FICI

1
stel e

An Introduction to Radar

1.1 BASIC RADAR

Radar is an electromagnetic system for the detection and location of reflecting objects
such as aircraft., ships, spacecraft. vehicles. people. and the natural environment h oper-
ates by radiating energy into space and detecting the echo signal reflected from an object, or
target. The reflected energy that is returned to the radar not only indicates the presence of a
target, but by comparing the received echo signal with the signal that was rrii mit. ter]. its
location can be determined along with other target-related information. Radar can perform
its function at long or ShOrt distance s and under conditions impervious to opt-F-

eel and infrared sensmrs. It can operate in darkness, haze, fog. rain, and snow- (Ls ability to
measure distance with high accuracy and in all weather is one of its mom importarn
attributes.
The basic principle of radar 1R illustrated in Pig. 1, I. A transmitter tin the upper left
portion of the figure) generates an electromagnetic siknal isuch as a short pulse of
sinewavei that is radiated into space by an antenna. A portion of the ratrismined energy is
intercepted by the target and reradiated in many directions. The [eradiation directed
back towards the radar is collected by the radar antenna, which delivers at to a receiver.
There it is processed to detect the presence of the target and determine Its location. A sin-
gle antenna is usually used on a time-shared basis for both transmitting and receiving
when the radar waveform Is is repetitive series of pulses. The range.. or distance. in a tar-
get is found by =last/ring the time it Lakes for the radar signal to travel LID the target and
return back to the radar. t Radar engineers use the term re.r.n0 to mean &mince, which is

1
 Transmitted signal

Antenna

Transmitter
Target

Receiver Eats signal

Target detection paid

information extraction Range to target

Figure 1.1 B051C principle of mdpr.

not the definition of range band in some dictionaries.*) The target's location in angle can be
found from the direction the narrow-beamwidth radar antenna points when the received echo
signal is of maximum amplitude. If the target is in motion. there is a shift in the fre-
quency of the echo signal due to the doppler effect. This frequency shift is proportional to
the velocity of the target relative to the radar (also called the radial velocity), The
doppler frequency shift is widely used in radar as the basis For separating desired mov-
ing targets from ruled (unwanted) "clutter" echoes reflected from the natural environment
such as land, sea, or rain. RalfAr can also provide information about the nature of the tar-
get being observed.
The term radar is a contraction of the words radio detection and ranging. The name
reflects the importance placed by the early workers in this field on the need for a device to
detect the presence of a target and to measure its range. Although modenn radar can
extract more information from a target's echo signal than its range, the measurement of
range is still one of its most important functions. There are no competitive techniques that
can accurately measure long ranges in both clear and adverse weather as well as can radar.

Range to a Target The most common radar signal, or waveform, is a series of short-
duration. somewhat rectangular-shaped pulses modulating a sinewave carrier. (This is
sometimes called a pulse (min.) The range to a target is determined by the time TR it takes the
radar signal to travel to the target and back_ Electromagnetic energy in free space trav-
els with the speed of light, which is c = 3 X 108 ruts. Thus the time for the signal to travel to a
target located at a range R and return back to the radar Is 21Vc. The range to a target is then

4WeErAtes New Collegiate Dictionary &gifts mow as 'the horizontal distance so which a pralechtip tom be pro
palled' or "he VIOnzontal distance banweitin o weapon and large' This is nal how the tErte is used in radar On
iha other lend, the dictionary defines r4n9it Roder. as 'an instrument 10 detiermloo Fke dailonce to a target:-

1 which is Os meaning in radar