Propagation of radio waves. Radar. A television. Development of communications. School encyclopedia

Since when transferring electromagnetic waves The receiver and transmitter are often located near the Earth's surface, then the shape and physical properties The earth's surface will significantly influence the propagation of radio waves. In addition, the propagation of radio waves will also be affected by the state of the atmosphere.

The ionosphere is located in the upper layers of the atmosphere. The ionosphere reflects waves with a wavelength λ>10 m. Let us consider each type of wave separately.

Ultrashort waves

Ultrashort waves - (λ< 10 м). Этот диапазон волн не отражается ионосферой, а проникает сквозь нее. Они не способны огибать земную поверхность, поэтому чаще всего используются для передачи сигнала на расстояния в пределах прямой видимости.

In addition, since they penetrate the ionosphere, they can be used to transmit signals to open space, to contact spaceships. Recently, attempts to detect other civilizations and transmit various signals to them have become more frequent. Various messages are sent mathematical formulas, information about the person, etc.

Short waves

The range of short waves is from 10 m to 100 m. These waves will be reflected from the ionosphere. They spread over long distances only due to the fact that they will be reflected many times from the ionosphere to the Earth, and from the Earth to the ionosphere. These waves cannot pass through the ionosphere.

We can emit a signal in South America, but take it, for example, in the center of Asia. This wave range appears to be sandwiched between the Earth and the ionosphere.

Medium and long waves

Medium and long waves - (λ significantly greater than 100 m). This wave range is reflected by the ionosphere. In addition, these waves bend well around the earth's surface. This occurs due to the phenomenon of diffraction. Moreover, the longer the wavelength, the more pronounced this bending will be. These waves are used to transmit signals over long distances.

Radar

Radar is the detection and determination of the exact location of an object using radio waves. A radar installation is called a radar or radar. Radar consists of receiving and transmitting parts. Highly directional waves are transmitted from the antenna.

The reflected waves are received either by the same antenna or by another. Since the wave is highly directional, we can talk about a radar beam. The direction to the object is defined as the direction of the beam at the moment when the reflected beam entered the receiving antenna.

Pulsed radiation is used to determine the distance to an object. The transmitting antenna emits waves in very short pulses, and the rest of the time it works to receive reflected waves.

Distance is determined by measuring the time it takes a wave to travel to an object and back. And since the speed of propagation of electromagnetic waves is equal to the speed of light, the following formula will be valid.

Electromagnetic waves of various ranges

Radio propagation

Electromagnetic waves used for radio communication are called radio waves. Radio waves are divided into groups.

When using electromagnetic waves for radio communications, both the source and receiver of radio waves are most often located near the earth's surface. Its shape and physical properties, as well as the state of the atmosphere, greatly influence the propagation of radio waves.

Layers of ionized gas in the upper parts of the atmosphere at an altitude of 100-300 km above the Earth's surface have a particularly significant influence on the propagation of radio waves. These layers are called ionosphere. Air ionization upper layers The atmosphere is caused by electromagnetic radiation from the Sun and the flow of charged particles emitted by the Sun.

The propagation of radio waves depends on the properties of the atmosphere. The lower, densest part of the atmosphere is called the troposphere and extends to an altitude of 10-12 km. Above is the stratosphere, the upper boundary of which lies at an altitude of 60-80 km. Next is the ionosphere, which is characterized by low gas density. Under the influence solar radiation gas molecules are ionized, that is, they break up into ions and free electrons. Ionized gas is electrically conductive and can reflect radio waves.

The ionosphere is heterogeneous; some of its layers are most strongly ionized. There are layers of the ionosphere D, E and F. The degree of ionization of the atmosphere depends on the intensity of solar radiation and changes at different times of the day and year.

Conductive electricity the ionosphere reflects radio waves with a wavelength λ > 10 m as usual metal plate. But the ability of the ionosphere to reflect and absorb radio waves varies significantly depending on the time of day and seasons (which is why radio communications, especially in the medium wavelength range (100-1000 m), are much more reliable at night and in winter).

