Radar

Radar

Overview

Radar is an object-detection system which uses radio wave
Radio Wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...

s to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

, ships, spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

, guided missiles, motor vehicle
Motor vehicle
A motor vehicle or road vehicle is a self-propelled wheeled vehicle that does not operate on rails, such as trains or trolleys. The vehicle propulsion is provided by an engine or motor, usually by an internal combustion engine, or an electric motor, or some combination of the two, such as hybrid...

s, weather formations
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...

, and terrain. The radar dish or antenna transmits pulses of radio waves or microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

s which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter.

Radar was developed in secret in nations across the world during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

.
Discussion
Ask a question about 'Radar'
Start a new discussion about 'Radar'
Answer questions from other users
Full Discussion Forum
 
Unanswered Questions
Recent Discussions
Encyclopedia

Radar is an object-detection system which uses radio wave
Radio Wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...

s to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

, ships, spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

, guided missiles, motor vehicle
Motor vehicle
A motor vehicle or road vehicle is a self-propelled wheeled vehicle that does not operate on rails, such as trains or trolleys. The vehicle propulsion is provided by an engine or motor, usually by an internal combustion engine, or an electric motor, or some combination of the two, such as hybrid...

s, weather formations
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...

, and terrain. The radar dish or antenna transmits pulses of radio waves or microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

s which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter.

Radar was developed in secret in nations across the world during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

. The term RADAR was coined in 1940 by the United States Navy
United States Navy
The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...

 as an acronym
Acronym and initialism
Acronyms and initialisms are abbreviations formed from the initial components in a phrase or a word. These components may be individual letters or parts of words . There is no universal agreement on the precise definition of the various terms , nor on written usage...

 for radio detection and ranging. The term radar has since entered English
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

 and other languages as the common noun radar, losing all capitalization.

The modern uses of radar are highly diverse, including air traffic control, radar astronomy
Radar astronomy
Radar astronomy is a technique of observing nearby astronomical objects by reflecting microwaves off target objects and analyzing the echoes. This research has been conducted for six decades. Radar astronomy differs from radio astronomy in that the latter is a passive observation and the former an...

, air-defense systems, antimissile systems
Close-in weapon system
A close-in weapon system , often pronounced sea-whiz, is a naval shipboard point-defense weapon for detecting and destroying at short range incoming anti-ship missiles and enemy aircraft which have penetrated the outer defenses....

; nautical radars to locate landmarks and other ships; aircraft anticollision systems; ocean surveillance
Research vessel
A research vessel is a ship designed and equipped to carry out research at sea. Research vessels carry out a number of roles. Some of these roles can be combined into a single vessel, others require a dedicated vessel...

 systems, outer space surveillance and rendezvous
Space rendezvous
A space rendezvous is an orbital maneuver during which two spacecraft, one of which is often a space station, arrive at the same orbit and approach to a very close distance . Rendezvous requires a precise match of the orbital velocities of the two spacecraft, allowing them to remain at a constant...

 systems; meteorological
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

 precipitation monitoring; altimetry and flight control systems; guided missile
Precision-guided munition
A precision-guided munition is a guided munition intended to precisely hit a specific target, and to minimize damage to things other than the target....

 target locating systems; and ground-penetrating radar
Ground-penetrating radar
Ground-penetrating radar is a geophysical method that uses radar pulses to image the subsurface. This nondestructive method uses electromagnetic radiation in the microwave band of the radio spectrum, and detects the reflected signals from subsurface structures...

 for geological observations. High tech radar systems are associated with digital signal processing
Digital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...

 and are capable of extracting objects from very high noise levels.

Other systems similar to radar have been used in other parts of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

. One example is "lidar
LIDAR
LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...

", which uses visible light from laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

s rather than radio waves.

History



As early as 1886, Heinrich Hertz showed that radio waves could be reflected from solid objects. In 1895 Alexander Popov
Alexander Stepanovich Popov
Alexander Stepanovich Popov was a Russian physicist who was the first person to demonstrate the practical application of electromagnetic waves....

, a physics instructor at the Imperial Russian Navy
Imperial Russian Navy
The Imperial Russian Navy refers to the Tsarist fleets prior to the February Revolution.-First Romanovs:Under Tsar Mikhail Feodorovich, construction of the first three-masted ship, actually built within Russia, was completed in 1636. It was built in Balakhna by Danish shipbuilders from Holstein...

 school in Kronstadt
Kronstadt
Kronstadt , also spelled Kronshtadt, Cronstadt |crown]]" and Stadt for "city"); is a municipal town in Kronshtadtsky District of the federal city of St. Petersburg, Russia, located on Kotlin Island, west of Saint Petersburg proper near the head of the Gulf of Finland. Population: It is also...

, developed an apparatus using a coherer
Coherer
The coherer was a primitive form of radio signal detector used in the first radio receivers during the wireless telegraphy era at the beginning of the twentieth century. Invented around 1890 by French scientist Édouard Branly, it consisted of a tube or capsule containing two electrodes spaced a...

 tube for detecting distant lightning strikes. The next year, he added a spark-gap transmitter
Spark-gap transmitter
A spark-gap transmitter is a device for generating radio frequency electromagnetic waves using a spark gap.These devices served as the transmitters for most wireless telegraphy systems for the first three decades of radio and the first demonstrations of practical radio were carried out using them...

. In 1897, while testing this in communicating between two ships in the Baltic Sea
Baltic Sea
The Baltic Sea is a brackish mediterranean sea located in Northern Europe, from 53°N to 66°N latitude and from 20°E to 26°E longitude. It is bounded by the Scandinavian Peninsula, the mainland of Europe, and the Danish islands. It drains into the Kattegat by way of the Øresund, the Great Belt and...

, he took note of an interference beat caused by the passage of a third vessel. In his report, Popov wrote that this phenomenon might be used for detecting objects, but he did nothing more with this observation.

The German Christian Huelsmeyer was the first to use radio waves to detect "the presence of distant metallic objects". In 1904 he demonstrated the feasibility of detecting a ship in dense fog but not its distance. He received a patent for his detection device in April 1904 and later a patent for a related amendment for determining the distance to the ship. He also received a British patent on September 23, 1904 for the first full radar application, which he called telemobiloscope.

In August 1917 Nikola Tesla
Nikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...

 outlined a concept for primitive radar units. He stated,
"...by their [standing electromagnetic waves] use we may produce at will, from a sending station, an electrical effect in any particular region of the globe; [with which] we may determine the relative position or course of a moving object, such as a vessel at sea, the distance traversed by the same, or its speed."


In 1922 A. Hoyt Taylor and Leo C. Young
Leo C. Young
Leo C. Young was an American radio engineer who had many accomplishments during a long career at the U.S. Naval Research Laboratory...

, researchers working with the U.S. Navy, discovered that when radio waves were broadcast at 60 MHz it was possible to determine the range and bearing of nearby ships in the Potomac River
Potomac River
The Potomac River flows into the Chesapeake Bay, located along the mid-Atlantic coast of the United States. The river is approximately long, with a drainage area of about 14,700 square miles...

. Despite Taylor's suggestion that this method could be used in darkness and low visibility, the Navy did not immediately continue the work. Serious investigation began eight years later after the discovery that radar could be used to track airplanes.

Before the Second World War, researchers in France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

, Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

, Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

, Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

, the Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...

