Radar Basics and concepts

Chapter 1

1. Elementary concepts

Radar is the name of an electronic system used

for the detection and location of objects. In the

"language" of radar the objects are called targets.

The word radar is an acronym for “Radio

Detection and Ranging”

A radar's function is intimately related to

properties and characteristics of electro- magnetic

waves as they interface with physical objects (the

targets). All early radars used radio waves, but

some modern radars today are based on optical

waves and the use of lasers. Thus the earliest

roots of radar can be associated with the

theoretical work of Maxwell that predicted

electromagnetic wave propagation.

1. Elementary concepts

The experimental work demonstrated that radio

waves could be reflected by physical objects. This

fundamental fact forms the basis by which radar

performs one of its main functions; by sensing the

presence of a reflected wave, the radar can

determine the existence of a target (the process

of detection).

Various early forms of radar devices were

developed between about 1903 and 1925 that

were also able to measure distance to a target

(called the target‘s range) besides detecting the

target's presence.

2. Fundamental elements of

Radar Transmitter with

transmitting antenna,

Receiver with receiving antenna,

The Channel

In general, the target is part of the propagation,

medium (also called the channel) between the transmission and reception locations.

The radar can detect the presence of a target by observing the

2.1 Types of Radar

a) Antenna locations

Monostatic, bistatic, multistatic

b) Types of the transmitted waveform s(t)

A continuous-wave (CW) type is one that

transmits continuously (usually with a constant

amplitude); it can contain frequency modulation

(FM), or can be constant-frequency.

When the transmitted waveform is pulsed, we

have a pulsed radar type.

In an analogous manner, active and passive

radars are types with and without transmitters,

respectively.

2.1 Types of Radarc) Radar Functions

Detection type, search type, terrain avoidance type, tracking type, and so forth.

To be noted that: The radar components in Fig. 1 might be located

on land or water (e.g., on a ship), in the earth's atmosphere (on an aircraft, missile, bomb, cannon shell, etc.), in free space (on a satellite or space vehicle), or even on other planets. Clearly there is almost no limitation on where a radar might be located. Its location does have an effect on operation because of the medium, or channel, in which the radar's waves must propagate.

2.2 Radar Medium The most elementary and simple radar medium is free

space.

The medium becomes more interesting if some target of interest exists in the free space (perhaps a space vehicle or satellite); this is the next most simple radar medium.

The next level of medium complexity would involve addition of unwanted targets, such as returns from a nearby planet's surface when the radar is close to the surface.

Next, the medium might contain an atmosphere with all its weather effects (rain, snow, etc.); this case might correspond to a surface-based radar that must contend with interference from a myriad of unwanted target signals, such as from land, forests, buildings, weather effects, and other propagation effects

2.3 General Block Diagram

2.4 Radar Frequencies

2.4 Radar Frequencies

2.5 FUNCTIONS PERFORMED The most important functions that a radar can perform

are :

1. Resolution: radar's ability to separate (resolve) one

desired target signal from another and to separate

desired from undesired target signals (noise and

clutter).

2. Detection: The detection function consists in sensing

the presence in the receiver of the reflected signal

from some desired target.

3. Measurement : Measurement of target range is

implicit in the name radar. However, modern radars

commonly measure much more than radial range;

they can measure a target's position in three-

dimensional space, its velocity vector (speed in three

space coordinates),angular direction, and vector

angular velocity (angle rates in two angle

2.6 OVERALL SYSTEM

CONSIDERATIONS

When designers are called

on to develop a new radar,

most considerations fall into

three broad classes, those

related to system choices,

those related to the

transmitting end of the

system, and those

concerning the receiving

end. Some of the more

important considerations in

making decisions are listed

here.

2.7 Target types

Point target (having small dimensions compared to the angular and range resolution of the radar)

Isolated targets that are too large to be point targets are often called extended targets. Extended targets can cause spreading in received pulses.

Still larger targets are called distributed targets. One class of examples includes earth surfaces such as forests, farms, oceans, and mountains. These are also called area targets. Another class of distributed target, also called a volume target includes rain, snow, sleet, hail, clouds, fog, smoke, and chaff.

2.8 Target types

Moving targets are those having motion relative

to the radar. If the radar is stationary on the

earth, natural targets such as forests or grassy

fields (vegetation in general) tend to have

relatively low-speed motions that tend to only

slightly spread the spectrum of the received

signals. Moving targets such as missiles, jet

aircraft, satellites, and cannon shells are often

fast enough to shift the spectrum of the received

signal by a significant (Doppler) amount in

frequency relative to the transmitted signal.

Some targets are called active if they radiate

energy on their own. All other targets are called

passive.

