See also: Radar (homonymy)

The radar is a system which uses the waves radio to detect and determine the distance and/or the speed from objects such as the Avion S, Bateau X, or the Pluie. A transmitter sends radio waves, which is reflected by the target and is detected by a receiver, often located at the same place as the transmitter. The position is estimated thanks to the time of return of the signal and the speed is measured starting from the change of frequency of the signal by Doppler effect.

The radar is used in many contexts: in Meteorology, for the Control of the air traffic, for the monitoring of the road traffic, by the soldiers, in astronautics, etc the word itself is a Néologisme coming from the Acronyme English: RA dio D etection has Nd R anging, which one can translate by “detection and estimate of the distance by radio waves” or more simply “radiolocation”. This acronym of American origin replaced the English initials previously used: " RDF" ( Radio Direction Finding ). Since, the word entered the usual language, loser thus his writing of initials.

History

• In 1864, James Clerk Maxwell describes the laws of the electromagnetism, which makes it possible for the first time to work on their source.

• In 1889, Heinrich Rudolf Hertz watch which the electromagnetic waves are reflected by metal surfaces.

• 20th century, several inventive, Scientific S, and Engineer S contributed to the development of the radar:

* Development of the radio and TSF (by Marconi, inter alia), therefore antennas.

* the theoretical bases of the radar date from the beginning of the 20th century with, in 1904, the deposit of the patent of the “Telemobiloskop” (Reichspatent NR. 165546) by German Christian Hülsmeyer, who showed the possibility of detecting the presence of boats in a very dense fog. By sending a wave using a multipolar antenna, its system noted the return since an obstacle with a dipolar Antenne without however being able to define some more than a approximate Azimut and at all its distance. It was thus the RAD (radio detection) but not AR (azimuth and ray).

*En 1917, Nikola Tesla establishes the theoretical principles (frequencies and levels of power) of the future “radar”

* In the years 1920: experiments of detection with antennas. It is necessary to solve problems of power and wavelength.

* In 1934, making following a systematic study of the Magnetron, tests on systems of detection by short waves are carried out in France by CSF (16 and 80 cm wavelength) according to the principles of Nikola Tesla. A patent is deposited (French patent n° 788795). Thus are born the “radars” with decimetre waves. The first equipped in 1934 the Oregon cargo liner, followed in 1935 by that of the steamer Normandy.

* In 1935, making following a patent deposited by Robert Watson-Watt (the inventor known as “official” of the radar ) (English patent GB593017), the first network of radars is ordered by the British and will bear code name Chain home .

* the Hungarian Zoltán Lajos Bay produced another of the first operational models in 1936 in the laboratory of the company Tungsram (Hungary).

• In a general way, one can consider that the radar was almost ready in its current form at the dawn of the Second world war. It however missed the operational experiment with the combat which pushed the engineers to find many improvements technical. Thus, the airborne radars were developed to give the possibility to the air weapon of proceeding to the bombardments and the hunting of night. One also carried out experiments on the Polarization. During the use of the radar in an operational way, the operators noted the presence of Artéfact S. For example, the operators of the radars microwaves of the allied armies noticed noise in the images. These noises proved to be echoes coming from Précipitation S (rain, snow, etc), which led to the development of the weather radars after the end of the engagements. The first techniques of jamming and electronic countermeasures are also developed.

• Since this war, the radars are used in many fields going of meteorology to the Astrométrie while passing by the air road control and .

• In the Fifties, the invention of the Radar to synthesis of opening paved the way towards obtaining images radar to very high-resolution.

• In 1965, Cooley and Tuckey (Re) discover the Transformée of fast Fourier which took all its interest especially when the Informatique started to become sufficiently powerful. This mathematical algorithm is at the base of the majority of the numerical radar treatments used today.

Disputed paternity of the invention

the extraordinary technical change caused by the radar in electronics was of course accompanied by an abundant flowering of memories, whose legend and technical nationalism always were unfortunately not absent. The basic principle of the radar belongs to the common inheritance of the physicists: what in the final analysis remains with the real credit of the technicians is measured with the effective manufacture of operational materials. (Maurice Big shot)

the creation of the radars was asserted by English who succeeded in spreading this idea in America, which was facilitated to them by the circumstances of the war and the occupation of France by the enemy. However the truth is different. (Emile Girardeau)

General description

A radar emits powerful waves, produced by a Oscillateur radio and transmitted by a antenna. Although the power of the emitted waves is large, the Amplitude returned signal is generally very small. Nevertheless, the radio operator signals are easily detectable electronically and can be amplified many times. There exist various ways of emitting these waves. The most used are:

• pulsated waves, where the radar emits an impulse and awaits the return.
• the radar with continuous emission, where one emits continuously starting from an antenna and one receives using one second.

By analyzing the reflected signal, it is possible to locate and identify the object responsible for the reflection, like calculating its rate of travel. The radar can detect objects having broad a Gamme of reflective properties, whereas the other types of signals, such as the its or the visible Lumière, ghost of these objects, would be too weak to be detected. Moreover, the radio waves can be propagated with weak a Atténuation through the air and various obstacles, the such Nuage S, the Brouillard or the Fumée, which absorb a light signal quickly. That makes possible detection and tracking under conditions which paralyze other technologies.

