Radar

:This article is about the device. For the fictional character in M*A*S*H (movie) and M*A*S*H (TV series), see Corporal Walter (Radar) O'Reilly.

Principles

Overview

Powerful radio waves are transmitted, and a receiver listens for any echoes. By analysing the reflected signal, the reflector can be located, and sometimes identified. Although the amount of signal returned is tiny, radio signals can easily be detected and amplified.Radar radio waves can be easily generated at any desired strength, detected at even tiny powers, and then amplified many times. Thus radar is suited to detecting objects at very large ranges where other reflections, like sound or visible light, would be too weak to detect.

Related Topics:
Radio - Echo - Amplified

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Reflection

The extent to which an object reflects or scatters radio waves called its radar cross section.

Related Topics:
Scatter - Radar cross section

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Electromagnetic waves reflect (scatter) from any large change in the dielectric or diamagnetic constants. This means that a solid object in air or vacuum, or other significant change in atomic density between object and what's surrounding it, will usually scatter radar (radio) waves. This is particularly true for electrically conductive materials such as metal and carbon fiber, making radar particularly well suited to the detection of aircraft and ships. Radar absorbing material, containing resistive and sometimes magnetic substances, is used on military vehicles to reduce radar reflection. This is the radio equivalent of painting something a dark color.

Related Topics:
Electromagnetic - Dielectric - Solid - Air - Vacuum - Atomic density - Electric - Metal - Carbon fiber - Aircraft - Ship

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Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave and the shape of the target. If the wavelength is much shorter than the target's size, the wave will bounce off in a way similar to the way light bounces from a mirror. If the wavelength is much longer than the size of the target, the target is polarized, like a dipole antenna. This is described by Rayleigh Scattering (like the blue sky). When the two length scales are comparable, there may be resonances. Early radars used very long wavelengths that were larger than the targets and received a vague signal, whereas some modern systems use shorter wavelengths (a few centimetres or shorter) that can image objects as small as a loaf of bread or smaller.

Related Topics:
Wave - Rayleigh Scattering - Resonance - Wavelength - Centimetre

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Radio waves reflect from curves and corners, in a way similar to glint from a rounded piece of glass. The most reflective targets for short wavelengths have 90° angles between the reflective surfaces. A structure consisting of three flat surfaces meeting at a single corner, like the corner on a box, will always reflect waves entering its opening directly back at the source. These so-called corner reflectors are commonly used as radar reflectors to make otherwise difficult-to-detect objects easier to detect, and are often found on boats in order to improve their detection in a rescue situation and reduce collisions. For generally the same reasons objects attempting to avoid detection will angle their surfaces in a way to eliminate inside corners and avoid surfaces and edges perpendicular to likely detection directions, which leads to "odd" looking stealth aircraft. These precautions do not completely eliminate reflection because of diffraction, especially at longer wavelengths. Half wavelength long wires or strips of conducting material such as chaff are very reflective but do not direct the scattered energy back toward the source.

Related Topics:
Curve - Corner - Glass - Reflective surface - Corner reflector - Stealth aircraft - Diffraction - Chaff

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Electromagnetic waves do not travel well underwater; thus for underwater applications, sonar, based on sound waves, has to be used instead of radar.

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Polarization

Polarization is the direction that the wave vibrates (more technically, the direction the electric field points). Radars use horizontal, vertical, and circular polarization to detect different types of reflections. For example, circular polarization is used to minimize the interference caused by rain. Linear polarization returns usually indicate metal surfaces, and help a search radar ignore rain. Random polarization returns usually indicate a fractal surface like rock or dirt, and are used by navigational radars.

Related Topics:
Polarization - Circular polarization - Linear polarization - Random - Rock - Dirt - Navigation

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Interference

Radar systems must overcome several different sources of unwanted signals in order to focus only on the actual targets of interest. These unwanted signals may originate from internal and external sources, both passive and active. The ability of the radar system to overcome these unwanted signals defines its signal-to-noise ratio (SNR) - the higher a system's SNR, the better it is in isolating actual targets from the surrounding noise signals.

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Noise

Signal noise is an internal source of signals, due to sub-optimal electonic components and mismatches in the electronic design (for a list of noise sources refer to the Signal noise article). Noise typically appears as an echo signal received in the radar receiver at a time when no actual radar echo is in fact returned. Therefore, most noise sources appear in the receiver and much effort is made to minimize these factors. Noise figure is a measure of the noise produced by a receiver compared to an ideal receiver, and this needs to be minimized.