Stable radio communication between remote points on the earth's surface beyond the line of sight is possible due to the reflection of waves from the ionosphere and the ability of radio waves to bend around the convex earth's surface (i.e. diffraction). Diffraction is more pronounced the longer the wavelength. Therefore, radio communication over long distances due to the waves bending around the Earth is possible only at wavelengths significantly exceeding 100 m ( average And long waves).

Radar- a field of science and technology that combines methods and means of detection, measuring coordinates, as well as determining the properties and characteristics of various objects based on the use of radio waves. A related and somewhat overlapping term is radio navigation, however, in radio navigation, a more active role is played by the object whose coordinates are measured, most often by determining its own coordinates. The main technical device of radar is a radar station.

There are active, semi-active, active with a passive response and passive RL. They are divided according to the radio wave range used, the type of probing signal, the number of channels used, the number and type of coordinates being measured, and the location of the radar installation.

Classification

There are two types of radar:

• Passive radar is based on receiving the object's own radiation
• With active radar, the radar emits its own probe pulse and receives it reflected from the target. Depending on the parameters of the received signal, the characteristics of the target are determined.

There are two types of active radar:

• With active response - the facility assumes the presence of a radio transmitter (transponder), which emits radio waves in response to a received signal. Active response is used to identify objects (friend or foe), remote control, as well as to receive additional information(for example, amount of fuel, type of object, etc.).
• With a passive response - the request signal is reflected from the object and is perceived at the receiving point as a response.

To view the surrounding space, the radar uses various ways review due to the movement of the directional beam of the radar antenna:

• circular
• sector
• helix view
• conical
• in a spiral
• "V" review
• linear (AWACS aircraft such as An-71 and A-50 (Russia-Ukraine) or American ones with the Avax system)

According to the type of radiation, radars are divided into

• Continuous wave radar
• Pulse radars

Operating principle

Radar is based on the following physical phenomena:

• Radio waves are scattered by electrical inhomogeneities encountered along the path of their propagation (objects with other electrical properties that differ from the properties of the propagation medium). In this case, the reflected wave, as well as the target radiation itself, makes it possible to detect the target.
• At large distances from the radiation source, we can assume that radio waves propagate rectilinearly and with constant speed, thanks to which it is possible to measure the range and angular coordinates of a target (Deviations from these rules, which are valid only to a first approximation, are studied by a special branch of radio engineering - Radio wave propagation. In radar, these deviations lead to measurement errors).
• The frequency of the received signal differs from the frequency of emitted oscillations when the receiving and emission points move mutually (Doppler effect), which makes it possible to measure the radial speeds of the target relative to the radar.
• Passive radar uses the emission of electromagnetic waves by observed objects, this can be thermal radiation, characteristic of all objects, active radiation created by the technical means of the object, or side radiation created by any objects with operating electrical devices.

Pulse radar method

With the pulsed radar method, transmitters generate oscillations in the form of short-term pulses, followed by relatively long pauses. Moreover, the pause duration is selected based on the radar range Dmax.

The essence of the method is as follows:

The transmitting device of the radar does not radiate energy continuously, but briefly, in strictly periodically repeating pulses, in the pauses between which the reflected pulses are received by the receiving device of the same radar. Thus, the pulsed operation of the radar makes it possible to separate in time a powerful probing pulse emitted by the transmitter and a much less powerful echo signal. Measuring the range to a target comes down to measuring the length of time between the moment the pulse is emitted and the moment it is received, that is, the time it takes the pulse to travel to the target and back.

Radar range

The maximum range of a radar depends on a number of parameters and characteristics of both the station’s antenna system and the generator and receiver of the system. In the general case, without taking into account power losses in the atmosphere, interference and noise, the range of the system can be determined as follows:

In the presence of noise and interference, the radar range decreases.

Impact of Interference

Effect of noise

Influence of the atmosphere

Atmospheric losses are especially large in the centimeter and millimeter ranges and are caused by rain, snow and fog, and in the millimeter range also by oxygen and water vapor. The presence of the atmosphere leads to a curvature of the propagation trajectory of radio waves (the phenomenon of refraction). The nature of refraction depends on the change in the refractive index of the atmosphere with changes in altitude. Because of this, the trajectory of radio waves is curved towards the earth's surface.

