Continuous Wave
The continuous-wave (CW) method uses the Dop-
pler effect to detect the presence and speed of an
object moving toward or away from the radar. The
system is unable to determine the range of the object
or to differentiate between objects that lie in the same
direction and are traveling at the same speed. It is
usually used by fire-control systems to track fast-
moving targets at close range.
Frequency Modulation
In the frequency-modulation (FM) method, energy
is transmitted as RF waves that continuously vary,
increasing and decreasing, from a fixed-reference fre-
quency. Measuring the difference between the fre-
quency of the returned signal and the frequency of the
radiated signal gives an indication of range. This
system works well with stationary or slowly moving
targets, but it is not satisfactory for locating fast-
moving objects. It is used in aircraft altimeters that
give a continuous reading of how high the aircraft is
above the Earth.
Pulse Modulation
In the pulse-modulation method, depending on the
type of radar, energy is transmitted in pulses that vary
from less than 1µ to 200µ. 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 resolu-
tion depends on the width of the transmitted pulse, the
types and sizes of the targets, and the efficiency of the
receiver and the 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 an-
tenna determines beamwidth. Two targets at the same
range must be separated by at least one beamwidth to
be distinguished as two objects.
Some external factors that affect radar perfor-
mance are operator skill; size, composition, angle, and
altitude of the target; possible electronic attack (EA)
activity; readiness of equipment (completed planned
maintenance system requirements); and weather con-
ditions.
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 large change in
the refraction index of the lowest few-hundred feet of
the 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 perfor-
mance. 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.
1-5
