Radar

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Radar is an object detection system that uses radio waves to determine the range, angle, or velocity of an object. It can be used to detect planes, ships, spacecraft, missiles, motor vehicles, weather formations, and terrain. The radar system consists of a transmitter that generates electromagnetic waves in radio or microwave domains, transmitting antennas, receiving antennas (often the same antenna used to transmit and receive) and the receiver and processor to determine the object property (s). Radio waves (pulsed or continuous) of the transmitter reflect the object and return to the receiver, providing information about the location and velocity of the object.

Radar was developed secretly for military use by some countries in the period before and during World War II. The key development is a magnetron cavity in the UK, which allows the creation of relatively small systems with sub-meter resolution. The term RADAR was created in 1940 by the United States Navy as an acronym for RA dio D etection A nd R anging or RA dio D the A nd R anging. The term radar has included English and other languages ​​as common nouns, missing all capital letters.

The use of modern radar is very diverse, including air and terrestrial traffic control, radar astronomy, air defense systems, antimisal systems, marine radar to find landmarks and other ships, aircraft anticollision systems, marine surveillance systems, aerospace surveillance and meeting systems, meteorological rains, altimetry and flight control systems, missile target search systems, ground penetrating radar for geological observation, and controlled radar with coverage for public health surveillance. High-tech radar systems are associated with digital signal processing, machine learning and are able to extract useful information from very high noise levels.

Other systems similar to radar utilize other parts of the electromagnetic spectrum. One such example is "lidar", which uses mostly infrared light from the laser rather than radio waves.

Video Radar

Histori

Eksperimen pertama

In early 1886, German physicist Heinrich Hertz showed that radio waves could be reflected from solid objects. In 1895, Alexander Popov, a physics instructor at the Russian Imperial Navy school in Kronstadt, developed a device using a coherent tube to detect lightning strikes from afar. The next year, he added a spark-gap transmitter. In 1897, while testing this equipment to communicate between two ships in the Baltic Sea, he noted interference disturbances caused by the passage of the third vessel. In his report, Popov writes that this phenomenon might be used to detect objects, but he does not do anything else with these observations.

German inventor Christian HÃÆ'¼lsmeyer was the first to use radio waves to detect "the presence of distant metal objects". In 1904, he demonstrated the feasibility of detecting ships in thick fog, but not its distance from the transmitter. He obtained a patent for the detector in April 1904 and then patent for an associated amendment to estimate the distance to the ship. He also obtained a British patent on September 23, 1904 for a full radar system, which he referred to as a telemobiloscope. It is operated at a 50 cm wavelength and pulsed radar signals are created through a split gap. The system already uses a classical antenna antenna horn with a parabolic reflector and is presented to German military officials on a practical test at the ports of Cologne and Rotterdam but is rejected.

In 1915, Robert Watson-Watt used radio technology to give advance warning to the aviators and during the 1920s went on to lead the formation of British research to make a lot of progress using radio techniques, including probing ionosphere and lightning detection at a distance. Through his lightning experiments, Watson-Watt became an expert in the use of radio directional search before diverting his questions to shortwave transmissions. Needing a suitable recipient for such a study, he told "new boy" Arnold Frederic Wilkins to conduct an extensive overview of the shortwave units available. Wilkins will choose the model of the Post Office General after recording his manual description of the "fading" effect (common term for interference at the time) when the plane flies over.

Across the Atlantic in 1922, after placing the transmitter and receiver on the opposite side of the Potomac River, US Navy researchers A. Hoyt Taylor and Leo C. Young found that ships passing through the jets caused the received signal to fade in and out. Taylor submitted the report, pointing out that this phenomenon might be used to detect the presence of the ship in low visibility, but the Navy did not immediately resume work. Eight years later, Lawrence A. Hyland at the Naval Research Laboratory (NRL) observed similar fading effects from passing aircraft; this revelation led to a patent application as well as a proposal for further intensive research on radio-echo signals from a moving target to take place at the NRL, where Taylor and Young were based at the time.

