Page 75 - Radar Technology Encyclopedia

BLINKING bridge, circular 65

appears and disappears, indicating, for example, that some The Bragg effect is the phenomenon of the interference
subsystem or the station is malfunctioning. SAL amplification of the x-rays reflected by the various atomic
Ref.: Johnston (1979), p. 56. planes of a crystal. It is observed when fulfilling the Bragg-
Wolf, condition which links the distance between the planes
A BLIP is “a deflection or a spot of contrasting luminescence
d, the wave length l, and the grazing angle, q:
on a radar display caused by the presence of a target.”
2dsinq = ml, m = 0, ± 1, ± 2...
Ref.: IEEE (1993), p. 116.
The Bragg effect explains the diffraction of x-rays on
The blip-scan ratio is the “fraction of scans for which a blip
crystals as well as the diffusion of light on inhomogeneities of
is observed at a given range.” (See also DETECTION prob-
the dielectric constant. In the latter case, the Bragg effect is
ability).
used when describing the modulation of light in acousto-opti-
Ref.: IEEE (1993), p. 116.
cal modulators (see Bragg cell).
BLOCKING is “the process of obscuring guidance signals By analogy to the x-ray phenomenon, the Bragg effect is
by active jamming.” used to describe the scattering of radar waves from the pattern
Ref.: Johnston (1979), p. 56. of regularly spaced waves on water (see CLUTTER, sea).
IAM
BLOOMING is “an increase in the blip size on the display as
a result of an increase in signal intensity or duration.” Ref.: Sivukhin (1985), p. 390, 610; Skolnik (1980), p. 480.
Bragg scattering in its original sense describes the propaga-
Ref.: IEEE (1993), p. 119.
tion of an optical beam when the incident beam is inclined to
BOLTZMANN’S CONSTANT is a universal physical con-
the normal at the Bragg angle. The Bragg scattering effect is
stant of proportionality that defines the Kelvin temperature
used in the Bragg-cell receiver. Bragg scattering also provides
scale and relates the energy of thermally generated noise to
the mechanism by which sea clutter is returned to over-the-
absolute temperature. Boltzmann’s constant k, named for its
horizon radars. (See also Bragg effect.) SAL
discoverer, Ludwig Boltzmann (1844–1906), is equal to Ref.: Long (1975), p. 84; Neri (1991), p. 297.
1.38 ´10 - 23 J/K, or W/Hz/K. PCH
Ref.: Van Nostrand (1983), pp. 401, 402; Jordan (1985), p. 3.16. BRIDGE, microwave. A microwave bridge is a directional
coupler in which the output power is equal in the two output
BORESIGHTING is “the process of aligning the electrical
branches, and there is a constant phase difference between the
and mechanical axes of a directional antenna system.”
voltages in them over the operating band.
Ref.: IEEE (1993), p. 123. A microwave bridge is characterized by its standing-
BOXCAR (see CIRCUIT, sample and hold). wave ratio, the coefficient of decoupling between the
branches, and the attenuation (see COUPLER, directional).
BRAGG
A microwave bridge may be constructed from different types
A Bragg cell is the basic element of the acoustic-optical mod- of transmission line (e.g., waveguide, coaxial, or microstrip).
ulator. It is an optically transparent crystal of lithium niobate Most widely used in radar are waveguide bridges of the slot-
or tellurium dioxide, on one side of which a piezoelectric con- ted, circular, or T-types. These are used in microwave amplifi-
verter is mounted. When the radio signals being analyzed ers and modulators, balanced mixers, monopulse radar sum-
arrive at the converter, traveling acoustic waves are excited. and-difference networks, and antenna commutators. IAM
These waves modulate the refraction coefficient of the crystal Ref.: Sazonov (1988), P. 96; Veselov (1988), p. 57.
and for a traveling diffraction grating. The Bragg cell is illu-
A balanced bridge is “a network with a minimum of two
minated by a parallel coherent laser beam that is incidental to
ports or terminal pairs capable of being operated in such a
the Bragg angle in relation to the direction of the propagation
manner that when power is fed into one port, by suitable
of the acoustic wave. As a result of the acoustic-optical inter-
adjustment of elements connected to one or more other ports,
action in the cell the laser beam is deflected, whereby the
zero output can be obtained at another port.” SAL
magnitude of the angle of deflection is directly proportional
IEEE (1993), p. 129.
to the frequency of the radio signal being analyzed. (See
Bragg effect.) A circular bridge consists of a ring that is 1.5 wavelengths in
Thanks to the broad momentary frequency band (all the circumference, and four branches located a quarter-wave-
way up to an octave in the 50-MHz to 2-GHz band) and the length from one another (Fig. B6). A circular bridge may be
sufficiently high-resolution capability in the acoustic-optical constructed from any type of transmission line.
receiver, several simultaneously existing analog and sampled A circular bridge is characterized by the decoupling of
signals of various types can be processed. Therefore, the the inputs 1 and 2. When a signal is presented at branches 1
Bragg cell is widely used in the construction of the intercept and 2, the sum of these signals is obtained at branch 4, and the
receivers used in electronic warfare means. IAM sum of the first signal and the second signal, shifted by 180°
relative to the first, results at branch 3. This property is used
Ref.: Zmuda (1994), Ch. 7.
to create balanced mixers, microwave amplifiers and modula-
tors, and multichannel phased-array radar receivers. The band