Page 88 - Radar Technology Encyclopedia

78 circulator, ferrite CLUTTER

Ferrite circulators are distinguished by the orientation of CLUTTER. Radar clutter is defined as “unwanted echoes,
the magnetization. In wave-rotation circulators the ferrite is typically from the ground, sea, rain or other precipitation,
magnetized along the direction of propagation, while in chaff, birds, insects, or aurora.” There is no single definition
phase-shift circulators the magnetization is perpendicular to because one user’s target is another’s clutter: to the radar
that direction. This determines the various requirements for meteorologist, precipitation is the target and aircraft are
the degree of magnetization and heat transfer. IAM clutter.
Ref.: Skolnik (1970), pp. 8.20–8.24; Lavrov (1974), p. 355; Popov (1980), The effects of clutter on target detection are to mask
p. 455. some targets as they pass through regions occupied by clutter
A phase-shift circulator is based on the creation of a nonre- echoes and generate false alarms that may distract attention
ciprocal phase shift. This occurs in a rectangular waveguide and draw resources from real targets. The effects on tracking
with a perpendicularly magnetized plate, in which waves and measurement are to generate false tracks and increase
propagating in opposite directions have different phase veloc- error in data on real targets. To reduce these effects, radars
ities. The common element in various designs of such a circu- rely on reducing the size of the spatial resolution cell to mini-
lator is the presence of nonreciprocal phase shifters in mize clutter input, providing velocity (doppler frequency)
rectangular waveguides that share a common wall. IAM resolution to filter clutter from the signal processor output
and applying constant-false-alarm-rate (CFAR) techniques to
Ref.: Skolnik (1970), p. 8.23; Popov (1980), p. 453.
the detection circuitry. In some cases the proper choice of fre-
A wave-rotation circulator operates by rotating the polariza-
quency or polarization can minimize clutter input to the radar.
tion plane of a wave in a waveguide with a magnetized ferrite
Even if all practical measures are taken to reject clutter, it
rod. A wave entering at one of the branches is rotated as it
is usually necessary to allow the CFAR circuits to raise the
passes the ferrite, which is magnetized along the direction of
detection threshold in regions of heavy clutter, compromising
propagation, resulting in the required transmission and block-
target detection.
ing at the other branches in the circulator. When the direction
Clutter sources are of three types: (1) discrete (or point)
of magnetization is reversed, the sequence of transfers within
clutter, described by specific locations and radar cross sec-
the circulator is also reversed. A wave-rotation circulator is
tions; (2) surface clutter, described by a dimensionless reflec-
distinguished by requiring a relatively low magnetization 0 2 2
tivity s (m of RCS per m area of surface); and (3) volume
level (on the order of several tens or hundreds of oersteds for 2 3
clutter, described by a reflectivity h (m of clutter per m of
v
circulators in the 3-cm waveband). Among its drawbacks are
volume). Other descriptive parameters include the probability
the difficulties associated with transferring heat from the
density function (pdf) of echo amplitude or power (see clut-
ferrite. IAM
ter (amplitude) distribution), the spectral distribution or
Ref.: Skolnik (1970), p. 8.21; Lavrov (1974), p. 357. temporal correlation function (see clutter spectrum), the
A Y-junction circulator is constructed with three rectangular spatial correlation function, the polarization and frequency
waveguides joined at 120° angle relative to one another, with dependence of reflectivity, the spatial distribution of clutter
a magnetized ferrite rod at the joint. With a certain magnetiza- sources, and dependence on weather and diurnal or seasonal
tion applied to the ferrite, it will reradiate a secondary wave factors.
with which the input wave from one of the branches is in The clutter power entering the radar depends not only on
phase at a second branch, and out of phase at the third branch. the RCS, s , of the clutter within the spatial resolution cell
c
If the magnetization is reversed, the sequence of branches is but also on the pattern-propagation factor, F , applicable to
c
reversed. the path between radar and clutter. This factor describes the
Y-junction circulators may also be constructed from transmission properties of the radar-clutter path and the
coaxial and strip lines. They are distinguished by their sim- antenna gains along that path. Most measurements and tabu-
plicity and low size and weight. The bandwidth for a lated values of surface-clutter RCS are actually values of
4
waveguide Y-junction circulator may reach 30%, and an s F , dependent on the measuring radar antenna and its loca-
c c
octave for a strip line device. IAM tion as well as on the stated parameters of frequency, polar-
Ref.: Lavrov (1974), p. 358; Gardiol (1984), p. 266. ization, grazing angle, and type of clutter. Especially at low
4
grazing angles, the propagation term in F may be the domi-
c
CLASSIFICATION OF TARGETS (see TARGET REC-
nant source of frequency dependence and broadened pdf.
OGNITION AND IDENTIFICATION.) DKB
CLIPPING refers to circuit operation in which the maximum Ref.: IEEE (1990), p. 8; Barton (1964), pp. 95–108, (1988), pp. 123–139,
(1991), pp. 5.14–5.21; Nathanson (1969), pp. 192–275; Skolnik (1980),
amplitude of a signal is limited to a predetermined value. In
pp. 470–512, (1990), pp. 12.1–13.40; Blake (1980), pp. 293–330; Long
digital signal processing, it causes quantization error and
(1983); Schleher (1991), pp. 19–54, 171–277; Currie (1992); Morchin
occurs when a quantized signal exceeds the saturation level of (1993), pp. 55–85.
the A/D converter or register. Clipping results in a broadening
Atmospheric clutter is defined as echoes from natural
of the signal spectrum and a narrowing of its correlation
function. SAL sources within some volume of the atmosphere, principally in
the lower troposphere, and is a form of volume clutter. The
Ref.: Nathanson (1990), p. 556.