Page 331 - Radar Technology Encyclopedia

321 RADAR radar, aerostat

are coverage, probabilities of detection and false alarm, reso- may operate autonomously or it may be cued by a long-range
lution, measurement errors, and throughput capacity. (See search radar located at or near a higher headquarters or, in the
PERFORMANCE.) Major subsystems affecting radar per- case of naval systems, aboard a ship near the fleet center.
formance are the antenna, transmitter, receiver, and signal Aerial target acquisition radars may be fixed or mobile.
processor. (See SUBSYSTEM.) Most radars operate in the The main requirements levied on them include long detection
radio frequency bands. (See FREQUENCY.) Active sensors range and the capability to acquire aerial targets at all possible
operating in the optical band are called laser radars, even operating altitudes. As a rule, modern aerial target acquisition
though the first letter of the radar acronym stands for radio, radars use automatic detection circuits, and some also initiate
indicating that lower band of frequencies. tracks automatically. Acquisition radars are sometimes used
There are many specific features that can be used in clas- also for air traffic control. The AN/FPS-7 and AN/TPS-59 are
sification of radars: examples of three-dimensional radars, while the AN/FPS-89
(1) Depending on the nature and location of the source of is an example of a height-finder. (See also air surveillance
electromagnetic radiation, which supplies information on the radar.) AIL, PCH
radar target and is called a radar signal, one can distinguish Ref.: Skolnik (1980), p. 153; Leonov. (1988), pp. 72–103.
active (or primary) radar, radar with active response (second-
active radar (see primary radar).
ary radar), or passive radar.
(2) Depending on maximum effective range, one can dis- An active radar missile seeker is a small, self-contained
tinguish short-range, horizon (direct visibility), or over-the- radar installed in a missile for acquiring a target and perform-
horizon radar. ing homing guidance on the energy returned from the missile-
(3) Depending on the number of channels used to deter- borne radar’s illumination of the target. The difference
mine angular coordinates in a given coordinate plane, one can between an active radar seeker and a semiactive radar seeker
include monopulse radar. is that the former provides its own source of radar energy (via
(4) Depending on the position of receiving and transmit- the transmitter), while the latter relies on passive reception of
ting antennas, one can distinguish monostatic or multistatic target-reflected energy provided by a separate, off-board, tar-
radar. get illumination and tracking radar.
(5) The term radar is applied to applications for solving Active radar seekers are used in air-to-air (AAM) and
scientific and practical problems in different branches of surface-to-air (SAM) defense missiles, as well as in tactical
human activity. For example, one refers to subsurface radar, air-to-surface (ASM) and surface-to-surface (SSM) weapon
radar astronomy, geographic research radar. AIL systems. Because of the physical limitations of weight, vol-
Ref.: IEEE (1993), p. 1051; Skolnik (1980, 1988, 1990). ume, and power attendant to installation in a tactical missile,
active radar seekers are generally used in the missile’s termi-
RADAR APPLICATIONS AND TYPES
nal guidance phase, acquiring the target during the last few
An above-the-horizon radar is one in which the target is seconds of flight after having been guided to the active seeker
observed over line-of-sight propagation paths that do not acquisition “basket” by one or more separate, midcourse
extend below the horizon. The term applies to the vast major- guidance modes. Active radar seekers give an AAM-
ity of radars, and hence is seldom used except to emphasize equipped aircraft the advantage of “fire-and-forget” soon
the distinction with over-the-horizon or subsurface radars. after launch, leaving the aircraft free to perform maneuvers
AIL that decrease its own vulnerability. (See also GUIDANCE,
Ref.: Dulevich (1978), p. 11. radar and HOMING, radar.) PCH
An acquisition radar in air defense systems is associated Ref.: Skolnik (1990), p. 19.15.
with an antiaircraft artillery (AAA) or surface-to-air missile Adaptive radar can be adapted to an unknown or changing
(SAM) battery or shipborne defense system that acquires tar- radar situation. As a rule, adaptive antenna arrays are used in
gets for hand-off to a separate target tracking radar. A 360° such radars. The fact that the characteristics of the targets and
azimuth scan usually is used as aerial target acquisition interference sources, their coordinates, number, and mutual
radars. They may be two-dimensional (measuring range and positioning change during the process of acquisition and
azimuth) or three-dimensional (measuring range, azimuth, determination of target coordinates, explains the requirement
and elevation). Height-finding radars may operate in associa- to create adaptive radars. A processing algorithm optimum
tion with 2D radars. Target parameters furnished by the for one situation produces a significant loss in others. There-
acquisition radar usually include range, azimuth, elevation or fore, the requirement arises to continually correct the process-
altitude, and target radial velocity (doppler). These target ing algorithm so it remains optimum in spite of changes in the
parameters are used by the tracking radar to initiate its own radar situation. This problem is solved in adaptive radars
local target acquisition mode. A air defense battery or ship- through use of special algorithms realized on a computer. AIL
borne system may have more than one target acquisition Ref.: Skolnik (1980), p. 332; (1990), p. 9.14; Lukoshkin (1983), p. 35.
radar, such as one radar dedicated to low-altitude targets
An aerostat radar (usually a search radar) is installed on a
(horizon search), and a second for medium-to-high- altitude
tethered balloon, increasing the detection range for low-alti-
targets. The target acquisition radar in an air defense battery