Continuous wave radar

They are mainly used to determine the radial speed of a moving object (uses the Doppler effect). The advantage of this type of radar is its low cost and ease of use, however, in such radars it is very difficult to measure the distance to an object.

Example: a simple radar for determining the speed of a car.

Main ideas and stages of development

As is known, the effect of reflection of radio waves was discovered by A.S. Popov in 1897. But technically, no one was able to use the amazing effect for “distant vision”: the waves scattered, and less than one billionth of them hit the location object. Practical work in the field of radar began in the 30s. Work was carried out almost simultaneously in the USSR, Germany, England and France. Naturally, the developments were kept secret. The main goal was to detect air attacks.

In the Soviet Union, awareness of the need for aircraft detection means free from the disadvantages of sound and optical surveillance led to the development of research in the field of radar. The idea proposed by the young artilleryman Pavel Oshchepkov received the approval of the high command: the People's Commissar of Defense of the USSR K. E. Voroshilov and his deputy, M. N. Tukhachevsky.

On January 3, 1934, an experiment was successfully carried out in the USSR to detect an aircraft using the radar method. An aircraft flying at an altitude of 150 meters was detected at a distance of 600 meters from the radar installation. The experiment was organized by representatives of the Leningrad Institute of Electrical Engineering and the Central Radio Laboratory. In 1934, Marshal Tukhachevsky wrote in a letter to the USSR government: “Experiments in detecting aircraft using an electromagnetic beam confirmed the correctness of the underlying principle.” The first experimental installation "Rapid" was tested in the same year; in 1936, the Soviet centimeter radar station "Storm" detected the aircraft from a distance of 10 kilometers. Work on radar was also started at the UPTI in Kharkov. The first radars in the USSR, adopted by the Red Army and mass-produced, were: RUS-1 - from 1939 and RUS-2 - from 1940.

In 1946, American experts Raymond and Hacherton, a former employee of the US Embassy in Moscow, wrote: “Soviet scientists successfully developed the theory of radar several years before radar was invented in England.”

The main factor limiting the technical characteristics of locators is low power received signal. In this case, the power of the received signal decreases as the fourth power of range, that is, to increase the range of the locator by 10 times, you need to increase the transmitter power by 10,000 times! Naturally, on this path we quickly came to limits, which were far from easy to overcome. Already at the very beginning of development, the fact was realized that it is not the power of the received signal itself that matters, but its visibility against the background noise of the receiver. Receiver noise reduction was also limited by natural noise in the receiver components, such as thermal noise. This impasse was overcome by increasing the complexity of methods for processing the received signal and the associated complication of the shape of the signals used. The development of radar as a scientific branch of knowledge went simultaneously with the development of cybernetics and now special research will be required to decide where exactly the first results were obtained. It should be noted the emergence of the concept of signal, which made it possible to abstract from specific physical processes in the receiver, such as voltage and current, and made it possible to solve the problems at hand as a mathematical problem of finding the best functional transformations of functions of time.

One of the first works in this area was the work of V. A. Kotelnikov on optimal reception signal, that is, the best signal processing method under noise conditions. As a result, it was proven that the quality of reception does not depend on the signal power, but on its energy, that is, the product of power and time, thus, there was a proven possibility of increasing the range by increasing the duration of signals, in the limit to continuous radiation. A significant step forward was the clear application in technology of methods of statistical decision theory (the Neyman-Pearson criterion) and the acceptance of the fact that a working device can work with a certain degree of probability. In order for the radar signal to be long enough to measure range and speed with high accuracy, complex signals were required, as opposed to simple radar pulses, changing any characteristics during the generation process. So. linear frequency modulation signals change the oscillation frequency during one pulse, phase shift keying signals change the phase of the signal in a stepwise manner, usually by 180 degrees. When creating complex signals, the concept of a signal uncertainty function was formulated, showing the relationship between the accuracy of range and speed measurements. The need to improve the accuracy of parameter measurement stimulated the development various methods filtering measurement results, for example, methods of optimal nonlinear filtering, which were a generalization of the Kalman filter to nonlinear problems. As a result of all these developments, theoretical radar has taken shape as an independent, highly mathematized branch of knowledge, in which formalized synthesis methods play a significant role, that is, design is carried out to a certain extent “at the tip of the pen.”