Just before World War II

Prior to the Second World War, researchers in Britain, France, Germany, Italy, Japan, the Netherlands, the Soviet Union and the United States, independently and in highly classified and advanced technology leading to modern radar versions. Australia, Canada, New Zealand and South Africa followed radar developments of the United Kingdom before the war, and Hungary produced radar technology during the war.

In France in 1934, following a systematic study on Split Anode Magnetron, the research arm of Compagnie GÃÆ' © © Tale © graphie Sans Fil (CSF) led by Maurice Ponte with Henri Gutton, Sylvain Berline and M. Hugon, began to develop a radio-tracer device, an aspect mounted on a naval ship Normandie in 1935.

During the same period, Soviet military engineer P. K. Oshchepkov, in collaboration with the Leningrad Electrophysical Institute, produced an experimental tool, RAPID, capable of detecting aircraft within 3 km of the receiver. The Soviets produced the first mass radar production of RUS-1 and RUS-2 Redut in 1939 but further development was slowed after Oshchepkov's capture and subsequent gulag penalty. In total, only 607 Redut stations were produced during the war. The first Russian air radar, Gneiss-2, began operation in June 1943 on a Pe-2 fighter. More than 230 Gneiss-2 stations were manufactured in late 1944. But the French and Soviet systems, featuring continuous wave operations that do not provide full performance are ultimately synonymous with modern radar systems.

The full radar evolved as a pulsating system, and the first basic equipment as demonstrated in December 1934 by American Robert M. Page, who worked at the Naval Research Laboratory. The following year, the United States Army successfully tested a primitive surface-to-surface radar to direct the battery floodlights at night. This design was followed by a pulsating system shown in May 1935 by Rudolf KÃÆ'¼nnhold and GEMA in Germany and then in June 1935 by the Air Ministry team led by Robert A. Watson-Watt in the United Kingdom.

In 1935, Watson-Watt was asked to assess the latest reports of German-based radio-based death rays and divert requests to Wilkins. Wilkins returned a series of calculations showing that the system was basically impossible. When Watson-Watt later asked what such a system might be, Wilkins recalled earlier reports about the plane causing radio interference. This revelation led to the Daventry Experiment February 26, 1935, using a powerful shortwave BBC transmitter as the source and setting of the GPO receiver in the field while a bomber flew around the site. When the plane was clearly detected, Hugh Dowding, Air Member for Supply and Research was very impressed with the potential of their system and funds were immediately provided for further operational development. Watson-Watt team patented the device in GB593017.

The development of the radar grew very rapidly on September 1, 1936 when Watson-Watt became Superintendent of a new company under the British Air Ministry, Bawdsey Research Station located at Bawdsey Manor, near Felixstowe, Suffolk. The work there resulted in the design and installation of aircraft detection and tracking stations called "Home Networks" along the eastern and southern coast of England in time for the outbreak of World War II in 1939. The system provided important information that helped the Royal Air Force win the Battle of Britain; without it, a large number of combat aircraft will always need to be in the air to respond quickly enough if enemy enemy detection relies solely on observations from land-based individuals. Also important is the "Dowding system" reporting and coordination to utilize radar information during initial radar deployment testing in 1936 and 1937.

Given all the funding and development support needed, the team produced a radar system that worked in 1935 and began to use. In 1936, the first five-Chain Home (CH) system operated and in 1940 stretched across England including Northern Ireland. Even by the standards of the times, CH is raw; Instead of broadcasting and receiving from the intended antenna, CH broadcasts the spotlight signal to the entire area in front of it, and then uses one of Watson-Watt's own radio searchers to determine the direction of the returning echo. This fact means that the CH transmitter must be much more robust and has a better antenna than a competitor system but allows for rapid recognition using existing technology.

During World War II

The key development is a magnetron cavity in the UK, which allows the creation of relatively small systems with sub-meter resolution. Britain shared the technology with the US during the 1940 Tadal Mission.