Technology of the radar

Components of a radar system

A radar is formed various components:

• the transmitting which generates the radio wave.
• On the radars with ultra high frequencies (frequencies higher than the gigahertz), it is a Guide of wave which brings the wave towards the antenna.
• the Duplexer, an electronic switch, directs the wave towards the antenna during the emission or the signal of return since the antenna towards the receiver at the time of the reception when one uses a radar monostatic. It thus makes it possible to use the same antenna for the two functions. It is paramount that it is synchronized, since the power of the emitted signal is about the megawatt what is too important for the receiver which, milked to him well signals of a power about a few nano-Watts. If the emitted impulse would be directed towards the receiver, this one would be destroyed instantaneously.
• the antenna whose role is to diffuse the electromagnetic wave towards the target with the minimum of loss. Its rate of travel, rotation and/or swinging, like its position, of rise as in azimuth, are controlled, that is to say mechanically, but sometimes also electronically (see the article Antenne network with ordering of phase). The antenna is requested as well in emission as in reception. These two functions can be however separate between two multistatic antennas in the case of radars.
• the receiving which receives the incidental signal (target - antenna - waveguide - duplexer), the fact of emerging noises parasitic radios, amplifies it, treats it;
• a stage of signal processing allowing to treat the rough signal in order to extract some from the data useful for the operator (detection, follow-up and identification of target; extraction of parameters weather, oceanographical, etc). The whole is controlled by the electronic system of the radar, programmed according to a software of survey. The data obtained are then posted with the users.

Radar monostatic, bistatic, multistatic

In the majority of the cases, the transmitter and the receiver of the radar share an electronics and a common antenna. One speaks then about radar monostatic. Nothing however prevents from considering a radar system where the transmitter and the receiver are separate; one speaks then about radar bistatic, or even about multistatic configuration, if there are a distinct transmitter and several receivers. One and the other configuration offer advantages and disadvantages:

• in a monostatic configuration, the division of electronics and antenna makes it possible to reduce the obstruction and the costs of synchronization between the transmitter and the receiver, which explains why the vast majority of the radars are monostatic. N the other hand, only the signal retrodiffused by the target is received by the radar. In addition, in a military context, the transmitter can be detected by the enemy and destroyed…
• in a bistatic configuration, the receiver is distinct from the transmitter and is completely passive, therefore less easily detectable by a potential enemy. In addition to specialized transmitters radar, it is also planned to use transmitters known as of opportunity like antenna-relays GSM, or of television, or satellites GPS of which the signal is diverted of their primary use to carry out at little cost and in a discrete way a work of measurement usually left with the radars. Lastly, the possibility of positioning the transmitter and the receiver at will makes it possible to explore other configurations of reflection making it possible to increase the volume of information available on the target. N the other hand, the use of a bistatic configuration requires a good synchronization between the transmitter and the receiver, and the use of a geometry of less commonplace acquisition. The radar low frequencies of Jindalee, in Australia, is an example of operational radar bistatic.

When one speaks about radar bistatic, one supposes implicitly that the transmitter and the receiver are really separate (either from the point of view of the distance, or from an angular point of view). If the transmitter and the receiver distinct (different antennas) but are physically located almost at the same place, the received signal is qualitatively close to a signal monostatic. One speaks thus about strongly bistatic configurations or slightly bistatic to integrate these two possibilities.

Generation of the wave

The transmitter with the site of the radar includes/understands: a permanent oscillator, an amplifier and a modulator. For the radars with ultra high frequencies, which form the vast majority of the radars in service, the generation of short and very energy impulses requires a technology which is different from that, say, of a broadcasting transmitter used in telecommunications. Thus, the generation of the wave is done in the following way:

• the permanent oscillating based on the technology of the tubes with cavity resonator, it can be a Klystron which have a very stable frequency, a Magnétron whose frequency varies in time, or other types of oscillators in solid state.

• the generating of impulse , or modulating, are electronic parts which produce the impulse radar starting from the wave continues produced by the oscillator. To some extent, they let pass the wave towards the amplifier during a very short amount of time (about the µ second). This makes it possible to concentrate the energy of the wave in this impulse (power about the Mr. Watt). There exist various kinds of switches of which most known is the Thyratron. The klystron can itself fulfill the roles of oscillator, of generator of impulse and amplifier.
• Once the wave is produced, the Guide of wave is charged to bring it towards the antenna with a loss of the weakest possible signal.

Beaches of frequencies used out of radar

General information

The Fréquence is mainly selected according to the application concerned. In a general way, a big wavelength (bands HF) will make it possible to benefit from the phenomena of Propagation and rebound on the Ionosphère, which makes it possible to carry to thousands of kilometers (case of the forward-scatter radar). In addition, only the objects whose typical size is at least of about size wavelength are visible. For example, a forest will be partially transparent for the big wavelengths (only the tree trunks are visible); while the forest will be opaque bandages X of them (only the Canopée will be visible), because the wavelength is about the size of the sheets and the branches. The size of the antenna also influences over the wavelength to use (and reciprocally).

The civil and military wavebands are allocated in an international way within the called World conference of the Radiocommunications every three years within the International union of Telecommunications (next conference in 2007), with also the participation of international agencies like NATO. The requests for band must be deposited a long time in advance insofar as the day orders of the conferences are generally fixed several years in advance. In addition, within a country, the kingly institutions can assume wavebands for the exclusive use of the military forces or police force. However, these institutions undergo increasingly important pressures on behalf of the industrialists insofar as civil new technologies (GSM, WiFi, etc) have a growing spectral occupation, but offering a very broad financial profit. The hour is thus with the co-operation between the various actors and a cohabitation (always very not succeeded) so as to limit jammings between the various applications. Always it is that the most adapted waveband from a applicatif point of view and that often should be found a compromise is not always available.

