Introduction to radar fundamentals

The term RADARis common in today's everyday language. You probably use it yourself when referring to a method of recording the speed of a moving object The term Radar is an acronym made up of the words radio detection and ranging. The term is used to refer to electronic equipment that detect the presence, direction, height, and distance of objects by using reflected electromagnetic energy. Electromagnetic energy of the frequency used for radar is unaffected by darkness and also penetrates weather to some degree, depending on frequency. It permits radar systems to determine the positions of ships, planes, and land masses that are invisible to the naked eye because of distance, darkness, or weather.

Modem radar systems are used for early detection of surface or air objects and pro­vide extremely accurate information on distance, direction, height, and speed of the ob­jects. Radar is also used to guide missiles to targets and direct the firing of gun systems. Other types of radar provide long-distance surveillance and navigation information.

Basic radar concepts

The electronics principle on which radar operates is very similar to the principle of sound-wave reflection. If you shout in the direction of a sound-reflecting object (like a rocky canyon or cave), you will hear an echo. If you know the speed of sound in air, you can then estimate the distance and general direction of the object. The time required for a return echo can be roughly converted to distance if the speed of sound is known. Radar uses electromagnetic energy pulses in much the same way. The radio-frequency (rf) energy is transmitted to and reflects from the reflecting object. Asmall portion of the energy is reflected and returns to the radar set. This returned energy is called an ECHO, just as it is in sound terminology. Radar sets use the echo to determine the direction and distance of the reflecting object.

NOTE:The terms TARGET, RETURN, ECHO, CONTACT, OBJECT, CONTACT are used interchangeably throughout this module to indicate a surface or airborne object that has been detected by a radar system.

Radar systems also have some characteristics in common with telescopes. Both provide only a limited field of view and require reference coordinate systems to define the positions of detected objects. If you describe the location of an object as you see it through a telescope, you will most likely refer to prominent features of the landscape.

Radar requires a more precise reference system. Radar surface angular measurements are normally made in a clockwise direction from TRUE NORTH or from the heading line of a ship or aircraft. The surface of the earth is represented by an imaginary flat plane, tangent (or parallel) to the earth's surface at that location. This plane is referred to as the HORIZONTAL PLANE. All angles in the up direction are measured in a second imaginary plane that is perpendicular to the horizontal plane.

This second plane is called the VERTICAL PLANE. The radar location is the cen­ter of this coordinate system. The line from the radar set directly to the object is referred to as the LINE OF SIGHT (los). The length of this line is called RANGE. The angle between the horizontal plane and the los is the ELEVATION ANGLE. The angle mea­sured clockwise from true north in the horizontal plane is called the TRUE BEARING or AZIMUTH angle. These three coordinates of range, bearing, and elevation describe the location of an object with respect to the antenna.

Radar classification and use

Radar systems, like cars, come in a variety of sizes and have different performance specifications. Some radar systems are used for air-traffic control at airports and others are used for long-range surveillance and early-warning systems. A radar system is the heart of a missile guidance system. Small portable radar systems that can be maintained and oper­ated by one person are available as well as systems that occupy several large rooms.

Radar systems are usually classified according to specific function and installation vehicle. Some common examples are listed below:

Function installation vehicle search — ground or land based Track — Airborne

Height-finder — Shipboard

Radar functions

No single radar system has yet been designed that can perform all of the many radar functions required by the military. Some of the newer systems combine several func­tions that formerly required individual radar systems, but no single system can fulfill all the requirements of modem warfare. As a result, modern warships, aircraft, and shore stations usually have several radar systems, each performing a different function.

One radar system, called SEARCH RADAR, is designed to continuously scan a volume of space to provide initial detection of all targets. Search radar is almost al­ways used to detect and determine the position of new targets for later use by TRACK RADAR. Track radar provides continuous range, bearing, and elevation data on one or more targets. Most of the radar systems used by the military are in one of these two categories, though the individual radar systems vary in design and capability.

Some radar systems are designed for specific functions that do not precisely fit into either of the above categories. The radar speed gun is an example of radar designed specifically to measure the speed of a target. The military uses much more complex radar systems that are adapted to detect only fast-moving targets such as aircraft. Since aircraft usually move much faster than weather or surface targets, velocity-sensitive radar can eliminate unwanted clutter from the radar indicator. Radar systems that detect and process only moving targets are called MOVING-TARGET INDICATORS (mti) and are usually combined with conventional search radar.

Another form of radar widely used in military and civilian aircraft is the RADAR ALTIMETER. Just as some surface-based radars can determine the height of a target, airborne radar can determine the distance from an aircraft to the ground. Many aircraft use radar to determine height above the ground. Radar altimeters usually use frequency-modulated signals.

Radar types

The preceding paragraphs indicated that radar systems are divided into types based on the designed use. This section presents the general characteristics of several com­monly used radar systems. Typical characteristics are discussed rather than the specific characteristics of any particular radar system.

