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Radar

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.

Table of contents

1 Electromagnetics 1.1 Reflection 1.2 Polarization 1.3 Frequency bands 2 Distance measurement 2.4 Transit time 2.5 Frequency modulation 3 Signals 4 Speed measurement 4.6 Doppler effect 4.7 Continuous wave 5 Position measurement 5.8 Early systems 5.9 Phased array 6 Types and uses of radar 7 Radar equation 8 History 8.10 1800s 8.11 1900s 8.12 Microwaves 8.13 United Kingdom and Germany 8.14 Chain Home 8.15 Later adaptations 8.16 Magnetron 8.17 German developments 8.18 Wuerzburg 8.19 Comparison 9 Specific radar systems 10 See also 11 Further reading 12 External links 13 Disambiguation

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 bands

Other users of the radio spectrum, such as the broadcasting and electronic countermeasures (ECM) industries, have replaced the traditional military designations with their own systems.

Radar Frequency Bands

Band Name	Frequency Range	Wavelength Range	Notes
HF	3-30 MHz	10-100 m	'high frequency'
P	below 300 MHz	1 m +	'P' for 'previous', applied retrospectively to early radar systems
VHF	50-330 MHz	0.9-6 m	very long range, ground penetrating; 'very high frequency'
UHF	300-1000 MHz	0.3-1 m	very long range (e.g. ballistic early warning), ground penetrating; 'ultra high frequency'
L	1-2 GHz	15-30 cm	long range air traffic control and surveillance; 'L' for 'long'
S	2-4 GHz	7.5-15 cm	terminal air traffic control, long-range weather; 'S' for 'short'
C	4-8 GHz	3.75-7.5 cm	a compromise (hence 'C') between X and S bands; weather
X	8-12 GHz	2.5-3.75 cm	missile guidance, marine radar, weather; in the USA the narrow range 10.525 GHz ±25 MHz is used for airport radar.
K	18-27 GHz	1.11-1.67 cm	from German kurtz, meaning 'short'; useless, except for detecting clouds, because of absorption by water vapour, so Ku and Ka were used instead for surveillance
Ku	12-18 GHz	1.67-2.5 cm	high-resolution mapping, satellite altimetry; frequency just under K band (hence 'u')
Ka	27-40 GHz	0.75-1.11 cm	mapping, airport surveillance; frequency just above K band (hence 'a')
mm	40-300 GHz	1 - 7.5 mm	'millimetre' band, subdivided as below
V	40-75 GHz	0.4 - 0.75 cm
W	75-110 GHz	0.27 - 0.4 cm

Distance measurement

Transit time

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

Signals

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. 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.

History

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 Source | Copyright