Names of the wavebands used out of radar

The name of the beaches of frequencies used in the world of the radars comes from the Second world war. Indeed, to keep secret the development of this system, the soldiers decided to give to these beaches code names which are remained of use since. They were adopted with the the United States by the Institute off electrical and electronics engineers (IEEE) and internationally by the International union of telecommunications . However, certain users of the radio bands, like the télédiffuseurs and the industry of the military countermeasures, replaced the traditional terms by their own identification.

Antennas

See also: Amorce=Pour more information, to see the article, radioelectric Antenna

In any general information, a antenna (radio or radar) can be seen like a Transducteur:

• on the one hand, used in emission, the antenna is used to convert an electrical energy which reigns on the surface of the wire or the plan of the antenna, into an electromagnetic wave which will be propagated in space.
• in addition, used in reception, the antenna converts an electromagnetic energy coming from a given direction of space, in an electrical energy which exists on the surface of the antenna and which, once collected and amplified, will form the received signal.

This energy transformation is not done without losses; thus, an antenna is characterized by a coefficient yield between 0 and 1, that one wishes to be highest possible.

If one wishes to use the radar to locate a target, it should be designed the antenna so that it does not receive the waves coming only from one privileged direction; this operation also has a beneficial side effect insofar as the antenna has the best carried as well in reception as in emission in this direction. The antenna thus is also characterized by its directivity and its " gain" maximum.

One will see further in this paragraph than the directivity of the antenna is influenced by the wavelength of the emitted signal and dimensions of the antenna; in certain applications (radar embarked on plane or satellite), dimensions of the antenna can be a strong constraint which must thus also be considered.

Telegraphic antennas

For technical reasons (the magnetron not being completely controlled yet), the first radars of the Second world war worked at low frequencies for which it was convenient to use telegraphic antennas. These antennas are well-known general public because their form is not at the base not different from that of the antennas of our radio stations or our televisions. According to the fitting of the bits composing the antenna, it is possible to obtain a more or less directing antenna. An antenna monobrin will be omnidirectional in the median plane of the antenna; on the contrary, an antenna Yagi is very directing in its main axis. The latter is celebrates it " antenna râteau" who is typically used in television.

Several use potential was explored during time. Thus, the system " Chain Home" British during the Second world war, was formed dipolar antennas which emitted in an omnidirectional way, and directional reception antennas. These last were made of two dipolar antennas placed at right angle. Indeed, for a dipolar antenna, the reception is maximum with right angle of the source of echoes, and minimal when the antenna points its direction. The operator radar can thus determine the direction of the signal while turning the antennas to determine this doublet max/min postings of his two antennas. First airborne radars, as the German radar Lichtenstein of the second world war were often made of networks of Yagi aerials assembled on the nose of the plane. These antennas added an additional trail to the plane, which is generally not desirable; however it was not possible to use less cumbersome antennas, because this one were not adapted to the low frequency which was then used.

The telegraphic antennas remain used nowadays for the radars with " basse" frequency (in lower part of a few hundred megahertzes, but there is no exact limit).

Antenna with opening

See also: Amorce=Pour more information, to see the article, parabolic Aerial

For the radars with ultra high frequencies, a traditional type of antenna is the antenna with opening. This antenna functions in the following way:

• the electromagnetic wave generated by the magnetron is led towards the antenna while following the way (1) through a Guide of wave (2);
• the guide of wave ends in a horn (3) which clarifies the surface of a plate or of a large-sized netting which acts as reflectors (4);
• the electric field $\backslash \; overrightarrow\; E\_0$ which is formed on the surface of this surface will give rise to in its turn an electromagnetic wave which will be propagated in space. To note that the field on the surface of the reflectors does not have any raison d'être constant in amplitude and direction.

If the “reflectors” is of parabolic form , and if the horn is located at the hearth of the parabola, then the rays reflected by surface will set out again roughly speaking in a parallel way towards the infinite one in the direction X , just like the bulb of a headlight of car is located at the metallized hearth of reflectors parabolic which reflects the luminous rays far on the road.

However, unlike the headlight of car, the size of surface forming the reflectors is relatively small in front of the wavelength of the emitted signal and it is then not possible to neglect the phenomena of diffraction. Each point of the surface of the reflectors will radiate like a point source, and the total field emitted in a point is the coherent sum of all the infinitesimal fields. All occurs as in the case from diffraction from a wave by an opening. In order to better feel the physique of the phenomenon, let us consider the following idealized case:

• the opening is plane and rectangular, of dimensions L (on the axis there) and L (on axis Z);
• the metal surface of the opening is perfectly conducting;
• the electric field generated by the horn on the surface of the antenna of constant direction and constant amplitude $E\_0$;

Either to measure the amplitude of the wave emitted in a direction located by the angles $\backslash \; phi$ (horizontal azimuth angle or layer) and $\backslash \; theta$ (Angle of elevation or site), and measured at a distance $r$ of the sufficiently large antenna so that the approximation of Fraunhofer or vérifée. The theory of diffraction shows that this one is worth:

$E\; (R,\; \backslash \; theta,\; \backslash \; phi)\; =E\_0\; \backslash \; cdot\; \backslash \; frac\; \{L\}\; \{\backslash \; lambda\; R\}\; \backslash \; cdot\; \backslash \; operatorname\; \{sinc\}\; \backslash \; left\; (\backslash \; pi\; \backslash \; frac\; \{L\}\; \{\backslash \; lambda\}\; \backslash \; cdot\; \backslash \; sin\; (\backslash \; phi)\; \backslash \; cdot\; \backslash \; cos\; (\backslash \; theta)\; \backslash \; right)\; \backslash \; operatorname\; \{sinc\}\; \backslash \; left\; (\backslash \; pi\; \backslash \; frac\; \{L\}\; \{\backslash \; lambda\}\; \backslash \; cdot\; \backslash \; sin\; (\backslash \; theta)\; \backslash \; right)$

In this expression, $\backslash \; rm\; sinc$ is the function cardinal Sinus defined by $\backslash \; sin\; (X)\; /x$. The maximum amplitude is obtained on axis X.

