Radar

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Radar is a recently-coined acronym for radio detection and ranging. It is a system used to detect, range (determine the distance) and map objects such as aircraft and rain. Strong radio waves are transmitted, and a receiver listens for reflected echoes. By analysing the reflected signal, the reflector can be located, and sometimes identified. Although the amount of signal returned is tiny, radio signals can easily be detected and amplified.

General Description

Radar radio waves can be easily generated at any desired strength, detected at even tiny powers, and then amplified many times. Thus radar is suited to detecting objects at very large ranges where other reflections, like sound or visible light, would be too weak to detect.

Electromagnetics

Radar sets attempt to reflect electromagnetic waves, notably radio waves and microwaves, from target objects. This reflection is then detected using a radio receiver.

Electromagnetic waves reflect from any large change in the dielectric or diamagnetic constants. This means that a solid object in air or vacuum, or other significant changes in atomic density, will usually reflect radar waves. This is particularly true of electrically-conductive materials such as metal, making radar particularly well suited to the detection of aircraft and ships.

Reflection

Radar waves reflect in a variety of ways depending on the size of the radio wave and the shape of the target. If the radio wave is much shorter than the reflector's size, the wave will bounce off in a way similar to the way light bounces from a mirror. Early radars used very long wavelengths that were larger than the targets and received a vague signal, whereas modern systems use shorter wavelengths (a few centimetres) that can image objects the size of a loaf of bread or larger.

Radio waves always reflect from curves and corners, in a way similar to glint from a rounded piece of glass. The most reflective targets have 90-degree angles between the reflective surfaces.

Polarization

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

Frequency

Radars in the frequency range of 2.4GHz are strongly absorbed by water. This is useful for a microwave oven but is typically avoided otherwise. Frequencies like those used in commercial TV (50-900 MHz) can pass through rock and earth, and are used for ground penetrating radar.

Distance Measurement

The easiest way to measure the range of an object is to broadcast a short pulse of radio signal, and then time how long it takes for the reflection to return. The distance is one-half the round trip time (because the signal has to travel to the target and then back to the receiver) divided by the speed of the signal, which in this case is the speed of light.

The receiver cannot detect the returned reflection (also just called a return) while the signal is being sent out – there's no way to tell if the signal it hears is the original or the return. This means that a radar has a distinct minimum range, which is the length of the pulse divided by the speed of light, divided by two. In order to detect closer targets you have to use a shorter pulse length.

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

These two effects tend to be at odds with each other, and it is not easy to combine both good short range and good long range in a single radar. This is because the short pulses needed for a good minimum range broadcast less total energy, making the returns much smaller and the target harder to detect. You could offset this by using more pulses, but this would shorten the maximum range again.

Signals

Each radar has a particular type of signal they use. Long range radars tend to use long pulses with long delays between then, and short range radars use smaller pulses with less time between them. This pattern of pulses and pauses is known as the Pulse Repetition Frequency (or PRF), and is one of the main ways to characterize a radar. As electronics have improved many radars now can change their PRF.

Speed Measurement

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

Doppler effect

However there is another effect that can be used to make much more accurate speed measurements, and do so almost instantly (no memory required), known as the Doppler effect. The Doppler effect is the change in frequency of any signal due to the finite speed at which the signal travels compared to the motion of the object. For instance, sound travels at the fairly slow speed of around 300m/s, which is why you hear the Doppler effect of an ambulance siren as it passes you at 3m/s or so. Although this results in a small 1% change in frequency, the human ear is very good at detecting this change.

In the case of radar the speed of light is much faster than sound and thus the resulting shift much smaller. However modern electronics are even better at detecting this change than the human ear is for sound. Speeds as slow as a few centimeters per second can be easily measured, an accuracy typically much better than for the measurement of distance. Practically every modern radar system uses this principle, and is generally referred to as Pulse Doppler Radar.

The major use of Doppler is to separate moving objects from clutter. It's common for Doppler radars to have a frequency range adjust control to reject low speeds. Another form color-codes returns by their speed.

