Page 214 - Radar Technology Encyclopedia

FUNCTION, random gain control, manual 204

These definitions show that determining the mean value where there is no overload of the receiver, is applied practi-
and variance of a random function requires specification only cally instantaneously to the controlled stages of the receiver.
of the first-order pdf, f (x ,x ,t ,t ). A further simplification Such control is practically noninertial.
1 1 2 1 2
can be achieved if one assumes that the random function x(t) In terms of speed we distinguish between fast (or low-
is a stationary random process. In this case the mean value inertial) and inertial systems of automatic gain control. In fast
2
m , and variance, s , become consistent values, invariant to systems, the time constant is comparable with the duration of
x
x
the time shift t = |t - t | and the correlation function depends the signal, while in inertial ones it significantly exceeds that.
2
1
only on the parameter t : Inertial systems of automatic gain control the gain depending
2 2 on the value of the signals from one selected target through
;
=
=
m t () m s t () s K t t ,( ) K t()
=
;
x x x x x 1 2 x
strobing of signals in the receiver. Such systems can be used
Since the basic task of any radar is target detection and in fire-control and missile-guidance radars.
measurement in a background of random interference (noise, Depending on the type of controlled stages, automatic
clutter, jamming), the concept of random functions is funda- gain control systems are divided into “forward” and “back”
mental to the theory of radar operation. It is widely used in systems. In the former, the gain coefficient of the low-fre-
the theory of radar detection, measurement, recognition, sig- quency amplification stages following the detector in the
nal processing, and so forth. SAL absence of feedback is controlled. A “back” system provides
Ref.: Barkat (1991), Ch. 2. great depth of adjustment as a result of control of the gain
coefficient of preceding stages of intermediate frequency.
FUSION (see DATA fusion).
Drawbacks of such adjustment include some inertia, so some
FUZE, radar. A radar fuze is a noncontact fuse mounted on a time is required to restore the sensitivity of the receiver after
missile (bomb, shell) based on compact, simple radar and exposure to strong interference. In addition, such control does
ensuring the explosion of missile warhead at the required dis- not protect the receiving circuit from saturation by pulsed
tance from the target, using the information in radar returns. interference. For purposes of stabilization of false alarms with
The basic proximity fuze is a CW homodyne device in which an increase in the intensity of noise at the input of the
commonly a single element is used as both oscillator and receiver, noise AGC is used. The regulating voltage is formed
mixer-detector. Typically, radar fuzes employ solid-state tech- on the basis of smoothing of the noise voltage in the range at
nology and they have to meet basic requirements of small which the appearance of useful signals is eliminated. Such an
size, low cost, and reliability under high acceleration. SAL automatic gain control with feedback is the most widely used
Ref.: Skolnik (1990), p. 14.20. in practically all types of radars.
In search radars, gain control may be used independently
of the value or presence of received signals. The gain is var-
ied as a function of time (generated in accordance with a spe-
G cific time program) and assures uniform amplification of
signals reflected from targets which are at different distances
GAIN is “the increase in signal power in transmission from from the radar. This is called sensitivity time control. (See
one point to another under stated conditions.” Gain is often SENSITIVITY.) IAM
expressed in decibels, Ref.: Skolnik (1970), p. 5.19; Popov (1980), pp. 14, 70, 222.
P 2 V 2
G dB = 10log ------ = 20log ------ diversity gain (see DIVERSITY).
P 1 V 1
Integration gain is that obtained from pulse integration,
where (P /P ) is power gain a (V /V ) is voltage gain in defined as a ratio of detectability factor of a single pulse to
1
2
1
2
dimensionless units. SAL that for each of n pulses. (See INTEGRATION gain). SAL
Ref.: Johnston (1979), p. 61. Ref.: Barton (1989), p. 71.
Automatic gain control (AGC) is a circuit that maintains Inverse gain refers to a jamming technique providing decep-
output signals of a receiver at a given level, without distortion tion or noise jamming with amplitude modulation in phase
of their shape, regardless of significant changes in amplitude opposition to that generated by the scanning radar beam. In
of the input signals. These changes can occur as a result of the simplest case the modulation is of the on-off type with the
change in the distance between the radar and the target, or off period coinciding with the maximum radar echo signal
from fading of the signal. The operation of AGC is based on a (Fig. G1). SAL
change in the gain coefficient of the receiver depending on
Ref.: Neri (1991), p. 380.
the value of the output signals under the action of a regulating
Manual gain control (MGC) is the mode in which the gain
voltage which is generated by the AGC system as a result of
of a receiver is controlled by the radar operator to make the
amplitude detection of output signals. The voltage of strong
dynamic range of the received signals match the dynamic
signals of powerful interference (instantaneous gain adjust-
range of the display. SAL
ment) may be used as the regulating voltage. In this case, a
bias voltage, which moves the operating point to a region Ref.: Currie (1987), p. 489.