The diagram of right-hand side gives the pace of the evolution of the power of the wave, standardized compared to the emitted maximum power, according to the site and of the layer (logarithmic scale). One sees appearing a central peak which represents the principal lobe radar, as well as secondary peaks representing of the secondary lobes . Here, the antenna has as dimensions 20 cm by 10 cm, which has the advantage of making the lobes quite visible; in reality, it can be desirable to have larger antennas to have a finer principal lobe (about the degree). The major part of the energy emitted or received by an antenna comes from the principal lobe; in particular, if a considered signal is received by the antenna, there will be a strong probability so that the target is in the direction given by the principal lobe. One however wishes to reduce the secondary lobes as much as possible, because they are not negligible. The reduction of the secondary lobes can be realized, for example, while being arranged so that the illumination of the reflectors is not constant any more, but important in the center and gently decreasing at the edges.

If $\backslash \; theta=0$, the whole of the angles for which the power is at least equal to half of the maximum power corresponds to the angles giving an argument higher than $\backslash \; tfrac1\; \backslash \; sqrt2$ in the first cardinal sine; numerically, the angular opening $R\_\; \backslash \; phi$ of this field is worth, for small openings:

$R\_\; \backslash \; phi\; \backslash \; approx\; 0,886\; \backslash \; frac\; \{\backslash \; lambda\}\; \{L\}$

It comes a similar relation if $\backslash ,\; \backslash \; phi=0$, by replacing L by L . It is seen that to reduce the angular opening of the antenna, there are two methods:

• either to increase the size of the antenna
• or to decrease the wavelength/to go up in frequency

It should be noted that the popularity of the antennas with opening decrease today in favor of the antennas patch and the antennas with slits (especially in the civil field), except in some applications where the power with the emission is important; however, the theory is not very different and the results stated above qualitatively remain valid.

Waveguide with slits

See also: Amorce=Pour more information, to see the article, Antenna with slits

In general, the signal coming from the transmitter moves in a Guide of wave in the transmitting antenna. It is however possible to transform the guide of wave itself into antenna in there piercing of the slits. The interference between the various slits indeed creates an owner of diffusion with an intense central peak and weaker secondary peaks in the direction according to which the slits are directed. One obtains thus a beam directional radar similar to that of a parabolic aerial.

This type of antenna has a good resolution according to its axis, but any in the perpendicular axis. It is then enough to make mechanically turn the guide of wave thus perforated on 360 degrees to obtain a sweeping of the horizon. This type of antenna is particularly used whenever one is interested only in what is in the plan swept without requiring a very high degree of accuracy. It is this type of antennas which one sees on the ships, along the tracks of the airports and in the ports and which resemble long Haut-parleur S placed horizontally and in rotation on a Mât. They very economic and are affected by the wind than of other types of antenna.

Antennas patch

See also: Amorce=Pour more information, to see the article, Antenna patch

The antennas patch consist of a printed circuit doubles face metallized. They have the advantage of being far from expensive, light and very flexible devices with the use. For that, they often find a use for the applications of imagery to synthetic antenna where they can be gone up in a way in conformity on the hull of a plane, a Drone, or embarked on a satellite. French radar RAMSES (Multispectral Airborne Radar of Study of the Signatures) uses for example such a technology. The results shown for the antennas with opening remain qualitatively valid for the antennas patch, i.e. the angular opening decreases when the dimension of the antenna increases and the wavelength decreases.

Antennas network with ordering of phase

See also: Amorce=Pour more information, to see the articles, Antenna network with ordering of phase, three-dimensional Radar with electronic sweeping

Another method used to diffuse the beam radar is that of the antennas network to ordering of phase. In this system, one divides the guide of wave coming from the transmitter into a very great number of under-guides of wave. The latter finish each one by a slit on a plate facing a direction. By controlling the phase of the wave passing in each one of these slits, one can create an owner of interferences who gives an emission in a particular direction. One can change the direction towards which the antenna emits without having to move this one: there is only to change the arrangement of the phases of the slits.

Like the change of arrangement is done electronically, one can carry out a sweeping of the horizon and vertical in a time much faster than would do it a parabolic aerial in mechanical rotation. One can even arrange the owner of emission so that two beams are had, which to create two virtual radars. However, the beam is not very precise in the direction shaving the plate and this is why one generally arranges three or four plates of this type in different directions to cover all volume around the radar. This gives a three-dimensional Radar to electronic sweeping.

The antennas network with ordering of phase were used in first during the Second world war but the limitations of the electronics of time did not make it possible to have results of good resolution. During the Cold war, a main effort was provided for their development, because the very fast targets like the fighter plans and the missiles move too quickly to be followed by the conventional systems. They are the heart of the Système of combat Aegis of the warships and the anti-missile system Patriot. They are used more and more, in spite of their high costs, in other fields where the speed of survey and the obstruction are critical, as aboard fighter plan. In the latter, they are very appreciated for their capacity to follow several targets. They were introduced there in first into the Mikoyan Mig-31. Its antenna with ordering of phase, the Zaslon SBI-16 , is regarded as most powerful of the antennas for fighter plans.

With the fall in the price of the electronic parts, this kind of antennas is spread more and more. Almost all the military systems of radar use this concept, because the additional costs are easily compensated by its versatility and its reliability (less moving parts). The antenna network with ordering of phase for radar also finds in the satellite S and one carries out even tests with the National Weather Service American for his use in the weather Radars. The parabolic aerial is still used in the general aviation and the other civil uses but that could change if the costs continue to decline.

