Radar Set

(also radar, radar system), the equipment used in the radar observation of a variety of objects (targets). The main units of a radar set are its transmitting and receiving apparatus, which may be located at the same point (combined radar sets) or separated from one another by a certain, usually large, distance (dual-position or multiposition radar sets). In the sets used in passive radar, there is no transmitter. The antenna may be common to the transmitter and receiver (in combined sets), or separate antennas may be used (in multiposition sets). An important component of the receiving apparatus is the cathode-ray-tube indicator, but in modern (mid-1970’s) radar sets, a digital computer, which automates many processing operations of the received signals, has become equally important. The major performance considerations of a radar set include its accuracy, resolution, noise immunity, and the limiting values of a number of parameters, for example, the maximum and minimum ranges, sectors scanned, and scanning periods. Other considerations are the set’s mobility, weight, size, electrical power intake, and service life. The number of people required to operate the set is also an important performance consideration.

Development. The first radar sets were used for the detection of aircraft. Five stationary pulse-radar sets were set up along the southwestern coast of Great Britain in 1936. They operated at relatively long (meter) wavelengths and were unwieldy and unable to detect aircraft flying at low altitudes. Nonetheless, a network of similar sets was rapidly emplaced along the British coast of the English Channel, and these sets demonstrated their effectiveness in repulsing the raids of the Luftwaffe during World War II. In the USA an experimental pulse-radar set was installed on a ship and subjected to comprehensive tests in 1937. Subsequently, efforts to create radar sets for various purposes were intensified in the USA, and by the early 1940’s, sets operating at centimeter wavelengths had been developed that could detect aircraft at great distances.

In the USSR, the first experiments on the radar detection of aircraft were conducted in 1934. Industrial production of radar sets for military use was begun in 1939. These sets (RUS 1), which used continuous-wave radiation modulated by audio frequencies, were placed along a certain line and were able to detect an aircraft that crossed the line. They were employed on the Karelian Isthmus during the Soviet-Finnish War of 1939–40 and in the Caucasus during the Great Patriotic War of 1941–45. The first pulse-radar installation was tested in 1937, and industrial production of pulse-radar sets (RUS 2, Redut) was begun in 1940. These sets, in which the transmitter and receiver were coupled to a common antenna, were installed together with a source of power supply in trucks. They could detect aircraft during a circular scanning at distances, depending on the altitude of the aircraft, of up to 150 km. In 1940, under the direction of Iu. B. Kobzarev, work was completed at the Physico-technical Institute in Leningrad on the construction of a stationary radar set for an antiaircraft defense system. The antennas were placed at a great height (20 m), which both increased the range (~250 km) and made it possible to detect relatively low-flying aircraft. During the Great Patriotic War, in addition to the Redut sets, production was begun on the reliable, portable Pegmatit set, which could be easily transported in packs and quickly set up anywhere. Subsequently, the Pegmatit sets were improved to the point where they were able to determine an aircraft’s altitude, in addition to its distance and azimuth. At the end of the war, radar sets were being improved through an increase in range and accuracy and through the automation of separate operations. This automation was seen, for example, in tracking systems, which measured the distance and tracked according to angular coordinates (in sets used for gun-laying), and in the use of computers (in sets used for instrument bombing).

After World War II, as aircraft attained increased altitudes, flying speeds, and maneuverability, it became necessary to develop radar sets capable of operating under the increased demands created by the necessity of tracking a large number of targets or of overcoming the possibility of jamming. The improved accuracy of coordinate measurements, owing in part to new methods of measuring, and the linking of radar sets both to computers and to integrated systems for the radar guidance of ballistic missiles, substantially altered the performance data of radar sets. These sets thus became the most important part in the automatic control of antiaircraft defense systems.

The development of rocketry and space technology during the 1950’s and 1960’s led to the creation of radar sets designed to cope with new problems. Various sets were also designed to meet the tasks of science and the national economy.

Figure 1. Scheme of the circular scanning of the earth’s surface with an airborne radar set

Principal types of radar sets. Radar sets are classified according to the specific tasks they perform, either independently or as part of a complex. Examples of such complexes are air-traffic control systems, antiaircraft defense systems involving the radar detection or guidance of missiles, systems for the radar search and rendezvous of space vehicles, and airborne radar sets designed for circular and sidewise scanning. The broad range of problems that radar is called upon to deal with, each with its own specific requirements, has led to a multiplicity of different radar sets. For instance, in order to improve the accuracy of antiaircraft fire, miniature radar sets are placed in the noses of antiaircraft missiles; these sets measure the distance between the missile and its target and activate, at a predetermined distance, the missile’s fuse. Another example is the airborne tail-warning radar that provides a warning, in the form of an automatic signal, when a second aircraft approaches from the rear.

