Page 323 - Radar Technology Encyclopedia

313 propagation in the troposphere pulse packet

quency-dependent character. (See ATTENUATION by clear
air.) IAM
Amplitude
Ref.:Bean (1968); Kravtsov (1983), pp. 83, 118, 185.
PULSE. A pulse is “a wave that departs from a first nominal
state, attains a second nominal state, and ultimately returns to
the first nominal state.” It is the basic waveform used in radar
t
applications, where the first and final state has zero amplitude
and the second state has a nominally constant amplitude. The t t r
ideal rectangular pulse is shown as a dashed line in Fig. P20, Figure P21 Pulse train.
with a typical real pulse as a solid line, characterized by a
leading edge with a rise time t , a trailing edge of duration t , a The pulse train is characterized by the pulse width t and
f
r
pulse droop from its initial value, and a ripple superimposed the pulse repetition interval (PRI) t , the latter being the recip-
r
on the top of the pulse. rocal of the pulse repetition rate (PRF), f . Since any discrete
r
approximation of a continuous time-domain function (sam-
Amplitude
Pulse pling of a single pulse or transmission of a pulse train) results
tr ripple t f Pulse droop in a periodic spectrum, the spectrum of a coherent pulse train
1 E 2 is periodic at f = 1/t , with an envelope determined by the
r
r
Leading E 1 spectrum of the individual pulse in the train (Fig. P22). The
edge width of the spectral lines, is the reciprocal of the duration of
Trailing
edge the train t and Fig. P22 is drawn for essentially infinite dura-
o
tion. PCH, SAL
Time, t
0 Ref.: IEEE (1993), pp. 1033, 1035; Skolnik (1980), p. 432; Brookner (1988),
p. 136.
Figure P20 Actual and ideal pulse shapes.
The pulse rise time is “the interval between the instants at
1
which the instantaneous value reaches specified lower and
t r
upper limits, namely, 10 and 90% of the peak pulse value
unless otherwise specified.” The pulse fall time t is defined
f
similarly. Pulse rise and falls times are important parameters
of pulse shape. A pulse with a fast rise time and a near-ideal
rectangular shape is necessary to realize the inherent range f
3 2 1 0 1 2 3
resolution of the pulse. A slow rise time makes accurate mea-
t t t t t t
surement of time-of-arrival difficult and also wastes transmit-
ter energy. Pulse rise time requirements also depend on the Figure P22 Spectrum of coherent pulse train.
type of transmitter tube used.
A coded pulse is a pulse employing intrapulse modulation
The slow decay of the top of the pulse from its initial
used for pulse compression or identification. (See WAVE-
value is called pulse droop, D:
FORM.) The corresponding device “for varying one or more
E – E 2 of the characteristics of a pulse or of a pulse train so as to
1
D = ------------------
E transmit information” is called a pulse coder, and the process-
1
There may also be oscillations on the top of the pulse, identi- ing device in the receiver is a pulse decoder. SAL
fied in Fig. P20 as pulse ripple. Ref.: IEEE (1993), p. 202.
The duration of the pulse is commonly called pulse A pulse-forming line (network) is “a passive electric net-
width, normally defined between the half-power points of the work in a radar modulator whose propagation delay deter-
leading and trailing edges. Pulse width is also called pulse mines the length of the modulation pulse.” SAL
length or pulse duration.
Ref.: IEEE (1990), p. 23.
Other than rectangular pulse shapes are possible, but
these normally are produced by pulse compression or other pulse integration (see INTEGRATION).
filtering in the receiver, rather than transmitted, because radar
pulse length (see PULSE).
transmitters operate most efficiently in the saturated mode,
producing constant output between the leading and trailing A pulse packet is “the volume of space occupied by a single
edges. The major pulse shapes and their corresponding spec- radar pulse. The dimensions of this volume are determined by
tra are shown in Table P5. the angular width of the beam, the duration of the pulse, and
A single pulse is seldom used in radar. Typically, a the distance from antenna.” In Russian literature, the term
sequence of pulses, termed a pulse train or pulse burst, is pulse packet refers to a train or burst of pulses. SAL
transmitted into each beam position (Fig. P21). Ref.: IEEE (1993), p. 1,031.