One generally distinguishes the active electronic antennas with sweeping from the passive antennas to electronic sweeping . In the case of the passive antennas with electronic sweeping, only one source generates the wave, which is then out of phase in an adequate way for each radiative element of the antenna. In the active antennas with electronic sweeping, the antenna is actually a whole of several (1000 to 1500, typically) under-antennas independent from/to each other and laying out each one of their own source. The advantage of this last approach is to be able to ensure the operation of the system after reconfiguration even if one of the under-antennas is defective. The radar RBE-2 which equips the French hunter Rafale is an example of radar with sweeping electonic with passive antenna. Radar AN/APG 77 equipping the American hunter F-22 is equipped with active antennas.

Synthetic antenna

See also: Amorce=Pour more information, to see the article, Radar with synthesis of opening

As its name indicates it, it is not a question strictly speaking of a physical antenna, but of a treatment applied to the rough signal received by the radar, at the end of the chain. By using an antenna on a carrier (plane or satellite) moving, one carries out the coherent summation of the signal received correspondent at the same point of space, over several successive moments, while arranging oneself so that the object remains in the principal lobe of the antenna over this duration. This summation increases the resolution of the image artificially, without to have to increase the physical size of the antenna. This solution has an unquestionable interest for radars embarked on satellite or plane, because it makes it possible to have good performances for a tiny weight and an obstruction.

Cooling agent of radar

The coolanol and the CAM (poly alpha olefin) are the two principal cooling agents used in the radar airborne. U.S. Navy having instituted an anti-pollution program to reduce toxic waste, Coolanol has been less of use for a few years. The CAM is lubricating synthetic composed of Ester S of Polyol, of antioxydant, inhibiters of rust and triazole a " yellow metal pacifier".

Principles of operation

Reflection

The electromagnetic waves are reflected by any significant change of the constants Diélectrique S or Diamagnétique S of the crossed medium. That means that solid object in the Air or the Vide, or any other significant change of the atomic density between the object and what surrounds it, disperses the Onde S radar. It is particularly true for the materials conducting S of electricity, the such metals and the Carbon fiber, which returns the radars very adapted to the detection of planes and boats.

The portion of the wave which is turned over to the radar by a target is called its Réflectivité. The propensity of the target to reflect or disperse these waves is called its cross Section. In fact, the waves radar disperse in ways different according to the Wavelength used, the form of the target and its composition:

• If the wavelength is much smaller than the size of the target, the wave will rebound above like the light on a Miroir. The cross section will depend in this case on the form on the target and its propriétées reflective.
• If the wavelength is much larger than the size of the target, the atoms of the latter will be polarized. I.e. the Charge S negative and positive in materials will be separate as in a dipolar Antenne. This is described by the model of the Diffusion Rayleigh which predicts the blue of the sky and the red one to lay down Sun. In this situation, the cross section proportional to the diameter of the target and its will be propriétées reflective.
• When the two lengths are comparable, it can occur resonances between the atoms of the target and the reflection behaves according to the Théorie of Mie, making the owner of réémission very variable.

The first radars used wavelengths much more important than the size of the targets and received a vague signal, while certain modern radars use shorter wavelengths (a few centimetres, even less) which can see smaller objects, like the rain or the insects.

The short radio waves are reflected by the Courbe S and of the acute Angle S like the light on a piece of Verre round. The most reflective targets for short wavelengths present Angle S of 90° between their reflective surfaces. A structure made up of three plane surfaces meeting in only one corner (for example the corner of a box) will always reflect the waves entering directly towards their source. These types of reflection are usually used as reflectors radar in order to detect more easily of the not easily detectable objects differently, and are often present on boats in order to improve their detection in the event of rescue and to reduce the collision risks.

For the same reasons, the objects wanting to avoid being detected will direct their surfaces in order to eliminate the interior corners and to avoid surfaces and edges perpendicular S to the current directions of detection. That led to furtive planes with the particular forms. These precautions do not completely eliminate the reflections because of the phenomenon of Diffraction, particularly for the big wavelengths. Cables being for length half the wavelength or of the conducting material bands (as the “spangles” of countermeasures radar) are very reflective but do not return the wave towards its source.

Another way of camouflaging itself is to use materials absorbing the waves of the radars, i.e. containing resistant substances ou/et Magnétique S. One use them on the military vehicles in order to reduce the reflection of the wave. It is to some extent equivalent to paint something of dark color in the Visible spectrum.

Calculation of the reflectivity

See also: Amorce=Pour more information, to see the article, equation of the radar

According to the equation radar, the power $\backslash ,\; P\_r$ turned over to the radar since the target is:

$P\_r\; \backslash \; propto$ Where $\backslash ,\; P\_t$ is the transmitted power, $\backslash ,\; R$ is the distance and $\backslash ,\; \backslash \; sigma^0$ is the cross section of the target.

The reflectivity being defined like $\backslash ,\; P\_r/P\_t$, one sees:

• That targets being at distances different but having the same characteristics from reflection will give extremely different echoes, and, in all the cases, much weaker than the emitted signal. This equation takes account only of the reduction in the power of the signal due to the distance and does not take account of the attenuation caused by the absorption of the crossed medium.

• That the reflectivity depends on the cross section which is found according to what one showed before. Other mathematical developments influence the cross section. Those include analyzes based at the same time on time and the frequency like the theory of the ondelettes and the Transformée of Chirplet. They use the fact that the targets moving of the radars are typically “singing” (i.e. they change their frequency according to time, like the song of a bird or bats).