Search radar

Search radar, as previously mentioned, continuously scans a volume of space and provides initial detection of all targets within that space. Search radar systems are fur­ther divided into specific types, according to the type of object they are designed to detect For example, surface-search, air-search, and height-finding radars are all types I of search radar.

Surface-Search Radar

A surface-search radar system has two primary functions: (1) the detection and determination of accurate ranges and bearings of surface objects and low-flying aircraft and (2) the maintenance of a 360-degree search pattern for all objects within line-of-sight distance from the radar antenna.

The maximum range ability of surface-search radar is primarily limited by the ra­dar horizon; therefore, higher frequencies are used to permit maximum reflection from small, reflecting areas, such as ship masthead structures and the periscopes of subma­rines. Narrow pulse widths are used to permit a high degree of range resolution at short ranges and to achieve greater range accuracy. High pulse-repetition rates are used to permit I maximum definition of detected objects. Medium peak power can be used to permit the detection of small objects at line-of-sight distances. Wide vertical-beam widths permit compensation for the pitch and roll of own ship and detection of low fly­ing aircraft. Narrow horizontal-beam widths permit accurate bearing determination and good bearing resolution.

Surface-search radar is used to detect the presence of surface craft and low flying aircraft and to determine their presence. Shipboard surface-search radar provides this type of information as an input to the weapons system to assist in the engagement of hostile targets by fire-control radar. Shipboard surface-search radar is also used exten­sively as a navigational aid in coastal waters and in poor weather conditions.

WORDLIST

Radar, radar set -радиолокационная станция (РЛС), радар

land masses - участки местности

height - высота (над поверхностью)

altitude -абсолютная высота; высота над

уровнем моря

electromagnetic energy pulse -электромагнитный импульс

echo -отраженный сигнал

return -отраженный сигнал, эхо-сигнал

reflecting object -отражающий объект

contact зд. -захват цели

airborne object -воздушный объект

detected object - обнаруженный объект

precise reference system -точная система координат

true north -истинный север

line of sight -линия прямой видимости,

линия визирования

elevation angle - угол возвышения; угол места цели

air traffic control (АТС) -управление воздушным движением

true bearing -истинный пеленг или азимут (цели)

long range surveillance -дальнее наблюдение

early warning system -раннее предупреждение,

дальнее обнаружение

missile guidance system - система наведения ракет

search radar - поисковая РЛС, РЛС обнаружения

height finder - высотомер

shipboard radar - корабельная РЛС

track radar -РЛС сопровождения

clutter - помеха, «местник»

moving target indicator -РЛ индикатор движущихся целей

radar altimeter -РЛ высотомер, радио высотомер

radar speed gun РЛ измеритель скорости

search pattern -поисковый обзор

pitch - угол наклона; зд. крен;

качение, качка

roll fire control radar - РЛС управления огнем

(к-л. систем вооружения)

radar horizon - горизонт РЛС

primary radar system -РЛС с пассивным ответом,

активная РЛС

secondary radar system -РЛС с активным ответом

EXPESSIONS

Radio detection and ranging — обнаружение цели и определение дальности до неё с помощью радиоволн.

То guide a missile to target (tgt)— направлять (наводить) ракету в цель

То direct the firing of gun systems — управлять огнём артиллерийских систем (артиллерии)

То estimate the distance — определять расстояние

То permit a high degree of range resolution at short ranges — обеспечивать высокую степень разрешения по дальности на малых расстояниях

High pulse repetition rate — высокая частота повторения сигнала (импульса) Wide vertical beam width — широкополосный вертикальный луч

Accurate bearing determination and good bearing resolution—. точное опре­деление (измерение пеленга) и хорошая разрешающая способность по пеленгу.

Velocity-sensitive radar can eliminate unwanted clutter from the radar indicator—РЛС, реагирующие на изменения скорости, позволяют удалять отметоки ненужных местных объектов с экрана локатора (индикатора).

EXERCISES

I. Give equivalents to the following words and word combinations:

Detect height and distance, поисковая РЛС, land masses, to guide missiles to target, угол наклона, to determine the positions of ships, управление воздушным движением, радиовысотомер, altitude, height finder, истинный пеленг, airborne object, дальнее наблюдение, shipboard radar, elevation angle, clutter, echo, track radar, contact.

II. Put 10 questions to the text and answer them:

a) How can you define the difference between a radar and radio set? Please, continue...

III. Translate into Russian the following text:

Air-Search Radar

Air-search radar systems initially detect and determine the position, course, and speed of air targets in a relatively large area. The maximum range of air-search radar can exceed 300 miles, and the bearing coverage is a complete 360-degree circle. Air-search radar systems are usually divided into two categories, based on the amount of position informa­tion supplied. Radar sets that provide only range and bearing information are referred to as two-dimensional, or 2D, radars. Radar sets that supply range, bearing, and height are called three-dimensional, or 3D, radars. Relatively low transmitter frequencies are used in 2D search radars to permit long-range transmissions with minimum attenuation. Wide pulse widths and high peak power are used to aid in detecting small objects at great dis­tances. Low pulse-repetition rates are selected to permit greater maximum range. A wide vertical-beam width is used to ensure detection of objects from the surface to relatively high altitudes and to compensate for pitch and roll of own ship. The output characteristics of specific air-search radars are classified; therefore, they will not be discussed.

