Radar equipment carried by commercial and military aircraft. These aircraft use airborne radar systems to assist in weather assessment and navigation. Military systems also provide other specialized capabilities such as targeting of hostile aircraft for air-to-air combat, detection and tracking of moving ground targets, targeting of ground targets for bombing missions, and very accurate terrain measurements for assisting in low-altitude flights. Airborne radars are also used to map and monitor the Earth's surface for environmental and topological study.

Radio communication employing amplitude modulation (AM) of a radio-frequency (RF) carrier wave as the means of conveying the desired intelligence. In AM, the amplitude of the carrier wave is made to vary in response to the fluctuations of a sound wave, television image, or other information to be conveyed. See also: Amplitude modulation; Radio

A conductor whose purpose is to emit and/or receive electromagnetic waves using the bulk oscillations of electric charges within the conductor. The conduction electrons in antennas are free to move throughout the conductor. The electrons form a bulk oscillating electric current in response to applied forces from an electric circuit. As a result, the electromagnetic field surrounding the current also oscillates, forming self-sustaining electromagnetic waves that travel away from the antenna. Because the oscillation of the wave is created by the oscillation of the current in the antenna, the frequency and the waveform of the wave equals the frequency and waveform of the current in the antenna. This fact makes antennas very useful in transmitting and receiving information using electromagnetic waves (see illustration). Due to the symmetry between how oscillating currents create electromagnetic waves and how electromagnetic waves create oscillating currents, the same antenna can be used to both generate and capture electromagnetic waves. This makes an antenna very useful in two-way wireless communications, where the same antenna can quickly switch between transmitting and receiving modes. See also: Electric charge; Electrical circuit; Electric current; Electrical conductor; Electromagnetic field; Electromagnetic radiation; Electromagnetic wave; Electron; Frequency (wave motion); Oscillation; Wave motion

A system that receives and processes television signals, as well as other digital forms of communications such as those related to high-speed Internet connectivity and cable telephony, from various sources and retransmits these signals through coaxial or optical-fiber (or fiber-optic) cables to subscribers' homes. The sources of the signals include broadcast transmissions, satellite-delivered programming, and local television studio productions. The facility that receives, processes, and retransmits the signals is called a headend.

The use of video cameras to transmit a signal to a particular limited audience at one or more specific locations via coaxial cables, telephone wires, fiber-optic strands, microwave radio systems, or communications satellites, as compared to openly broadcast television intended for the general public. The term closed-circuit television (CCTV) is slowly falling into disuse and currently refers primarily to older analog video technology used for such functions as monitoring, surveillance, and process monitoring. These are discussed below in more depth. This older CCTV equipment is based on the analog television standards that have been in widespread use since about 1940 and are also described below. This equipment is being replaced by digital television (DTV) and the widely popular high-definition digital television (HDTV). In addition, DTV, coupled with the spread of the Internet, has now made commonplace the use of web cameras (webcams) and Internet Protocol (IP)–based cameras. These are causing what traditionally has been called CCTV to slowly disappear and be replaced by similar web-based applications. For example, the video teleconferencing systems of the 1990s have been made obsolete by applications such as Skype. See also: Internet; Teleconferencing; Television; Television standards; Coaxial cable; Communications cable; Communications satellite; Microwave; Optical communications; Telephone service

A radar system used to measure the relative velocity of targets and their position. The operation of these systems is based on the fact that the Doppler frequency shift in the target echo is proportional to the radial component of target velocity. In the United States, the National Weather Service operates a network of 160 weather radars (NEXRADs; Next Generation Weather Radars; Fig. 1). NEXRADs have the capability of measuring both reflectivity and radial velocity of the hydrometeors (rain drops, snowflakes, hail) throughout the storm volume, enabling forecasters to infer the internal circulations (wind speed) within the storms. See also: Doppler effect; Radar; Radar meteorology

Radio transmission accomplished by variation of the frequency aspect of a radio-frequency (RF) carrier signal by an input signal (called the modulating signal). The frequency-modulation (FM) technique was invented by Major Edwin A. Armstrong in the 1930s. FM radio broadcasting offers a number of advantages over amplitude-modulation (AM) radio broadcasting, most significantly higher audio fidelity and greater immunity to atmospheric and human-made sources of interference.

A remote-sensing device that transmits and receives radio or microwave radiation for the purpose of detecting and measuring weather phenomena. Radar is an acronym for "radio detection and ranging." Many types of radar are used in meteorology. One prominent type is Doppler radar, used to determine air motions (for example, to detect tornadoes). Airborne Doppler radar plays a vital role in meteorological research. Multiparameter radar provides information on the phase (ice or liquid), shape, and size of hydrometeors—the term for any condensation or deposition of atmospheric water vapor that forms in the air or on Earth's surface or is blown by wind off of Earth's surface. Weather radar instruments are also deployed in orbit above Earth on satellites. Further examples of meteorological radar applications include detection of hail, estimation of rainfall rates, observation of the clear-air atmosphere to monitor wind patterns, and studies of the electrification processes in thunderstorms that generate lightning discharges. See also: Lightning; Meteorology; Radar; Satellite (spacecraft); Thunderstorm; Weather; Weather forecasting and prediction; Weather observation

Electromagnetic energy with wavelengths in free space ranging roughly from 0.1 to 100 cm, with corresponding frequencies ranging from 0.3 to 300 gigahertz (GHz). On the electromagnetic spectrum, microwaves represent the frequency band between radio waves and infrared light (Fig. 1). Microwave wavelengths on the order of millimeters are more specifically called millimeter (mm) waves. See also: Electromagnetic radiation; Electromagnetism; Frequency (wave motion); Light; Infrared radiation; Submillimeter-wave technology; Wave (physics); Wave motion; Wavelength

The study of those properties of microwaves which are analogous to the properties of light waves in optics. The fact that microwaves and light waves are both electromagnetic waves, the major difference being that of frequency, already suggests that their properties should be alike in many respects. But the reason microwaves behave more like light waves than, for instance, very low frequency waves for electrical power (50 or 60 Hz) is primarily that the microwave wavelengths are usually comparable to or smaller than the ordinary physical dimensions of objects interacting with the waves.