Polarization

In the signal emitted by the radar, the Electric field is perpendicular to the direction of propagation, and the direction of this electric field is the polarization of the wave. The radars use a vertical, horizontal polarization and circular to detect various types of reflections.

• For example, circular polarization is used to minimize the interferences caused by the rain.
• a linear polarization generally indicates metal surfaces, and helps a radar of research to be unaware of the rain.
• a random polarization generally indicates a surface Fractale, for example of the rock or ground, and is used by the radars of navigation.

Interferences

There exist many sources of signals malvenus, that the radars must be able to be unaware of more or less, in order to focus itself only on the interesting targets. These signals malvenus can have origins internal and external, passive and active. The capacity of a radar to overcome these harmful effects defines its Rapport signal on noise (SNR): the larger the SNR is, the more the radar can effectively separate a target from the interfering signals surrounding.

Noise

The noise is an internal source of random variations of the signal, that all the components electronic S generate in a way inherent in various degrees. The noise typically seems made up of random variations superimposed on the signal of echo received by the radar, which is that one seeks. The lower the power of the desired signal is, the more it is difficult to distinguish it noise (to try to hear a murmur close to a Route encumbered is similar). Thus, the most importunate sources of noise appear on the level of the receiver and much of efforts are made to minimize these factors. The Facteur of noise is a measurement of the noise produced by a receiver compared with that produced by an ideal receiver, and this ratio must be minimal.

The noise is also generated by external sources, mainly by natural thermal radiations of the Environnement surrounding the target of the radar. In the case of the modern radars, thanks to high efficiencies of their receivers, the internal noise is lower or equal to the noise of the external environment, except if the radar is pointed towards a clear sky, in which case the environment is so cold that it generates very few thermal Bruit.

Unwanted echos

The unwanted echos are returns coming from targets which are by definition uninteresting for the operator radar. The causes of these echoes are:

• Of the natural objects such as the ground, the Sea, the Precipitation S (such as the Rain, the Snow or the Hail), the sandstorms, the animals (particularly birds), turbulences atmospheric, and other atmospheric effects (for example falls of Meteor S or reflections on the Ionosphere).

• Of the objects manufactured by the man such as the buildings or of the metal spangles released intentionally like countermeasures in the electronic war.
• supports of the Guide of wave on the basis of the antenna towards the horn of emission located at the focal point of the parabola. In a posting radar like pi, these undesirable echoes will resemble points very brilliant in the center of posting.
• Of the reflections coming from ways by multiple reflections on a target. Thus, the beam radar strikes a target and as the emitted wave is considered in all the directions, a part can be considered on another target and turn over to the radar. As the time put for this second reflection to reach the radar is longer than the direct return, it will be placed at the bad place. One can thus obtain two targets instead of one.
• Of the echoes of abnormal propagation in the atmosphere. Indeed, the way which the beam radar must traverse is calculated starting from a normal structure of the atmosphere. If the temperature varies from standard differently, the beam will be deviated abnormally. If the temperature increases with altitude (inversion of temperature), the beam is deviated towards the ground and one has a very strong return of this last.
• Of the echoes coming from the ionospheric reflections/refractions. This type of parasites is particularly difficult to identify, since it is moving and behaves same manner as the wanted targets, thus creating a Fantôme.
• Of the visible very reflective objects through a secondary lobe of the antenna, whereas the antenna points towards a less reflective zone. One will then see a phantom in the direction where point the principal lobe.

It should be noted that what is an undesirable echo for some can however be the sought-after goal by others. Thus the operators with aviation want to eliminate all from which one comes to speak but the meteorologists consider that the planes are noise and want to keep only the signals coming from precipitations.

The unwanted echos are regarded as a source of interferences passivates, since they are detected only in answer to the signals emitted by the radar. There exist several ways of eliminating these echoes. Several of these methods rest on the fact that these echoes tend to be stationary during sweepings of the radar. Thus, by comparing successive surveys radar, the desired target will be mobile and all the stationary echoes could be eliminated. The sea returns can be reduced by using a horizontal polarization, while the rain is reduced with a circular polarization (note that the weather radars wish to obtain the opposite effect, thus using a horizontal polarization in order to detect precipitations). The other methods aim at increasing the signal ratio on noise.

Method CFAR ( Constant False-Alarm Rate , sometimes called AGC for Automatic Gain Control ) rests on the fact that the echoes due to the parasites are much more than those due to the target. The profit of the receiver is automatically adjusted in order to maintain a level constant of the unwanted echos visible. The targets having a return more important than the parasites will easily come out from the latter, even if the weaker targets are lost in the noise. In the past, the CFAR was controlled electronically and also affected all probed volume. Now, the CFAR is controlled by computer and can be regulated differently in each zone of posting. Thus it adapts to the level of the unwanted echos according to the distance and the azimuth.

One can also use masks of known areas of permanent unwanted echos (e.g. mountains) or incorporate a chart of the surroundings of the radar to eliminate all the echoes having an origin located under the level from the ground or with the top a certain height. To reduce the returns of the supports of the horn of emission without decreasing the range, it is necessary to adjust the dumb period between the moment when the transmitter sends an impulse and the moment when the receiver is activated, in order not to take account of internal returns to the antenna.

Jamming

The Brouillage radar refers at the frequencies radios originating in sources external with the radar, emitting at the frequency of the radar and thus masking the interesting targets. The jamming can be intentional (a device anti-radars in the case of a electronic Guerre) or not desired (for example in the case of allied forces using of the material which emits in the same frequency band). Jamming is regarded as a source of interferences activates, since it is caused by elements external with the radar and generally without bond with the signals of the radar.