, the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

, the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

, and the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, independently and in great secrecy, developed technologies that led to the modern version of radar. Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

, and South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...

 followed prewar Great Britain, and Hungary
Hungary
Hungary , officially the Republic of Hungary , is a landlocked country in Central Europe. It is situated in the Carpathian Basin and is bordered by Slovakia to the north, Ukraine and Romania to the east, Serbia and Croatia to the south, Slovenia to the southwest and Austria to the west. The...

 had similar developments during the war.

In 1934 the Frenchman Émile Girardeau
Emile Girardeau
Émile Girardeau was a French engineer, famous for being the first person to patent the original system of frequencies that is used today and known as the radar...

 stated he was building an obstacle-locating radio apparatus "conceived according to the principles stated by Tesla" and obtained a patent for a working system, a part of which was installed on the Normandie
SS Normandie
SS Normandie was an ocean liner built in Saint-Nazaire, France for the French Line Compagnie Générale Transatlantique. She entered service in 1935 as the largest and fastest passenger ship afloat; she is still the most powerful steam turbo-electric-propelled passenger ship ever built.Her novel...

 liner in 1935.

During the same year, the Soviet military engineer P.K.Oschepkov, in collaboration with Leningrad Electrophysical Institute
Saint Petersburg State Electrotechnical University
Saint Petersburg State Electrotechnical University founded in 1886, and is one of the oldest higher education institutions in Saint Petersburg....

, produced an experimental apparatus, RAPID, capable of detecting an aircraft within 3 km of a receiver. The French and Soviet systems, however, had continuous-wave operation and could not give the full performance that was ultimately at the center of modern radar.

Full radar evolved as a pulsed system, and the first such elementary apparatus was demonstrated in December 1934 by American Robert M. Page
Robert Morris Page
Robert Morris Page was an American physicist who was a leading figure in the development of radar technology. Later, Page served as the Director of Research for the U.S. Naval Research Laboratory.-Life and career:...

, working at the Naval Research Laboratory. The following year, the United States Army
United States Army
The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...

 successfully tested a primitive surface to surface radar to aim coastal battery search lights at night. This was followed by a pulsed system demonstrated in May 1935 by Rudolf Kühnhold
Rudolf Kühnhold
Rudolf Kühnhold was an experimental physicist who is often given credit for initiating research that led to the Funkmessgerät in Germany.-Early life:...

 and the firm GEMA
GEMA
GEMA may be:* Georgia Emergency Management Agency* * gema, a General Purpose Macro Processor* Gesellschaft für musikalische Aufführungs- und mechanische Vervielfältigungsrechte, or GEMA, a performance rights organisation in Germany...

 in Germany and then one in June 1935 by an Air Ministry
Air Ministry
The Air Ministry was a department of the British Government with the responsibility of managing the affairs of the Royal Air Force, that existed from 1918 to 1964...

 team led by Robert A. Watson Watt
Robert Watson-Watt
Sir Robert Alexander Watson-Watt, KCB, FRS, FRAeS is considered by many to be the "inventor of radar". Development of radar, initially nameless, was first started elsewhere but greatly expanded on 1 September 1936 when Watson-Watt became...

 in Great Britain. Later, in 1943, Page greatly improved radar with the monopulse technique
Monopulse radar
Monopulse radar is an adaptation of conical scanning radar which sends additional information in the radar signal in order to avoid problems caused by rapid changes in signal strength. The system also makes jamming more difficult...

 that was used for many years in most radar applications.

The British were the first to fully exploit radar as a defence against aircraft attack. This was spurred on by fears that the Germans were developing death ray
Death ray
The death ray or death beam was a theoretical particle beam or electromagnetic weapon of the 1920s through the 1930s that was claimed to have been invented independently by Nikola Tesla, Edwin R. Scott, Harry Grindell Matthews, and Graichen, as well as others...

s. The Air Ministry asked British scientists in 1934 to investigate the possibility of propagating electromagnetic energy and the likely effect. Following a study, they concluded that a death ray was impractical but that detection of aircraft appeared feasible. Robert Watson Watt's team demonstrated to his superiors the capabilities of a working prototype and then patented the device. It served as the basis for the Chain Home
Chain Home
Chain Home was the codename for the ring of coastal Early Warning radar stations built by the British before and during the Second World War. The system otherwise known as AMES Type 1 consisted of radar fixed on top of a radio tower mast, called a 'station' to provide long-range detection of...

 network of radars to defend Great Britain. In April 1940, Popular Science
Popular Science
Popular Science is an American monthly magazine founded in 1872 carrying articles for the general reader on science and technology subjects. Popular Science has won over 58 awards, including the ASME awards for its journalistic excellence in both 2003 and 2004...

 showed an example of a radar unit using the Watson-Watt patent in an article on air defence, but not knowing that the U.S. Army and U.S. Navy were working on radars with the same principle, stated under the illustration, "This is not U.S. Army equipment." Also, in late 1941 Popular Mechanics had an article in which a U.S. scientist conjured what he believed the British early warning system on the English east coast most likely looked like and was very close to what it actually was and how it worked in principle.

The war precipitated research to find better resolution, more portability, and more features for radar, including complementary navigation systems like Oboe
Oboe (navigation)
Oboe was a British aerial blind bombing targeting system in World War II, based on radio transponder technology. Oboe accurately measured the distance to an aircraft, and gave the pilot guidance on whether or not they were flying along a pre-selected circular route. The route was only 35 yards...

 used by the RAF's Pathfinder
Pathfinder (RAF)
The Pathfinders were elite squadrons in RAF Bomber Command during World War II. They located and marked targets with flares, which a main bomber force could aim at, increasing the accuracy of their bombing...

.

Applications


The information provided by radar includes the bearing and range (and therefore position) of the object from the radar scanner. It is thus used in many different fields where the need for such positioning is crucial. The first use of radar was for military purposes: to locate air, ground and sea targets. This evolved in the civilian field into applications for aircraft, ships, and roads.

In aviation
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...

, aircraft are equipped with radar devices that warn of obstacles in or approaching their path and give accurate altitude readings. The first commercial device fitted to aircraft was a 1938 Bell Lab unit on some United Air Lines aircraft. They can land in fog at airports equipped with radar-assisted ground-controlled approach
Ground-controlled approach
In aviation a ground-controlled approach , is a type of service provided by air-traffic controllers whereby they guide aircraft to a safe landing in adverse weather conditions based on radar images...

 systems, in which the plane's flight is observed on radar screens while operators radio landing directions to the pilot.

Marine radar
Marine radar
Marine radars are x-band or s-band radar to provide bearing and distance of ships and land targets in vicinity from own ship for collision avoidance and navigation at sea...

s are used to measure the bearing and distance of ships to prevent collision with other ships, to navigate and to fix their position at sea when within range of shore or other fixed references such as islands, buoys, and lightships. In port or in harbour, vessel traffic service
Vessel Traffic Service
A vessel traffic service is a marine traffic monitoring system established by harbour or port authorities, similar to air traffic control for aircraft...

 radar systems are used to monitor and regulate ship movements in busy waters. Police forces use radar gun
Radar gun
A radar speed gun is a small doppler radar unit used to measure the speed of moving objects, including vehicles, pitched baseballs, runners and other moving objects. Radar speed guns may be hand-held, vehicle-mounted or static...

s to monitor vehicle speeds on the roads.

