RANGE/BEARING/ALTITUDE
Radar systems use the reference coordinates to
provide early detection of surface and air targets,
resulting in extremely accurate information on di-
stance, direction, height, and speed of the targets. The
visual radar data required to determine a target’s posi-
tion and to track the target is usually displayed on a
specially designed cathode-ray tube (CRT) installed
in a unit known as a plan position indicator (PPI).
Some types of radar are also used to guide mis-
siles to targets and to direct the firing of gun systems;
other types of radar provide long-distance surveillance
and navigation information.
Range and bearing (and in the case of aircraft, alti-
tude) are necessary to determine target movement. It
is very important that you understand the limitations
of your radar system in the areas of range, bearing,
and altitude.
Range
The radar measurement of range (or distance) is
possible due to the properties of radiated electromag-
netic energy. This energy normally travels through
space in a straight line, at a constant speed, and varies
only slightly due to atmospheric and weather condi-
tions.
MINIMUM RANGE.— Radar duplexers alter-
nately 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. Therefore, any
reflected pulses from close targets that return before
the receiver is connected to the antenna will be un-
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, and receiver sensitivity pulse-
repetition rate (PRR).
The peak power of the pulse determines what
maximum range the pulse can travel to a target and
still return a usable echo. A usable echo is the
smallest signal detectable by a receiver that can be
processed and presented on an indicator.
The PRR will determine the frequency that the
indicator is reset to the zero range. With the leading
edge of each transmitted pulse, the indicator time base
used to measure the returned echoes is reset, and a
new sweep appears on the screen. If the transmitted
pulse is shorter than the time required for an echo to
return, that target will be indicated at a false range in
a different sweep. For example, the interval between
pulses is 610 µsec with a repetition rate of 1,640
pulses per second. Within this time, the radar pulse
can go out and come back a distance equal to 610
µsec x 164 yards per µsec, or 100,000 yards, which
becomes the scope’s sweep limit. Echoes from targets
beyond this distance appear at a false range. Whether
an echo is a true target or a false target can be deter-
mined by simply changing the PRR.
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. 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 antenna
height, the antenna beamwidth, and the antenna rota-
tion rate. A higher antenna will create a longer radar
horizon, which allows a greater range of detection.
Likewise, a more-concentrated beam has a greater
range capability, since it provides higher energy
density per unit area. Also, because the energy beam
strikes each target more times, a slower antenna rota-
tion provides stronger echo returns and a greater de-
tection range for the radar.
1-3
