Misplaced Pages

#645354

41-468: The Zaslon is a pulse-Doppler radar with a passive electronically scanned array (PESA) antenna and digital signal processing. The antenna used by the Zaslon is actually a multi-channel system comprising two separate electronically controlled arrays, an X band radar with 1700 emitters and a L band transponder with 64 emitters brought together into a single antenna. The antenna has a diameter of 1.1 meters and

82-474: A coherent oscillator with very little noise. Phase noise reduces sub-clutter visibility performance by producing apparent motion on stationary objects. Cavity magnetron and crossed-field amplifier are not appropriate because noise introduced by these devices interfere with detection performance. The only amplification devices suitable for pulse-Doppler are klystron , traveling wave tube , and solid state devices. Pulse-Doppler signal processing introduces

123-407: A phase-shift on the transmit pulse that can produce signal cancellation. Doppler has maximum detrimental effect on moving target indicator systems, which must use reverse phase shift for Doppler compensation in the detector. Doppler weather effects (precipitation) were also found to degrade conventional radar and moving target indicator radar, which can mask aircraft reflections. This phenomenon

164-404: A 1.4 m diameter (larger) antenna, with 50% to 100% better performance than Zaslon. In April 1994 it was used with an R-37 to hit a target at 300 km distance. It has a search range of 400 km for a 19/20 m RCS target and can track 24 targets at once, engaging six (282 km for 5 m). Target speed increased from Mach 5 to Mach 6, improving possibility of firing through the land. The MiG-31

205-1111: A blind velocity. Ringing artifacts pose a problem with search, detection, and ambiguity resolution in pulse-Doppler radar. Look-down Look for Look-down on one of Misplaced Pages's sister projects : [REDACTED] Wiktionary (dictionary) [REDACTED] Wikibooks (textbooks) [REDACTED] Wikiquote (quotations) [REDACTED] Wikisource (library) [REDACTED] Wikiversity (learning resources) [REDACTED] Commons (media) [REDACTED] Wikivoyage (travel guide) [REDACTED] Wikinews (news source) [REDACTED] Wikidata (linked database) [REDACTED] Wikispecies (species directory) Misplaced Pages does not have an article with this exact name. Please search for Look-down in Misplaced Pages to check for alternative titles or spellings. You need to log in or create an account and be autoconfirmed to create new articles. Alternatively, you can use

246-408: A phenomenon called scalloping. The name is associated with a series of holes that are scooped-out of the detection performance. Scalloping for pulse-Doppler radar involves blind velocities created by the clutter rejection filter. Every volume of space must be scanned using 3 or more different PRF. A two PRF detection scheme will have detection gaps with a pattern of discrete ranges, each of which has

287-404: A vulnerability region in pulse-amplitude time-domain radar . Non-Doppler radar systems cannot be pointed directly at the ground due to excessive false alarms, which overwhelm computers and operators. Sensitivity must be reduced near clutter to avoid overload. This vulnerability begins in the low-elevation region several beam widths above the horizon, and extends downward. This also exists throughout

328-455: Is 5 km to 50 km. Range and velocity cannot be measured directly using medium PRF, and ambiguity resolution is required to identify true range and speed. Doppler signals are generally above 1 kHz, which is audible, so audio signals from medium-PRF systems can be used for passive target classification. Radar systems require angular measurement. Transponders are not normally associated with pulse-Doppler radar, so sidelobe suppression

369-572: Is aimed above the horizon to avoid an excessive false alarm rate, which renders systems vulnerable. Aircraft and some missiles exploit this weakness using a technique called flying below the radar to avoid detection ( nap-of-the-earth ). This flying technique is ineffective against pulse-Doppler radar. Pulse-Doppler provides an advantage when attempting to detect missiles and low observability aircraft flying near terrain, sea surface, and weather. Audible Doppler and target size support passive vehicle type classification when identification friend or foe

410-579: Is also known as clutter rejection. Rejection velocity is usually set just above the prevailing wind speed (10 to 100 mph or 20 to 160 km/h). The velocity threshold is much lower for weather radar . | Doppler frequency × C 2 × transmit frequency | > velocity threshold . {\displaystyle \left\vert {\frac {{\text{Doppler frequency}}\times C}{2\times {\text{transmit frequency}}}}\right\vert >{\text{velocity threshold}}.} In airborne pulse-Doppler radar,