Meteorologists use radar to monitor precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

. It has become the primary tool for short-term weather forecasting and to watch for severe weather
Severe weather
Severe weather phenomena are weather conditions that are hazardous to human life and property.- Examples Include :Severe weather can occur under a variety of situations, but three characteristics are generally needed: a temperature or moisture boundary, moisture, and , instability in the...

 such as thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...

s, tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...

es, winter storm
Winter storm
A winter storm is an event in which the dominant varieties of precipitation are formed that only occur at low temperatures, such as snow or sleet, or a rainstorm where ground temperatures are low enough to allow ice to form...

s, precipitation types, etc. Geologist
Geologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...

s use specialised ground-penetrating radar
Ground-penetrating radar
Ground-penetrating radar is a geophysical method that uses radar pulses to image the subsurface. This nondestructive method uses electromagnetic radiation in the microwave band of the radio spectrum, and detects the reflected signals from subsurface structures...

s to map the composition of the Earth's crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

.

Principles


A radar system has a transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

 that emits radio waves
Radio waves
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves,...

 called radar signals in predetermined directions. When these come into contact with an object they are usually reflected
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

 and/or scattered
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...

 in many directions. Radar signals are reflected especially well by materials of considerable electrical conductivity—especially by most metals, by seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...

, by wet land, and by wetland
Wetland
A wetland is an area of land whose soil is saturated with water either permanently or seasonally. Wetlands are categorised by their characteristic vegetation, which is adapted to these unique soil conditions....

s. Some of these make the use of radar altimeter
Radar altimeter
A radar altimeter, radio altimeter, low range radio altimeter or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft...

s possible. The radar signals that are reflected back towards the transmitter are the desirable ones that make radar work. If the object is moving either closer or farther away, there is a slight change in the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 of the radio waves, caused by the Doppler effect
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...

.

Radar receivers are usually, but not always, in the same location as the transmitter. Although the reflected radar signals captured by the receiving antenna are usually very weak, these signals can be strengthened by the electronic amplifier
Electronic amplifier
An electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...

s. More sophisticated methods of signal processing
Signal processing
Signal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...

 are also used in order to recover useful radar signals.

The weak absorption of radio waves by the medium through which it passes is what enables radar sets to detect objects at relatively long ranges—ranges at which other electromagnetic wavelengths, such as visible light, infrared light, and ultraviolet light, are too strongly attenuated. Such things as fog, clouds, rain, falling snow, and sleet that block visible light are usually transparent to radio waves. Certain radio frequencies that are absorbed or scattered by water vapor, raindrops, or atmospheric gases (especially oxygen) are avoided in designing radars except when detection of these is intended.

Radar relies on its own transmissions rather than light from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 or the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, or from electromagnetic waves emitted by the objects themselves, such as infrared wavelengths (heat). This process of directing artificial radio waves towards objects is called illumination, regardless of the fact that radio waves are invisible to the human eye or cameras.

Reflection


Electromagnetic waves
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

 reflect (scatter) from any large change in the dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...

 or diamagnetic
Diamagnetism
Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the...

 constants. This means that a solid object in air
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 or a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

, or other significant change in atomic density between the object and what is surrounding it, will usually scatter radar (radio) waves. This is particularly true for electrically conductive materials, such as metal and carbon fiber, making radar well suited to the detection of aircraft and ships. Radar absorbing material, containing resistive
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

 and sometimes magnetic
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...

 substances, is used on military vehicles to reduce radar reflection
Stealth technology
Stealth technology also termed LO technology is a sub-discipline of military tactics and passive electronic countermeasures, which cover a range of techniques used with personnel, aircraft, ships, submarines, and missiles, to make them less visible to radar, infrared, sonar and other detection...

. This is the radio equivalent of painting something a dark color so that it cannot be seen through normal means.

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
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

. If the wavelength is much longer than the size of the target, the target may not be visible because of poor reflection. Low Frequency radar technology is dependent on resonances for detection, but not identification, of targets. This is described by Rayleigh scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...

, an effect that creates the Earth's blue sky and red sunset
Sunset
Sunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west...

s. When the two length scales are comparable, there may be resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

s. 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 centimeters or shorter) that can image objects as small as a loaf of bread.

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
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

. A structure consisting of three flat surfaces meeting at a single corner, like the corner on a box, will reflect waves entering its opening directly back at the source. These so-called corner reflector
Corner reflector
A corner reflector is a retroreflector consisting of three mutually perpendicular, intersecting flat surfaces, which reflects waves back directly towards the source, but shifted . Unlike a simple mirror, they work for a relatively wide-angle field of view. The three intersecting surfaces often have...

s 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
Stealth aircraft
Stealth aircraft are aircraft that use stealth technology to avoid detection by employing a combination of features to interfere with radar as well as reduce visibility in the infrared, visual, audio, and radio frequency spectrum. Development of stealth technology likely began in Germany during...

. These precautions do not completely eliminate reflection because of diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

, especially at longer wavelengths. Half wavelength long wires or strips of conducting material, such as chaff
Chaff (radar countermeasure)
Chaff, originally called Window by the British, and Düppel by the Second World War era German Luftwaffe , is a radar countermeasure in which aircraft or other targets spread a cloud of small, thin pieces of aluminium, metallized glass fibre or plastic, which either appears as a cluster of secondary...

, 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 cross section
Radar cross section is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected.An object reflects a limited amount of radar energy...

.

Radar equation


The power Pr returning to the receiving antenna is given by the equation:


where
  • Pt = transmitter power
  • Gt = gain
    Gain
    In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...

     of the transmitting antenna
  • Ar = effective aperture (area) of the receiving antenna
  • σ = radar cross section
    Radar cross section
    Radar cross section is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected.An object reflects a limited amount of radar energy...

    , or scattering coefficient, of the target
  • F = pattern propagation factor
  • Rt = distance from the transmitter to the target
  • Rr = distance from the target to the receiver.


In the common case where the transmitter and the receiver are at the same location, Rt = Rr and the term Rt² Rr² can be replaced by R4, where R is the range.
This yields:

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 small.

The equation above with F = 1 is a simplification for vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

 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.

Doppler effect



Ground-based radar systems used for detecting speeds rely on the Doppler effect. The apparent frequency (f) of the wave changes with the relative position of the target. The doppler equation is stated as follows for (the radial speed of the observer) and (the radial speed of the target) and frequency of wave :


However, the change in phase of the return signal is often used instead of the change in frequency. Only the radial component of the speed is available. Hence when a target is moving at right angle to the radar beam, it has no relative velocity, while one parallel to it has maximum recorded speed even if both might have the same real absolute motion.

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, linear and circular polarization to detect different types of reflections. For example, circular polarization
Circular polarization
In electrodynamics, circular polarization of an electromagnetic wave is a polarization in which the electric field of the passing wave does not change strength but only changes direction in a rotary type manner....

 is used to minimize the interference caused by rain. Linear polarization
Linear polarization
In electrodynamics, linear polarization or plane polarization of electromagnetic radiation is a confinement of the electric field vector or magnetic field vector to a given plane along the direction of propagation...

 returns usually indicate metal surfaces. Random polarization returns usually indicate a fractal
Fractal
A fractal has been defined as "a rough or fragmented geometric shape that can be split into parts, each of which is a reduced-size copy of the whole," a property called self-similarity...

 surface, such as rocks or soil, and are used by navigation radars.