Radar sets are also classified according to their location as ground-based, marine, airborne, or satellite. They are then subdivided according to their technical characteristics. One subdivision, based on carrier frequency (operating wavelength range), groups the sets into meter, decimeter, centimeter, millimeter, and other ranges. A second subdivision depends on the mode of operation and groups sets into, for example, pulse, continuous-wave, coherent-pulse, and noncoherent-pulse radars. A third subdivision depends on the parameters of the set’s most important units—transmitter, receiver, antenna, and system for processing received signals—or on other performance data.

The radar sets known as gunlaying sets determine with a high degree of accuracy the azimuth angle, elevation angle, and distance of aerial, marine, and ground objects. The advent of these sets revolutionized antiaircraft artillery. A marked improvement in the accuracy of coordinates, primarily of the angular coordinates, became possible with the introduction of the centimeter wavelength range. This range enabled the antennas of gunlaying radar sets to transmit highly directional radio waves. In so doing, the efficient utilization of the transmitted power in the desired direction was greatly increased, and to a considerable extent it became possible to overcome the effects of the earth, local objects, and other types of radar interference.

The use of the centimeter range permitted the development of airborne radar sets designed for a circular scanning of the earth’s surface (Figure 1). These sets played an important part during World War II in instrument bombing and in the search for and destruction of submarines at sea. It was characteristic of these sets that they were able to discriminate perfectly between separate details on the earth’s surface, such as bridges, structures, and railroads; on the ocean, they could detect such things as submarine periscopes.

The availability of the centimeter range also led to the creation of search and control radar sets for detecting aircraft and vectoring interceptors. The interceptors, by using data obtained from a ground-based, early-warning radar set or by operating independently, could detect an aircraft and simultaneously measure its distance, azimuth, and altitude coordinates (for example, by the V-beam scan). Two antennas are used in this method, one of which has a beam pattern with a narrow azimuth angle but a wide angle in the vertical plane, while the other antenna has the same beam pattern but is deflected from the vertical plane by an angle of 45°. As the antennas are rotated together, the azimuth and distance of an object are determined by the first antenna, and the elevation, by the time that elapses before the object is picked up by the second antenna.

Sidewise-scanning radar sets, which are designed for such tasks as aerial mapping and reconnaissance, have a high resolution that makes for high definition of the radar display image. This is accomplished either by substantially increasing the size of the antenna positioned along the fuselage of the aircraft, thereby increasing the resolution by some ten times in comparison with the panoramic, circular-scanning radar sets, or by synthesizing the antenna aperture using special hardware (Figure 2). This second method makes it possible to approach the resolving power of optical means of observation, since when the apertures are altered, the resolution does not depend on either the distance to the object or the wavelength of the scanning signal. In radar sets having specially designed antenna apertures, use is often made of complex optical systems with both multichannel (with respect to distance) processing of signals and a coherent storage of the signals in each channel. Combining these systems with photographic equipment permits a high-quality depiction of the information.

The radar sets used in the antiballistic-missile (ABM) systems that protect major cities and industrial targets (in the USA, according to the foreign press) form complexes that include sets for detection, tracking, and target identification, as well as sets for guiding antimissile missiles. These sets operate for the most part in the centimeter or, more rarely, decimeter wavelength ranges. This complex of sets contains hundreds of transmitters, each with a pulse power of 0.1 to 1 watt, a phased array controlled by a digital computer, and thousands of parametric

Figure 2. Airborne sidewise scanning

amplifiers in the input circuits of the receivers. There are projects under way in other countries for ground-based ABM systems that use powerful lasers against targets. Such systems must operate in conjunction with equipment for automatically tracking and focusing the high-intensity laser beam. This equipment may also include a radar set for an imprecise tracking that can provide rough data about an approaching target, a second radar set using lasers for a precise tracking of the target, and a system for distinguishing the true target from any false targets. Owing to the feasibility of obtaining a narrow beam and to the small size of radar sets using lasers, it has been proposed to use these sets also on space vehicles and satellites.