Doppler measures the speed only along the direction from the reflection to the radar antenna. In order to measure the object's true speed and direction, the radar set or operator had to remember a return's location. Military organizations traditionally used a manual plotting board for this purpose. Computers in the radar systems have made this even more convenient.

Continuous Wave

It is possible to make a radar without any pulsing, known as a Continuous Wave Radar (or CW), by sending out a very pure signal of a known frequency. Return signals from targets are shifted away from this base frequency via the Doppler effect, so they can be picked up at another antenna even if it is physically close to the broadcaster.

The main advantage of the CW radars is that they have no pulsing, and thus no minimum or maximum ranges (although the broadcast strength imposes a practical limit on the latter) as well as maximizing power on the target. However they also have the disadvantage of only being able to detect moving targets, as motionless ones (along the line of sight) will not cause a Doppler shift and the signal from such a target will be filtered out. Such systems thus find themselves being used at either end of the range spectrum, as radio-altimeters at the close-range end (where the range may be a few feet) and long distance early-warning radars at the other.

CW radars have the disadvantage that they cannot measure distance, because there are no pulses to time. In order to correct for this problem, the signal can be changed in frequency subtly over time. When a reflection is received the frequencies can be examined, and by knowing when in the past that particular frequency was sent out, you can do a range calculation similar to using a pulse. It is generally not easy to make a broadcaster that can send out random frequencies cleanly, so instead these Frequency Modulated Continuous Wave Radar (FMCW), use a smoothly varying "ramp" of frequencies up and down. For this reason they are also known as a chirped radar.

Position Measurement

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

Early systems

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

Phased array

Another method of steering is used in phased array radar. It uses the radio signal's interference with itself. One can broadcast a signal from mulitple antennas. The result is a single beam with the waves in the rest of space cancelling each other. In order to point the beam, computer-controlled delay lines adjust the delay to each antenna. Instead of constructing a single large antenna, such a system has a number of small omni-directional antennas referred to as elements, usually arranged in a flat plate.

Phased array radars require no physical movement. The beam can be steered by electronically adjusting the delay lines to each antenna. This means that the beam can scan at thousands of degrees per second, fast enough to irradiate 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.

Phased array radars were originally used for missile defense. On ships, they are the heart of the Aegis combat system, and are increasingly used in other areas because the lack of moving parts makes them more reliable, and sometimes permits a much larger effective antenna.

Types and uses of radar

"Search radars" scan a wide area with pulses of short radio waves. The usually scan the area two to four times a minute. The waves are usually less than a meter long. Ships and planes are metal, and reflect radio waves. The radar measures the distance to the reflector by measuring the time from emission of a pulse to reception, and dividing by the speed of light. To be accepted, the received pulse has to lie within a period of time called the range gate. The radar determines the direction because the short radio waves behave like a search light when emitted from the reflector of the radar set's antenna.
"Targeting radars" use the same principle but scan a much smaller area far more often, usually several times a second or more, where a search radar might scan a few times per minute. Some targeting radars have a range gate that can track a target, to eliminate clutter and electronic counter-measures.
"Radar proximity fuses" are attached to anti-aircraft artillery shells or other explosive devices, and detonate the device when it approaches a large object. They use a small rapidly pulsing omnidirectional radar, usually with a powerful battery that has a long storage life, and a very short operational life. The fuses used in anti-aircraft artillery have to be mechanically designed to accept fifty thousand gravities of acceleration, yet still be cheap enough to throw away.
"Weather radars" can resemble search radars. These radar use radio waves with horizontal, dual (horizontal and vertical), or circular polarization. The frequency selection of weather radar is a performance compromise between precipitation reflectivity and attenuation due to atmospheric water vapor. Some weather radar uses doppler to measure wind speeds.
"Navigational radars" resemble search radar, but use very short waves that reflect from earth and stone. They are common on commercial ships and long-distance commercial aircraft.
"General purpose radars" are increasingly being substituted for pure navigational radars. These generally use navigational radar frequencies, but modulate the pulse so the receiver can determine the type of surface of the reflector. The best general-purpose radars distinguish the rain of heavy storms, as well as land and vehicles. Some can superimpose sonar and map data from GPS position.
"Radar altimeters" measure an aircraft's true height above ground.
Air traffic control uses Primary and Secondary Radars
- Primary radar is a "classical" radar which reflects all kind of echoes, including aircraft and clouds.
- Secondary radar emit pulses and listen for special answer of digital data emitted by an Aircraft Transponder as an answer. Transponders emit different kind of data like a 4 octal ID (mode A), the onboard calculated altitude (mode C) or the Callsign (not the flight number) (mode S). Military use transponders to establish the nationality and intention of an aircraft, so that air defenses can identify possibly hostile radar returns.