Beam path and range



The radar beam would follow a linear path in vacuum, but it really follows a somewhat curved path in the atmosphere because of the variation of the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 of air, that is the radar horizon
Radar horizon
The radar Horizon is a critical area of performance for aircraft detection systems that is defined by the distance at which the radar beam raise enough above the Earth surface to make detection of a target at low level impossible. It is associated with the low elevation region of performance and...

. Even when the beam is emitted parallel to the ground, it will rise above it as the Earth curvature
Figure of the Earth
The expression figure of the Earth has various meanings in geodesy according to the way it is used and the precision with which the Earth's size and shape is to be defined. The actual topographic surface is most apparent with its variety of land forms and water areas. This is, in fact, the surface...

 sinks below the horizon. Furthermore, the signal is attenuated by the medium it crosses, and the beam disperses.

The maximum range of a conventional radar can be limited by a number of factors:
  • Line of sight, which depends on height above ground.
  • The maximum non-ambiguous range which is determined by the pulse repetition frequency
    Pulse repetition frequency
    Pulse repetition frequency or Pulse repetition rate is the number of pulses per time unit . It is a measure or specification mostly used within various technical disciplines Pulse repetition frequency (PRF) or Pulse repetition rate (PRR) is the number of pulses per time unit (e.g. Seconds). It...

    . The maximum non-ambiguous range is the distance the pulse could travel and return before the next pulse is emitted.
  • Radar sensitivity and power of the return signal as computed in the radar equation. This includes factors such as environmentals and the size (or radar cross section) of the target.

Noise


Signal noise is an internal source of random variations in the signal, which is generated by all electronic components. 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. Noise figure
Noise figure
Noise figure is a measure of degradation of the signal-to-noise ratio , caused by components in a radio frequency signal chain. The noise figure is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard...

 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, 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.

There will be also flicker noise
Flicker noise
Flicker noise is a type of electronic noise with a 1/ƒ, or pink power density spectrum. It is therefore often referred to as 1/ƒ noise or pink noise, though these terms have wider definitions...

 caused by electrons transit, but depending on 1/f, flicker noise will be much lower than thermal noise when the frequency is high. Hence, in pulse radar, the system will be heterodyne
Heterodyne
Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden where high frequency signals are converted to lower frequencies by combining two frequencies. Heterodyning is useful for frequency shifting information of interest into a useful...

.

Interference


Radar systems must overcome 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
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...

 (SNR). SNR is defined as the ratio of a signal power to the noise power within the desired signal.

In less technical terms, SNR compares the level of a desired signal (such as targets) to the level of background noise. The higher a system's SNR, the better it is in isolating actual targets from the surrounding noise signals.

Clutter


Clutter refers to radio frequency (RF) echoes returned from targets which are uninteresting to the radar operators. Such targets include natural objects such as ground, sea, precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

 (such as rain, snow or hail), sand storm
Sand Storm
Sand Storm is a 1992 action game in which you are shooting an anti-air ballistic weapon. The premise of the game is similar to that of Sabotage. You must defend your home base from planes , Patriot Missiles, and Scuds. If a missile or bomb touches the ground, some of the buildings in the base are...

s, animals (especially birds), atmospheric turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...

, and other atmospheric effects, such as ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

 reflections, meteor
METEOR
METEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...

 trails, and three body scatter spike
Three body scatter spike
A hail spike or three body scatter spike is an artifact on a weather radar display indicative of large hail. They are identified by a spike of weak reflectivity echoes that extend out from a thunderstorm, and away from the radar site.- Cause :...

. Clutter may also be returned from man-made objects such as buildings and, intentionally, by radar countermeasures such as chaff
Chaff (radar countermeasure)
Chaff, originally called Window by the British, and Düppel by the Second World War era German Luftwaffe , is a radar countermeasure in which aircraft or other targets spread a cloud of small, thin pieces of aluminium, metallized glass fibre or plastic, which either appears as a cluster of secondary...

.

Some clutter may also be caused by a long radar waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...

 between the radar transceiver and the antenna. In a typical plan position indicator
Plan position indicator
The plan position indicator , is the most common type of radar display. The radar antenna is usually represented in the center of the display, so the distance from it and height above ground can be drawn as concentric circles...

 (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 a diffused transmit pulse reflected before it leaves the antenna. 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 scan 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
Circular polarization
In electrodynamics, circular polarization of an electromagnetic wave is a polarization in which the electric field of the passing wave does not change strength but only changes direction in a rotary type manner....

 (note that meteorological radars wish for the opposite effect, therefore using linear polarization
Linear polarization
In electrodynamics, linear polarization or plane polarization of electromagnetic radiation is a confinement of the electric field vector or magnetic field vector to a given plane along the direction of propagation...

 to detect precipitation). Other methods attempt to increase the signal-to-clutter ratio.

The most effective clutter reduction technique is pulse-Doppler radar
Pulse-doppler radar
Pulse-Doppler is a 4D radar system capable of detecting both target 3D location as well as measuring radial velocity . It uses the Doppler effect to avoid overloading computers and operators as well as to reduce power consumption...

. Doppler separates clutter from aircraft and spacecraft using a frequency spectrum, so individual signals can be separated from multiple reflectors located in the same volume using velocity differences. This requires a coherent transmitter.
Another technique is moving target indicator that subtracts the receive signal from two successive pulses using phase to reduce signals from slow moving objects. This can be adapted for systems that lack a coherent transmitter, such as time-domain pulse-amplitude radar.

Constant False Alarm Rate
Constant false alarm rate
Constant false alarm rate detection refers to a common form of adaptive algorithm used in radar systems to detect target returns against a background of noise, clutter and interference.Other detection algorithms are not adaptive...

, a form of Automatic Gain Control
Automatic gain control
Automatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...

 (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.
Clutter may also originate from multipath echoes from valid targets caused by ground reflection, atmospheric ducting or ionospheric reflection
Ionospheric reflection
Ionospheric reflection is a bending, through a complex process involving reflection and refraction, of electromagnetic waves propagating in the ionosphere back toward the Earth....

/refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

 (e.g. Anomalous propagation
Anomalous propagation
Anomalous propagation includes different forms of electromagnetic wave propagation that are not encountered in a standard atmosphere. While technically the term includes propagation with larger losses than in standard atmosphere, in practical applications it is most often meant to refer to cases...

). This clutter type is especially bothersome since it appears to move and behave like other normal (point) targets of interest. In a typical scenario, an aircraft echo is 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 eliminating it on the basis of jitter
Jitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...

 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. In newer Air Traffic Control radar equipment, algorithms are used to identify the false targets by comparing the current pulse returns, to those adjacent, as well as calculating return improbabilities.

Jamming


Radar jamming refers to radio frequency signals originating from sources outside the radar, transmitting in the radar's frequency and thereby masking targets of interest. Jamming may be intentional, as with an electronic warfare
Electronic warfare
Electronic warfare refers to any action involving the use of the electromagnetic spectrum or directed energy to control the spectrum, attack an enemy, or impede enemy assaults via the spectrum. The purpose of electronic warfare is to deny the opponent the advantage of, and ensure friendly...

 tactic, or unintentional, as with 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
Line-of-sight propagation
Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...

 from the jammer to the radar (mainlobe jamming). Jammers have an added effect of affecting radars along other lines of sight through the radar receiver's sidelobes (sidelobe jamming).