Related Topics:
Signal noise - Noise figure

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Clutter

Clutter refers to actual radio frequency (RF) echos returned from targets which are by definition uninteresting to the radar operators in general. Such targets mostly include natural objects such as ground, sea, rain/snow/hail and other precipitation forms, sand storms, animals (esp. birds), atmospheric turbulences, and other atmospheric effects (ionosphere reflections, meteor trails etc.). Clutter may also be returned from man-made objects such as buildings and chaff (this latter cause being intentional).

Related Topics:
Precipitation - Turbulence - Ionosphere - Meteor - Chaff

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It should be noted that while some clutter sources may be undesirable for some radar applications (e.g., storm clouds for air-defence radars), they may be desirable for others (meteorological radars in this example). Clutter is considered a passive interference source, since it only appears in response to radar signals sent by the radar.

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There are several methods of detecting and neutralizing clutter. Many of these methods rely on the fact that clutter tends to appear static between radar scans. Therefore, when comparing subsequent scans echos, desirable targets will appear to move and all stationary echos can be eliminated. Sea clutter can be reduced by using horizontal polarization, while rain is reduced with circular polarization (note that meteorological radars wish for the opposite effect, therefore using linear polarization the better to detect percipitation). Other methods attempt to increase the signal-to-clutter ratio.

Related Topics:
Circular polarization - Linear polarization - Signal-to-clutter ratio

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CFAR (Constant False-Alarm Rate, sometimes called Automatic Gain Control, or AGC) is a method relying on the fact that clutter returns far outnumber echoes from targets of interest. The receiver's gain is automatically adjusted to maintain a constant level of overall visible clutter. While this does not help detect targets masked by stronger surrounding clutter, it does help to distinguish strong target sources. In the past, radar AGC was electronically controlled and affected the gain of the entire radar receiver. As radars evolved, AGC became computer-software controlled, and affected the gain with greater granularity, in specific detection cells.

Related Topics:
CFAR - Automatic Gain Control

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Clutter may also originate from multipath echos from valid targets due to ground reflection, atmospheric ducting or ionospheric reflection/refraction. This specific clutter type is especially bothersome, since it appears to move and behave like nother normal (point) targets of interest, thereby creating a ghost. In a typical scenario, an aircraft echo is multipath-reflected from the ground below, appearing to the receiver as an identical target below the correct one. The radar may try to unify the targets, reporting the target at an incorrect height, or worse - eliminating it on the basis of jitter or a physical impossibility. These problems can be overcome by incorporating a ground map of the radar's surroundings and eliminating all echoes which appear to originate below ground or above a certain height.

Related Topics:
Multipath - Atmospheric ducting - Ionospheric reflection - Refraction - Jitter

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Jamming

Radar jamming refers to RF signals originating from sources outside the radar, transmitting in the radar's frequency and thereby masking targets of interest. Jamming may be intentional (as an anti-radar electronic warfare (EW) tactic) or unintentional (e.g., by friendly forces operating equipment that transmits using the same frequency range). Jamming is considered an active interference source, since it is initiated by elements outside the radar and in general unrelated to the radar signals.

Related Topics:
Radar jamming - Electronic warfare

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Jamming is problematic to radar since the jamming signal only needs to travel one-way (from the jammer to the radar receiver) whereas the radar echos travel two-ways (radar-target-radar) and are therefore significantly reduced in power by the time they return to the radar receiver. Jammers therefore need be much less powerful than their jammed radars in order to effectively mask targets along the line of sight from the jammer to the radar (Mainlobe Jamming). Jammers have an added effect of affecting radars along other line-of-sights, due to the radar receiver's sidelobes (Sidelobe Jamming).

Related Topics:
Line of sight - Sidelobe

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While mainlobe jamming cannot generally be overcome, sidelobe jamming can be overcome by reducing receiving sidelobes in the radar design and by using an omnidirectional antenna to disregard non-mainlobe signals. Other anti-jamming technique are frequency hopping and polarization. See Electronic counter-counter-measures for details.

Related Topics:
Omnidirectional antenna - Frequency hopping - Polarization - Electronic counter-counter-measures

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