Page 218 - Radar Technology Encyclopedia

GUIDANCE, radar GYROTRON 208

Radar guidance of missiles can be further differentiated tice. When the field intensity is less than critical but close to
on the basis of whether or not the guided missile is equipped it, as a result of the heterogeneity of the material of the semi-
with an on-board target sensor or “seeker,” in which case it is conductor, a separate area of electrons with less mobility
said to use radar homing guidance. Nonhoming radar guid- occurs. This is a so-called domain, a thin layer of negative
ance techniques (i.e., no onboard seeker), include command space charge, slowly moving toward the positive electrode. In
guidance and radar beam-riding guidance. In a command the remaining part of the semiconductor, the intensity remains
guidance system, one or more radars track both the target and less than critical and other domains are not formed. The
the missile under control, and the tracking data are used to domains occur in series in the vicinity of the cathode and
compute missile autopilot commands that are transmitted determine the current pulses in the external circuit with a
(uplinked) to the missile either continuously or at a high data period approximately equal to the transit time of the electrons
rate. In a beam-rider guidance scheme, the target is tracked by in the sample. The Gunn effect serves as the basis of the Gunn
an external radar and the missile, equipped with its own diode. (See DIODE.) IAM
antenna and receiver system, senses its position relative to the Ref.: Fink (1982), p. 9.72; Gassanov (1988), p. 186.
center of the beam. Missile course corrections are made in
GYROTRON. A gyrotron is a microwave device using a rel-
proportion to the amount of lateral deviation of the missile
ativistic electron beam and converting constant electron
from beam center, thus the name beam rider.
energy to microwave energies in an intense electromagnetic
Radar guidance using missileborne seekers can be cate-
field. It has good prospects for production; at millimeter-
gorized in terms of the source of the sensed radar energy, and
wave frequencies, where peak powers much higher than those
all such seekers are said to use homing guidance. Passive
can be obtained with conventional millimeter-wave tubes
radar seekers, also referred to as antiradiation homing seekers
using previous techniques. Achievable and predicted gyrotron
sense the energy emitted from a target radar. An active seeker
power levels over a range of frequencies are given in Table
is a self-contained radar that transmits its own radar signal,
G1 and Fig. G3. There are different types of gyrotron amplifi-
detects and tracks the target, and homes in on the reflected
ers configurations, the main being the gyromonotron,
energy from the target. In a semiactive guidance system, an
gyroklystron, gyro-TWT, gyro-BWO, and gyrotwystron
external radar, the illuminator radar, tracks and illuminates
based on the corresponding parent configurations, which are
the target. Part of the target-reflected energy is received by a
shown in Fig. G4. Other terms used for gyrotron are cyclotron
passive antenna and receiver system on board the missile,
resonance maser or electron-cyclotron maser. SAL
which uses the signal for homing guidance to intercept.
Modern medium-to-long-range defensive missiles (e.g., Ref.: Ewell (1981), pp. 76–79; Currie (1987), pp. 466–470; Brookner (1988),
p. 334; Flyagin, V. A., IEEE Trans. MTT-25, No 6, 1977.
50 to 100 km), may use a combination of guidance modes.
For example, a typical multimode guided missile might
employ inertial or command-inertial guidance for the initial
and midcourse phases of flight, with a transition to semiactive
or active radar homing for the terminal phase. Such a combi-
nation takes advantage of the best features of each mode.
Command or command-inertial guidance accuracy deterio-
rates with range (or flight time), while radar homing accuracy
improves as the missile-to-target range decreases. (See
RADAR, missile guidance.) PCH
Ref.: Skolnik (1990), Ch. 19; James (1986).
A GULL is “a floating radar reflector used to simulate sur-
face targets for deceptive purposes.” SAL
Ref.: Johnston (1979), p. 61.
GUNN EFFECT. The Gunn effect, experimentally discov-
ered in 1963 by J. Gunn, entails the onset of microwave oscil-
lations in a gallium arsenide (GaAs) conductor in the
presence of a high field intensity (on the order of 3 kW/cm). It
is explained by the effect of the field on the mobility of the
carriers, conditioned by the presence of two energetic zones
of electronic conductivity. Under the effect of the critical val-
Figure G3 Gyrotron state-of-the-art capabilities (from Ewell,
ues of the field intensity the electrons begin to move to the
1981, Fig. 2-43, p. 77).
upper zone, where their mobility is decreased as a result of
the strong interaction with the field of the GaAs crystal lat-