The Science Of Remote Sensing

Remote sensing is the collection and measurement of information by a device not in physical contact with what it is observing. Common remote sensing devices include eyes, cameras, binoculars, microscopes, telescopes, video cameras, and satellites. When a 35-mm camera takes a photograph, for instance, a hard-copy print of the object is the output. If the picture were of a house, the photo interpreter would gain useful information such as the shape of the house, number of floors, number of windows, color, and landscaping. Using the photo, an interpreter is able to gather all this information without ever physically touching the house.

Remote sensing devices allow an interpreter to see objects at a distance or to see small objects better. Earth scientists use remote sensing to gather information about the Earth. The photographs they use are images that can be obtained from many types of remote sensing devices and that offer unique views of the Earth's surface unattainable in any other way. The most common platforms today are airplanes and satellites.

Satellites allow the Earth's surface, atmosphere, and oceans to be observed from space. Similar to humans, satellites have sensors that serve as their eyes. Satellites can see better than humans, however, because their sensors can detect much more of the Sun's electromagnetic energy, that is, all the energy that comes from the Sun. This energy travels through space from the Sun to the Earth and is composed of several ranges of wavelengths.

The shortest wavelengths in the spectrum are gamma rays, X-rays, and ultraviolet rays. Humans cannot see this energy (X-rays are what doctors use to take pictures of bones, and ultraviolet rays are what cause sunburn). As the wavelengths lengthens, visible light appears. These are the wavelengths that humans can see, but it is a tiny portion of the entire spectrum. Visible light can be broken into blue, green, and red light. Wavelengths longer than those of the visible spectrum cannot be seen by humans. These include infrared radiation, microwave radiation, and radio wavelengths. Scientists refer to these groups as bands (blue band, green band, red band, infrared band, microwave band, etc.).

Even though humans cannot see this energy, remote sensing imaging systems can. The various bands of wavelengths can be used to see different things. Most satellites can see objects in several different bands, and each band gives an image interpreter different information about the landscape. Some remote sensors can detect and record more than

200 different spectral bands. These types of sensors create "hyperspec-tral" imagery and are very useful to geologists looking for specific mineral deposits. Such bands are very narrow and highly discriminating.

Equally important as the wavelengths in which the remote sensor can "see" is image resolution, remote sensing system's ability to record and display fine detail. The smaller the resolution, the more detailed the image. When a digital image is acquired, it consists of rows and columns of numeric data. Each space on the resultant grid is a cell called a pixel (short for picture element). There is one numeric value for each pixel in each band, the size of which determines its resolution. If a satellite image has a 98-foot (30-m) pixel resolution, the smallest object in the image directly observable is at least 98 feet (30 m) in size. Different sensors on satellites have different pixel resolutions. The type of resolution scientists use for a project depends on what they need to see. If they wish to study large areas in general detail, then images with larger pixels work well. If they need to see specific detailed imagery, an image with smaller, more detailed pixels works well.

Computers are able to interpret the data contained in the imagery using sophisticated image analysis and classification software that sees the images as different spectral bands and looks at the numerical values of each pixel in each band to classify the image according to those patterns of numerical values. The computer can discriminate much better than the human eye, which can differentiate only a limited number of colors or tones. A computer can differentiate hundreds of values.

Image processing software performs many diverse functions. It can correct geometric properties so that an image better represents the ground it depicts. It can enhance, evaluate, and identify features based on principles of contrast and texture. It can display images in multiple classes based on multispectral ranges. It can look at different combinations of spectral bands to show highly diverse information and create data to be used.

Two basic types of remote sensing systems are used, active and passive. Passive systems record the energy emitted from the Earth. LANDSAT, QuickBird, and weather satellites are examples of passive systems—they merely record what they see reflected from the Earth's surface. Active systems generate their own energy, send out signals, and then record how they interact with the surface of the Earth. Active systems can operate from both aircraft and satellites.

An example of an active system is radar, a distinct form of remote sensing imagery. It transmits a microwave signal and then receives its reflection as the basis for forming digital or pictorial images of the Earth's surface. The radar contains a transmitter that sends repetitive pulses of microwave energy at a given frequency. It also has a receiver that accepts the reflected signal received by the antenna, then filters and amplifies it. It is the antenna array that transmits the narrow beam of microwave energy. Finally, a recorder logs and displays the signal as an image. Radar is the same technology as the radar guns highway patrol officers use to check a driver's speed.

Earth scientists use remote sensing for a wide variety of applications. It is useful for geologic applications, such as identifying the physical and chemical properties of rocks, understanding the relationships between plant cover at the Earth's surface and the structure of underlying rock, and studying faults, drainage systems, coastlines, and mountain systems. It can help hydrologists study water bodies, rivers, and streams and understand environmental effects from things such as drought, pollution, and global warming.

Remote sensing can also help archaeologists locate ancient ruins, which give paleoclimatologists clues to past climatic conditions. Remote sensors can detect irrigation ditches filled with sediment because they hold more moisture and have a different temperature than the surrounding soil. The ground above a buried stone wall may be slightly hotter than the surrounding terrain because the stone absorbs more heat. Radar waves can actually penetrate the ground to see what is under the soil.

Remote sensing can also be used as a discovery technique, since a computer can be programmed to look for distinctive signatures of energy emitted by a known site or feature in areas where surveys have not been conducted. Such signatures serve as recognition features, like fingerprints. Characteristics such as elevation, distance from water, distance between sites or cities, corridors, and transportation routes can help predict the location of potential archaeological sites. There are several examples of how remote sensing has been used to make archaeological and paleoclimatological discoveries:

• The Maya causeway (old remnants of trade routes used by the ancient Mayan culture) was detected through analysis of wavelengths in the infrared portion of the electromagnetic spectrum. Even though old paths may not be visible to the human eye, they appear different from vegetation in the infrared portion of the electromagnetic spectrum. Because they are discernable with imaging equipment, these ancient routes can be discovered, mapped, analyzed, and explored.

• A laser device called LIDAR (light detection and ranging) is a remote sensing system primarily used to collect topographic data that has been used to detect eroded footpaths that still affect the topography in many parts of the world. It is used extensively by NOAA and NASA to document subtle topographic changes in landscapes. LIDAR senses what is on the ground through using laser pulses similar to radar technology (which uses radio waves instead of light). LIDAR has proven highly successful in identifying archaeological sites.

• In 1982, radar from the space shuttle penetrated the sand of the Sudanese desert and revealed ancient watercourses. Because radar has the ability to sense beneath the surface of sand dunes, P-band (the 430-MHz wavelength range) microwave sensors have also detected ancient river drainage patterns, enabling researchers to link ancient settlement patterns with existing natural resources.

Some locations in the world where a better understanding of ancient climate has been gained through the use of remote sensing technology include Darfur in the Middle East, the Saraswati River in India, primeval forests in the United States, the Safsaf Oasis in Egypt, and the central Sahara in Africa.

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