Radar Equation

The amount of power P_r returning to the receiving antenna is given by the radar equation:

P_{r}={{P_{t}G_{t}A_{r}\sigma } \over {{(4\pi )}^{2}R_{t}^{2}R_{r}^{2}}}

where

P_t = transmitter power,
G_t = gain of transmitting antenna,
A_r = area of receiving antenna,
σ = scattering coefficient of target,
R_t = distance from transmitter to target,
R_r = distance from target to receiver.

In the common case where the transmitter and receiver are at the same location, R_t = R_r and the term R_t² R_r² can be replaced by R⁴, where R is the range. This shows that the received power declines as the fourth power of the range, which means that the reflected power from distant targets is very, very small.

History

1800s

In 1887 the German physicist Heinrich Hertz began experimenting with radio waves in his laboratory. He found that radio waves could be transmitted through some materials, and were reflected by others. Although this was predicted earlier by James Clerk Maxwell, his work was not widely known as the time and much of the research became known through Hertz. In fact today we use his name as the SI unit of frequency, the hertz, often abbreviated to Hz.

1900s

Some years later a German engineer, Chistian Huelsmeyer, proposed the use of radio echoes to avoid collisions in marine navigation. In 1904 he completed construction of a device he called the telemobiloscope, which consisted of a simple spark gap aimed using a funnel-shaped metal antenna. When a reflection was seen by the two straight antennas attached to the receiver, a bell sounded. Although very simple, the system could detect shipping accurately up to about 3 km. Nevetheless the naval world seemed uninterested in his invention, and it was not put into production.

Modern radar

The invention of modern radar is generally credited to Sir Robert Watson-Watt. In 1915 he joined the Royal Aircraft Factory at Ditton Park as a meteorologist, where he attempted to use radio signals generated by lightning strikes to map out thunderstorms. The difficulty in pinpointing the direction of these high-speed signals led to the use of rotating directional antennas, and in 1923 the use of oscilloscopes in order to display them in 2-D. At this point the only missing part of a functioning radar was the broadcaster.

Nikola Tesla, in August 1917, first established principles regarding frequency and power level for the first primitive RADAR units in 1934. In the 1917 The Electrical Experimenter, Tesla stated the principles of modern military radar in detail. Tesla's study of high voltage, high frequency alternating currents led to this development. Tesla had formed the concept of using radio waves to detect objects at a distance.

Tesla stated,

"For instance, 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."

Tesla proposed to use electromagnetic waves to determine the relative position, speed, and course of a moving object and other modern concepts of radar. Tesla had proposed that it might help find submarines (for which it isn't well-suited), though it was first applied successfully to find aircraft (after their later proliferation) and surface ships during World War II. Emil Girardeau, working with the first French radar systems, stated he was building radar systems "conceived according to the principles stated by Tesla".

In 1934, Watson-Watt was well established in the area of radio, and was approached by H.E. Wimperis from the Air Ministry, who asked about the use of radio to produce a 'death ray'. While he knew this to be unlikely, he did mention Meanwhile attention is being turned to the still difficult, but less unpromising, problem of radio detection and numerical considerations on the method of detection by reflected radio waves will be submitted when required. Watson-Watt and his assistant Arnold Wilkins published a report on the topic in February 1935, titled The Detection of Aircraft by Radio Methods.