451-412: Is fixed in position with a scanning sector of ±70 degrees in azimuth and +70/−60 degrees in elevation. The X-band components of the radar uses reciprocal ferrite phase shifters that allow the radar to position beams in around 1.2 ms. This high performance is one of the big advantages of phased array radars compared with the previous generation of mechanically scanned arrays which take seconds to perform

SECTION 10

#1732855387646

492-410: Is low (above horizon with clear skies). The antenna type is an important consideration for multi-mode radar because undesirable phase shift introduced by the radar antenna can degrade performance measurements for sub-clutter visibility. The signal processing enhancement of pulse-Doppler allows small high-speed objects to be detected in close proximity to large slow moving reflectors. To achieve this,

533-410: Is not available from a transponder signal . Medium pulse repetition frequency (PRF) reflected microwave signals fall between 1,500 and 15,000 cycle per second, which is audible. This means a helicopter sounds like a helicopter, a jet sounds like a jet, and propeller aircraft sound like propellers. Aircraft with no moving parts produce a tone. The actual size of the target can be calculated using

574-505: Is one of only a few aircraft able to intercept and destroy cruise missiles flying at extremely low heights. Pulse-Doppler radar A pulse-Doppler radar is a radar system that determines the range to a target using pulse-timing techniques, and uses the Doppler effect of the returned signal to determine the target object's velocity. It combines the features of pulse radars and continuous-wave radars , which were formerly separate due to

615-472: Is required for practical operation. Tracking radar systems use angle error to improve accuracy by producing measurements perpendicular to the radar antenna beam. Angular measurements are averaged over a span of time and combined with radial movement to develop information suitable to predict target position for a short time into the future. The two angle error techniques used with tracking radar are monopulse and conical scan . Pulse-Doppler radar requires

656-535: Is the angle offset between the antenna position and the aircraft flight trajectory. Surface reflections appear in almost all radar. Ground clutter generally appears in a circular region within a radius of about 25 miles (40 km) near ground-based radar. This distance extends much further in airborne and space radar. Clutter results from radio energy being reflected from the earth surface, buildings, and vegetation. Clutter includes weather in radar intended to detect and report aircraft and spacecraft. Clutter creates

697-487: Is the phase shift induced by range motion. Rejection speed is selectable on pulse-Doppler aircraft-detection systems so nothing below that speed will be detected. A one degree antenna beam illuminates millions of square feet of terrain at 10 miles (16 km) range, and this produces thousands of detections at or below the horizon if Doppler is not used. Pulse-Doppler radar uses the following signal processing criteria to exclude unwanted signals from slow-moving objects. This

738-504: Is to reduce the transmitted power while achieving acceptable performance for improved safety of stealthy radar. Pulse-Doppler techniques also find widespread use in meteorological radars , allowing the radar to determine wind speed from the velocity of any precipitation in the air. Pulse-Doppler radar is also the basis of synthetic aperture radar used in radar astronomy , remote sensing and mapping. In air traffic control , they are used for discriminating aircraft from clutter. Besides

779-464: The article wizard to submit a draft for review, or request a new article . Search for " Look-down " in existing articles. Look for pages within Misplaced Pages that link to this title . Other reasons this message may be displayed: If a page was recently created here, it may not be visible yet because of a delay in updating the database; wait a few minutes or try the purge function . Titles on Misplaced Pages are case sensitive except for

820-524: The frequency ambiguity resolution process. The range resolution is the minimal range separation between two objects traveling at the same speed before the radar can detect two discrete reflections: range resolution = C PRF × ( number of samples between transmit pulses ) . {\displaystyle {\text{range resolution}}={\frac {C}{{\text{PRF}}\times ({\text{number of samples between transmit pulses}})}}.} In addition to this sampling limit,

861-581: The MiG-31BM: The onboard radar complex of the MiG-31BM can track 24 airborne targets at one time, 6 of which can be simultaneously attacked by R-33S missiles. The MiG-31M, MiG-31D, and MiG-31BM standard aircraft have an upgraded Zaslon-M radar, with larger antenna and greater detection range (said to be 400 km (250 mi) against AWACS -size targets) and the ability to attack multiple targets — air and ground — simultaneously. The Zaslon-M has