Mainlobe jamming can generally only be reduced by narrowing the mainlobe solid angle
Solid angle
The solid angle, Ω, is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large that object appears to an observer looking from that point...

 and cannot 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
Omnidirectional antenna
In radio communication, an omnidirectional antenna is an antenna which radiates radio wave power uniformly in all directions in one plane, with the radiated power decreasing with elevation angle above or below the plane, dropping to zero on the antenna's axis. This radiation pattern is often...

 to detect and disregard non-mainlobe signals. Other anti-jamming techniques are frequency hopping and polarization.

Transit time



One way to measure the distance to an object is to transmit a short pulse of radio signal (electromagnetic radiation) and measure the time it takes for the reflection to return. The distance is one-half the product of the round trip time (because the signal has to travel to the target and then back to the receiver) and the speed of the signal. Since radio waves travel at the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

, accurate distance measurement requires high-performance electronics.
In most cases, the receiver does not detect the return while the signal is being transmitted. Through the use of a duplexer, the radar switches between transmitting and receiving at a predetermined rate.
A similar effect imposes a maximum range as well. In order to maximize range, longer times between pulses should be used, referred to as a pulse repetition time, or its reciprocal, pulse repetition frequency.

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. 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. As electronics have improved many radars now can change their pulse repetition frequency, thereby changing their range. The newest radars fire two pulses during one cell, one for short range (10 km / 6 miles) and a separate signal for longer ranges (100 km /60 miles).

The distance resolution
Optical resolution
Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...

 and the characteristics of the received signal as compared to noise depends on the shape of the pulse. The pulse is often modulated
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

 to achieve better performance using a technique known as pulse compression
Pulse compression
Pulse compression is a signal processing technique mainly used in radar, sonar and echography to increase the range resolution as well as the signal to noise ratio...

.

Distance may also be measured as a function of time. The radar mile is the amount of time it takes for a radar pulse to travel one nautical mile
Nautical mile
The nautical mile is a unit of length that is about one minute of arc of latitude along any meridian, but is approximately one minute of arc of longitude only at the equator...

, reflect off a target, and return to the radar antenna. Since a nautical mile is defined as 1,852 meters, then dividing this distance by the speed of light (299,792,458 meters per second), and then multiplying the result by 2 yields a result of 12.36 microseconds in duration.

Frequency modulation


Another form of distance measuring radar is based on frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...

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

This technique can be used in continuous wave radar and is often found in aircraft radar altimeter
Radar altimeter
A radar altimeter, radio altimeter, low range radio altimeter or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft...

s. In these systems a "carrier" radar signal is frequency modulated in a predictable way, typically varying up and down with a sine wave
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...

 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 using a simple beat frequency modulator that produces an audio frequency tone from the returned signal and a portion of the transmitted signal.

Since the signal frequency is changing, by the time the signal returns to the aircraft the transmit frequency has changed. The amount of frequency shift is used to measure distance.

The modulation index riding on the receive signal is proportional to the time delay between the radar and the reflector. The amount of that frequency shift becomes greater with greater time delay. The measure of the amount of frequency shift is directly proportional to the distance traveled. That distance can be displayed on an instrument, and it may also be available via the transponder. This signal processing is similar to that used in speed detecting Doppler radar. Example systems using this approach are AZUSA
AZUSA
AZUSA refers to a ground-based radar tracking system installed at Cape Canaveral, Florida and the NASA Kennedy Space Center. AZUSA dated back to the early 1950s and was named after the southern California town where the system was devised....

, MISTRAM
MISTRAM
MISTRAM was a high-resolution tracking system used by the United States Air Force to provide highly detailed trajectory analysis of rocket launches....

, and UDOP
UDOP
The UDOP multistatic radar and multiradar system utilizes Doppler radar for missile tracking and trajectory measurement. A target is illuminated at 450 MHz. Five receiving stations, located along the baselines with the lengths from , receive signals from the target's transponder at 900 MHz. These...

.

A further advantage is that the radar can operate effectively at relatively low frequencies. This was important in the early development of this type when high frequency signal generation was difficult or expensive.

Terrestrial radar uses low-power FM signals that cover a larger frequency range. The multiple reflections are analyzed mathematically for pattern changes with multiple passes creating a computerized synthetic image. Doppler effects are used which allows slow moving objects to be detected as well as largely eliminating "noise" from the surfaces of bodies of water.

Speed measurement


Speed
Speed
In kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...

 is the change in distance to an object with respect to time. Thus the existing system for measuring distance, combined with a memory capacity to see where the target last was, is enough to measure speed. At one time the memory consisted of a user making grease pencil
Grease pencil
The grease pencil, a wax writing tool also known as a wax pencil, china marker, , is made of hardened colored wax and is useful for marking on hard, glossy non-porous surfaces such as porcelain, glass, polished stone, plastic, ceramics and other glazed, lacquered or polished surfaces, as well as...

 marks on the radar screen and then calculating the speed using a slide rule
Slide rule
The slide rule, also known colloquially as a slipstick, is a mechanical analog computer. The slide rule is used primarily for multiplication and division, and also for functions such as roots, logarithms and trigonometry, but is not normally used for addition or subtraction.Slide rules come in a...

. Modern radar systems perform the equivalent operation faster and more accurately using computers.

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
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...

. Most modern radar systems use this principle into doppler radar
Doppler radar
A Doppler radar is a specialized radar that makes use of the Doppler effect to produce velocity data about objects at a distance. It does this by beaming a microwave signal towards a desired target and listening for its reflection, then analyzing how the frequency of the returned signal has been...

 and pulse-doppler radar
Pulse-doppler radar
Pulse-Doppler is a 4D radar system capable of detecting both target 3D location as well as measuring radial velocity . It uses the Doppler effect to avoid overloading computers and operators as well as to reduce power consumption...

 systems (weather radar
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...

, millitary radar, etc...). 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 the line of sight cannot be determined by using the Doppler effect alone, but it can be determined by tracking the target's azimuth over time.

It is possible to make a doppler radar without any pulsing, known as a continuous-wave radar
Continuous-wave radar
Continuous-wave radar is a type of radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects.Continuous wave radar uses Doppler, which renders the radar immune to interference from large stationary objects and slow moving...

 (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. CW radar is typically used by traffic enforcement to measure vehicle speed quickly and accurately where range is not important.

When using a pulsed radar, the variation between the phase of successive returns gives the distance the target has moved between pulses, and thus its speed can be calculated.
Other mathematical developments in radar signal processing include time-frequency analysis
Time-frequency analysis
In signal processing, time–frequency analysis comprises those techniques that study a signal in both the time and frequency domains simultaneously, using various time–frequency representations...

 (Weyl Heisenberg or wavelet
Wavelet
A wavelet is a wave-like oscillation with an amplitude that starts out at zero, increases, and then decreases back to zero. It can typically be visualized as a "brief oscillation" like one might see recorded by a seismograph or heart monitor. Generally, wavelets are purposefully crafted to have...

), as well as the chirplet transform
Chirplet transform
In signal processing, the chirplet transform is an inner product of an input signal with a family of analysis primitives called chirplets.-Similarity to other transforms:...

 which makes use of the fact that radar returns from moving targets typically "chirp" (change their frequency as a function of time).