The main points in the confrontation with aviation were:

• The use of passive masking interference in the form of pieces of foil sprayed in the air that reflect radio waves to hide airplanes and helicopters. The response to this was the introduction of moving target selection systems in radars, which, based on the Doppler effect, distinguishes moving aircraft from relatively stationary foil.
• The development of technologies for constructing aircraft and ships that reduce the power of signals reflected back to the radar, called Stealth. For this purpose, special absorbing coatings and a special shape are used that reflects the incident radio wave not back, but in a different direction.

see also

Links

• Bistatic radar [ unreputable source?]

Notes

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See what “Radar” is in other dictionaries:

Radar... Spelling dictionary-reference book

Detection and location of misc. objects using radio technology. devices. The first radars stations (radars), also called radars or radars, appeared in Great Britain, the USSR and the USA at the end. 1930s Operating principle… … Physical encyclopedia

- (from radio... and lat. locatio location) a field of science and technology, the subject of which is the observation of various objects (targets) by radio engineering methods: their detection, recognition, determination of their location and speed, etc.; the process itself... ... Big Encyclopedic Dictionary

Radar is a collection scientific methods and technical means used to determine the coordinates and characteristics of an object via radio waves. The object under study is often called a radar target (or simply a target).

Radio equipment and tools designed to perform radar tasks are called radar systems, or devices (radar or RLU). The fundamentals of radar are based on the following physical phenomena and properties:

• In the propagation medium, radio waves encountering objects with different electrical properties are scattered by them. The wave reflected from the target (or its own radiation) allows radar systems to detect and identify the target.
• At large distances, the propagation of radio waves is assumed to be rectilinear, with a constant speed in a known medium. This assumption makes it possible to reach the target and its angular coordinates (with a certain error).
• Based on the Doppler effect, the radial velocity of the emission point relative to the RLU is calculated from the frequency of the received reflected signal.

Historical reference

The ability of radio waves to reflect was indicated great physicist G. Hertz and the Russian electrical engineer back in late XIXcentury. According to a patent from 1904, the first radar was created by the German engineer K. Hulmeier. The device, which he called a telemobiloscope, was used on ships plying the Rhine. In connection with the development, the use of radar looked very promising as an element. Research in this area was carried out by advanced specialists from many countries around the world.

In 1932, he described the basic principle of radar in his works Researcher LEFI (Leningrad Electrophysical Institute) Pavel Kondratyevich Oshchepkov. Them, in collaboration with colleagues B.K. Shembel and V.V. Tsimbalin was demonstrated in the summer of 1934 prototype radar installation that detected a target at an altitude of 150 m at a distance of 600 m. Further work to improve radar equipment was reduced to increasing their range and increasing the accuracy of determining the location of the target.

Nature electromagnetic radiation targets allows us to talk about several types of radar:

• Passive radar explores its own radiation (thermal, electromagnetic, etc.), which generates targets (missiles, airplanes, space objects).
• Active with active response is carried out if the object is equipped with its own transmitter and interaction with it occurs according to the “request-response” algorithm.
• Active with passive response involves the study of a secondary (reflected) radio signal. in this case it consists of a transmitter and a receiver.
• Semi-active radar- This special case active, in the case when the receiver of reflected radiation is located outside the radar (for example, it is structural element homing missile).

Each type has its own advantages and disadvantages.

Methods and equipment

According to the method used, all radar equipment is divided into continuous and pulsed radiation radars.

The first contain a transmitter and a radiation receiver that operate simultaneously and continuously. The first radar devices were created using this principle. An example of such a system is a radio altimeter (an aircraft instrument that determines the distance aircraft from the surface of the earth) or a radar known to all motorists to determine the speed limit of a vehicle.

With the pulse method electromagnetic energy is emitted in short pulses over a period of several microseconds. Afterwards, the station works only for reception. After capturing and registering the reflected radio waves, the radar transmits a new pulse and the cycles are repeated.

