Module 18—Radar Principles
Pages i - ix
1-1 to 1-10
, 1-11 to 1-20
1-21 to 1-30
, 1-31 to 1-40
1-41 to 1-45
, 2-1 to 2-10
2-11 to 2-20
, 2-21 to 2-30
2-31 to 2-40
,2-41 to 2-51
3-1 to 3-10
, 3-11 to 3-20
3-21 to 3-23
, 4-1 to 4-10
4-11 to 4-20
, 4-21 to 4-26
AI-1 to AI-11
, AII-1 to AII-2
Index-1 to 3
Table 1-1.—Table of Equipment Indicators
|TABLE OF EQUIPMENT INDICATORS||Miscellaneous|
|Type of Equipment|
B—Underwater mobile, submarine
G—General ground use
W—Water surface and under water combination Z—Piloted and pilotless airborne vehicle combination
|A—invisible light, heat|
G—Telegraph or Teletype
I—Interphone and public address
J—Electromechanical or Inertial wire covered
N—Sound in air
Q—Sonar and underwater sound
S—Special types, magnetic, etc., or combinations of types
V—Visual and visible light
W—Armament (peculiar to armament, not otherwise covered)
X—Facsimile or television
C—Communications (receiving and transmitting
D—Direction finder reconnaissance and/or surveillance
E—Ejection and/or release
G—Fire control, or search- light directing
H—Recording and/or reproducing (graphic meteorological and sound)
test assemblies (including tools)
N—Navigational aids (including altimeters, beacons, compasses, racons, depth sounding, approach and landing)
Q—Special, or combination of purposes
R—Receiving, passive detecting
S—Detecting and/or range and bearing, search
W—Automatic flight or remote control
X—Identification and recognition
Y—Surveillance (search detect, and multiple target tracking) and control (both fire control and air control)
X—Facsimile or television
|X, Y, Z—Changes in|
voltage, phase, or frequency
Figure 1-23.—Joint service classification system.
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 modern 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 of the type discussed earlier in the chapter.
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, 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 of 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 a 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. For example, a common shipboard surface-search radar has the following design specifications:
• Transmitter frequency 5,450-5,825 MHz
• Pulse width .25 or 1.3 microseconds
• Pulse-repetition rate between 625 and 650 pulses per second
• Peak power between 190 and 285 kW
• Vertical beam width between 12 and 16 degrees
• Horizontal beam width 1.5 degrees
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. A typical surface-search radar antenna is shown in figure 1-24.
Figure 1-24.—Surface-search radar.
Q32. What type of radar provides continuous range, bearing, and elevation data on an object?
Q33. Radar altimeters use what type of transmission signal?
Q34. A surface-search radar normally scans how many degrees of azimuth?
Q35. What limits the maximum range of a surface-search radar?
Q36. What is the shape of the beam of a surface-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. As mentioned earlier in this chapter, 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. (3D radar will be covered in the next section.) The coverage pattern of a typical 2D radar system is illustrated in figure 1-25. A typical 2D air-search radar antenna is shown in figure 1-26.
Figure 1-25.—2D radar coverage pattern.
Figure 1-26.—2D air-search radar.
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.
Height-Finding Search Radar
The primary function of a height-finding radar (sometimes referred to as a three-coordinate 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 (figure 1-27). This is because of its high accuracy and because the maximum ranges are only slightly less than those available from 2D radar.
Figure 1-27.—3D air-search 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
Q37. Air-search radar is divided into what two basic categories?
Q38. What position data are supplied by 2D search radar?
Q39. Why do 2D air-search radars use relatively low carrier frequencies and low pulse-repetition rates?
Q40. Why is the range capability of 3D radar usually less than the range of 2D 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 pattern. 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. A typical fire-control radar antenna is shown in figure 1-28. In this example the antenna used to produce a narrow beam is covered by a protective radome.
Figure 1-28.—Fire-control radar.
A radar system that provides information used to guide a missile to a hostile target is called
GUIDANCE RADAR. Missiles use radar to intercept targets in three basic ways: (1) Beam-rider missiles
follow a beam of radar energy that is kept continuously pointed at the desired target; (2) homing missiles detect and home in on radar energy reflected from the target; the reflected energy is provided by a radar transmitter either in the missile or at the launch point and is detected by a receiver in the missile; (3) passive homing missiles home in on energy that is radiated by the target. Because target position must be known at all times, a guidance radar is generally part of, or associated with, a fire-control tracking radar. In some instances, three radar beams are required to provide complete guidance for a missile. The beam- riding missile, for example, must be launched into the beam and then must ride the beam to the target. Initially, a wide beam is radiated by a capture radar to gain (capture) control of the missile. After the missile enters the capture beam, a narrow beam is radiated by a guidance radar to guide the missile to the target. During both capture and guidance operations, a tracking radar continues to track the target. Figure 1-29 illustrates the relationships of the three different radar beams.
Figure 1-29.—Beam relationship of capture, guidance, and track beams.
Q41. Fire-control tracking radar most often radiates what type of beam?
Q42. Tracking radar searches a small volume of space during which phase of operation?
Q43. What width is the pulse radiated by fire-control tracking radar?
Q44. Which beam of missile-guidance radar is very wide?
CARRIER-CONTROLLED APPROACH (CCA) AND GROUND-CONTROLLED APPROACH (GCA) RADAR
CARRIER-CONTROLLED APPROACH and GROUND-CONTROLLED APPROACH radar systems are essentially shipboard and land-based versions of the same type of radar. Shipboard CCA radar systems are usually much more sophisticated systems than GCA systems. This is because of the movements of the ship and the more complicated landing problems. Both systems, however, guide aircraft to safe landing under conditions approaching zero visibility. By means of radar, aircraft are detected and observed during the final approach and landing sequence. Guidance information is supplied to the pilot in the form of verbal radio instructions, or to the automatic pilot (autopilot) in the form of pulsed control signals.
Airborne radar is designed especially to meet the strict space and weight limitations that 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.
The following paragraphs summarize the important points of this chapter.
RADAR is an electronic system that uses reflected electromagnetic energy to detect the presence and position of objects invisible to the eye.
TARGET POSITION is defined in reference to true north, the horizontal plane, and the vertical plane.
TRUE BEARING is the angle between true north and the line of sight to the target, measured in a clockwise direction in the horizontal plane.
ELEVATION ANGLE is the angle between the horizontal plane and the line of sight, measured in the vertical plane.
RANGE is the distance from the radar site to the target measured along the line of sight. The concepts are illustrated in the figure.
RANGE to any target can be calculated by measuring the time required for a pulse to travel to a target and return to the radar receiver and by dividing the elapsed time by 12.36 microseconds.
The MINIMUM RANGE of a radar system can be calculated from the formula:
The MAXIMUM RANGE of a pulse radar system depends on the CARRIER FREQUENCY, PEAK POWER, PULSE-REPETITION FREQUENCY, and RECEIVER SENSITIVITY.
PULSE-REPETITION TIME is the time between the beginning of one pulse and the beginning of the next pulse and is the reciprocal of prf.
AMBIGUOUS RETURNS are echoes from targets that exceed the prt of the radar system and result in false range readings. The maximum (unambiguous) range for a radar system can be determined by the formula:
The PEAK POWER of a radar system is the total energy contained in a pulse. Peak power is obtained by multiplying the maximum power level of a pulse by the pulse width.
Since most instruments are designed to measure AVERAGE POWER over a period of time, prt must be included in transmitter power measurements. The formula for average power is:
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