Pulse-Doppler signal processing


Pulse-Doppler signal processing
Pulse-Doppler signal processing
Pulse-Doppler signal processing is a radar performance enhancement strategy that allows small high-speed objects to be detected in close proximity to large slow moving objects. Detection improvements on the order of of 1,000,000:1 are common...

 includes frequency filtering in the detection process. The space between each transmit pulse is divided into range cells or range gates. Each cell is filtered independently much like the process used by a spectrum analyzer
Spectrum analyzer
A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals...

 to produce the display showing different frequencies. Each different distance produces a different spectrum. These spectra are used to perform the detection process. This is required to achieve acceptable performance in hostile environments involving weather, terrain, and electronic countermeasures.

The primary purpose is to measure both the amplitude and frequency of the aggregate reflected signal from multiple distances. This is used with weather radar
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...

 to measure radial wind velocity and precipitation rate in each different volume of air. This is linked with computing systems to produce a real-time electronic weather map. Aircraft safety depends upon continuous access to accurate weather radar information that is used to prevent injuries and accidents. Weather radar uses a low PRF. Coherency requirements are not as strict as those for military systems because individual signals ordinarily do not need to be separated. Less sophisticated filtering is required, and range ambiguity processing is not normally needed with weather radar in comparison with military radar intended to track air vehicles.

The alternate purpose is "look-down/shoot-down
Look-down/shoot-down
Look-down/shoot-down is a capability a radar system is said to possess if it is able to detect, track and put a weapon onto an air target moving below the horizon as seen by the radar...

" capability required to improve military air combat survivability. Pulse-Doppler is also used for ground based surveillance radar required to defend personnel and vehicles. Pulse-Doppler signal processing increases the maximum detection distance using less radiation in close proximity to aircraft pilots, shipboard personnel, infantry, and artillery. Reflections from terrain, water, and weather produce signals much larger than aircraft and missiles, which allows fast moving vehicles to hide using nap-of-the-earth
Nap-of-the-earth
Nap-of-the-earth is a type of very low-altitude flight course used by military aircraft to avoid enemy detection and attack in a high-threat environment....

 flying techniques and stealth technology
Stealth technology
Stealth technology also termed LO technology is a sub-discipline of military tactics and passive electronic countermeasures, which cover a range of techniques used with personnel, aircraft, ships, submarines, and missiles, to make them less visible to radar, infrared, sonar and other detection...

 to avoid detection until an attack vehicle is too close to destroy. Pulse-Doppler signal processing incorporates more sophisticated electronic filtering that safely eliminates this kind of weakness. This requires the use of medium pulse-repetition frequency with phase coherent hardware that has a large dynamic range. Military applications require medium PRF which prevents range from being determined directly, and range ambiguity resolution
Range ambiguity resolution
Range ambiguity resolution is a technique used with medium Pulse repetition frequency radar to obtain range information for distances that exceed the distance between transmit pulses.This signal processing technique is required with pulse-Doppler radar....

 processing is required to identify the true range of all reflected signals. Radial movement is usually linked with Doppler frequency to produce a lock signal that cannot be produced by radar jamming signals. Pulse-Doppler signal processing also produces audible signals that can be used for threat identification.

Reduction of interference effects


Signal processing
Signal processing
Signal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...

 is employed in radar systems to reduce the radar interference effects. Signal processing techniques include moving target indication
Moving target indication
Moving target indication is a mode of operation of a radar to discriminate a target against clutter. In contrast to another mode, stationary target indication, it takes an advantage of the fact that the target moves with respect to stationary clutter. The most common approach takes advantage of...

, Pulse-Doppler signal processing
Pulse-Doppler signal processing
Pulse-Doppler signal processing is a radar performance enhancement strategy that allows small high-speed objects to be detected in close proximity to large slow moving objects. Detection improvements on the order of of 1,000,000:1 are common...

, moving target detection processors, correlation with secondary surveillance radar
Secondary surveillance radar
Secondary surveillance radar is a radar system used in air traffic control , that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude...

 targets, space-time adaptive processing
Space-time adaptive processing
Space-time adaptive processing is a signal processing technique most commonly used in radar systems. It involves adaptive array processing algorithms to aid in target detection. Radar signal processing benefits from STAP in areas where interference is a problem...

, and track-before-detect
Track-before-detect
In radar technology and similar fields, track-before-detect is a concept according to which a signal is tracked before declaring it a target...

. Constant false alarm rate
Constant false alarm rate
Constant false alarm rate detection refers to a common form of adaptive algorithm used in radar systems to detect target returns against a background of noise, clutter and interference.Other detection algorithms are not adaptive...

 and digital terrain model processing are also used in clutter environments.

Plot and track extraction


Radar video returns on aircraft can be subjected to a plot extraction process whereby spurious and interfering signals are discarded. A sequence of target returns can be monitored through a device known as a plot extractor. The non relevant real time returns can be removed from the displayed information and a single plot displayed. In some radar systems, or alternatively in the command and control system to which the radar is connected, a radar tracker
Radar tracker
A radar tracker is a component of a radar system, or an associated command and control system, that associates consecutive radar observations of the same target into tracks...

 is used to associate the sequence of plots belonging to individual targets and estimate the targets' headings and speeds.

Engineering



A radar's components are:
  • A transmitter
    Transmitter
    In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

     that generates the radio signal with an oscillator such as a klystron
    Klystron
    A klystron is a specialized linear-beam vacuum tube . Klystrons are used as amplifiers at microwave and radio frequencies to produce both low-power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for modern...

     or a magnetron and controls its duration by a modulator.
  • A waveguide
    Waveguide
    A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...

     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
    Receiver (radio)
    A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...

    . Knowing the shape of the desired received signal (a pulse), an optimal receiver can be designed using a matched filter
    Matched filter
    In telecommunications, a matched filter is obtained by correlating a known signal, or template, with an unknown signal to detect the presence of the template in the unknown signal. This is equivalent to convolving the unknown signal with a conjugated time-reversed version of the template...

    .
  • An electronic section that controls all those devices and the antenna to perform the radar scan ordered by 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 omnidirectional broadcast antennas
Omnidirectional antenna
In radio communication, an omnidirectional antenna is an antenna which radiates radio wave power uniformly in all directions in one plane, with the radiated power decreasing with elevation angle above or below the plane, dropping to zero on the antenna's axis. This radiation pattern is often...

, 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 angle
Right angle
In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles...

s 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
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

 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
Inverse-square law
In physics, an inverse-square law is any physical law stating that a specified physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity....

 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 use a steerable parabolic
Parabola
In mathematics, the parabola is a conic section, the intersection of a right circular conical surface and a plane parallel to a generating straight line of that surface...

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

Parabolic reflectors can be either symmetric parabolas or spoiled parabolas:
Symmetric parabolic antennas produce a narrow "pencil" beam in both the X and Y dimensions and consequently have a higher gain. The NEXRAD
NEXRAD
NEXRAD or Nexrad is a network of 159 high-resolution Doppler weather radars operated by the National Weather Service, an agency of the National Oceanic and Atmospheric Administration within the United States Department of Commerce...

 Pulse-Doppler weather radar uses a symmetric antenna to perform detailed volumetric scans of the atmosphere. Spoiled parabolic antennas produce a narrow beam in one dimension and a relatively wide beam in the other. This feature is useful if target detection over a wide range of angles is more important than target location in three dimensions. Most 2D surveillance radars use a spoiled parabolic antenna with a narrow azimuthal beamwidth and wide vertical beamwidth. This beam configuration allows the radar operator to detect an aircraft at a specific azimuth but at an indeterminate height. Conversely, so-called "nodder" height finding radars use a dish with a narrow vertical beamwidth and wide azimuthal beamwidth to detect an aircraft at a specific height but with low azimuthal precision.

