   #copyright

Radar

2007 Schools Wikipedia Selection. Related subjects: Engineering

   This long range RADAR antenna, known as ALTAIR, is used to detect and
   track space objects in conjunction with ABM testing at the Ronald
   Reagan Test Site on the Kwajalein atoll.
   Enlarge
   This long range RADAR antenna, known as ALTAIR, is used to detect and
   track space objects in conjunction with ABM testing at the Ronald
   Reagan Test Site on the Kwajalein atoll.

   RADAR is a system that uses radio waves to determine and map the
   location, direction, and/or speed of both moving and fixed objects such
   as aircraft, ships, motor vehicles, weather formations and terrain. A
   transmitter emits radio waves, which are reflected by the target and
   detected by a receiver, typically in the same location as the
   transmitter. Although the radio signal returned is usually very weak,
   radio signals can easily be amplified, so radar can detect objects at
   ranges where other emissions, such as sound or visible light, would be
   too weak to detect. Radar is used in many contexts, including
   meteorological detection of precipitation, air traffic control, police
   detection of speeding traffic, and by the military.

   The term RADAR was coined in 1941 as an acronym for Radio Detection and
   Ranging. This acronym of American origin replaced the previously used
   British abbreviation RDF (Radio Direction Finding). The term has since
   entered the English language as a standard word, radar, losing the
   capitalization in the process.

History

   Several inventors, scientists, and engineers contributed to the
   development of radar. The use of radio waves to detect "the presence of
   distant metallic objects via radio waves" was first implemented in 1904
   by Christian Hülsmeyer, who demonstrated the feasibility of detecting
   the presence of a ship in dense fog, but not its distance. He received
   a Reichspatent patent Nr. 165546 for his pre-radar device in April and
   on November 11, 1904 the patent 169154 an amendment of his patent for
   ranging that is indirectly related to his device. He received a patent
   (GB13170) in England for his telemobiloscope on September 22 1904.

   Prior to the Second World War, developments by the Americans, the
   Germans, the French (French Patent n° 788795 in 1934), and the British
   (British Patent GB593017 by Robert Watson-Watt in 1935), led to the
   first real radars. Hungarian Zoltán Bay produced a working model by
   1936 at the Tungsram laboratory in the same vein.

   The war precipitated the research to find better resolution, more
   portability, more features for that new defensive weapon. Post-war
   years have seen the use of radar in fields as diverse as air traffic
   control, weather monitoring, astrometry and road speed control.

Principles

Reflection

   Brightness can indicate reflectivity as in this 1960 weather radar
   image. The radar's frequency, pulse form, and antenna largely determine
   what it can observe.
   Enlarge
   Brightness can indicate reflectivity as in this 1960 weather radar
   image. The radar's frequency, pulse form, and antenna largely determine
   what it can observe.

   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
   the 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 fibre, 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 colour.

   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 is reflected by a mirror. If the
   wavelength is much longer than the size of the target, the target is
   polarized (positive and negative charges are separated), like a dipole
   antenna. This is described by Rayleigh scattering, an effect that
   creates the Earth's blue sky and red sunsets. 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.

   Short 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 to
   reduce collisions. For similar 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. The extent to which an object reflects or scatters radio waves
   is called its radar cross section.

Radar equation

   The amount of power P[r] returning to the receiving antenna is given by
   the radar equation:

          P_r = {{P_t G_t A_r \sigma F^4}\over{{(4\pi)}^2 R_t^2R_r^2}}

   where*'
     * P[t] = transmitter power
     * G[t] = gain of the transmitting antenna
     * A[r] = effective aperture (area) of the receiving antenna
     * σ = radar cross section, or scattering coefficient, of the target
     * F = pattern propagation factor
     * R[t] = distance from the transmitter to the target
     * R[r] = distance from the target to the receiver.

   In the common case where the transmitter and the receiver are at the
   same location, R[t] = R[r] and the term R[t]^2 R[r]^2 can be replaced
   by R^4, where R is the range. This yields:

          P_r = {{P_t G_t A_r \sigma}\over{{(4\pi)}^2 R^4}}

   This shows that the received power declines as the fourth power of the
   range, which means that the reflected power from distant targets is
   very, very small. (This is the basis of that old chesnut, old radar men
   never die, they just fade away as the fourth power.)

