We stated earlier that radar is used to determine the
distance and direction to and the height of distant
objects. These three pieces of information are known,
respectively, by the standard terms range, bearing, and
altitude. The use of these standard terms allows anyone
interested in a specific target to establish its position
quickly and accurately. Radar operators determine a
targets range, bearing, and altitude by interpreting its
position displayed on a specially designed cathode-ray
tube (CRT) installed in a unit known as a plan position
While most radars are used to detect targets, some
types are used to guide missiles to targets and to direct
the firing of gun systems; other types provide
long-distance surveillance and navigation information.
Range and bearing (and in the case of aircraft,
altitude) are necessary to determine target movement.
To be a successful radar operator, you must understand
the capabilities and limitations of your radar system in
determining range, bearing, and altitude.
The radar measurement of range (or distance) is
p o s s i b l e d u e t o t h e p r o p e r t i e s o f r a d i a t e d
electromagnetic energy. This energy normally travels
through space in a straight line, at a constant speed, and
varies only slightly due to atmospheric and weather
The frequency of the radiated energy
causes the radar system to have both a minimum
effective range and a maximum effective range.
M I N I M U M R A N G E . R a d a r d u p l e x e r s
alternately switch the antenna between the transmitter
and the receiver so that one antenna can be used for
both functions. The timing of this switching is critical
to the operation of the radar and directly affects the
minimum range of the radar system. A reflected pulse
will not be received during the transmit pulse and
subsequent receiver recovery time.
range of a radar, therefore, is the minimum distance
between the radars antenna and a target at which a
radar pulse can be transmitted, reflected from the
target, and received by the radar receiver.
antenna is closer to the target than the radars minimum
range, any pulse reflected from the target will return
before the receiver is connected to the antenna and will
not be detected.
MAXIMUM RANGE.The maximum range of
a pulse-radar system depends on carrier frequency;
peak power of the transmitted pulse; pulse-repetition
frequency (PRF) or pulse-repetition rate (PRR) (PRF
and PRR are synonymous terms.); and receiver
sensitivity, with PRF/PRR as the primary limiting
The peak power of a pulse determines how far the
pulse can travel to a target and still return a usable echo.
A usable echo is the weakest signal that a receiver can
detect, process, and present on a display.
The PRR determines the rate at which the range
indicator is reset to zero. As the leading edge of each
pulse is transmitted, the indicator time base used to
measure the returned echo is reset, and a new sweep
appears on the screen.
RANGE ACCURACY.The shape and width of
the radio-frequency (RF) pulse influences minimum
range, range accuracy, and maximum range. The ideal
pulse shape is a square wave that has vertical leading
and trailing edges.
The vertical edge provides a
definite point from which to measure elapsed time on
the indicator time base.
A sloping trailing edge
lengthens the pulsewidth.
A sloping leading edge
provides no definite point from which to measure
elapsed time on the indicator time base.
Other factors affecting range are the antennas
height, beamwidth, and rotation rate. A higher antenna
will create a longer radar horizon, allowing a greater
range of detection.
An antenna with a narrow
beamwidth, provides a greater range capability, since it
provides more concentrated beam with a higher energy
density per unit area. A slower antenna rotation rate,
providing more transmitted pulses during the sweep,
allows the energy beam to strike each target more
times, providing stronger echo returns and a greater
From the range information, the operator knows
the distance to an object.
He now needs bearing
information to determine where the target is in
reference to the ship.
Radar bearing is determined by the echos signal
strength as the radiated energy lobe moves past the
t a r g e t .
S i n c e s e a r c h r a d a r a n t e n n a s m o v e
continuously, the point of maximum echo return is
determined either by the detection circuitry as the
beam passes the target or visually by the operator.
Weapons control and guidance radar antennas are
positioned to the point of maximum signal return and