Radar

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 particularily true of electrically-conductive materials such
as metal, making radar particularily 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 later) 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
        o Primary radar is a "classical" radar which reflects all kind of
          echoes, including aircraft and clouds.
        o 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 Pr 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^2R_r^2}}]

where

   * Pt = transmitter power,
   * Gt = gain of transmitting antenna,
   * Ar = area of receiving antenna,
   * σ = 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 Rt² Rr² 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 "Koenigsberg" 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 adaptions

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 lacksidasical 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

   * X-band Radar
   * Millimeter Cloud Radar
   * Over the Horizon Radar in Australia
   * Doppler radar as weather radar