Page 296 - Radar Technology Encyclopedia

NETWORK noise, video noise leveling 286

NETWORK as the fluctuating sum of many small vectors changing ran-
domly in relative phase. Amplitude noise can include periodic
all-pass time-delay network (see DELAY LINE).
components and typically is divided into low-frequency and
array (antenna) feed network (see FEED). high-frequency amplitude noise, depending on its spectral
characteristics. Amplitude noise, which is the reason for scin-
beam-forming network (see FEED, beam-forming net- tillation error, is termed amplitude fluctuation. (See FLUC-
work). TUATION; RCS fluctuation.) SAL
digital beam-forming network (see FEED, beam-forming Ref.: IEEE (1993), p. 34; Skolnik (1990), p. 18.34.
network). Angle noise is “the noise-like variation in the apparent angle
of arrival of a signal received from a target, caused by a
An electrical delay network is one with dispersive charac-
change in phase and amplitude of target-scattering sources
teristics used in analog pulse compression technology. Typi-
and including angular components of both glint and scintilla-
cally, it is used as an electrical-network class of linear FM
tion error.” Random variation in observed angular location of
waveform generators and has linear delay-versus-frequency
the target results from a change with time in the apparent
characteristics. The most common representatives of delay
location of the target with respect to a reference point, which
network are all-pass network (a four terminal lattice network
is usually a center of gravity of the reflectivity distribution
with constant gain at all frequencies and a phase shift varying
along the target. In some cases this location can fall at a point
with the square of frequency to ensure the constant delay
completely outside the extremities of the target. Angle noise
slope; this the low-frequency device using lumped elements),
typically has Gaussian distribution and the errors caused by
a folded-type meander line, which is the microwave analog of
angular noise are inversely proportional to range. The angle-
all-pass network, and waveguides operating near its cutoff
noise phenomenon affects all types of radars but it is mainly
frequency. SAL
of concern for precision tracking radars operating at medium
Ref.: Skolnik (1990), p. 10.13.
and short range. The main measures for angle noise reduction
NOISE is unwanted electromagnetic energy with random are proper choice of servo bandwidth and AGC characteris-
parameters that interferes with the ability of radar to perform tics. (See also ERROR.) SAL
in an ideal, error-free mode. Typically, the noise is repre- Ref.: IEEE (1993), p. 39; Skolnik (1990), p. 18.37.
sented as a random process, and its presence at radar input has Antenna noise is the result of the noise received by antennas
the fundamental importance for estimation of radar perfor- from external sources (primarily, because of atmospheric
mance in any mode of operation: detection, measurement, radiation) and the internal noise generated inside antenna and
and discrimination. Classification of noise can be based on microwave feed structures (primarily generated in the ohmic
different factors: components: resistive conductors and imperfect insulators).
(1) From the standpoint of its origin, noise is classified as Typically, it is described by the antenna noise temperature.
target noise (caused by radar target, its shape and motion (See TEMPERATURE.) SAL
peculiarities), external (environmental) noise (caused by the
Ref.: Skolnik (1990), p. 2.28.
environment in which radar signal propagates) or internal
Atmospheric noise [atmospherics] is a noise component
noise (caused by radar hardware, primarily by the antenna
produced by radiation from lightning strokes (not to be con-
and receiver).
fused with atmospheric absorption noise). The spectrum of
(2) From the standpoint of its spectral characteristics, as
atmospheric noise falls off rapidly with increasing frequency,
white or colored.
and for frequencies above 50 MHz has no significant effect
(3) From the standpoint of its distribution as Gaussian or
on radar. (See also ATMOSPHERICS.) SAL
non-Gaussian (often Rayleigh).
The main characteristics of noise are noise power and power Ref.: Skolnik (1980), p. 463.
density. When noise is represented as a random process it Atmospheric absorption noise is produced by the atmo-
may be described with the parameters typical of this kind of sphere by the same phenomena that result in attenuation of
random function representation: mathematical expectation, the electromagnetic waves. The resulting noise has at the
variance and correlation function. (See FUNCTION, ran- radar input a spectral density DN = kT (1 - 1/L ), where T a
0
a
a1
dom.) Pure noise typically has zero mathematical expectation is the ambient temperature of the atmospheric gases contrib-
and a delta correlation function (uncorrelated). SAL uting the loss (T » 290K) and L is the one-way loss
a
a1
Ref.: Bendat (1958). through the atmosphere. SAL
Ambient noise (see environmental noise). Ref.: Skolnik (1980), p. 461.
Automatic video noise leveling is a “CFAR technique in
Amplitude noise is the fluctuation of echo signal amplitude
which the receiver gain is readjusted to maintain a constant
caused by the complex shape of the target and its changing
video noise level.” In this case the noise level is sampled at
aspect angle, excluding the effects of changing target range. It
the receiver output at the end of each range sweep prior to the
is the result of echo signal amplitude modulation by interfer-
next transmission and the resulting gain is fixed throughout
ence between scatterers on the target, and can be represented