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Radar is an 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 any
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.
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
Transit Time
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.
Frequency Modulation
Another form of distance measuring radar, used in some aircraft radar altimiters, frequency modulates (with a sine wave at
audio frequencies) a continuously transmitted UHF signal transmitted downard. A separate receiver detects the frequency
difference between the transmitted and recieved signals. The frequency of this detected signal is related in a predictable manner
to the length of the signal path and is subseqently displayed on an instrument. This signal processing is similar to that used in
speed detecting doppler radar. See also the section on Continuous Wave radar below.
Signals
Each radar has a particular type of signal they use. Long range radars tend to use
long pulses with long delays between them, and short range radars use smaller pulses with less time between them. This pattern of
pulses and pauses is known as the Pulse Repetition Frequency (or PRF), and is one of the main ways to characterize a radar. As electronics have improved many radars
now can change their PRF.
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 300 m/s,
which is why you hear the Doppler effect of an ambulance siren as it passes you at 3 m/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 receive
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.
- "Mapping radars" are used to scan a large region for remote
sensing and geography applications. They generally use synthetic aperture radar, which limits them to relatively
static targets, normally terrain.
Radar Equation
The amount of power Pr returning to the receiving antenna is given by the radar equation:
-
where
- Pt = transmitter power,
- Gt = gain of transmitting antenna,
- Ar = area of receiving antenna,
- = scattering coefficient of
target,
- Rt = distance from transmitter to target,
- Rr = distance from target to receiver.
In the common case where the transmitter and receiver are at the same location, Rt = Rr and the term
Rt˛ Rr˛ 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 different types of materials, and were reflected by others. Although this was predicted earlier by James Clerk Maxwell, his work was not widely known at the time and
much of the research became known through Hertz.
1900s
By the 1900s a German engineer,
Chistian
Huelsmeyer, proposed the use of radio echoes to avoid collisions. He invented 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.
Nikola Tesla, in August
1917, proposed principles regarding frequency and power levels for primitive RADAR units.
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. In the 1917 The
Electrical Experimenter, Tesla stated the principles in detail.
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 also proposed the use of
standing electromagnetic waves along with pulsed reflected waves to determine the relative position, speed, and course
of a moving object and other modern concepts of radar.
Tesla had first proposed that radio location might help find submarines (for
which it is not well-suited) with a fluorescent screen indicator, though it was first applied successfully to locate 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". Tesla first established principles regarding frequency and power level for the
first primitive RADAR units in 1934.
On February 12, 1935, Robert Watson-Watt sent a memo of a proposed RADAR system to the
Air Ministry, entitled "Detection and location of aircraft by radio
methods". The invention of modern radar is generally credited to Sir 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.
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.
United Kingdom and Germany
At this point both the United Kingdom 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 the United Kingdom, 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
See also
Further reading
External links
Disambiguation
Radar is also a fictional character in the M*A*S*H novel, movie and TV series. See: Corporal Walter (Radar) O'Reilly.
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