   The equation above with F = 1 is a simplification for vacuum without
   interference. The propagation factor accounts for the effects of
   multipath and shadowing and depends on the details of the environment.
   In a real-world situation, pathloss effects should also be considered.

   Other mathematical developments in radar signal processing include
   time-frequency analysis ( Weyl Heisenberg or wavelet), as well as the
   chirplet transform which makes use of the fact that radar returns from
   moving targets typically "chirp" (change their frequency as a function
   of time, as does the sound of a bird or bat).

Polarization

   In the transmitted radar signal, the electric field is perpendicular to
   the direction of propagation, and this direction of the electric field
   is the Polarization of the wave. 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, such as rocks or soil, and
   are used by navigational radars.

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.

Noise

   Signal noise is an internal source of random variations in the signal,
   which is inherently generated to some degree by all electronic
   components (for a list of noise sources refer to the Signal noise
   article). Noise typically appears as random variations superimposed on
   the desired echo signal received in the radar receiver. The lower the
   power of the desired signal, the more difficult it is to discern it
   from the noise (similar to trying to hear a whisper while standing near
   a busy road). Therefore, the most important 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.

   Noise is also generated by external sources, most importantly the
   natural thermal radiation of the background scene surrounding the
   target of interest. In modern radar systems, due to the high
   performance of their receivers, the internal noise is typically about
   equal to or lower than the external scene noise. An exception is if the
   radar is aimed upwards at clear sky, where the scene is so cold that it
   generates very little thermal noise.

Clutter

   Clutter refers to actual radio frequency (RF) echoes returned from
   targets which are by definition uninteresting to the radar operators in
   general. Such targets mostly include natural objects such as ground,
   sea, precipitation (such as rain, snow or hail), sand storms, animals
   (especially birds), atmospheric turbulence, and other atmospheric
   effects (such as ionosphere reflections and meteor trails). Clutter may
   also be returned from man-made objects such as buildings and,
   intentionally, by radar countermeasures such as chaff.

   Some clutter may also be caused by a long waveguide between the radar
   transceiver and the antenna. In a typical PPI radar with a rotating
   antenna, this will usually be seen as a "sun" or "sunburst" in the
   centre of the display as the receiver responds to echoes from dust
   particles and misguided RF in the waveguide. Adjusting the timing
   between when the transmitter sends a pulse and when the receiver stage
   is enabled will generally reduce the sunburst without affecting the
   accuracy of the range, since most sunburst is caused by diffused
   transmit pulse reflected before it leaves the antenna.

   While some clutter sources may be undesirable for some radar
   applications (such as 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.

   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 echoes,
   desirable targets will appear to move and all stationary echoes 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 precipitation). Other
   methods attempt to increase the signal-to-clutter ratio.

   CFAR (Constant False-Alarm Rate, a form of 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.
   Radar multipath echoes from an actual target cause ghosts to appear.
   Enlarge
   Radar multipath echoes from an actual target cause ghosts to appear.

   Clutter may also originate from multipath echoes 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 other 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.

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.

   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 echoes travel two-ways (radar-target-radar) and are therefore
   significantly reduced in power by the time they return to the radar
   receiver. Jammers therefore can be much less powerful than their jammed
   radars and still 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).

   Mainlobe jamming can generally only be reduced by narrowing the
   mainlobe solid angle, and can never fully be eliminated when directly
   facing a jammer which uses the same frequency and polarization as the
   radar. Sidelobe jamming can be overcome by reducing receiving sidelobes
   in the radar antenna design and by using an omnidirectional antenna to
   detect and disregard non-mainlobe signals. Other anti-jamming
   techniques are frequency hopping and polarization. See Electronic
   counter-counter-measures for details.

   Interference has recently become a problem for C-band (5.66  GHz)
   meteorological radars with the proliferation of 5.4 GHz band WiFi
   equipment.