In April 1940, Popular Science shows an example of a radar unit that uses Watson-Watt patents in an article on air defense. Also, by the end of 1941 Popular Mechanics had an article in which a US scientist speculated on the British early warning system on the east coast of England and approached what it was and how it worked. Watson-Watt was sent to the United States in 1941 to advise on air defense after the Japanese attack on Pearl Harbor. Alfred Lee Loomis organized the Radiation Laboratory in Cambridge, Massachusetts that developed the technology in 1941-45. Then, in 1943, the Page greatly improved the radar with monopulse techniques used for years in most radar applications.

The war speeds up research to find better resolution, more portability, and more features for radar, including complementary navigation systems such as Oboe used by Pathfinder RAF.

Maps Radar

Apps

The information provided by the radar includes the bearing and the range (and therefore position) of the object from the radar scanner. It is thus used in various fields where the need for such a position is essential. The first radar use was for military purposes: to search for air, land and sea targets. It evolved in the civil field into applications for aircraft, ships, and roads.

In flight, the aircraft can be equipped with radar devices that warn planes or other obstacles in or near their tracks, display weather information, and provide accurate altitude readings. The first commercial device plugged into the plane was Bell Lab 1938 units on several United Air Lines aircraft. The aircraft can land in fog at airports equipped with a radar-controlled detection system in which the position of the aircraft is observed on the radar screen by the operator directing the landing instructions to the pilot, maintaining the aircraft in the approach path determined to the runway. Military combat aircraft are usually equipped with air-to-air indicating radar, to detect and target enemy aircraft. In addition, larger special military aircraft carry strong air radar to observe air traffic in large areas and direct fighter jets towards targets.

Sea radar is used to measure bearings and ship distances to prevent collisions with other vessels, to navigate, and to improve their position at sea when within reach of a beach or other fixed reference such as island, buoy, and illumination. At ports or at harbors, radar traffic service systems of vessels are used to monitor and regulate the movement of ships in busy waters.

Meteorologists use radar to monitor rainfall and wind. It has become a major tool for short-term weather forecasts and oversees severe weather such as storms, tornadoes, winter storms, rain types, etc. Geologists use special ground penetrating radar to map out the composition of the Earth's crust. Police use radar guns to monitor the speed of vehicles on the road. Smaller radar systems are used to detect human movement. Examples are breathing pattern detection for sleep monitoring and hand and finger movement detection for computer interaction. Automatic door opening, light activation and intruder sensing are also common.

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Principles

radar signal

The radar system has a transmitter that emits radio waves called radar signals in a predetermined direction. When these contacts with their objects are usually reflected or scattered in many directions. But some of them absorb and penetrate to the target to some extent. The radar signal is reflected very well by sufficient electrical conductivity materials - mainly by most metals, by sea water and by wet soils. Some of these allow the use of radar altimeter. The radar signal that is reflected back to the transmitter is the desired signal that makes the radar work. If the object is moving either towards or away from the transmitter, there is little equivalent change in the frequency of the radio waves, caused by the Doppler effect.

The radar receiver is usually, but not always, in the same location as the transmitter. Although the reflected radar signal captured by the receiving antenna is usually very weak, they can be amplified by electronic amplifiers. More sophisticated signal processing methods are also used to recover useful radar signals.

The weak absorption of radio waves by the media it passes is what allows the radar sets to detect objects over a relatively long range - ranges in which other electromagnetic wavelengths, such as visible light, infrared light, and ultraviolet light, are so attenuated strongly.. Weather phenomena such as fog, clouds, rain, snow, and hail that blocks visible light are usually transparent to radio waves. Certain radio frequencies absorbed or dispersed by water vapor, rain, or atmospheric gases (especially oxygen) are avoided in radar design, unless the detection is intended.

Illumination

Radar depends on its own transmission rather than light from the Sun or the Moon, or from the electromagnetic waves emitted by the object itself, such as the infrared (heat) wavelength. The process of directing artificial radio waves to this object is called illumination , although radio waves are not visible to the human eye or optical cameras.