Jamming poses problems with the radars since the signals of jamming need to traverse only one to go (of the jammer to the receiver of the radar) whereas the echoes of the radar traverse a return ticket (radar-target-radar) and are thus much less powerful once of return to the receiver. The jammers require thus much less to be powerful that the radars in order to effectively mask the sources along the Field of view since the jammer towards the radar (jamming of the principal lobe). The jammers have an additional effect on the radars located along other fields of view, because of the secondary lobes of the receiver of the radar (jamming of the side lobes).

The jamming of the principal lobe can generally be only reduced by reducing its solid angle, and can never be completely eliminated if the jammer is located directly vis-a-vis the radar and if it uses the same frequencies and polarization that the radar. The jamming of the secondary lobes can be surmounted by reducing the secondary lobes of reception in the design of the antenna of the radar and by using an antenna Unidirectionnel it in order to detect and be unaware of all the signals not intended for the principal lobe. Work is also undertaken currently on the antennas with active electronic sweeping in order to enable them to dynamically reposition their secondary lobes in the event of jamming. Lastly, one can quote other techniques antijamming: the frequency hopping and polarization for example. To refer to electronic against-against-measurements for more details.

The interferences recently became a problem for the weather radars of Bande C (5,66 GHz) because of the proliferation of the equipment WiFi to 5,4 GHz.

Treatment of the signals radar

Measure of distance

Time of return of the signal

A manner of measuring the distance to an object is to emit a short impulse of radio signal, and to measure the time which the wave takes to return after having been considered. The distance is half of the time of return of the wave (because the signal must go to the target then to return) multiplied by the speed of the signal (which is close speed of light in the vacuum if the crossed medium is the atmosphere).

When the antenna is at the same time transmitting and receiving (what is the case more running), the antenna cannot detect the considered wave (also called return ) while the signal is emitted - one cannot know if the measured signal is the original or the return. That implies that a radar has a minimal range, which is half of the duration of the impulse multiplied by speed of light. To detect closer targets, it is necessary to use one duration of shorter impulse.

A similar effect imposes same manner a maximum range. If the return arrives when the following impulse is emitted, once again the receiver cannot make the difference. The maximum range is thus calculated by:

$x\; =\; \backslash \; frac\; \{C\; \backslash \; Delta\; T\}\; \{2\}$ where C is speed of light and $\backslash \; Delta\; t$ is time between two impulses

The form of the impulse exploits the capacity of the radar to distinguish two close objects (concept of To be able of resolution). See the article devoted to with the Compression of impulse for more details.

This form of emission is used by the radars with impulsions.

Frequency modulation

Another way of measuring the distance to the radar is to use a frequency modulation of a radar with continuous emission. The wave is emitted by an antenna and is received by one second antenna since same electronics cannot emit and receive at the same time. In this case the signal emitted at time T has a frequency has but a fréquence' B' at later time You . The emitted signal with T which strikes a target and returns to the radar will thus have a frequency different from that emitted at this time by the radar. By making the difference between the two frequencies, one can deduce the distance covered, return ticket, between the radar and the target. One generally uses a sinusoidal variation of frequencies which it is easy to gauge and the comparison between the two frequencies is made by using the inter-frequential beats. This technique is used for a long time in the altimeters to measure the altitude of flight and can be used in the radars like the speed sensors of the road police force.

This form of emission is used by the radars with emission continue.

Velocity measurement

There exist various methods to measure the rate of travel of a target:

• oldest consists in noting its position at one moment X , using a soft lead pencil, on posting radar. At one moment Y , one remakes the same thing and the difference of the two positions divided by past time entre' X' and Y gives the rate of travel.

• One can also note the variation of frequency of the wave emitted compared to that received when one emits continuously at a fixed frequency. It is the use of the Effect Doppler. Like one does not make vary the emitted frequency, one cannot however define the position of the target in this manner. Moreover, there is only the radial component with the radar speed. For example, a target moving perpendicular to the beam radar will not cause change of frequency whereas the same target moving towards the radar at the same speed causes a maximum change.
• most current of the methods is to use an alternative of the Doppler effect with a radar with impulses. In this case, one notes the difference in phase between the successive impulses ghost of the target. This method makes it possible to determine the radial speed AND the position of the target.

Doppler speed with radar with impulses

Instead of measuring the difference in frequency between the emitted wave and that received, which can be too tiny for electronics, one uses the difference of phase between two successive impulses returning from the same probed volume (even of pulsated waves). Between each impulse, the targets move slightly and are struck by the wave with a part slightly different from its cycle. It is this difference in phase which the radar notes with the return.

The intensity of an impulse after a return ticket is given by: $I\; =\; I\_0\; sin\; \backslash \; left\; (\backslash \; frac\; \{4\; \backslash \; pi\; x\_0\}\; \{\backslash \; lambda\}\; \backslash \; right)\; =\; sin\; \backslash \; left\; (\backslash \; phi\_0\; \backslash \; right)$

$O\; \backslash \; serious\; \{U\}:\; \backslash \; quad\; \backslash \; begin\; \{boxes\}\; x\_0\; =\; distance\; \backslash \; radar-target\; \backslash \; \backslash \; \backslash \; lambda\; =\; length\; \backslash \; of\; wave\; \backslash \; \backslash \; \backslash \; Delta\; T\; =\; time\; \backslash \; between\; \backslash \; two\; \backslash \; impulses\; \backslash \; end\; \{boxes\}.$

The intensity of a subsequent impulse ghost of the same probed volume but where the targets slightly moved is given by:

$I\; =\; I\_0\; sin\; \backslash \; left\; (\backslash \; frac\; \{4\; \backslash \; pi\; (x\_0\; +\; v\; \backslash \; Delta\; T)\}\{\backslash \; lambda\}\; \backslash \; right)\; =\; I\_0\; sin\; \backslash \; left\; (\backslash \; phi\_0\; +\; \backslash \; Delta\; \backslash \; phi\; \backslash \; right)$

Thus $\backslash \; Delta\; \backslash \; phi\; =\; \backslash \; left\; (\backslash \; frac\; \{4\; \backslash \; pi\; v\; \backslash \; Delta\; T\}\; \{\backslash \; lambda\}\; \backslash \; right)$

$v\; =\; speed\; \backslash \; of\; the\; \backslash \; target\; \backslash \; =\; \backslash \; frac\; \{\backslash \; lambda\; \backslash \; Delta\; \backslash \; phi\}\; \{4\; \backslash \; pi\; \backslash \; Delta\; T\}$

As one obtains only the radial component of displacement, it thus should be followed to know the angle which its true direction of displacement with the ray with the radar forms. Thereafter, a simple trigonometrical calculation gives the true speed of the target.

Doppler dilemma

We interest maintaining in maximum speed that one can measure without ambiguity. As one can determine starting from a sine only one angle ranging between - $\backslash \; pi$ and +$\backslash \; pi$, one cannot measure an high speed with:

$Vitesse\_\; \{max\}\; =\; \backslash \; pm\; \backslash \; frac\; \{\backslash \; lambda\}\; \{4\; \backslash \; Delta\; T\}$

It is what is called the speed of Nyquist. To obtain a better determination the speed of the targets, it is necessary to send very brought closer impulses, therefore with $\backslash ,\; \backslash \; very\; small\; Delta\; t$. But it is also known that the range in reflectivity is directly proportional to $\backslash \; Delta\; t$, which requires large a $\backslash \; Delta\; t$ to be sure position of the echoes returning by far without ambiguity. This dilemma Doppler limit thus the range useful of the Doppler radars for impulses.

Reduction of the interferences

The Treatment of the signal is necessary to eliminate the interferences (due to sources radio others that of the radar) as well as the unwanted echos. The following techniques are used:

• Elimination while following only the echoes which move.
• Filtering of the echoes by using their Doppler speed: the unwanted echos and the interferences generally have null speeds.
• Correlation with secondary radars of monitoring: it is about a device which sends since the target a signal when he receives a beam radar. This signal identifies the target and, according to the case, its altitude and its speed.
• adaptif Process time-space: by using an antenna with ordering of pulsated phase and Doppler speeds that one obtains some, one can analyze the average owner of the frequencies and emphasize some the peak which indicates the target.
• Rate of false alarm constant: it is a question of determining the continual average noise level in each point of posting radar and of keeping only the echoes having a return higher than this one.
• digital Mask of the ground which makes it possible to eliminate the echoes which would come from under the level from the ground.

Applications

The first operational uses of the radar took place during the Second world war in order to detect from the coast the approach of air formations, and ships, as well by the the United Kingdom as by the German forces.

The radars have today a very large variety of applications in many fields:

• military: radars of detection and air monitoring on the ground or embarked (on hunters for the aerial combat and System of Detection and Airborne Command on planes of guet ( Airborne Warning And Control System ( AWACS ) in English); radars of day before on the surface on the warships; radars of landing; identification radar (IF); homing heads of missiles; radars of terrestrial detection; artillery radars; jammers radar; satellites radar of observation of the ground;
• aeronautical: control air traffic; guidance of approach of airport; radars of altimetry; radars of navigation;
• maritime: radar of navigation; anti-collision radars; beacons radars; Transponder radar
• meteorology: detection of precipitations (rain, Snow, Grésil, Hail, etc) and of cloudy formations. The most recent radars use the Doppler effect and are thus able to evaluate the speed of these particles. Certain radars use the polarizations vertical and horizontal to give an idea of the mixture of forms of the probed particles what, associated with their intensity, can indicate the type of precipitation.
• circulation and road safety: control speed of the cars (see Cinémomètre), the traditional model on the roads of France is the Miradop ( semi nor ruffle dar DOP pler) used by the brigades of gendarmerie. They are placed on the Autoroute S, in the zones where the Véhicule S can roll at an high speed to the authorized Maximum speed. Radars of retreat on cars, Radar of assistance to emergency braking (ACC Adaptive Cruise Control);
• scientific: embarked on satellite for the observation of the Earth, the level of the oceans…

To deepen

• General information
• Radar with synthesis of opening
• three-dimensional Radar with electronic sweeping
• Radar with side antenna
• Moving Target Indicator
• Radar of continuation
• Equation of the radar
• airborne Radars
• weather Radar
• Profileur of winds
• space Radars
• Altimeter radar, to see Altimeter
• Diffusiometer
• naval Radars
• Radars of air control
• primary education Radar
• secondary Radar
• Radar of road control
• Automatic radar
• Automatic radar in France
• Study of the basement
• GPR

See too

Related articles

• Laser Effect Doppler
• Teledetection
• Lidar
• Maser
• Radome

External bonds

• short History of the radar
• history of the radar, facts: Patent French 1934
• ''' The inventor Christian Hülmeyer ''' by Site 100 years of Radar
• simple Explanations of the radar
• the Radar - Sir Robert WATSON-WATT - Atoms: N°1 - March 1946 and N°3 - May 1946

Random links:

Lawsuit of Moscow | Paternalism | Precede (typography) | Nozomi Tsuji | Marc Leval | Etymology of Radar