Radar signal processing

Distance measurement

Transit time

   Pulse radar
   Enlarge
   Pulse radar
   Enlarge

   Principle of radar distance measurement using pulse round trip time.
   One way to measure the distance to an object is to transmit a short
   pulse of radio signal, and measure the time it takes for the reflection
   to return. The distance is one-half the product of round trip time
   (because the signal has to travel to the target and then back to the
   receiver) and the speed of the signal. where c is the speed of light in
   a vacuum, and τ is the round trip time. For radar, the speed of signal
   is the speed of light, making the round trip times very short for
   terrestrial ranging. Accurate distance measurement requires
   high-performance electronics.

   The receiver cannot detect the return while the signal is being sent
   out – there is no way to tell if the signal it hears is the original or
   the return. This means that a radar has a distinct minimum range, which
   is the length of the pulse multiplied by the speed of light, divided by
   two. In order to detect closer targets one must use a shorter pulse
   length.

   A similar effect imposes a specific maximum range as well. If the
   return from the target comes in when the next pulse is being sent out,
   once again the receiver cannot tell the difference. In order to
   maximize range, one wants to use longer times between pulses, the
   inter-pulse time.

   These two effects tend to be at odds with each other, and it is not
   easy to combine both good short range and good long range in a single
   radar. This is because the short pulses needed for a good minimum range
   broadcast have less total energy, making the returns much smaller and
   the target harder to detect. This could be offset by using more pulses,
   but this would shorten the maximum range again. So each radar uses a
   particular type of signal. Long range radars tend to use long pulses
   with long delays between them, and short range radars use smaller
   pulses with less time between them. This pattern of pulses and pauses
   is known as the Pulse Repetition Frequency (or PRF), and is one of the
   main ways to characterize a radar. As electronics have improved many
   radars now can change their PRF.

Frequency modulation

   Another form of distance measuring radar is based on frequency
   modulation. Frequency comparison between two signals is considerably
   more accurate, even with older electronics, than timing the signal. By
   changing the frequency of the returned signal and comparing that with
   the original, the difference can be easily measured.

   This technique can be used in radar systems, and is often found in
   aircraft radar altimeters. In these systems a "carrier" radar signal is
   frequency modulated in a predictable way, typically varying up and down
   with a sine wave or sawtooth pattern at audio frequencies. The signal
   is then sent out from one antenna and received on another, typically
   located on the bottom of the aircraft, and the signal can be
   continuously compared.

   Since the signal frequency is changing, by the time the signal returns
   to the aircraft the broadcast has shifted to some other frequency. The
   amount of that shift is greater over longer times, so greater frequency
   differences mean a longer distance, the exact amount being the "ramp
   speed" selected by the electronics. The amount of shift is therefore
   directly related to the distance travelled, and can be displayed on an
   instrument. This signal processing is similar to that used in speed
   detecting Doppler radar. See the article on continuous wave radar for
   more information.

Speed measurement

   Speed is the change in distance to an object with respect to time. Thus
   the existing system for measuring distance, combined with a little
   memory to see where the target last was, is enough to measure speed. At
   one time the memory consisted of a user making grease-pencil marks on
   the radar screen, and then calculating the speed using a slide rule.
   Modern radar systems perform the equivalent operation faster and more
   accurately using computers.

   However, if the transmitter's output is coherent (phase synchronized),
   there is another effect that can be used to make almost instant speed
   measurements (no memory is required), known as the Doppler effect. Most
   modern radar systems use this principle in the pulse-doppler radar
   system. Return signals from targets are shifted away from this base
   frequency via the Doppler effect enabling the calculation of the speed
   of the object relative to the radar. The Doppler effect is only able to
   determine the relative speed of the target along the line of sight from
   the radar to the target. Any component of target velocity perpendicular
   to this line of sight cannot be determined by Doppler alone---tracking
   the target's azimuth over time must be used.

   It is also possible to make a radar without any pulsing, known as a
   continuous-wave radar (CW radar), by sending out a very pure signal of
   a known frequency. CW radar is ideal for determining the radial
   component of a target's velocity, but it cannot determine the target's
   range. CW radar is typically used by traffic enforcement to measure
   vehicle speed quickly and accurately where range is not important.

Reduction of interference effects

   Signal processing is employed in radar systems to reduce the
   interference effects. Signal processing techniques include moving
   target indication (MTI), pulse doppler, moving target detection (MTD)
   processors, correlation with secondary surveillance radar (SSR) targets
   and space-time adaptive processing (STAP). Constant false alarm rate
   (CFAR) and digital terrain model (DTM) processing are also used in
   clutter environments.

