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University of Nebraska–Lincoln

Remote Sensing Glossary

Reference Information for Virtual Nebraska

Terms, Definitions and Concepts

R

R&D

Research and Development.

radiant
  1. In optics, the point or object from which light proceeds.
  2. In geometry, a straight line proceeding from a given point, or fixed pole, about which it is conceived to revolve.
  3. In astronomy, the point in the heavens from which a shower of meteors seems to proceed.
radiation

Energy transfer in the form of electromagnetic waves or particles that release energy when absorbed by an object.

radiation budget

A measure of all the inputs and outputs of radiative energy relative to a system, such as Earth. See Earth Radiation Budget Experiment.

radiative cooling

Cooling process of the Earth's surface and adjacent air, which occurs when infrared (heat) energy radiates from the surface of the Earth upward through the atmosphere into space. Air near the surface transfers its thermal energy to the nearby ground through conduction, so that radiative cooling lowers the temperature of both the surface and the lowest part of the atmosphere.

radiative transfer

Theory dealing with the propagation of electromagnetic radiation through a medium.

radioactive

Giving off or capable of giving off radiant energy in the form of particles or rays, as in alpha, beta, and gamma rays.

radiometer

An instrument that quantitatively measures electromagnetic radiation. Weather satellites carry radiometers to measure radiation from snow, ice, clouds, bodies of water, the Earth's surface, and the sun.

radiosonde

A balloon-borne instrument that measures meteorological parameters from the Earth's surface up to 20 miles in the atmosphere. The radiosonde measures temperature, pressure, and humidity, and transmits or "radios" these data back to Earth. Upper air winds also are determined through tracking of the balloon ascent.

Radiosonde observations generally are taken twice a day (0000 and 1200 UTC) around the globe. NOAA's National Weather Service (NWS) operates a network of about 90 radiosonde observing sites in the U.S. and its territories. When the balloons burst, radiosondes return to Earth on a parachute. Approximately 25 percent are recovered and returned to NWS for reconditioning and reuse.

radio spectrum

The complete range of frequencies or wave lengths of electromagnetic waves, specifically those used in radio and television.

radio wave

An electrical impulse sent through the atmosphere at radio frequency.

rain forest

An evergreen woodland of the tropics distinguished by a continuous leaf canopy and an average rainfall of about 100 inches per year. Rain forests play an important role in the global environment. The Earth sustains life because of critical balances and interactions among many factors. Were there not processes at work that limit the effects of other essential processes, Earth would become uninhabitable. Destruction of tropical rain forests reduces the amount of leaf area in the tropics, and consequently the amount of carbon dioxide absorbed, causing increases in levels of carbon dioxide and other atmospheric gases. It is estimated that cutting and burning of tropical forests contributes about 20 percent of the carbon dioxide added to the atmosphere each year. The World Resources Institute and the International Institute for Environment and Development have reported that the world's tropical forests are being destroyed at the rate of fifty-four acres per minute, or twenty-eight million acres lost annually. Rain forest destruction also means the loss of a wide spectrum of biological life, erosion of soil, and possible desertification.

rain gauge

Calibrated container that measures the amount of rainfall during a specific period of time.

RAM

Random Access Memory. Computers use two types of memory, RAM and ROM. RAM is the computer's working area, the primary location where the microprocessor stores the information it needs. The designation "random access" stems from the microprocessor's ability to access information in memory randomly by knowing its location, or address, rather than hunting through memory sequentially from beginning to end. Because information in RAM is stored electronically, accessing data stored in RAM is much faster than getting that data from a mechanical storage device such as a disk drive. But because it is stored electronically, all information in RAM is temporary (which is why you must store it on a more permanent storage capability, such as a disk). Compare with ROM.

real time

As it happens.

receiver sensitivity

The ability of a receiver to detect weak signals through the noise level of the receiving system, which includes the antenna and internal thermal noise of the receiver. See signal-to-noise ratio.

reflection

The return of light or sound waves from a surface. If a reflecting surface is plane, the angle of reflection of a light ray is the same as the angle of incidence.

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remapping

Flattening the Earth into a standard map projection. When the spherical Earth is photo-graphed by satellites, areas lying near the outer edge of the picture are distorted. Remapping rectifies the distortion.

remote sensing

The technology of acquiring data and information about an object or phenomena by a device that is not in physical contact with it. In other words, remote sensing refers to gathering information about the Earth and its environment from a distance, a critical capability of the Earth Observing System.

For example, spacecraft in low-Earth orbit pass through the outer thermosphere, enabling direct sampling of chemical species there. These samples have been used extensively to develop an understanding of thermospheric properties. Explorer-17, launched in 1963, was the first satellite to return quantitative measurements of gaseous stratification in the thermosphere. However, the mesosphere and lower layers cannot be probed directly in this way--global observations from space require remote sensing from a spacecraft at an altitude well above the mesopause. The formidable technological challenges of atmospheric remote sensing, many of which are now being overcome, have delayed detailed study of the stratosphere and mesosphere by comparison with thermospheric research advances.

Some remote-sensing systems encountered in everyday life include the human eye and brain, and photographic and video cameras.

resolution

A measure of the ability to separate observable quantities. In the case of imagery, it describes the area represented by each pixel of an image. The smaller the area represented by a pixel, the more accurate and detailed the image. APT has a resolution of 4 km, i.e., each pixel represents a square, 4 km on each side. HRPT has a resolution of 1.1 km at nadir (4 km at edge of scan), and WEFAX of 8 km. See resolution cell.

resolution cell

The smallest unit of area in an image of discrete elements. The area represented by a pixel.

retrograde orbit

An east-to-west orbit of Earth (Earth spins west to east). See prograde orbit.

revolution

Process of the Earth circling the sun in its orbit. Revolution determines the seasons, and the length of the year. In addition, differences in seasons occur because of Earth's inclination (tilt on its axis) of about 23.5 degrees as it revolves around the sun. Compare with rotation.

right ascension of ascending node (aka =, RAAN or RA of Node)

One of six Keplerian elements, it indicates the rotation of the orbit plane from some reference point. Two numbers orient an orbital plane in space; inclination is the first, this is the second. After specifying inclination, an infinite number of orbital planes are possible. The intersection of the equatorial plane and the orbital plane (see diagram, line of nodes) must be specified by a location on the equator that fully defines the orbital plane. The line of nodes occurs in two places. However, by convention, only the ascending node (where the satellite crosses the equator going from south to north) is specified. The descending node (where the satellite crosses the equator going from north to south) is not.

Because the Earth spins, conventional latitude and longitude points are not used to separate where the lines of node occur. Instead, an astronomical coordinate system is used, known as the right-ascension/declination coordinate system, which does not spin with the Earth. Right ascension of ascending node is an angle, measured at the center of the Earth, from the vernal equinox to the ascending node. For example, draw a line from the center of the Earth to the point where the satellite crossed the equator (going from south to north). If this line points directly at the vernal equinox, then RAAN = 0 degree.

ROM

Read Only Memory. Refers to the computer memory chips that contain information the computer uses (along with system files) throughout the system, including the information it needs to get itself started. Information in ROM is permanent; it doesn't vanish when the power is turned off. Compare with RAM.

rotation

Process of the Earth turning on its axis. Rotation determines day and night, and the length of the day. Compare with revolution.