2.9 Basic Radar Parameters

Range, angular, velocity, size, shape,…

measurement accuracy

Range resolution

Velocity resolution

Angular resolution

(carrier Frequency, pulse repetition frequency ,

pulse length, power of the transmitting wave, etc.

all the parameters will be integrated in the radar

equation later …to sketch the effect of each of

one)

2.10 Radar Displays A radar display

is a device for visual presentation of target information to an operator, who may be involved with on-line operation or with maintenance of the unit. Most displays use a cathode-ray tube (CRT) which are typically labeled by names such as A-scope, B-scope and C

2.10 Radar Displays

A PPI displays refer to plan-position indicator, can

have several variations.

2.10 Radar Display (LABO)

2.11 RADAR'S WAVEFORM,

POWER, AND ENERGY

Radar’s Waveform : the radar's transmitted

waveform s(t) it is the signal at the output

terminals of the transmitter

s(t) may be modulated in amplitude and in

frequency with time.

a(t) is due to amplitude modulation, (t) due to

frequency modulation,

))(cos()()( 00 ttwtats

Pulse repetition interval : PRI

Waveform equations

)sin()2

cos()( 00 twAtwAts

elsewhere

ttrect

2/2/

0

1)(

where w0 is the transmitted frequency,

In the case of CW s(t) has the following form (no frequency

modulated) :

In the case of pulsed radar, the above signal s(t) is the

carrier signal and it is amplitude modulated by the the

rectangular function rect(t):

Where is the pulse length,

s(t) is then equal to:

)sin()()sin()2/()( 00

0 twtatwATitrectAtsN

iR

Peak and Average Transmitted

Powers

In the case of pulsed radar system

Energy = Power x time

R

peakaverT

PP

peakaver PP

2.12 Range of the pulsed radar

system

The range of a target depends on

the round trip transit time

Velocity of the wave (c )

Where Tr is the receiving time.

The basic measurement of Pulsed radar is the

target range, CW radar can detect the ranging of

the target if it is frequency modulated (see later)

2rTc

R

2.12 Range of the pulsed radar

system

2.14 Range ambiguity (pulsed radar)

The maximal distance of the target to be received

without ambiguity Far target means distance of

round trip high, the time of received echo is

high…

If this time < T no-ambiguity

Else distance ambiguity or called also range

ambiguity

Where T is the pulse repetition interval.

2.13 Range resolution The range resolution of a radar is the ability to

resolve closely spaced targets along the same lineof sight. Two targets along the same line of sight fromthe radar antenna will produce two distinguishableblips on the display if they are separated by adistance equal to or greater than the range resolution.If, however, the separation is less than the rangeresolution, the two targets will not be resolved, andwill appear as a single blip.

The degree of range resolution depends on the widthof the transmitted pulse, the types and sizes oftargets, and the efficiency of the receiver andindicator. Pulse width is the primary factor in rangeresolution.

Two targets along the same line of sight from theradar are resolved if they are separated by a distanceequal or greater to R.

2.13 Range resolution

R=?

Two received pulses A,

and B

at times tA, and tB ,

calculate the

acceptable difference

time?, deduce the

range reslution

accordingly.

B

ccR

22

2.13 Range resolution

3. CW Radar

In CW radar, the transmitter transmits

continuously. CW systems can achieve

considerable maximum ranges without the high

peak-power levels required in pulse radar. CW

radar systems are generally simpler, less costly,

and more compact than pulsed radar systems.

Although an unmodulated CW radar is unable to

measure range, it can easily determine the

relative speed of a target using the Doppler

effect.

3.1 Doppler frequency Radars use Doppler frequency to

extract target radial velocity (range rate), as well as to distinguish between moving and stationary targets or objects, such as clutter. The Doppler phenomenon describes the shift in the centerfrequency of an incident waveform due to the target motion with respect to the source of radiation. Depending on the direction of the target’s motion, this frequency shift may be positive or negative. Where range measurement is required, the CW signal is frequency modulated before transmission (to be explained later)

3.1 Doppler frequency

Radars use Doppler frequency to extract target

radial velocity (range rate), as well as to

distinguish between moving and stationary

targets or objects, such as clutter. The Doppler

phenomenon describes the shift in the center

frequency of an incident waveform due to the

target motion with respect to the source of

radiation. Depending on the direction of the

target’s motion, this frequency shift may be

positive or negative

3.1 Doppler frequency

3.1 Doppler frequency

Consider first the case where the target is fixe, if

the transmitted signal is St(t)=Asin(Wot), what is

the expression of Sr(t).

Take the case where the target is moving toward

the radar station

The doppler frequency is given by:

Where is the angle between the target moving

direction and the ligne of sight, V*cos() is the

radial velocity of the target,

0

cos2

vfd