Page 101 - Radar Technology Encyclopedia

compression ratio CFAR, cell-averaging (CACFAR) 91

collapsing loss (see LOSS, collapsing). The connector usually does not allow turning of the con-
nected sections of the circuit. For these purposes, and also for
COMPRESSION (see PULSE compression).
connection of transmission lines of other types, connections
Compression ratio, in a radar that uses pulse-compression of circuits in the form of joints are used. IAM
techniques, is the ratio of the uncompressed-pulse width to Ref.: IEEE (1993), p. 244; Sazonov (1988), p. 53.
the compressed-pulse width (see also PULSE compression).
CONOPULSE is a hybrid angle-tracking system combining
PCH
monopulse and conical scan techniques. Two squinted beams
CONDUCTANCE is “that physical property of the element, are rotated or nutated in space in a conical scan manner. The
device, branch, network, or system that is the factor by which received signals are processed either with monopulse pro-
the mean square voltage must be multiplied to give the corre- cessing followed by conical scan or vice versa. The advantage
sponding power lost by dissipation as heat or as other perma- claimed for the conopulse technique is that, like monopulse,
nent radiation or loss of electromagnetic energy from the amplitude fluctuations do not affect the angular accuracy,
circuit.” SAL while only two receivers are required instead of three used in
Ref.: IEEE (1993), p. 236. a conventional monopulse tracker. With modern solid-state
technology, it can be easier to realize the third receiver than to
Eigenconductance is the conductance of each input of a net-
arrange proper scanning of a pair of squinted beams. Some-
work determined when there is a short circuit of all the other
times conopulse is called scan with compensation. SAL
inputs in the form of the ratio of the current to the voltage
exciting the given input. Ref.: IEEE (1990), p. 9; Barton (1988), p. 420; Skolnik (1980), p. 165.
Eigenconductances are the diagonal elements of the CONSTANT FALSE-ALARM RATE (CFAR) is “a prop-
matrix of conductances. This matrix is most often used in the erty of threshold or gain control devices that maintain an
designs of multielement antennas for the calculation of the approximately constant rate of false target detections when
mutual influence of individual radiators on one another. SAL the noise, and/or clutter levels, and/or ECM (electronic coun-
Ref.: Sazonov (1988), p. 79. termeasures) into the detector are variable.” CFAR techniques
Negative conductance is a falling portion of the dynamic or are used in reception and signal processing to avoid increased
static voltage-current curve, in which the voltage and current false-alarm rates in the presence of jamming, clutter residue,
increments have opposite signs. Negative conductance can be or other interference sources. The two fundamental
used for generation of electromagnetic energy in such devices approaches are to adapt the detection threshold of a given test
as tunnel diodes, Gunn diodes, IMPATT diodes, and also tra- cell to the environment as determined by (1) statistics of ref-
ditional active devices such as electron tubes and transistors. erence cells surrounding the test cell (e.g., cell-averaging
IAM CFAR or nonparametric CFAR) or (2) time statistics of past
observations in the test cell itself (e.g., clutter mapping). In
Ref.: IEEE (1993), p. 839; Popov (1980), p. 273.
the first method, statistical parameters of the amplitudes in
CONFUSION AREA. The effective confusion area is the the reference cells (e.g., mean, standard deviation) may be
chaff cross section that equals the radar cross section of a par- used to set a threshold, or a nonparametric method (e.g., rank
ticular aircraft at a particular frequency. SAL order) may be used. The reference cells may be separated
Ref.: Johnston (1979), p. 58. from the test cell in time delay (range), doppler shift, angle, or
CONNECTOR, microwave. A microwave connector is “a some combination of these coordinates. Nonlinear processing
coupling device employed to connect conductors of one cir- of reference cells (e.g., clipping or editing the largest sam-
cuit or transmission elements with those of another circuit as ples) may precede the estimation of statistical parameters.
transmission elements,” usually based on pin and socket con- The use of CFAR circuits protects against excessive
tacts. The basic requirements for the connectors are retention numbers of false alarms, but at the cost of suppression of tar-
of matching and power-handling capability of the circuit with gets lying within regions of strong interference. DKB
minimum induced attenuation of power and the absence of Ref.: IEEE (1993), p. 247; Skolnik (1980), pp. 392–395, (1990), pp. 3.46–
parasitic radiation. In high-frequency joints for flexible coax- 3.54; Barton (1988), pp. 88–94; Nitzberg (1992), Ch. 9.
ial cables (see coaxial transmission lines), the contacts are amplitude-discrimination CFAR (see cell-averaging
produced using spring-clips and plugs held in the joint by CFAR).
external threaded connections or other locking devices.
Cell-averaging CFAR (CACFAR) is a technique in which
Matching in the connectors depends greatly on the connection
the threshold is controlled by the average of received ampli-
of the cable, and as a standing-wave ratio of 1.05 to 1.15
tudes in cells surrounding the test (target detection) cell, as a
when it is done correctly. Connectors for rigid coaxial lines
means of controlling the false-alarm rate. A typical method
are made without supporting dielectric washers using pin and
uses the integrated outputs of several range cells (Fig. C35),
jack-type contacts of internal and external conductors. In a
which are summed and multiplied by a CFAR constant K to
waveguide transmission line it is typically a mechanical
establish the detection threshold. In a doppler radar, the refer-
device (excluding an adapter) that electrically joins separable
ence cells in range may be replaced by filters adjacent in fre-
parts of a waveguide.