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A mathematical entity consisting of a set of values (the carrier set) and a collection of operations that manipulate them. For example, the Integer abstract data type consists of a carrier set containing the positive and negative whole numbers and…

Signal processing is a discipline that deals with the extraction of information from signals. The devices that perform this task can be physical hardware devices, specialized software codes, or combinations of both. In recent years the complexity of…

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…

A well-defined procedure to solve a problem. As such, an algorithm describes how to solve a problem (Fig. 1). The study of algorithms is a fundamental area of computer science. In writing a computer program to solve a problem, a programmer expresses…

A book that contains astronomical or meteorological data arranged according to days, weeks, and months of a given year and may also include diverse information of a nonastronomical character. This article is restricted to astronomical and…

Any device which measures the height of an aircraft. The two chief types are the pressure altimeter, which measures the aircraft's distance above sea level, and the radar altimeter, which measures distance above the ground. Pressure Altimeter A…

Two-way radio communications by individuals as a leisure-time activity. Amateur, or “ham,” radio is defined by international treaty as a “service of self-training, intercommunications, and technical investigation carried on by amateurs; that is, by…

The process or result of the process whereby the amplitude of a carrier wave is changed in accordance with a modulating wave. This broad definition includes applications using sinusoidal carriers, pulse carriers, or any other form of carrier, the…

A device for moving the frequency of an information signal, which is generally at baseband (such as an audio or instrumentation signal), to a higher frequency, by varying the amplitude of a mediating (carrier) signal. The motivation to modulate may…

A device for recovering information from an amplitude-modulated (AM) electrical signal. Such a signal is received, usually at radio frequency, with information impressed in one of several forms. The carrier signal may be modulated by the information…

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…

A computer or computational device in which the problem variables are represented as continuous, varying physical quantities. An analog computer implements a model of the system being studied. The physical form of the analog may be functionally…

A device for converting the information contained in the value or magnitude of some characteristics of an input signal, compared to a standard or reference, to information in the form of discrete states of a signal, usually with numerical values…

Modulation in which the instantaneous angle varies in proportion to the modulating waveform. When used in an analog manner, it is referred to as either frequency modulation (FM) or phase modulation (PM), depending upon whether the instantaneous…

Sea-level rise is caused by adding mass (water) and heat to the ocean. The Antarctic ice sheet represents a potentially large source of water. Knowing the speed and direction (velocity) at which the Antarctic ice sheet is flowing to the ocean is…

The device that couples the transmitter or receiver network of a radio system to space. Radio waves are used to transmit signals from a source through space. The information is received at a destination which in some cases, such as radar, can be…

A mathematical model of computing. The versatility of modern computers and their applications to the study of complex and dynamic systems, such as financial markets, cellular growth, and communication networks, raise questions about the ultimate…

The use of computers to automate decision tasks. A decision task is any task requiring the generation or selection of options. There is an increasing demand for autonomous systems that can perform various complex tasks without human control or…

The automatic control of the intermediate frequency in radio, television, or radar receivers, to correct for variations of the frequency of the transmitted carrier or the local oscillator in the receiver. For many reasons the frequency of the…

The automatic maintenance of a nearly constant output signal level of an amplifier circuit in the presence of a variable input signal level by adjusting its gain in inverse proportion to the input signal level. Automatic gain control (AGC) is also…

The collection, storage, and management of huge amounts of digital information. Databases falling into the realm of big data often begin in the hundreds of terabytes but can range up to petabytes (1015) or greater in size.The main characteristics…

Technologies that employ a distinctive, measurable biological or behavioral characteristic that can reliably distinguish one individual from another (biometric) for authentication, identification, and verification of individuals. Biometric…

An anchored or moored floating object, other than a lightship, intended as an aid to navigation. Buoys are the most numerous of all engineered aids to navigation, with some 20,000 in United States waters alone.A great variety of buoys are in use.…

Cameras in mobile phones or smartphones that can capture images and videos.The cellphone, the world’s favorite must-have gadget, has been with us for nearly a half-century, while experiencing continuous and often phenomenal improvements. Much of the…

The process of converting scanned images of machine-printed or handwritten text (numerals, letters, and symbols) into a computer-processable format; also known as optical character recognition (OCR). Systems A typical OCR system (Fig. 1) contains…

A model for offering computing as a commonly purchased service (that is, a utility) for storing data and providing access to computing resources (for example, programs) via the Internet, instead of on a local computer. The notion of a “cloud” of…

An artificial device that circumnavigates the Earth, receiving and retransmitting radio-frequency signals to multiple points on Earth. The communications hardware includes antennas, receivers, signal processing, and power amplifiers. The performance…

An approach to designing intelligent systems using mathematical algorithms. It consists of the fields of neural networks, evolutionary computation, and fuzzy sets (Fig. 1). Computational intelligence has been used to enable self-driving cars and…

The practice of designing computer systems. Just as the architecture of a building describes its overall structural concept and principal features, the architecture of a computer system consists of a description of the overall layout and major…

The process of protecting against threats to computing systems. A threat is an event that can cause harm to computers, data or programs, or computations (Fig. 1). A failure of computer security occurs because of a vulnerability or weakness in a…

Complex systems in which computers play a major role. While complex physical systems and sophisticated software systems can help people to lead healthier and more enjoyable lives, reliance on these systems can also result in loss of money, time, and…

The evaluation of performance, from the perspectives of both developers and users, of complex systems of hardware and software. Modern computer-based information systems have become increasingly complex because of networking, distributed computing,…

Programmable computers integrated with electrically powered machines that perform household chores. Domestic robots, also known as home or service robots, are essentially programmable computers integrated with electrically powered machines that…

A burst of electromagnetic energy. In a military context—and the subject of this article—an electromagnetic pulse (EMP) is a transient electromagnetic signal produced by a nuclear explosion in or above Earth's atmosphere (Fig. 1). Though not…

The functions associated with managing the information assets of an enterprise, typically a business, company, or organization. Many companies are taking the view that information (see illustration) is an asset of the enterprise in much the same way…