Reflection

If electromagnetic waves traveling through a material meet with another material, having different dielectric constants or diamagnetic constants from the first, the waves will bounce or spread from the boundary between the materials. This means that a solid object in air or in a vacuum, or a significant change in the atomic density between the object and what surrounds it, will normally propagate radar waves (radio) from its surface. This is especially true for electrically conductive materials such as metals and carbon fibers, making the radar particularly suitable for detecting aircraft and ships. Radar absorbent materials, containing resistive and sometimes magnetic materials, are used in military vehicles to reduce radar reflections. This is a radio equivalent to painting something dark so it can not be seen by the eye at night.

Radar waves spread in various ways depending on the size (wavelength) of radio waves and the shape of the target. If the wavelength is much shorter than the target size, the waves will bounce in a manner similar to the light reflected by the mirror. If the wavelength is longer than the target size, the target may not be visible due to poor reflection. Low-frequency radar technology relies on resonance for detection, but not identification, of the target. This is explained by Rayleigh scattering, an effect that creates the Earth's blue sky and red sunsets. When two long scales are proportional, there may be resonance. Initial radar uses very long wavelengths that are larger than the target and thus receive vague signals, while many modern systems use shorter wavelengths (few centimeters or less) that can describe objects as small as bread.

Short radio waves reflect from curves and angles in a manner similar to flashes from round glass pieces. The most reflective target for short wavelengths has a 90 ° angle between reflective surfaces. The angular reflector consists of three flat surfaces that meet like the inner corners of the box. The structure will reflect the waves entering its opening directly back to the source. They are usually used as radar reflectors to make objects that are difficult to detect more easily detected. The angle reflector on the boat, for example, makes it more detectable to avoid collisions or during rescue. For similar reasons, objects intended to avoid detection will have no angle or inner surface and edges perpendicular to the direction of possible detection, leading to a "weird" looking stealth aircraft. This precaution does not completely eliminate reflection due to diffraction, especially at longer wavelengths. Half wavelength wires or strips of conducting material, such as chaff, are highly reflective but do not direct the energy that is scattered back towards the source. The extent to which an object reflects or transmits radio waves is called a radar cross section.

The radar equation

Kekuatan P _r kembali ke antena penerima diberikan oleh persamaan:

${\ displaystyle P_ {r} = {\ frac {P_ {t} G_ {t} A_ {r} \ sigma F ^ {4}} {{(4 \ pi) )} ^ {2} R_ {t} ^ {2} R_ {r} ^ {2}}}}  ÂÂ$

dimana

• P _t = daya pemancar
• G _t = penguatan antena pemancar
• A _r = bukaan efektif (area) dari antena penerima; ini juga dapat dinyatakan sebagai ${\ displaystyle {G_ {r} \ lambda ^ {2}} \ over {4 \ pi}}  ÂÂ$ , di mana

• ${\ displaystyle \ lambda}  ÂÂ$ = mentransmisikan panjang gelombang
• G _r = perolehan antena penerima

• ? = radar cross section, atau koefisien hamburan, dari target
• F = faktor propagasi pola
• R _t = jarak dari pemancar ke target
• R _r = jarak dari target ke penerima.

Dalam kasus umum di mana pemancar dan penerima berada di lokasi yang sama, R _t = R _r dan istilah R _t ² R _r ² dapat digantikan oleh R ⁴ , di mana R adalah kisarannya. Hasil ini:

${\ displaystyle P_ {r} = {{P_ {t} G_ {t} A_ {r} \ sigma F ^ {4}} \ over {{(4 \ pi) )} ^ {2} R ^ {4}}}.}  ÂÂ$

This indicates that the received power decreases as the fourth power of the range, which means that the power received from the distant target is relatively small.

Additional filtering and pulse integration modify the radar equation slightly for pulse-Doppler pulse performance, which can be used to increase detection range and reduce transmit power.

The above equation with F = 1 is a simplification for transmission in a no-hassle free space. Propagation factors contribute to multipath effects and shadowing and depend on the details of the environment. In real-world situations, pathloss effects should also be considered.

Doppler effect

Frequency shifts are caused by motions that change the amount of wavelength between the reflector and the radar. This can decrease or improve the performance of the radar depending on how it affects the detection process. For example, Moving Target Indication can interact with Doppler to generate signal cancellation at certain radial velocities, which degrade performance.