The radar sets for tracking artificial earth satellites and measuring the satellites’ trajectories differ primarily in design and in the number of parameters they measure. In the simplest, or single-parameter, sets, only the Doppler frequency is measured, and from the nature of the frequency variation at the site of the set, the period of rotation and other parameters of the satellite’s orbit are determined. A satellite’s orbit can be accurately determined by positioning a number of radar sets, operating in the centimeter wavelength range, along the flight path. These sets can be accurate pulse-radar sets used as range-only radars, which interrogate a transponder placed on board the satellite. Any instability in the time delay of the transponder’s response is relatively small. The radar sets have parabolic antennas and with conical scanning can determine the angular coordinates of a satellite with an accuracy within several minutes of arc; under monopulse operation, the accuracy is within 1 min of arc. While these three-parameter sets can be regarded as a sophistication of the gunlaying set, they differ in the design of the principal automatic range-finder channel and the multiplicity of the scales. They also maintain a high accuracy in the range tracking; for example, the error in range tracking of an object moving at cosmic speeds is within 10 m.

With pulse modulation, it is possible for several radar sets to work simultaneously with a single transponder. Four-parameter radar sets are used in conjunction with a coherent-pulse transponder that is placed on board the space vehicle; here, an added measurement, that of the vehicle’s radial velocity, is provided through the simpler mode of continuous oscillations. The retention of pulse modulation in the measurement of radial velocity from the Doppler frequency requires that the radar set operate in a coherent-pulse mode. In place of a simple magnetron transmitter, a superhigh-frequency power amplifier, such as a klystron amplifier, and a more complex coherent-pulse transponder must be used.

Radar sets that measure six parameters of an object’s motion, namely, its distance and two angular coordinates and the quantities derived from these—the radial and two angular velocities—are used, for example, when these parameters are determined from one point along the active flight leg of a missile or space vehicle. The complexity of these sets is due to the construction of many channels for the accurate phase measurement of the angular coordinates, an accuracy of about 10 sec of arc.

Radar sets for tracking satellites at altitudes extending to several hundred km and for measuring satellite trajectories can also be based on precise direction finders that operate in the decimeter wavelength range and have much simpler (nontrack-ing) antennas for phase-angle measuring channels; in this wavelength range the effective receiving area of the antennas is sufficient. The transmitters that are on board the satellites operate with continuous oscillations.

In order to track satellites at distances of about 40,000 km (stationary satellites or those of the Molniia type, which have elliptical orbits), radar sets having fully steerable parabolic antennas are used. These sets operate in the decimeter wavelength range when tracking according to the flight program and in the centimeter range when tracking automatically.

The radar sets used in astronomy, which measure the distance to a planet, the parameters of the planet’s motion, and other physical characteristics, are distinguished by a large effective receiving area of the antenna, a high transmitter power, and a high sensitivity in the receiving apparatus. The duration of the probing signal for such sets is limited by the time required for the radio waves to reach the planet and return. This time, for example, is about 5 min for Venus, some 10 min for Mars, and about 1 hr for Jupiter. Thus, in the radar set that researchers at the Institute of Radio Engineering and Electronics of the Academy of Sciences of the USSR used to study Mars, the distance measurements were made by a phase method from the envelope of oscillations having a carrier frequency of 768 megahertz, which was amplitude-modulated by oscillations having frequencies of 3 and 4 hertz. The radial component of the velocity was measured by the Doppler method at the carrier frequency. The signal received during the observation sessions was stored on a tape recorder, and the envelope delay of the signal was determined (in the process of repeated reproduction after a session) using a correlation method, that is, from the maximum output signal of a correlation meter receiving the reference signal with various delays. The value of the Doppler shift in frequency was determined by selective electrical filters covering specific resonant frequencies.

The over-the-horizon radar sets, which operate (in the USA, according to information in the foreign press) in the decameter (shortwave) range of wavelengths, are used for observations at distances of several thousand km. These sets are used, for example, to provide an early warning of the launching of ballistic missiles and to approximate their coordinates, to detect nuclear explosions, to observe various regions of the ionosphere, and to track satellites. They are placed in ground-based installations and are equipped with large, complex multielement arrays and with high-power transmitters having a peak power signal of tens of megawatts. As a rule these sets are of the dual-position or multiposition types. Over-the-horizon sets typically have a multichannel design (for example, with 120 and more channels in a frequency range of 4 to 6 megahertz). It is possible with these sets to operate at various pulse widths and pulse repetition frequencies and to correspondingly adjust the frequency bandwidth of the receiver and other characteristics. In this way, the mode of operation can be obtained that is optimal for the state of the ionosphere and the nature of the given task.