radar introduction monostatic bistatic range

1. Chapter 1
Radar Basics and concepts

2. 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.

3. 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.

4. 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

5. 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.

6. 2.1 Types of Radar
c) 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.

7. 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

8. 2.3 General Block Diagram

9. 2.4 Radar Frequencies

10. 2.4 Radar Frequencies

11. 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

12. 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.

13. 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.

14. 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.

15. 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)

16. 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

17. 2.10 Radar Displays
 A PPI displays refer to plan-position indicator, can
have several variations.

18. 2.10 Radar Display (LABO)

19. 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
( ) ( )cos( ( ) ) 0 0 s t  a t w t  t 
 s(t) may be modulated in amplitude and in
frequency with time.
 a(t) is due to amplitude modulation, (t) due to
frequency modulation,

20. Pulse repetition interval : PRI

21. Waveform equations
In the case of CW s(t) has the following form (no frequency
modulated) :

( ) cos( 0 0s t  A w t   A w t
) sin( )
2
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):
t
where w0 is the transmitted frequency,
elsewhere
rect t
/ 2 / 2
1
0
( )
  
  

Where  is the pulse length,
s(t) is then equal to:
N
0 s t A rect t i T A w t a t w t
( )    (  / 2   )  sin( ) 
( )sin( ) R 0
0
i



22. Peak and Average Transmitted
Powers
 In the case of pulsed radar system
P P

 
aver peak T
R
 Energy = Power x time
aver peak P  P

23. 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 )
c 
Tr
2
R

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)

24. 2.12 Range of the pulsed radar
system

25. 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.

26. 2.13 Range resolution
 The range resolution of a radar is the ability to
resolve closely spaced targets along the same line
of sight. Two targets along the same line of sight from
the radar antenna will produce two distinguishable
blips on the display if they are separated by a
distance equal to or greater than the range resolution.
If, however, the separation is less than the range
resolution, the two targets will not be resolved, and
will appear as a single blip.
 The degree of range resolution depends on the width
of the transmitted pulse, the types and sizes of
targets, and the efficiency of the receiver and
indicator. Pulse width is the primary factor in range
resolution.
 Two targets along the same line of sight from the
radar are resolved if they are separated by a distance

27. 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
c c
R



 

2 2

28. 2.13 Range resolution

29. 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.

30. 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. Where
range measurement is required, the
CW signal is frequency modulated
before transmission (to be
explained later)

31. 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

32. 3.1 Doppler frequency

33. 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:
v
2 cos
  

0

fd
Where  is the angle between the target moving
direction and the ligne of sight, V*cos() is the
radial velocity of the target,