Page 84 - Radar Technology Encyclopedia

74 CEPSTRUM channel, radar

The CEPSTRUM is a function used in nonlinear signal pro- The overall RCS of dipoles randomly oriented after dispersal
cessing. The term is a “half-inverse” of the word spectrum. is about
2
The most common forms are the cepstrum power and the s = 0.18l N
=
complex cepstrum, related to the Fourier transform F of the where l wavelength and N is the total numbers of dipoles.
signal by the relations SAL
Ref.: Nathanson (1969), p. 223.
2
K t() = Fln F s t ()[{ ] } 2
p
Delayed opening chaff is chaff that blooms at a specific
=
K t() Fln F s t (){[{ }]}
elapsed time after it is dispensed.
Ref.: Johnston (1979), p. 58.
respectively. The argument of the cepstrum has the units of
time. jammer-illuminated chaff (see JAFF).
A feature of a cepstrum analyzer is its ability to perform
Rope chaff is a type of chaff consisting of one or more rope
the deconvolution of signals (i.e., the operation which is the
elements, which are long rolls of metallic foil or wire
inverse of the convolution of two functions and makes it pos-
designed for broadband, low-frequency response. SAL
sible to extract the convolved signals).
Ref.: Johnston (1979), pp. 58, 66; Neri (1991), p. 396.
Cepstral processing is performed using the fast Fourier
transform (FFT) in digital computers, charge-coupled CHANNEL
devices, or Fourier processors based on surface acoustic wave
A difference channel is “a part of monopulse receiver dedi-
techniques.
cated to the amplification, filtering, and other processing of a
The cepstrum power is used to detect and classify sig-
difference signal, which is generated by comparison of sig-
nals, to determine pulse duration, and to measure the pulse
nals received by two (or two sets of) antenna beams, and indi-
repetition frequency. The complex cepstrum, on the other
cating the departure of the target from the boresight axis. Also
hand, makes it possible to reconstruct signals distorted by
a signal path through a monopulse receiver for processing a
multipath propagation and to process radar images. IAM
difference signal that is commonly generated by comparison
Ref.: Childers, D. G., Skinner, D. P., and Merait, R. C., “The Cepstrum: A
of two or more signals received by two antenna beams.” (See
Guide to Processing,” Proc. IEEE 65, no. 10, 1977, pp. 1428–1443.
MONOPULSE.) SAL
CHAFF is “strips of lightweight metal or metallic material Ref.: IEEE (1993), p. 343.
that are dispersed in large numbers (bundles) into surveillance
A guard channel is “one or more auxiliary processing chan-
or observation volume of a radar to reflect impinging signals
nels to control the main processing channel in order to reject
and simulate a true target.” It typically consists of strips of
interference that is partly in, but not centered on, the main
aluminum foil or metal-coated fibers. Each bundle of chaff
channel. Guard channels may be displaced in time (range),
may contain thousands of individual reflectors whose lengths
Doppler frequency, carrier frequency, or angle. Sometimes
are related to the wavelength of the radar. Chaff for use at HF
called 'guard gates,' 'guard bands,' or 'sidelobe blanking.'
and VHF bands is called rope (or rope chaff).
Used against range gate stealers, velocity gate stealers, chaff,
As a passive electronic countermeasures technique, chaff
sidelobe jamming, and to enhance apparent angle resolution
may be used in three distinct ways:
in IFF. May use auxiliary displays.” SAL
(1) A localized chaff puff or burst can simulate a true tar-
Ref.: IEEE (1993), p. 576.
get, serving as a decoy for radars not having doppler
clutter rejection capability, and contributing to over- An image channel is the path of image-frequency signals
load in the data leaving the radar. (those displaced by the intermediate frequency on the oppo-
(2) A chaff corridor can be laid behind the lead aircraft in site side of the local oscillator signal from the intended
a raid, or fired in rockets forward from the lead air- response) into the mixer of the superheterodyne receiver. IAM
craft, masking aircraft within the corridor. Ref.: Van Voorhis (1948), p. 17; Popov (1980).
(3) A chaff cloud may be dispensed over a large region, The in-phase channel is the channel with its signal in phase
masking the actual attack corridor of a subsequent with the coherent (reference) oscillator, in a quadrature-chan-
raid. SAL, DKB nel processor. IAM
Ref.: IEEE (1993), p. 173; Schleher (1986), p. 133; Barton (1988), p. 194; Ref.: Skolnik (1980), p. 119.
Nathanson (1990), p. 295.
The quadrature channel is the channel with 90° phase rela-
Chaff dipoles are chaff elements, typically lightweight metal
tively to the coherent (reference) oscillator in a quadrature-
(e.g., aluminum) or aluminum-plated glass or Mylar dipoles,
channel processor. SAL
which are dispensed in the atmosphere to reflect radar energy
Ref.: Skolnik (1980), p. 119
imitating the real target with large RCS. The chaff dipoles are
intended to resonate at the frequencies of victim radars, A radar channel is a channel in which the generation, trans-
requiring that the length of the dipole be about one-half radar mission, reception, and processing of radar information is
wavelength. Often the dipoles in a package are cut to different performed. The radar channel includes the target, the propa-
lengths to cover an entire radar band or several radar bands. gation path, and the radar itself. The radar data is formed