostile nation, it may be necessary to employ remote-sensing equipment and techniques. These typically involve views from the air or from space, which require the use of electromagnetic radiation (EMR) across a wide spectrum. Though the information rewards can be high, intelligence services using electromagnetic (EM) scanners in space must deal with a variety of challenges in data collection and analysis.

Electromagnetic radiation from the sun. Light from the sun is electromagnetic radiation, and it contains both electric and magnetic components. The direction of propagation for an electromagnetic wave is mutually perpendicular with directions of its electrical and magnetic fields, whereas the electrical field might be thought of as the x -axis on a Cartesian coordinate plane, and the magnetic field the y -axis, the direction of wave propagation is the z -axis.

About 30% of the electromagnetic radiation from the sun that reaches Earth is reflected back into space unchanged, without entering Earth's atmosphere. This is due to the planet's albedo—its reflective power, or the proportion of incoming radiation that it reflects. Another 25% of solar radiation is absorbed by the atmosphere, while about 45% is absorbed at the planetary surface by living and non-living materials. This energy is later re-radiated to space in degraded form, that is, at a longer wavelength.

The atmosphere and its current conditions have a powerful effect on the amount of visible light reflected, and this—along with the loss of electromagnetic energy from the sun—places constraints on the observational abilities of remote-sensing equipment.

Detecting images. There is a continuous distribution of electromagnetic energy levels, from extremely low to extremely high, that together constitutes the entire electromagnetic spectrum of energy. At the lowest level are radio waves, then microwaves (the section of the spectrum across which television transmission takes place). Higher in frequency and energy levels are such forms of light as infrared, visible, and ultraviolet. Still higher are x rays, and highest of all are gamma rays, which have an extremely small wavelength and extremely high frequency.

Of most interest in remote sensing are the energy levels near the middle of the spectrum: microwaves, infrared, visible light, and ultraviolet light. Remote sensing satellites measure the EMR reflected from features on Earth back into space. Photographic cameras on remote-sensing satellites are capable of detecting light from the near infrared to the near ultraviolet. Remote sensing equipment typically divides the infrared portion into relatively low-energy near-infrared images, and higher energy thermal infrared images. The satellite may have a thermal scanner that operates in the thermal infrared portion, or a multi-spectral scanner operating across a range from ultraviolet to thermal infrared. There may also be passive microwave and active radar systems operating in the microwave portion of the spectrum.

Scanning and processing images. Satellites equipped with multi-spectral scanners can make precise measurements across a number of narrow bands. These scanners may be of the oscillating or "wisk-broom" type, which scan along a line perpendicular to that of the satellite's trajectory, or of the "push-broom" type, which detect entire scan lines at once.

These multi-spectral scanners record electromagnetic radiation as electrical signals, convert them to digital format, and transmit the information to an Earth receiving station. The latter interprets various numbers as brightness values on a gray scale. Depending on the needs of the observing agency, images may be adjusted for resolution. For example, spatial resolution improves the detail for smaller objects, while radiometric resolution allows for the greatest levels of contrast. The analyzing agency may, in the digital image processing phase, add false color to enhance the readability of images for specific data—for example, red to indicate heat levels at areas where weapons are being tested.



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"Earthshots: Satellite Images of Environmental Change (U.S. Geological Survey)." < http://edcwww.cr.usgs.gov/earthshots/slow/tableofcontents > (February 26, 2003).

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