Sea-based radar systems, semi-active homive radar, active homing radar, weather radar, military aircraft, and astronomical radar depend on the Doppler effect to improve performance. It produces information about target speed during the detection process. It also allows small objects to be detected in environments that contain much larger slow moving objects.

Doppler shift depends on whether the radar configuration is active or passive. Active radar sends the reflected signal back to the receiver. Passive radar depends on the object that sends the signal to the receiver.

Pergeseran frekuensi Doppler untuk radar aktif adalah sebagai berikut, di mana ${\ displaystyle F_ {D}}  ÂÂ$ adalah frekuensi Doppler, ${\ displaystyle F_ {T}}  ÂÂ$ adalah frekuensi transmisi, ${\ displaystyle V_ {R}}  ÂÂ$ adalah kecepatan radial, dan ${\ displaystyle C}  ÂÂ$ adalah kecepatan cahaya:

${\ displaystyle F_ {D} = 2 \ kali F_ {T} \ times \ left ({\ frac {V_ {R}} {C}} \ right)}  ÂÂ$ .

Radar pasif berlaku untuk penanggulangan elektronik dan astronomi radio sebagai berikut:

${\ displaystyle F_ {D} = F_ {T} \ times \ left ({\ frac {V_ {R}} {C}} \ right)}  ÂÂ$ .

Only radial component of the relevant speed. When the reflector moves at right angles to the radar jets, it has no relative speed. Vehicles and weather moving parallel to radar rays produce maximum Doppler frequency shifts.

Ketika frekuensi transmisi ( ${\ displaystyle F_ {T}}  ÂÂ$ ) berdenyut, menggunakan frekuensi ulangi pulsa ${\ displaystyle F_ {R}}  ÂÂ$ , spektrum frekuensi yang dihasilkan akan mengandung frekuensi harmonik di atas dan di bawah ${\ displaystyle F_ {T}}  ÂÂ$ dengan jarak ${\ displaystyle F_ {R}}  ÂÂ$ . Akibatnya, pengukuran Doppler hanya tidak ambigu jika pergeseran frekuensi Doppler kurang dari setengah ${\ displaystyle F_ {R}}  ÂÂ$ , disebut frekuensi Nyquist, karena frekuensi yang dikembalikan sebaliknya tidak dapat dibedakan dari pergeseran frekuensi harmonik di atas atau di bawah, sehingga membutuhkan:

${\ displaystyle | F_ {D} | & lt; {\ frac {F_ {R}} {2}}}  ÂÂ$

Atau saat mengganti dengan ${\ displaystyle F_ {D}}  ÂÂ$ :

${\ displaystyle | V_ {R} | & lt; {\ frac {F_ {R} \ kali {\ frac {C} {F_ {T}}}} {4} }}  ÂÂ$

For example, Doppler weather radar with a 2 kHz pulse and 1 GHz transmission frequency can reliably measure the weather speed up to at most 150 m/s (340 mph), so it can not reliably determine the radial velocity of a 1,000 m/s (2,200 mph).

Polarization

In all electromagnetic radiation, the electric field is perpendicular to the direction of propagation, and the direction of the electric field is the wave polarization. For transmitted radar signals, polarization can be controlled to produce different effects. Radar uses horizontal, vertical, linear, and circular polarization to detect different types of reflections. For example, circular polarization is used to minimize disruption caused by rain. Linear linear polarization usually indicates a metal surface. Returns of random polarization usually indicate fractal surfaces, such as rocks or soils, and are used by navigation radars.

Limiting factor

Path and range of blocks

The radar rays will follow a linear path in a vacuum, but it actually follows a slightly curved path in the atmosphere because of the variations in the air-refractive index, called the radar horizon. Even when the beam is emitted parallel to the ground, it will rise above it because the curvature of the Earth sank beneath the horizon. Furthermore, the signal is attenuated by the medium it is crossed, and the file is spread.

