Page 163 - Radar Technology Encyclopedia

DUTY FACTOR [CYCLE, RATIO] eclipsing, deterministic 153

E
Transmitter Antenna
ECHO, radar. A radar echo is the portion of energy of the
transmitted pulse that is reflected to a receiver. SAL
Ref.: IEEE (1990), p. 15.
angel echo (see ANGEL).
T/R T/R
An echo box is “a high-Q resonant cavity that stores part of
the transmitted pulse power and feeds the resulting exponen-
tially decaying power into the receiver after completion of the
Receiver
pulse transmission.” As a result of their simplicity and reli-
Limiter
ability, echo boxes were widely used as built-in test equip-
ment during the 1940s and 1950s but have subsequently been
Figure D61 Gas-tube duplexer (after Skolnik, 1990, Fig. 4.2a, replaced with electronic test equipment. SAL
p. 4.4)
Ref.: IEEE (1990), p. 15.

average power P = P × t/t = P ×D , the duty factor is the An echo intensifier is a device typically mounted on the tar-
t
av
r
t
u
ratio of P to peak power P : D = P /P . SAL get and used to increase abnormally the amplitude of the
av
t
u
t
av
Ref.: IEEE (1993), p. 395. reflected signal. SAL
Ref.: Johnston (1979), p. 58.
DYNAMIC RANGE. Dynamic range is “the difference, in
decibels, between the overload level and the minimum echo reduction (see RADAR CROSS SECTION reduc-
acceptable signal level in a system or transducer.” In radar tion).
applications, this concept is typically applied to receiver
A false echo device is a device that produces an echo that dif-
dynamic range. SAL
fers from the one normally observed in some essential param-
Ref.: IEEE (1993), p. 397.
eter (in character, time, etc.). It is used to prevent accurate
Dynamic range compression is the reduction in dynamic target recognition. SAL
range of the input signal to match it with the dynamic range Ref.: Johnston (1979), p. 60.
of signals processors and filters, as their dynamic ranges are
ground echo (see CLUTTER, land).
typically less than that of the input. Theoretically, compres-
sion of the dynamic range of signal amplitudes is optimum at A mantle-shaped echo is an angel echo having a U- or V-
the output of a filter. However in practice it is advisable to shape and is associated with meteorological effects in the
perform compression at the input of the filter. This makes it atmosphere. SAL
possible to obtain greater compression, to avoid additional Ref.: Skolnik (1980), p. 511.
equipment errors when multichannel filters are used, and in a
meteorological echo (see CLUTTER).
number of other cases to normalize interference intensity
simultaneously. A multiple-[nth] time-around echo is one that arrives at the
Dynamic range compression is done by a nonlinear cir- radar with delay greater than the pulse repetition interval
cuit with a logarithmic amplitude characteristic or by one that (PRI), leading to a possible ambiguous range measurement
places a strict restriction of the amplitude of the input signal when a constant-PRI is used. (See AMBIGUITY, range.)
(e.g., a limiter). Compensation of the spread of levels of the When the echo arrives with a delay from one to two PRIs, it is
input signal due to interference occurs as a result of normal- called a second-time-around echo. SAL
ization of their intensity. One of the basic methods of normal- Ref.: Skolnik (1980), p. 3.
izing intensity of interference is to change the threshold level
A ring echo is an echo that starts as a point on radar display,
or control the gain of the processing channel with a fixed
and then forms a rapidly expanding ring. Typically, it is an
threshold value. (See GAIN, automatic gain control.) IAM
angel echo associated with birds flying away from roosting
Ref.: Barton (1964), p. 196; Sloka (1970), p. 125.
areas. SAL
Instantaneous dynamic range is the range between the Ref.: Skolnik (1980), p. 512.
noise level and the saturation level in a receiver when the gain
sea echo (see CLUTTER, sea).
of the receiver stages is constant. Typically, it is less than the
total receiver dynamic range because the gain of receiver ECLIPSING is “the loss of information on radar echoes dur-
stages may be varied. SAL ing intervals when the receiver is blanked because of the
Ref.: Currie (1987), p. 495. occurrence of a transmitter pulse. Numerous such blanking
can occur in radars having high-pulse-repetition frequencies.”
The eclipsing effect applies to pulsed radars (Fig. E1), as in
CW radars reception can be accomplished all the time and no