chapter 1-generality
DESCRIPTION
radar introduction monostatic bistatic rangeTRANSCRIPT
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