The maximum range of conventional radars can be limited by a number of factors:

• Scattered, which depends on the height above the ground. This means that without direct line of sight, the path of light is blocked.
• The maximum distance is not ambiguous, determined by the frequency of pulse repetition. The ambiguous maximum distance is the distance that the pulse can pass and return before the next pulse is transmitted.
• The radar sensitivity and the return signal strength as calculated in the radar equation. These include factors such as environmental conditions and the size (or radar cross section) of the target.

Noise

Noise signal is the internal source of random variation in the signal, which is generated by all electronic components.

The reflected signal decreases rapidly as distance increases, so noise introduces a range of radar coverage. Noise level and signal to noise ratio are two different performance measures that affect range performance. Reflectors too far produce too few signals to exceed the noise floor and can not be detected. Detection requires signals that exceed the noise floor by at least the ratio of signals to noise.

Noise usually appears as a random variation superimposed on the desired echo signal received at the radar receiver. The lower the signal strength you want, the harder it is to distinguish it from noise. The noise figure is the measure of noise generated by the receiver compared to the ideal receiver, and this needs to be minimized.

Noise shots are generated by electrons traveling across the discontinuities, which occur in all detectors. The firing sound is the dominant source in most recipients. There will also be flicker noise caused by the transit of electrons through the amplification device, which is reduced using heterodyne amplification. Another reason for heterodyne processing is that for fixed fractional bandwidth, the instantaneous bandwidth increases linearly in frequency. This allows increased range resolution. The only exception to the heterodyne radar system (downconversion) is the ultra-broadband radar. Here one cycle, or transient wave, is used similarly to UWB communication, see UWB Channel list.

Noise is also generated by external sources, the most important natural background natural radiation around the desired target. In modern radar systems, internal noise is usually equal or lower than external noise. An exception is if the radar is directed upward in a clear sky, where the landscape is so "cold" that it produces little thermal noise. The thermal noise is given by k _B TB , where T is the temperature, B is the bandwidth (the post match filter) ) and k _B are Boltzmann's constants. There is an interesting intuitive interpretation of this relationship in the radar. Suitable screening allows all energy received from the target to be compressed into one tray (be it a range, Doppler, elevation, or azimuth bin). On the surface it will be seen that within a certain time interval one can obtain perfect detection, error free. To do this, just compress all the energy into very small time slices. What limits this approach in the real world is that, while time can be arbitrarily divisible, it is not currently. The quantum of electrical energy is the electron, and the best that can be done is to match filtering all the energy into one electron. Because electrons move at a certain temperature (Plank spectrum) this noise source can not be eroded further. We see that the radar, like all macro-scale entities, is strongly influenced by quantum theory.

Noise is a random signal and the target is not. Signal processing can take advantage of this phenomenon to reduce noise levels using two strategies. The type of signal integration used with moving target indications can increase the noise to ${\ displaystyle {\ sqrt {2}}}$ for each stage. The signal can also be split between several filters for pulse-Doppler signal processing, which reduces noise floor by the number of filters. This improvement depends on coherence.

Interference

The radar system must overcome unwanted signals to focus on the desired target. These unwanted signals can come from internal and external sources, either passive or active. The ability of the radar system to overcome these unwanted signals defines the signal-to-noise (SNR) ratio. SNR is defined as the ratio of signal strength to noise power in the desired signal; this compares the desired target signal level with the background noise level (atmospheric noise and noise generated inside the receiver). The higher the SNR system the better it is to distinguish the true target from the noise signal.

Clutter

Clutter refers to the radio frequency (RF) echoes back from targets that do not appeal to radar operators. These targets include natural objects such as land, sea, and when not assigned to meteorological purposes, deposition (such as rain, snow or hail), sand storms, animals (especially birds), atmospheric turbulence, and other atmospheric effects, such as ionosphere reflection, traces of meteors, and hail spikes. Clutter can also be returned from man-made objects such as buildings and, intentionally, by radar countermeasures such as chaff.