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, examples include conical scan, unidirectional sector scan, lobe 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 its 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
Phased array
In wave theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna 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
Aegis combat system
The Aegis Combat System is an integrated naval weapons system developed by the Missile and Surface Radar Division of RCA, and now produced by Lockheed Martin...

 and the Patriot Missile System
MIM-104 Patriot
The MIM-104 Patriot is a surface-to-air missile system, the primary of its kind used by the United States Army and several allied nations. It is manufactured by the Raytheon Company of the United States. The Patriot System replaced the Nike Hercules system as the U.S. Army's primary High to Medium...

 and are increasingly used in other areas because the lack of moving parts makes them more reliable, which 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 common. Almost all modern military radar systems are based on phased arrays, where the small additional cost is 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 valued for use in aircraft since they can track multiple targets. The first aircraft to use a phased array radar was the B-1B Lancer. The first aircraft fighter to use phased array radar was the Mikoyan MiG-31
Mikoyan MiG-31
The Mikoyan MiG-31 is a supersonic interceptor aircraft developed to replace the MiG-25 "Foxbat". The MiG-31 was designed by the Mikoyan design bureau based on the MiG-25.-Development:...

. The MiG-31M's SBI-16 Zaslon phased array radar is considered to be the world's most powerful fighter radar.
Phased-array interferometry
Interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...

 or, aperture synthesis
Aperture synthesis
Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection...

 techniques, using an array of separate dishes that are phased into a single effective aperture, are not typically used for radar applications, although they are widely used in radio astronomy
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...

. Because of the thinned array curse
Thinned array curse
The thinned array curse is a theorem in electromagnetic theory of transmitters. It states that a transmitting aperture which is synthesized by a coherent phased array of smaller apertures that are spaced apart will have a smaller minimum beam spot size The thinned array curse (sometimes, sparse...

, such arrays of multiple apertures, when used in transmitters, result in narrow beams at the expense of reducing the total power transmitted to the target. In principle, such techniques used could increase the spatial resolution, but the lower power means that this is generally not effective. Aperture synthesis
Synthetic aperture radar
Synthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...

 by post-processing of motion data from a single moving source, on the other hand, is widely used in space and airborne radar systems.

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. They have been adopted in the United States by the Institute of Electrical and Electronics Engineers
Institute of Electrical and Electronics Engineers
The Institute of Electrical and Electronics Engineers is a non-profit professional association headquartered in New York City that is dedicated to advancing technological innovation and excellence...

 and internationally by the International Telecommunication Union
International Telecommunication Union
The International Telecommunication Union is the specialized agency of the United Nations which is responsible for information and communication technologies...

. 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
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...

 and electronic countermeasures
Electronic countermeasures
An electronic countermeasure is an electrical or electronic device designed to trick or deceive radar, sonar or other detection systems, like infrared or lasers. It may be used both offensively and defensively to deny targeting information to an enemy...

 industries, have replaced the traditional military designations with their own systems.
Radar frequency bands
Band nameFrequency rangeWavelength rangeNotes
HF
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...

3–30 MHz 10–100 m
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

coastal radar systems, over-the-horizon radar
Over-the-horizon radar
Over-the-horizon radar, or OTH , is a design concept for radar systems to allow them to detect targets at very long ranges, typically up to thousands of kilometers...

 (OTH) radars; 'high frequency'
P < 300 MHz 1 m+ 'P' for 'previous', applied retrospectively to early radar systems
VHF 30–300 MHz 1–10 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
Ballistic Missile Early Warning System
The United States Air Force Ballistic Missile Early Warning System was the first operational ballistic missile detection radar. The original system was built in 1959 and could provide long-range warning of a ballistic missile attack over the polar region of the Northern Hemisphere. They also...

), ground penetrating, foliage penetrating; 'ultra high frequency'
L
L band
L band refers to four different bands of the electromagnetic spectrum: 40 to 60 GHz , 1 to 2 GHz , 1565 nm to 1625 nm , and around 3.5 micrometres .-NATO L band:...

1–2 GHz 15–30 cm
Centimetre
A centimetre is a unit of length in the metric system, equal to one hundredth of a metre, which is the SI base unit of length. Centi is the SI prefix for a factor of . Hence a centimetre can be written as or — meaning or respectively...

Long range air traffic control and surveillance
Surveillance
Surveillance is the monitoring of the behavior, activities, or other changing information, usually of people. It is sometimes done in a surreptitious manner...

; 'L' for 'long'
S
S band
The S band is defined by an IEEE standard for radio waves with frequencies that range from 2 to 4 GHz, crossing the conventional boundary between UHF and SHF at 3.0 GHz. It is part of the microwave band of the electromagnetic spectrum...

2–4 GHz 7.5–15 cm Moderate range surveillance, Terminal air traffic control, long-range weather, marine radar; 'S' for 'short'
C
C band
The C band is a name given to certain portions of the electromagnetic spectrum, including wavelengths of microwaves that are used for long-distance radio telecommunications. The IEEE C-band - and its slight variations - contains frequency ranges that are used for many satellite communications...

4–8 GHz 3.75–7.5 cm Satellite transponders; a compromise (hence 'C') between X and S bands; weather; long range tracking
X
X band
The X band is a segment of the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of X band is rather indefinitely set at approximately 7.0 to 11.2 gigahertz . In radar engineering, the frequency range is specified...

8–12 GHz 2.5–3.75 cm Missile
Missile
Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...

 guidance, marine radar
Marine radar
Marine radars are x-band or s-band radar to provide bearing and distance of ships and land targets in vicinity from own ship for collision avoidance and navigation at sea...

, weather, medium-resolution mapping and ground surveillance; in the USA the narrow range 10.525 GHz ±25 MHz is used for airport
Airport
An airport is a location where aircraft such as fixed-wing aircraft, helicopters, and blimps take off and land. Aircraft may be stored or maintained at an airport...

 radar; short range tracking. Named X band because the frequency was a secret during WW2.
Ku
Ku band
The Kμ band is a portion of the electromagnetic spectrum in the microwave range of frequencies. This symbol refers to —in other words, the band directly below the K-band...

12–18 GHz 1.67–2.5 cm high-resolution
K
K band
K band designates certain portions of the electromagnetic spectrum, in either the microwave domain or in the infrared domain. The microwave K bands are used primarily for radar and satellite communications while the infrared K band is used for astronomical observations.-NATO K band:The NATO K band...

18–24 GHz 1.11–1.67 cm from German
German language
German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....

 kurz, meaning 'short'; limited use due to absorption by water vapour
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

, so Ku and Ka 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.
Ka
Ka band
The Ka band covers the frequencies of 26.5–40 GHz. The Ka band is part of the K band of the microwave band of the electromagnetic spectrum. This symbol refers to "K-above" — in other words, the band directly above the K-band...

24–40 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 frequency ranges depend 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.
V
V band
The V band of the electromagnetic spectrum ranges from 50 to 75 GHz. The V band is not heavily used, except for millimeter wave radar research and other kinds of scientific research...