SECTION 20

#1732855387646

902-636: The Russians even removing the radome of the fighter to allow the Zaslon's revolutionary antenna to be seen. Also at Paris was the US F-117 Nighthawk (revolutionary for its use of stealth technology). Zaslon uses an Argon-15A computer (first airborne digital computer designed in USSR by Research Institute of Computer Engineering (NICEVT, currently NII Argon). Adopted in 1981 RP-31 N007 backstop (Russian - Zaslon ). The basic differences between other versions and

943-501: The Soviet airspace at low altitude (through Terrain masking / clutter (radar) ), without being detected. The radar was a landmark in aviation since it was the first time a PESA radar (previously found only on ground-based systems and the B-1 strategic bomber ) had been installed in a jet fighter. The Zaslon radar was publicly unveiled at the 1991 Paris Airshow with its associated MiG-31 interceptor,

984-558: The United States Air Force, and later for the Lockheed YF-12 . The US's first pulse-Doppler radar, the system had look-down/shoot-down capability and could track one target at a time. It became possible to use pulse-Doppler radar on aircraft after digital computers were incorporated in the design. Pulse-Doppler provided look-down/shoot-down capability to support air-to-air missile systems in most modern military aircraft by

1025-405: The above conventional surveillance applications, pulse-Doppler radar has been successfully applied in healthcare, such as fall risk assessment and fall detection, for nursing or clinical purposes. The earliest radar systems failed to operate as expected. The reason was traced to Doppler effects that degrade performance of systems not designed to account for moving objects. Fast-moving objects cause

1066-510: The angle of the antenna (or similar means) to determine the bearing. However, this only worked when the radar antenna was not pointed down; in that case the reflection off the ground overwhelmed any returns from other objects. As the ground moves at the same speed but opposite direction of the aircraft, Doppler techniques allow the ground return to be filtered out, revealing aircraft and vehicles. This gives pulse-Doppler radars " look-down/shoot-down " capability. A secondary advantage in military radar

1107-458: The audible signal. Ambiguity processing is required when target range is above the red line in the graphic, which increases scan time. Scan time is a critical factor for some systems because vehicles moving at or above the speed of sound can travel one mile (1.6 km) every few seconds, like the Exocet , Harpoon , Kitchen , and air-to-air missiles . The maximum time to scan the entire volume of

1148-498: The complexity of the electronics . The first operational pulse-Doppler radar was in the CIM-10 Bomarc , an American long range supersonic missile powered by ramjet engines, and which was armed with a W40 nuclear weapon to destroy entire formations of attacking enemy aircraft. Pulse-Doppler systems were first widely used on fighter aircraft starting in the 1960s. Earlier radars had used pulse-timing in order to determine range and

1189-924: The duration of the transmitted pulse could mean that returns from two targets will be received simultaneously from different parts of the pulse. The velocity resolution is the minimal radial velocity difference between two objects traveling at the same range before the radar can detect two discrete reflections: velocity resolution = C × PRF 2 × transmit frequency × filter size in transmit pulses . {\displaystyle {\text{velocity resolution}}={\frac {C\times {\text{PRF}}}{2\times {\text{transmit frequency}}\times {\text{filter size in transmit pulses}}}}.} Pulse-Doppler radar has special requirements that must be satisfied to achieve acceptable performance. Pulse-Doppler typically uses medium pulse repetition frequency (PRF) from about 3 kHz to 30 kHz. The range between transmit pulses

1230-448: The mid 1970s. Pulse-Doppler systems measure the range to objects by measuring the elapsed time between sending a pulse of radio energy and receiving a reflection of the object. Radio waves travel at the speed of light , so the distance to the object is the elapsed time multiplied by the speed of light, divided by two – there and back. Pulse-Doppler radar is based on the Doppler effect , where movement in range produces frequency shift on

1271-477: The phase of each transmitted pulse for comparison to returned echoes. Early examples of military systems includes the AN/SPG-51 B developed during the 1950s specifically for the purpose of operating in hurricane conditions with no performance degradation. The Hughes AN/ASG-18 Fire Control System was a prototype airborne radar/combination system for the planned North American XF-108 Rapier interceptor aircraft for