Radar engineering

   Radar components
   Enlarge
   Radar components

   A radar has different components:
     * A transmitter that generates the radio signal with an oscillator
       such as a klystron or a magnetron and controls its duration by a
       modulator.
     * A waveguide that links the transmitter and the antenna.
     * A duplexer that serves as a switch between the antenna and the
       transmitter or the receiver for the signal when the antenna is used
       in both situations.
     * A receiver.
     * An electronic section that controls all those devices and the
       antenna to perform the radar scan ordered by a software.
     * A link to end users.

Antenna design

   Radio signals broadcast from a single antenna will spread out in all
   directions, and likewise a single antenna will receive signals equally
   from all directions. This leaves the radar with the problem of deciding
   where the target object is located.

   Early systems tended to use omni-directional broadcast antennas, with
   directional receiver antennas which were pointed in various directions.
   For instance the first system to be deployed, Chain Home, used two
   straight antennas at right angles for reception, each on a different
   display. The maximum return would be detected with an antenna at right
   angles to the target, and a minimum with the antenna pointed directly
   at it (end on). The operator could determine the direction to a target
   by rotating the antenna so one display showed a maximum while the other
   shows a minimum.

   One serious limitation with this type of solution is that the broadcast
   is sent out in all directions, so the amount of energy in the direction
   being examined is a small part of that transmitted. To get a reasonable
   amount of power on the "target", the transmitting aerial should also be
   directional.

Parabolic reflector

   More modern systems used a steerable parabolic "dish" to create a tight
   broadcast beam, typically using the same dish as the receiver. Such
   systems often combined two radar frequencies in the same antenna in
   order to allow automatic steering, or radar lock.

Types of Scan

   Primary Scan – A scanning technique where the main antenna aerial is
   moved to produce a scanning beam, examples include circular scan,
   sector scan etc

   Secondary Scan – A scanning technique where the antenna feed is moved
   to produce a scanning beam, example include conical scan,
   unidirectional sector scan, loge switching etc.

   Palmer Scan – A scanning technique that produces a scanning beam by
   moving the main antenna and its feed. A Palmer Scan is a combination of
   a Primary Scan and a Secondary Scan.

Slotted waveguide

   Applied similarly to the parabolic reflector the slotted waveguide is
   moved mechanically to scan and is particularly suitable for
   non-tracking surface scan systems, where the vertical pattern may
   remain constant. Owing to lower cost and less wind exposure, shipboard,
   airport surface, and harbour surveillance radars now use this in
   preference to the parabolic antenna.

Phased array

   Another method of steering is used in a phased array radar. This uses
   an array of similar aerials suitably spaced, the phase of the signal to
   each individual aerial being controlled so that the signal is
   reinforced in the desired direction and cancels in other directions. If
   the individual aerials are in one plane and the signal is fed to each
   aerial in phase with all others then the signal will reinforce in a
   direction perpendicular to that plane. By altering the relative phase
   of the signal fed to each aerial the direction of the beam can be moved
   because the direction of constructive interference will move. Because
   phased array radars require no physical movement the beam can scan at
   thousands of degrees per second, fast enough to irradiate and track
   many individual targets, and still run a wide-ranging search
   periodically. By simply turning some of the antennas on or off, the
   beam can be spread for searching, narrowed for tracking, or even split
   into two or more virtual radars. However, the beam cannot be
   effectively steered at small angles to the plane of the array, so for
   full coverage multiple arrays are required, typically disposed on the
   faces of a triangular pyramid (see picture).

   Phased array radars have been in use since the earliest years of radar
   use in World War II, but limitations of the electronics led to fairly
   poor accuracy. Phased array radars were originally used for missile
   defense. They are the heart of the ship-borne Aegis combat system, and
   the Patriot Missile System, and are increasingly used in other areas
   because the lack of moving parts makes them more reliable, and
   sometimes permits a much larger effective antenna, useful in fighter
   aircraft applications that offer only confined space for mechanical
   scanning.

   As the price of electronics has fallen, phased array radars have become
   more and more common. Almost all modern military radar systems are
   based on phased arrays, where the small additional cost is far offset
   by the improved reliability of a system with no moving parts.
   Traditional moving-antenna designs are still widely used in roles where
   cost is a significant factor such as air traffic surveillance, weather
   radars and similar systems.

