determining target size, strength, and location (fig.
2-7).
RADAR PULSE CHARACTERISTICS
Radar pulses travel at the speed of light (186,000
miles per second). Thus, the distance to a target can
easily be calculated by monitoring a pulse’s elapsed
time from transmission until its return. Half the
distance traveled by the pulse determines the target’s
range from the antenna.
Pulse Length
Pulse length (or pulse duration) is the
measurement taken from the leading to trailing edge of
a pulse and is a good indicator of the amount of power
contained within the pulse (fig. 2-7). Generally, longer
pulses emitted from a radar return more power, thus
increased target information and data reliability.
Longer pulses have the disadvantage in that fine details
within the return echo may be lost. Pulse length is
usually expressed in microseconds, but is also
measured in kilometers. The WSR-88D incorporates a
variable pulse length that may be as short as 1.57
microseconds (1,545 feet). Important aspects ofaradar
pulse include minimum range, range resolution, and
pulse repetition frequency.
MINIMUM RANGE.—Pulse length determines
a radar’s minimum range or how close a target can get
to the antenna without adversely affecting operations.
Minimum radar range is defined as any distance
greater than one-half the pulse length. In other words,
targets more than one-half pulse length from the
antenna can be correctly processed, while approaching
targets that get too close pose serious problems. If
targets come within one-half pulse length or less of the
antenna, the pulse’s leading edge will strike the target
and return before the radar can switch into its receive
mode. Some portion of the return energy is lost and the
radar may become confused and discard the pulse.
RANGE RESOLUTION.—A radar’s resolution
is its ability to display multiple targets clearly and
separately. Range resolution refers to targets oriented
along the beam axis as viewed from the antenna’s
position. Longer pulses have poorer range resolution.
Targets too close together lose definition and become
blurred. They must be more than one-half pulse length
apart or they will occupy the pulse simultaneously and
appear as a single target. The problem of range
resolution will be discussed in more detail later.
PULSE REPETITION FREQUENCY
(PRF).—PRF is the rate at which pulses are
transmitted (per second). It controls a radar’s
maximum effective range by dictating the duration of
its listening time. Increased PRF speeds the rate at
which targets are repeatedly radiated. This increased
sampling results in greater target detail, but the
maximum range of the radar is reduced because of the
shorter periods between pulses. The WSR-88D can
emit anywhere from 318 to 1304 pulses per second. It
has a maximum range of approximately 250 nautical
miles (nmi).
Listening Time
Following the transmission of each pulse, the radar
switches to receive mode awaiting its return. This
break in transmission is appropriately called "listening
time."
When pulses do not return during their
prescribed listening time, the radar assumes no targets
were encountered and that the pulse has continued on
its outward direction.
Listening time determines a radar’s maximum
effective range as it, in effect, limits the distance a pulse
can travel. If listening time is reduced, pulses can cover
less distance and effective range is decreased. Thus, a
50-percent reduction in listening time cuts maximum
radar range in half. Only targets within the maximum
effective range are detectable.
Figure 2-7.—Radar Pulses
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