Page 207 - Radar Technology Encyclopedia
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197 filter, Wiener frequency agility
S = x + n , where n is the noise vector. The impulse metric line of sight. In the troposphere, these fluctuations are
response of the Wiener filter is derived from the solution of primarily caused by irregularities in the water-vapor content,
the Wiener-Hopf integral equation: especially in the vicinity of clouds but also in the clear air.
t The rms value of angle-of-arrival fluctuations in clear air is a
few tens of microradians, increasing to over a hundred micro-
ò ( t ) B t– t ' ) t= B ( t t )
'""
h t ' +
+
(
d
sx
e
x
e
radians on paths through clouds. These fluctuations are inde-
0
pendent of radar wavelength. Ionospheric fluctuations also
where t is extrapolation time. occur, and these vary as the square of wavelength. DKB
e
The impulse response of the Wiener filter is Ref.: Barton (1969), App. D, (1988), pp. 309–313.
æ h t() t0 antenna-scanning fluctuation (see MTI, limitations to per-
,
³
h = ç
w
è 0 t 0 formance).
<
,
The frequency response is Path-length fluctuations occur along atmospheric paths as a
˜ result of random variations in refractive index of the tropo-
Bsx f ()
=
(
Hf () ---------------exp j2pft ) sphere and ionosphere. Tropospheric fluctuations are inde-
e
˜
Bx f () pendent of radar wavelength and typically amount to a few
˜ ˜ millimeters when observed over periods of minutes, with
where Bsf f () and Bx f ()
are Fourier transforms of B (t) and
sx greater drifts over longer periods. Ionospheric fluctuations are
B (t). The Wiener filter is a filter-extrapolator. IAM
x
in the order of centimeters for 0.3m wavelength, varying with
Ref.: Farina (1985), vol. 1, p. 83; Korostelev (1987), p. 219.
the square of wavelength. DKB
The yttrium-iron-garnet (YIG) filter uses a monocrystal of Ref.: Barton (1969), App. D, (1988), p. 313.
iron-yttrium garnet, which possesses ferromagnetic properties
FOLLOWER
and low losses. They are used as dispersion and directional
filters (see also ferrite filter). A cathode follower is an electron tube amplifier with a load
Dispersion properties arise in the presence of an external in a cathode circuit, which repeats the shape and phase of the
nonhomogeneous magnetic field in electromagnetic, spin, input voltage. The cathode follower has a high input resis-
and acoustic waves having different types of propagation. tance, low input capacitance, low output resistance, and high
The required linearity of dispersion is obtained through ratio- stability. The gain coefficient of the cathode follower is less
nal distribution of intensity of the constant magnetic field than one.
along the length of the crystal, through control of propagation The cathode follower is used in pulsed amplifiers (video
of the acoustic wave using toroidal plates of aluminum- amplifiers) operating with a low-ohm active-capacitance
yttrium garnet, by changing the tilt of the edges of the crystal, load; and also for matching of a high-ohm source with a low-
and so forth. ohm load; for example, when a transmission line matched at
YIG filters are used for analog matched filtering of linear the terminal is connected. IAM
frequency-modulated waveforms with bandwidths of hun- Ref.: Terman (1955), p. 357; Popov (1980), p. 171; Fradkin (1969), p. 70.
dreds of megahertz, providing a compression factor in the
An emitter follower circuit is a transmitter amplifier having
hundreds. IAM
the common-collector configuration. The performance and
Ref.: Shirman (1974), p. 149.
usage is analogous to the cathode follower. IAM
FLIP-FLOP (see TRIGGER). Ref.: Zherebtsov (1989); Fradkin (1969), p. 184.
FLUCTUATION. The term fluctuation is applied to a unsys- FREQUENCY is the rate at which a periodic phenomenon is
tematic (typically random) deviation of any physical quantity repeated. In radar, one considers first the radio frequency of
from its nominal value. In radar it is applied to radar echoes to the transmitted carrier, which is the rate at which the sine
describe the changes in RCS of complex targets (RCS fluctu- wave representing the electromagnetic field is repeated. Other
ations) and effects of irregularities in atmospheric refractive frequencies used to describe radar operation are the interme-
index (angle-of-arrival fluctuations). Other types of fluctua- diate frequency, referring to the downconverted signal in a
tions can include instability of carrier frequency, antenna superheterodyne receiver; the video frequency components,
scanning, rotation of the polarization plane, and so forth. The which represent the baseband pulse; and the pulse repetition
main issue is the RCS fluctuation, usually described in terms frequency, which is the rate at which transmitter pulses are
of the Swerling target models (see RCS fluctuation). These repeated. DKB
lead to degradation of probability of detection and to errors in
Frequency agility refers to the capability of a radar to change
radar measurement. SAL
its transmitted carrier frequency on a pulse-to-pulse or
Ref.: Skolnik (1962), p. 50; Popov (1980), p. 457; Vasin (1977), pp. 84–88.
burst-to-burst basis, as opposed to frequency diversity, which
Angle-of-arrival fluctuations of radar signals are the result refers to a radar’s use of several complementary transmis-
of irregularities in the refractive index of the atmosphere, sions at different frequencies. Frequency agility and diversity
which cause the path of the echo wave to vary from the geo- are basically ECCM techniques that force a potential jammer