   Phased array radars are also valued for use in aircraft, since they can
   track multiple targets. The first aircraft to use phased array radar
   was the Mikoyan MiG-31. The MiG-31M's SBI-16 Zaslon phased array radar
   is considered to be the world's most powerful fighter radar.

Frequency bands

   The traditional band names originated as code-names during World War II
   and are still in military and aviation use throughout the world in the
   21st century. They have been adopted in the United States by the IEEE,
   and internationally by the ITU. Most countries have additional
   regulations to control which parts of each band are available for
   civilian or military use.

   Other users of the radio spectrum, such as the broadcasting and
   electronic countermeasures ( ECM) industries, have replaced the
   traditional military designations with their own systems.

   CAPTION: Radar frequency bands

   Band Name Frequency Range Wavelength Range Notes
   HF 3-30 MHz 10-100 m coastal radar systems, over-the-horizon (OTH)
   radars; 'high frequency'
   P < 300 MHz 1 m+ 'P' for 'previous', applied retrospectively to early
   radar systems
   VHF 50-330 MHz 0.9-6 m very long range, ground penetrating; 'very high
   frequency'
   UHF 300-1000 MHz 0.3-1 m very long range (e.g. ballistic missile early
   warning), ground penetrating, foliage penetrating; 'ultra high
   frequency'
   L 1-2 GHz 15-30 cm long range air traffic control and surveillance; 'L'
   for 'long'
   S 2-4 GHz 7.5-15 cm terminal air traffic control, long range weather,
   marine radar; 'S' for 'short'
   C 4-8 GHz 3.75-7.5 cm Satellite transponders; a compromise (hence 'C')
   between X and S bands; weather
   X 8-12 GHz 2.5-3.75 cm missile guidance, marine radar, weather,
   medium-resolution mapping and ground surveillance; in the USA the
   narrow range 10.525 GHz ±25 MHz is used for airport radar. Named X band
   because the frequency was a secret during WW2.
   K[u] 12-18 GHz 1.67-2.5 cm high-resolution mapping, satellite
   altimetry; frequency just under K band (hence 'u')
   K 18-27 GHz 1.11-1.67 cm from German kurz, meaning 'short'; limited use
   due to absorption by water vapour, so K[u] and K[a] were used instead
   for surveillance. K-band is used for detecting clouds by
   meteorologists, and by police for detecting speeding motorists. K-band
   radar guns operate at 24.150 ± 0.100 GHz.
   K[a] 27-40 GHz 0.75-1.11 cm mapping, short range, airport surveillance;
   frequency just above K band (hence 'a') Photo radar, used to trigger
   cameras which take pictures of license plates of cars running red
   lights, operates at 34.300 ± 0.100 GHz.
   mm 40-300 GHz 7.5 mm - 1 mm millimetre band, subdivided as below. The
   letter designators appear to be random, and the frequency ranges
   dependent on waveguide size. Multiple letters are assigned to these
   bands by different groups. These are from Baytron, a now defunct
   company that made test equipment.
   Q 40-60 GHz 7.5 mm - 5 mm Used for Military communication.
   V 50-75 GHz 6.0 - 4 mm Very strongly absorbed by the atmosphere.
   E 60-90 GHz 6.0 - 3.33 mm
   W 75-110 GHz 2.7 - 4.0 mm used as a visual sensor for experimental
   autonomous vehicles, high-resolution meteorological observation, and
   imaging.

Radar modulators

   Modulators are sometimes called pulsers and act to provide the short
   pulses of power to the magnetron. This technology is known as Pulsed
   power. In this way, the transmitted pulse of RF radiation is kept to a
   defined, and usually very short, duration. Modulators consist of a high
   voltage pulse generator formed from a HV supply, a pulse forming
   network or line (PFN) and a high voltage switch such as a thyratron.

   A klystron tube is an amplifier, so it can be modulated by its low
   power input signal.

Radar Coolant

   Coolanol and PAO (poly alpha olefin) are the two main coolants used to
   cool Airborne Radar equipment.