Some chaos can also be caused by long waveguide radars between radar transceivers and antennas. In a typical plan position (PPI) positioning radar with a rotating antenna, this will usually be seen as a "sun" or "sunburst" in the center of the screen when the receiver responds to echoes of dust and misguided RF particles in the waveguide. Adjust the time between when the transmitter sends the pulse and when the activated receiver stage will generally reduce the sunburst without affecting the accuracy of the range, since most of the sunburst caused by the scattered transmission pulse is reflected before leaving the antenna. Clutter is considered a source of passive interference, as it only appears in response to radar signals sent by the radar.

Clutter is detected and neutralized in several ways. Clutter tends to appear static between radar scans; on the next scanning echo, the desired target will appear to move, and all stationary echoes can be removed. Sea clutter can be reduced by using horizontal polarization, while rain decreases with circular polarization (meteorological radar wants the opposite effect, and therefore uses linear polarization to detect precipitation). Another method tries to improve the signal-to-clutter ratio.

Clutter moves with wind or stationary. Two common strategies for increasing the size or performance in a cluttered environment are:

• Indicates a moving target, which integrates consecutive and
pulses
• Doppler processing, which uses filters to separate interference from desired signals.

The most effective interference reduction technique is the Doppler-pulse radar. Doppler separates interference from aircraft and spacecraft using frequency spectrum, so that individual signals can be separated from multiple reflectors located in the same volume using speed difference. This requires a coherent transmitter. Another technique uses a moving target indicator that reduces the signal reception of two consecutive pulses using a phase to reduce the signal from a slow-moving object. This can be adapted for systems that do not have coherent transmitters, such as domain-domain-frequency-amplitude radar.

A constant false alarm rate, a form of automatic control (AGC) reinforcement, is a method that relies on a far greater number of returns than the desired echo. The receiver gains are automatically adjusted to maintain a constant level of overall visible disturbance. While this does not help detect targets disguised by the clutter around the stronger, it helps distinguish a strong target source. In the past, AGC radar was electronically controlled and affected the acquisition of all radar receivers. As radar evolves, AGC becomes computer-controlled software and affects profits with greater detail in certain detection cells.

Clutter can also be derived from multipath echoes from valid targets caused by soil reflection, atmospheric ducting or ionospheric refraction/refraction (eg, anomalous propagation). This type of disorder is very disturbing because it seems to move and behave like other normal (interest) targets. In a typical scenario, the echoes of the airplane are reflected from the underground, appearing to the receiver as identical targets below the correct one. Radar may try to unite targets, report targets at the wrong height, or eliminate them on the basis of jitter or physical impossibility. Terrain bounce jamming utilizes this response by amplifying the radar signal and directing it down. These problems can be overcome by inserting a soil map of the radar environment and eliminating any echoes that seem to come underground or above a certain height. Monopulse can be improved by changing the elevation algorithm used at low elevation. In newer air traffic control radar equipment, the algorithm is used to identify false targets by comparing back current pulses with adjacent ones, as well as calculating the impossibility of returns.

Jamming

Radar jamming refers to radio frequency signals coming from sources outside the radar, transmitting in radar frequencies and thus covering the desired target. Jamming may be deliberate, as with electronic warfare tactics, or unintentional, as with friendly army operation equipment that transmits using the same frequency range. Jamming is considered an active source of interference, since it is initiated by elements outside the radar and is generally unrelated to radar signals.

Jamming is problematic with radar because interference signals only need to travel one way (from jammer to radar receiver) while radar reverberates goes both ways (radar-target-radar) and therefore significantly reduces power when they return to the radar receiver. Therefore, Jammers can be much weaker than jammed radar and still effectively cover the targets along the line of sight from the jammer to the radar ( mainlobe jamming ). Jammers have the additional effect of affecting the radar along other lines of sight through sidelob radar receivers ( sidelobe jamming ).

Mainlobe jamming can generally only be reduced by narrowing the solid corner of the mainlobe and can not be completely eliminated when dealing directly with jammers that use the same frequency and polarization as the radar. Sidelobe interference can be overcome by reducing sidelob reception in radar antenna design and by using omnidirectional antennas to detect and ignore non-mainlobe signals. Other anti-jamming techniques are frequency hopping and polarization.

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Radar signal processing

Distance measurement

Transit time

One way to

Source of the article : Wikipedia