Radar operating modes

There are two main modes of operation of radar stations and devices. The first is scanning the space. It is carried out according to a strictly defined system. With a sequential review, the movement of the radar beam can be circular, spiral, conical, or sectoral. For example, an antenna array can rotate slowly in a circle (azimuth) while simultaneously scanning in elevation (tilting up and down). With parallel scanning, the review is carried out by a beam of radar beams. Each has its own receiver, and several information streams are processed at once.

The tracking mode implies that the antenna is constantly aimed at the selected object. To rotate it in accordance with the trajectory of a moving target, special automated tracking systems are used.

Algorithm for determining range and direction

The speed of propagation of electromagnetic waves in the atmosphere is 300 thousand km/s. Therefore, knowing the time spent by the broadcast signal to cover the distance from the station to the target and back, it is easy to calculate the distance of the object. To do this, it is necessary to accurately record the time the pulse was sent and the moment the reflected signal was received.

Highly directional radar is used to obtain information about the location of the target. Determination of azimuth and elevation (elevation angle or elevation) of an object is carried out by an antenna with a narrow beam. Modern radars use phased antenna arrays (PAA) for this purpose, capable of setting a narrower beam and differing high speed rotation. As a rule, the process of scanning space is performed by at least two beams.

Basic system parameters

From tactical and technical characteristics equipment largely depends on the efficiency and quality of the tasks being solved.

Tactical radar indicators include:

• The viewing area is limited to a minimum and maximum range target detection, permissible azimuth angle and elevation angle.
• Resolution in range, azimuth, elevation and speed (the ability to determine the parameters of nearby targets).
• Measurement accuracy, which is measured by the presence of gross, systematic or random errors.
• Noise immunity and reliability.
• The degree of automation of extraction and processing of the incoming flow of information data.

The specified tactical characteristics are laid down when designing devices through certain technical parameters, among which:

At the combat post

Radar is universal tool, which has become widespread in the military sphere, science and national economy. The areas of use are steadily expanding due to the development and improvement of technical means and measurement technologies.

The use of radar in the military industry makes it possible to solve important problems of surveillance and control of space, detection of air, ground and water mobile targets. Without radars it is impossible to imagine equipment used for information support navigation systems and gun fire control systems.

Military radar is a basic component of the strategic missile attack warning system and integrated missile defense.

Radio astronomy

Radio waves sent from the surface of the earth are also reflected from objects in the near and deep space, as well as from near-Earth targets. Many space objects could not be fully explored only using optical instruments, and only the use of radar methods in astronomy made it possible to obtain rich information about their nature and structure. Passive radar was first used to study the Moon by American and Hungarian astronomers in 1946. Around the same time, radio signals from outer space were also accidentally received.

In modern radio telescopes, the receiving antenna has the shape of a large concave spherical bowl (similar to the mirror of an optical reflector). The larger its diameter, the weaker the signal the antenna can receive. Often radio telescopes operate in a complex manner, combining not only devices located close to each other, but also those located at different continents. Among most important tasks modern radio astronomy - the study of pulsars and galaxies with active nuclei, the study of the interstellar medium.

Civil application

In agriculture and forestry, radar devices are indispensable for obtaining information on the distribution and density of vegetation, studying the structure, parameters and types of soils, and timely detection of fires. In geography and geology, radar is used to perform topographic and geomorphological work, determine the structure and composition of rocks, and search for mineral deposits. In hydrology and oceanography, radar methods are used to monitor the condition of the country's main waterways, snow and ice cover, and map the coastline.

Radar is indispensable assistant meteorologists. The radar can easily determine the state of the atmosphere at a distance of tens of kilometers, and based on the analysis of the data obtained, a forecast of changes is made weather conditions in one area or another.

Development prospects

For a modern radar station, the main evaluation criterion is the ratio of efficiency and quality. Efficiency refers to generalized performance characteristics equipment. Creating a perfect radar is a complex engineering, scientific and technical task, the implementation of which is only possible using the latest achievements electromechanics and electronics, computer science and computer technology, energy.

According to experts, in the near future the main functional units of stations of various levels of complexity and purpose will be solid-state active phased array antennas (phased array antennas), transforming analog signals to digital. Development computing complex will allow you to fully automate the control and basic functions of the radar, providing the end user with a comprehensive analysis of the information received.