between the horizontal plane and the LOS is the
ELEVATION ANGLE. The angle measured
clockwise from true north in the horizontal plane is
called the TRUE BEARING or AZIMUTH angle.
Information based on these terms describes the location
of an object with respect to the antenna, giving the
operator data on range, bearing, and altitude.
Using the coordinate system discussed above, radar
systems provide early detection of surface or air objects,
giving extremely accurate information on distance,
direction, height, and speed of the objects. The visual
radar data required to determine a targets position and
to track the target is usually displayed on a specially
designed cathode-ray tube (crt) installed in a unit known
as a planned position indicator (ppi).
Radar is also used to guide missiles to targets and to
direct the firing of gun systems. Other types of radar
provide long-distance surveillance and navigation
Bearing and range (and in the case of aircraft,
altitude) are necessary to determine target movement.
It is very important that you understand the limitations
of your radar system in the areas of range, hewing, and
Radar measurement of range (or distance) is made
possible because of the properties of radiated
electromagnetic energy. This energy normally travels
through space in a straight line, at a constant speed, and
will vary only slightly because of atmospheric and
weather conditions. The range to an object, in nautical
miles, can be determined by measuring the elapsed time
(in microseconds) during the round trip of a radar pulse
and dividing this quantity by the number of
microseconds required for a radar pulse to travel 2
nautical miles (12.36). In equation form this is:
range (nautical miles) =
MINIMUM RANGE. Radar duplexers
alternately switch the antenna between the transmitter
and 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 undetected.
MAXIMUM RANGE. The maximum range of a
pulse radar system depends upon carrier frequency peak
power of the transmitted pulse, pulse repetition
frequency (prf), or pulse repetition rate (prr), and
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
sec with a repetition rate of 1640 pulses per second.
Within this time the radar pulse can go out and come
back a distance equal to 610 sec 164 yards per sec, or
100,000 yards, which becomes the scopes 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 determined by simply changing the prr.
RANGE ACCURACY. The shape and width of
the 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 pulse width. A
sloping leading edge provides no definite point from
which to measure elapsed time on the indicator time
Other factors affecting range are the antenna height,
antenna beam width, and antenna rotation 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 would strike each target more
times, a slower antenna rotation provides stronger echo
returns and a greater detection range for the radar.
Given the range information, the operator knows the
distance to an object, but information on bearing is still
required to determine in which direction from the ship
the target lies.