   The U.S. Navy has instituted a program for Pollution Prevention (P2) to
   reduce or eliminate the volume and toxicity of waste, air emissions,
   and effluent discharges. Because of this Coolanol is used less often
   today.

   PAO is a synthetic lubricant composition is a blend of a polyol ester
   admixed with effective amounts of an antioxidant, yellow metal pacifier
   and rust inhibitors. The polyol ester blend includes a major proportion
   of poly(neopentyl polyol) ester blend formed by reacting poly(
   pentaerythritol) partial esters with at least one C7 to C12 carboxylic
   acid mixed with an ester formed by reacting a polyol having at least
   two hydroxyl groups and at least one C8-C10 carboxylic acid.
   Preferably, the acids are linear and avoid those which can cause odours
   during use. Effective additives include secondary arylamine
   antioxidants, triazole derivative yellow metal pacifier and an amino
   acid derivative and substituted primary and secondary amine and/or
   diamine rust inhibitor.

   A synthetic coolant/lubricant composition, comprising an ester mixture
   of:

   50 to 80 weight percent of poly(neopentyl polyol) ester formed by
   reacting a poly(neopentyl polyol) partial ester and at least one linear
   monocarboxylic acid having from 6 to 12 carbon atoms, and 20 to 50
   weight percent of a polyol ester formed by reacting a polyol having 5
   to 8 carbon atoms and at least two hydroxyl groups with at least one
   linear monocarboxylic acid having from 7 to 12 carbon atoms, the weight
   percents based on the total weight of the composition.

Radar functions and roles

   Radar display commonly found on ships
   Enlarge
   Radar display commonly found on ships

Detection and search radars

     * Early Warning (EW) Radar Systems
          + Early Warning Radar
          + Ground Control Intercept (GCI) Radar
          + Airborne Early Warning (AEW)
          + Over-the-Horizon (OTH) Radar
     * Target Acquisition (TA) Radar Systems
          + Surface-to-Air Missile (SAM) Systems
          + Anti-Aircraft Artillery (AAA) Systems
     * Surface Search (SS) Radar Systems
          + Surface Search Radar
          + Coastal Surveillance Radar
          + Harbour Surveillance Radar
          + Antisubmarine Warfare (ASW) Radar
     * Height Finder (HF) Radar Systems
     * Gap Filler Radar Systems

Threat radars

     * Target Tracking (TT) Systems
          + AAA Systems
          + SAM Systems
          + Precision Approach Radar (PAR) Systems
     * Multi-Function Systems
          + Fire Control (FC) Systems
               o Acquisition Mode
               o Semiautomatic Tracking Mode
               o Manual Tracking Mode
          + Airborne Intercept (AI) Radars
               o Search Mode
               o TA Mode
               o TT Mode
               o Target Illumination (TI) Mode
               o Missile Guidance (MG) Mode

Missile guidance systems

     * Air-to-Air Missile (AAM)
     * Air-to-Surface Missile (ASM)
     * SAM Systems
     * Surface-to-Surface Missiles (SSM) Systems

Battlefield and reconnaissance radar

     * Battlefield Surveillance Systems
          + Battlefield Surveillance Radars
     * Countermortar/Counterbattery Systems
          + Shell Tracking Radars
     * Air Mapping Systems
          + Side Looking Airborne Radar (SLAR)
          + Synthetic Aperture Radar (SAR)

Air Traffic Control and Navigation

     * Air Traffic Control Systems
          + Air Traffic Control (ATC) Radars
          + Secondary Surveillance Radar (SSR) (Airport Surveillance
            Radar)
          + Ground Control Approach (GCA) Radars
          + PAR Systems
     * Distance Measuring Equipment (DME)
     * Radio Beacons
     * Identification Friend or Foe (IFF) Systems
          + IFF Interrogator
          + IFF Transponder
     * Altimeter (AL) Radar Systems
     * Terrain-Following Radar (TFR) Systems

Space and range instrumentation Radar systems

     * Space (SP) Tracking Systems
     * Range Instrumentation (RI) Systems
     * Video Relay/Downlink Systems
     * Space-Based Radar

Weather-sensing Radar systems

     * Weather radar

          + Doppler weather radar

     * Wind profilers

   Retrieved from " http://en.wikipedia.org/wiki/Radar"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