40–75 GHz 4.0–7.5 mm Very strongly absorbed by atmospheric oxygen, which resonates at 60 GHz.
W
W band
The W band of the microwave part of the electromagnetic spectrum ranges from 75 to 110 GHz. It sits above the U.S. IEEE designated V band in frequency, yet overlaps the NATO designated M band...

75–110 GHz 2.7–4.0 mm used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.
UWB
Ultra-wideband
Ultra-wideband is a radio technology that can be used at very low energy levels for short-range high-bandwidth communications by using a large portion of the radio spectrum. UWB has traditional applications in non-cooperative radar imaging...

1.6–10.5 GHz 18.75 cm – 2.8 cm used for through-the-wall radar and imaging systems.

Radar modulators


Modulators
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

 act to provide the waveform of the RF-pulse. There are two different radar modulator designs:
  • high voltage switch for non-coherent keyed power-oscillators These modulators consist of a high voltage pulse generator formed from a high voltage supply, a pulse forming network
    Pulse forming network
    A Pulse Forming Network accumulates electrical energy over a comparatively long time, then releases the stored energy in the form of a relatively square pulse of comparatively short duration for various pulsed power applications...

    , and a high voltage switch such as a thyratron
    Thyratron
    A thyratron is a type of gas filled tube used as a high energy electrical switch and controlled rectifier. Triode, tetrode and pentode variations of the thyratron have been manufactured in the past, though most are of the triode design...

    . They generate short pulses of power to feed the e.g. magnetron
    Cavity magnetron
    The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field. The 'resonant' cavity magnetron variant of the earlier magnetron tube was invented by John Randall and Harry Boot in 1940 at the University of...

    , a special type of vacuum tube that converts DC (usually pulsed) into microwaves. This technology is known as pulsed power
    Pulsed power
    Pulsed power is the term used to describe the science and technology of accumulating energy over a relatively long period of time and releasing it very quickly thus increasing the instantaneous power.-Overview:...

    . In this way, the transmitted pulse of RF radiation is kept to a defined, and usually, very short duration.
  • hybrid mixers, fed by a waveform generator and an exciter for a complex but coherent
    Coherence (physics)
    In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....

     waveform. This waveform can be generated by low power/low-voltage input signals. In this case the radar transmitter must be a power-amplifier, e.g. a klystron tube or a solid state transmitter. In this way, the transmitted pulse is intrapulse-modulated and the radar receiver must use pulse compression
    Pulse compression
    Pulse compression is a signal processing technique mainly used in radar, sonar and echography to increase the range resolution as well as the signal to noise ratio...

     technique.

Radar coolant


Coolanol (silicate ester) was used in several military radars in the 1970s. However, it is hygroscopic, leading to formation of highly flammable alcohol. The loss of a U.S. Navy aircraft in 1978 was attributed to a silicate ester fire. Coolanol is also expensive and toxic.
The U.S. Navy has instituted a program named Pollution Prevention
Pollution prevention
Pollution prevention ' describes activities that reduce the amount of pollution generated by a process, whether it is consumer consumption, driving, or industrial production...

 (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 blend of a polyol ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

 mixed with effective amounts of an antioxidant
Antioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...

, yellow metal pacifier and rust inhibitors. Effective additives include secondary arylamine antioxidants, triazole
Triazole
Triazole refers to either one of a pair of isomeric chemical compounds with molecular formula C2H3N3, having a five-membered ring of two carbon atoms and three nitrogen atoms.The two isomers are:*1,2,3-Triazole 100px*1,2,4-Triazole 100px...

 derivative yellow metal pacifier and an amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 derivative and substituted primary and secondary amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...

 and/or diamine rust inhibitor.

See also



  • Acronyms and abbreviations in avionics
    Acronyms and abbreviations in avionics
    -A:*ACARS: Aircraft Communications Addressing and Reporting System.*ACAS: Airborne Collision Avoidance System.*ACP: Audio Control Panel.*ACS: Audio Control System.*ADAHRS: Air Data and Attitude Heading Reference System.*ADC: Air Data Computer....


Definitions
  • Amplitude-comparison monopulse
    Amplitude-Comparison Monopulse
    Amplitude monopulse direction finding refers to a common technique employed in radar systems to improve the accuracy with which the direction of arrival of a pulse can be estimated.-Approach:...

  • Constant false alarm rate
    Constant false alarm rate
    Constant false alarm rate detection refers to a common form of adaptive algorithm used in radar systems to detect target returns against a background of noise, clutter and interference.Other detection algorithms are not adaptive...

  • Sensitivity Time Control
    Sensitivity Time Control
    Sensitivity Time Control is used to attenuate the very strong signals returned from nearby ground clutter targets in the first few range gates of a Radar receiver. Without this signal attenuation, the receiver would routinely saturate due to the strong signals....


Hardware
  • Radar engineering details
  • Klystron
    Klystron
    A klystron is a specialized linear-beam vacuum tube . Klystrons are used as amplifiers at microwave and radio frequencies to produce both low-power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for modern...

  • Cavity magnetron
    Cavity magnetron
    The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field. The 'resonant' cavity magnetron variant of the earlier magnetron tube was invented by John Randall and Harry Boot in 1940 at the University of...

  • Radio
    Radio
    Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

  • Traveling-wave tube
  • Crossed-field amplifier
    Crossed-field amplifier
    A crossed-field amplifier is a specialized vacuum tube, first introduced in the mid-1950s and frequently used as a microwave amplifier in very-high-power transmitters....

  • Gallium arsenide
    Gallium(III) arsenide
    Gallium arsenide is a compound of the elements gallium and arsenic. It is a III/V semiconductor, and is used in the manufacture of devices such as microwave frequency integrated circuits, monolithic microwave integrated circuits, infrared light-emitting diodes, laser diodes, solar cells and...


Similar detection and ranging methods
  • LIDAR
    LIDAR
    LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...

  • LORAN
    LORAN
    LORAN is a terrestrial radio navigation system using low frequency radio transmitters in multiple deployment to determine the location and speed of the receiver....

  • Sonar
    Sonar
    Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...


Historical radars
  • List of radars
  • SCR-270 radar
    SCR-270 radar
    The SCR-270 was one of the first operational early warning radars. It was the U.S. Army's primary long-distance radar throughout World War II and was deployed around the world...

  • H2S radar
    H2S radar
    H2S was the first airborne, ground scanning radar system. It was developed in Britain in World War II for the Royal Air Force and was used in various RAF bomber aircraft from 1943 to the 1990s. It was designed to identify targets on the ground for night and all-weather bombing...


Further reading

  • Burch, David F., Radar For Mariners, McGraw Hill, 2005, ISBN 978-0-07-139867-1.
  • Kaiser, Gerald, Chapter 10 in "A Friendly Guide to Wavelets", Birkhauser, Boston, 1994.
  • Kouemou, Guy (Ed.): Radar Technology. InTech, 2010, ISBN 978-953-307-029-2, (Radar Technology - Free Open Access Book | InTechOpen).
  • Younghusband, Eileen., Not an Ordinary Life. How Changing Times Brought Historical Events into my Life, Cardiff Centre for Lifelong Learning, Cardiff, 2009., ISBN 987-0-9561156-9-0 (Pages 36–67 contain the experiences of a WAAF radar plotter in WWII.)
  • Younghusband, Eileen., One Woman's War. Cardiff. Candy Jar Books. 2011. ISBN 978-0-9566826-2-8

External links