Air-search radar systems are used as early-warning devices because they can detect approaching enemy aircraft or missiles at great distances. In hostile situations, early de­tection of the enemy is vital to a successful defense against attack. Antiaircraft defenses in the form of shipboard guns, missiles, or fighter planes must be brought to a high degree of readiness in time to repel an attack. Range and bearing information, provided by air-search radars, used to initially position a fire-control tracking radar on a target. Another function of the air-search radar system is guiding combat air patrol (CAP) air­craft to a position suitable to intercept an enemy aircraft In the case of aircraft control, the guidance information is obtained by the radar operator and passed to the aircraft by either voice radio or a computer link to the aircraft.

IV. Translate into English:

РЛС— это радиоэлектронное устройство, предназначенное для обнаруже­ния и определения местоположения объектов в воздухе, на море и на суше путем облучения их радиоволнами. Современные радары не только обнаруживают от­дельные объекты, но и указывают расстояние до них, определяют их положение в пространстве, размер и форму, скорость и направление движения. Существует большое количество различных радиолокационных станций, но все они состоят из следующих основных элементов: передатчика, приёмника, антенны, синхрони­затора (timer) и, конечно, источника питания.

По способу локации РЛС различают активные или с активным ответом и пас­сивные станции.

Действие активной РЛС основано на излучении в направлении объекта пря­мого радиолокационного сигнала и приеме отраженного от объекта эхо-сигнала. Примерами станций с активным ответом являются радиолокационный маяк и ра­бота системы опознавания «свой-чужой». А радиолокационная станция с пассив­ным ответом не имеет передающего устройства, и в качестве сигналов от объектов используется их естественное излучение.

ADDITIONAL MATERIALS:

HEIGHT-FINDING SEARCH RADAR

The primary function of a height-finding radar (sometimes referred to as a three-ordinate or 3D radar) is that of computing accurate ranges, bearings, and altitudes of aircraft targets detected by air-search radars. Height-finding radar is also used by the ship's air controllers to direct CAP aircraft during interception of air targets. Modern 3D radar is often used as the primary air-search radar. This is because of its high accuracy and because the maximum ranges are only slightly less than those available from 2D radar.

The range capability of 3D search radar is limited to some extent by an operating frequency that is higher than that of 2D radar. This disadvantage is partially offset by higher output power and a beam width that is narrower in both the vertical and horizontal planes.

The 3D radar system transmits several narrow beams to obtain altitude coverage and, for this reason, compensation for roll and pitch must be provided for shipboard installations to ensure accurate height information.

Applications of height-finding radars include the following: — Obtaining range, bearing, and altitude data on enemy aircraft and missiles to assist in the control of CAP aircraft — Detecting low-flying aircraft — Determining range to distant land mass­es — Tracking aircraft over land — Detecting certain weather phenomena — Tracking weather balloons — Providing precise range, bearing, and height information for fast, accurate initial positioning of fire-control tracking radars

TRACKING RADAR

Radar that provides continuous positional data on a target is called tracking radar. Most tracking radar systems used by the military are also fire-control radar; the two names are often used interchangeably.

Fire-control tracking radar systems usually produce a very narrow, circular beam. Fire-control radar must be directed to the general location of the desired target because of the narrow-beam partem. This is called the DESIGNATION phase of equip­ment operation. Once in the general vicinity of the target, the radar system switches to the ACQUISITION phase of operation. During acquisition, the radar system searches a small volume of space in a prearranged pattern until the target is located. When the target is located, the radar system enters the TRACK phase of operation. Using one of several possible scanning techniques, the radar system automatically follows all target motions. The radar system is said to be locked on to the target during the track phase. The three sequential phases of operation are often referred to as MODES and are com­mon to the target-processing sequence of most fire-control radars.

Typical fire-control radar characteristics include a very high prf, a very narrow pulse width, and a very narrow beam width. These characteristics, while providing ex­treme accuracy, limit the range and make initial target detection difficult.

AIRBORNE RADAR

Airborne radar is designed especially to meet the strict space and weight limitations mat are necessary for all airborne equipment Even so, airborne radar sets develop the same peak power as shipboard and shore-based sets.

As-with shipboard radar, airborne radar sets come in many models and types to serve many different purposes. Some of the sets are mounted in blisters (or domes) that form part of the fuselage; others are mounted in the nose of the aircraft.

In fighter aircraft, the primary mission of a radar is to aid in the search, interception, and destruction of enemy aircraft. This requires that the radar system have a tracking feature. Airborne radar also has many other purposes. The following are some of the general classifications of airborne radar, search, intercept and missile control, bombing, navigation, and airborne early warning.