Pulse Modulation
With the pulse modulation method, depending on
the type of radar, energy is transmitted in pulses that vary
from less than 1 microsecond to 200 microseconds. The
time interval between transmission and reception is
computed and converted into a visual indication of range
in miles or yards.
Pulse radar systems can also be
modified to use the Doppler effect to detect a moving
object. The Navy uses pulse modulation radars to a
great extent.
FACTORS AFFECTING RADAR
PERFORMANCE
Radar accuracy is a measure of the ability of a radar
system to determine the correct range, bearing, and in
some cases, altitude of an object. The degree of
accuracy is primarily determined by the resolution of the
radar system and atmospheric conditions.
Range Resolution
Range resolution is the ability of a radar to resolve
between two targets on the same bearing, but at slightly
different ranges.
The degree of range resolution
depends on the width of the transmitted pulse, the types
and sizes of targets, and the efficiency of the receiver
and indicator.
Bearing Resolution
Bearing, or azimuth, resolution is the ability of a
radar system to separate objects at the same range but at
slightly different bearings. The degree of bearing
resolution depends on radar beamwidth and the range of
the targets. The physical size and shape of the antenna
determines beamwidth. Two targets at the same range
must be separated by at least one beamwidth to be
distinguished as two objects.
Earlier in this chapter, we talked about other internal
characteristics of radar equipment that affect range
performance. But there are also external factors that
effect radar performance. Some of those are the skill of
the operator; size, composition, angle, and altitude of the
target; possible electronic-countermeasure (ECM)
activity; readiness of equipment (completed PMS
requirements); and weather conditions
Atmospheric Conditions
Several conditions within the atmosphere can have
an adverse effect on radar performance. A few of these
are temperature inversion, moisture lapse, water
droplets, and dust particles.
Either temperature inversion or moisture lapse,
alone or in combination, can cause a huge change in the
refraction index of the lowest few-hundred feet of
atmosphere. The result is a greater bending of the radar
waves passing through the abnormal condition. The
increased bending in such a situation is referred to as
DUCTING, and may greatly affect radar performance.
The radar horizon may be extended or reduced,
depending on the direction in which the radar waves are
bent. The effect of ducting is illustrated in figure 1-3.
Water droplets and dust particles diffuse radar
energy through absorption, reflection, and scattering.
This leaves less energy to strike the target so the return
echo is smaller. The overall effect is a reduction in
usable range. Usable range varies widely with weather
conditions.
The higher the frequency of the radar
system, the more it is affected by weather conditions
such as rain or clouds.
All radar systems perform the same basic functions
of detection, so, logically, they all have the same basic
equipment requirements. Next, we will talk about that
basic radar system.
BASIC RADAR SYSTEMS
Radar systems, like other complex electronics
systems, are composed of several major subsystems and
many individual circuits.
Although modern radar
systems are quite complicated, you can easily
understand their operation by using a basic block
diagram of a pulsed radar system.
FUNDAMENTAL RADAR SYSTEM
Since most radars used today are some variation of
the pulse radar system, the units we discuss in this
section will be those used in a pulse radar. All other
Figure 1-3.—Ducting effect on the radar wave.
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