Microwaves

Meanwhile in Germany, Hans Eric Hollmann had been working for some time in the field of microwaves, which were to later become the basis of almost all radar systems. In 1935 he published Physics and Technique of Ultrashort Waves, which was then picked up by researchers around the world. At the time he had been most interested in their use for communications, but he and his partner Hans-Karl von Willisen had also worked on radar-like systems.

In the autumn of 1934 their company, GEMA, built the first commercial radar system for detecting ships. Operating in the 50 cm range it could detect ships up to 10 km away, similar in purpose to Huelsmeyer's earlier device. In the summer of 1935 a pulse radar was developed with which they could spot the ship the Königsberg 8 km away, with an accuracy of up to 50 m, enough for gun-laying. The same system could also detect an aircraft at 500 m altitude at a distance of 28 km. The military implications were not lost this time around, and construction of land and sea-based versions took place as Freya and Seetakt.

England and Germany

At this point both England and Germany knew of the ongoing efforts by their "competition". Both nations were intensely interested in the other's developments in the field, and engaged in an active campaign of espionage and false leaks about their respective equipment. But it was only in Britain that the usefulness of the system became obvious, so while the German systems had the edge technologically (operating on much shorter wavelengths) only Britain started true mass deployment of both the radars and the control systems needed to support them.

Chain Home

Shortly before the outbreak of World War II several radar stations known as Chain Home (or CH) were constructed in the south of England. As one might expect from the first radar to be deployed, CH was a simple system. The broadcast side was formed from two 300' (100 m) tall steel towers strung with a series of cables between them. The output of a powerful 50 MHz radio of about 200 kW (up to 800 kW in later models) was fed into these cables, pulsed at about 50 times a second. A second set of 240' (73 m) tall wooden towers were used for reception, with a series of crossed antennas at various heights up to 215' (65 m). In fact most stations had more than one set of each antenna, tuned to operate at different frequencies.

The CH radar was read with an oscilloscope. When a pulse was sent out into the broadcast towers, the scope was triggered to start its beam moving horizontally across the screen very rapidly. The output from the receiver was amplified and fed into the vertical axis of the scope, so a return from an aircraft would deflect the beam upward. This formed a spike on the display, and the distance from the left side - measured with a small scale on the bottom of the screen - would give the distance to the target. By rotating the receiver antennas to make the display disappear, the operator could determine the direction (this is the reason for the cross shaped antennas), the size of the vertical displacement indicated something of the number of aircraft involved, and by comparing the strengths returned from the various antennas up the tower, you could determine the height.

CH proved highly effective during the Battle of Britain, and are often credited with allowing the RAF to defeat the much larger Luftwaffe forces. Whereas the Luftwaffe had to hunt all over to find the RAF fighters, the RAF on the other hand knew exactly where the Luftwaffe bombers where, and could converge all of their fighters on them.

Very early in the battle the Luftwaffe made a series of small raids on a few of the stations, but they were eventually returned to operation in a few days. In the meantime the operators took to broadcasting radar-like signals from other systems in order to fool the Germans into believing that the systems were still operating. Eventually they gave up trying to bomb them. The Luftwaffe apparently never understood the importance of radar to the RAF's efforts, or they would have assigned them a much higher priority -- it is clear they could have knocked them out continually if they wished.

In order to avoid the CH system the Luftwaffe adopted other tactics. One was to approach Britain at very low levels, below the sight line of the radar stations. This was countered to some degree with a series of shorter range stations built right on the coast, known as Chain Home Low (CHL). These radars had originally been intended to use for naval gun-laying an known as Coastal Defense (CD), but their narrow beams also meant they could sweep an area much closer to the ground without seeing the reflection of the ground (or water) itself. Unlike the larger CH systems, CHL had to have the broadcast antenna itself turned, as opposed to just the receiver. This was done manually on a pedal-crank system run by WAAFs until more reliable motorized movements were installed in 1941.