Zaslon - Misplaced Pages Continue

1312-499: The radar to separate the reflections from multiple objects located in the same volume of space by separating the objects using a spread spectrum to segregate different signals: v = target speed = λ Δ Θ 4 π Δ t , {\displaystyle v={\text{target speed}}={\frac {\lambda \Delta \Theta }{4\pi \Delta t}},} where Δ Θ {\displaystyle \Delta \Theta }

1353-502: The return signals from at least 3 different PRFs can be processed out to the maximum anticipated detection range. This is known as dwell time . Antenna motion for pulse-Doppler must be as slow as radar using MTI . Search radar that include pulse-Doppler are usually dual mode because best overall performance is achieved when pulse-Doppler is used for areas with high false alarm rates (horizon or below and weather), while conventional radar will scan faster in free-space where false alarm rate

1394-957: The returned signal has a phase difference, or phase shift , from pulse to pulse. This causes the reflector to produce Doppler modulation on the reflected signal. Pulse-Doppler radars exploit this phenomenon to improve performance. The amplitude of the successively returning pulse from the same scanned volume is I = I 0 sin ⁡ ( 4 π ( x 0 + v Δ t ) λ ) = I 0 sin ⁡ ( Θ 0 + Δ Θ ) , {\displaystyle I=I_{0}\sin \left({\frac {4\pi (x_{0}+v\Delta t)}{\lambda }}\right)=I_{0}\sin(\Theta _{0}+\Delta \Theta ),} where So Δ Θ = 4 π v Δ t λ . {\displaystyle \Delta \Theta ={\frac {4\pi v\Delta t}{\lambda }}.} This allows

1435-555: The same functions as a phased array. The detection performance of the Zaslon radar is stated to be 200 km against a target with a radar cross section (RCS) of 16 m, the radar can track up to 10 targets while engaging 4 of those at any one time with either R-33 radar guided or R-40 , R-60 IR-guided air-to-air missiles. The Zaslon was the Soviet Union's second look-down/shoot-down radar. This made it much harder for United States Air Force aircraft and cruise missiles to penetrate

1476-435: The signal reflected from the target. Doppler frequency = 2 × transmit frequency × radial velocity C . {\displaystyle {\text{Doppler frequency}}={\frac {2\times {\text{transmit frequency}}\times {\text{radial velocity}}}{C}}.} Radial velocity is essential for pulse-Doppler radar operation. As the reflector moves between each transmit pulse,

1517-477: The sky must be on the order of a dozen seconds or less for systems operating in that environment. Pulse-Doppler radar by itself can be too slow to cover the entire volume of space above the horizon unless fan beam is used. This approach is used with the AN/SPS 49(V)5 Very Long Range Air Surveillance Radar, which sacrifices elevation measurement to gain speed. Pulse-Doppler antenna motion must be slow enough so that all

1558-445: The transmitter must be coherent and should produce low phase noise during the detection interval, and the receiver must have large instantaneous dynamic range . Pulse-Doppler signal processing also includes ambiguity resolution to identify true range and velocity. The received signals from multiple PRF are compared to determine true range using the range ambiguity resolution process. The received signals are also compared using

1599-598: The velocity threshold is offset by the speed of the aircraft relative to the ground. | Doppler frequency × C 2 × transmit frequency − ground speed × cos ⁡ Θ | > velocity threshold , {\displaystyle \left\vert {\frac {{\text{Doppler frequency}}\times C}{2\times {\text{transmit frequency}}}}-{\text{ground speed}}\times \cos \Theta \right\vert >{\text{velocity threshold}},} where Θ {\displaystyle \Theta }

1640-508: The volume of moving air associated with weather phenomenon. Pulse-Doppler radar corrects this as follows. Clutter rejection capability of about 60 dB is needed for look-down/shoot-down capability, and pulse-Doppler is the only strategy that can satisfy this requirement. This eliminates vulnerabilities associated with the low-elevation and below-horizon environment. Pulse compression and moving target indicator (MTI) provide up to 25 dB sub-clutter visibility. An MTI antenna beam

1681-510: Was adapted for use with weather radar in the 1950s after declassification of some World War II systems. Pulse-Doppler radar was developed during World War II to overcome limitations by increasing pulse repetition frequency . This required the development of the klystron , the traveling wave tube , and solid state devices. Early pulse-dopplers were incompatible with other high power microwave amplification devices that are not coherent , but more sophisticated techniques were developed that record

Zaslon - Misplaced Pages Continue

#645354