Later adaptations

Similar systems were later adapted with a new display to produce the Ground Controlled Intercept stations starting in late 1941. In these systems the antenna was rotated mechanically, followed by the display on the operators console. That is, instead of a single line across the bottom of the display from left to right, the line was rotated around the screen at the same speed as the antenna was turning.

The result was a 2-D display of the air around the station with the operator in the middle, with all the aircraft appearing as dots in the proper location in space. These so-called Plan Position Indicators (PPI) dramatically simplified the amount of work needed to track a target on the operator's part. Such a system with a rotating, or sweeping, line is what most people continue to associate with a radar display.

Rather than avoid the radars, the Luftwaffe took to avoiding the fighters by flying at night and in bad weather. Although the RAF was aware of the location of the bombers, there was little they could do about them unless the fighter pilots could see the opposing planes. However, just this eventuallity had already been forseen, and Watson-Watt (likely at the urging of Tizzard) had already started work on a miniaturized radar system suitable for aircraft, the so-called AI (airborne interception) set. Initial sets were available in 1941 and fitted to Bristol Blenheim aircraft, replaced quickly with the better performing Bristol Beaufighter, which quickly put an end to German night- and bad-weather bombing over England.

Magnetron

The next major development in the history of radar was the invention of the cavity magnetron by Randall and Boot of Birmingham University in early 1940. This was a small device which generated much more powerful microwaves than previous devices, which in turn allowed for the detection of much smaller objects and the use of much smaller antennas. The secrecy of the device was so high that it was decided in 1940 to move production to the USA, which resulted in the creation of the MIT Radiation Lab to develop the device further.

German developments

German developments mirrored those in England, but it appears radar received a much lower priority until later in the war. The Freya was in fact much more sophisticated than its CH counterpart, and by operating in the 1.2 m wavelength (as opposed to ten times that for the CH) the Freya was able to be much smaller and yet offer better resolution. Yet by the start of the war only eight of these units were in operation, offering much less coverage.

However the Germans did not have an airborne system of any sort deployed until 1942, leaving them with the problem of having to get their fighters into that 300m range solely with ground-based equipment. To fill this need another system known as Wuerzburg was deployed, starting in 1941.

Wuerzburg

Unlike other systems, the Wuerzburg was mounted on a highly directional parabolic antenna that was sensitive in only one direction. This made it useless for finding the targets, but once guided to one by an associated Freya it could track it with extreme accuracy: later models were accurate to 0.2 degrees or less. In order to do this the radar sent out two lobes and the return of each was shown on the display. By keeping the returns from both the same strength, the operator kept the Wuerzburg pointed directly at the target.

The downfall of the German radar network was that it could only track a single aircraft per Wuerzburg. In fact the system required two Wuerzburgs per interception, one for the target, and one for the fighter. This meant that as a raid developed, only a few night fighters could be directed at any one time, as only a small number of the eventual 5,000 Wuerzburgs would be within their 25 km range at any one time.

Comparison

Compared to the British PPI systems, the German system was far more labour intensive. This problem was compounded by the lackadaisical approach to command staffing. It was several years before the Luftwaffe had a command and control system nearly as sophisticated as the one set up by Watt before the war, after seeing the confusion too much information caused during one test.

German airborne radar units followed a similar pattern. Early Lichtenstein BC units were not deployed until 1942, and as they operated on the 2 m wavelength they required large antennas. By this point in the war the British had become experts on jamming German radars, and when a BC-equipped Ju 88 night fighter landed in England one foggy night, it was only a few weeks before the system was rendered completely useless. By late 1943 the Luftwaffe was starting to deploy the greatly improved SN-2, but this required huge antennas that slowed the planes as much as 50 km/h. Jamming the SN-2 took longer, but was accomplished. A 9 cm wavelength system known as Berlin was eventually developed, but only in the very last months of the war.

Specific radar systems

External links

Early history of radar
Anecdotal account of the carriage of the world's first high power cavity magnetron from Britain to the US during WW2.