RADAR is
abbreviated as Radio detection and ranging. Radar is an electromagnetic system,
which generates EM signals to detect and to find the distance from system to objects
(or) targets near to it.
How it can
detect?
As said
earlier radar transmits the EM signal from an antenna, these signals spread out
in space and when they hit any object they come back to receiver, present in
the same antenna. Returned signal is called ECHO signal and the direction it
came from provides the direction of object with respect to radar system.
How it
determines the Range (distance)?
Radar system
is capable of determining the distance from system to object. we know that
distance(R) = speed(c) * time(t).
R=(CTr)/2--->eq.1;
where c=velocity of signal
Tr=pulse repetition
time period.
In the
denominator 2 is placed, as we calculated the time for to and fro motion.
This range
equation does not provide the correct distance from system to object, so many
factors have to be included like radar cross section, aperture gain, gain of
antenna(G), the power transmitted from the antenna(Pt),minimum power detectable signal(Smin),by this radar equation is developed.Even this is not perfect.
Many other factors like noise need to be included,as noise is the chief
facor limiting receiver sensitivity,it needs to be described.Even if you
operate in noise free region,then also noise is entered which is due to thermal
motion of conduction electrons in the ohmic portions of reciever,this is called
johnson noise or thermal noise.So the total noise power at the output of
receiver is equal to thermal noise power multiplied by noise figure(Fn).The minimum power
Smin will become as
Smin=KToBnFn(So/No),replacing in the
above equation will give you the better range valve.
*If a signal is transmitted from radar,it may strike the object at multiple
locations, due to this many pulses are usually returned from a object on each
scan and can be used to improve detection.The process of summing all the radar echo pulses to improve the
detection is called integration of radar pulses.The integration activity can be
performed before the second detector(in the radar block diagram) and after the
second detector.The integration performed before is called predetection or
coherent detection and after is called postdetection or noncoherent detection.Predetection
is mostly adopted as it is capable of
storing the phase information for further purpose like finding the range in CW
radar,but postdetection is not entertained as it destroys the phase informaion
for further access.
The integration effeciancy is Ei(n)=((S/N)1/n(S/N)n)
Where
n=no of pulses integrated
(S/N)1 =signal to noise
ratio of a single pulse required to produce given probability of detection(n=1)
(S/N)1 =signal to noise
ratio per pulse required to produce
same probability of detection when n
pulses are integrated.
Placing the above one in the radar equation we get better results.
*The transmitted power Pt in the radar equation is called peak power.This is not the
instantaneous power of a radar wave.The average radar power Pav is ratio of average
transmitter power over pulse repetition period.
Pav=(Ptt/Tp) Tp=pulse repetition period
Replacing the above one in radar equation will give better results.
*Even in the final equation,we havent considered the losses take place in
the system,these reduce the signal to noise ratio at the receiver output.These
losses are classified depending upon whether they are predictable or not.The
losses that are predictable are antenna beam shape losses,plumbing
losses,collapsing losses and the losses that aren't predictable are operator
losses,integration of radar pulses,the losses due to field degradation.
Plumbing losses are the losses that are experienced in the transmission.
Antenna beam shape losses include the pulses that are returned from target
are received correctly or not by aperture of receiver antenna.
Collapsing losses occur in radar that is capable of integrating additional
noises samples along with the wanted signal,the additional noise results in a
degradation.
*Now we look on another type of radar called CW radar.The difference
between the CW radar and the pulse type of radar what we have discussed
is,suppose if strong signal has to be separated from weak echo(normally 10-18 times that of
transmitted power) then it is better to use continuous wave rather than pulse
wave as carrier.
Upto now we had considered that the object(target) is in rest.Now if the
target is in motion then relative velocity exists between radar and the
target,an apparent shift in frequency will result,this is doppler effect and is
basis of CW radar.the doppler frequency is given by
Fd=2VrF0/c ------->eq.2 where c=velocity of light
Vr=relative velocity
F0=Transmitted
frequency.
So the frequency that is received at the receiver will be F0 +(or)- Fd.
When the object is approaching towards radar system we have F0+Fd and when the object
is moving away from radar system then we have F0-Fd.By this we will know Fd as we know F0(transmitted
frequency), and
if it is placed in the above equation(eq.2),we get the relative velocity(Vr) existed between
radar system and target.
*The major disadvantage with the CW radar is that it cannot measure the
range,this can be overcome by modulating the CW carrier.The inability of simple
CW radar to measure is related to the relatively narrow spectrum of its
transmitted waveform and a widely used technique to broaden the spectrum of CW
radar is to frequency modulate the carrier.This type of radar is called
FMCW(frequency modulated continuous wave) radar.
In this type of radar the transmitter frequency is changed as a function
of time in a known manner which is transmitted and echo is received after time
T=2R/c(from eq.1).This echo signal frequency,along with transmitter signal
frequency is fed to mixer and the output is beat frequency Fb. suppose if there is
no doppler frequency shift i.e no relative motion existed between radar and
target,the beat frequency is equal to range frequency Fr=(df0/dt)*T
Fr=2Fm*(2R/c)----->eq.3
We know the returned frequency at the receiver Fr,modulating
frequency Fm,velocity
of light c,placing all these we get the Range(R).
We calculated the range when the target is in rest,for suppose if target
is in motion,then we get two Fb(up)=Fr+Fd
Fb(down)=Fr-Fd
Above two equations are added to get the Fr,which is placed in the above equation(eq.3)
to get the range(R).
If suppose multiple frequencies are resulted from the target,then narrow
band pass filters are used at the receiver to separate individual frequencies.
*Now we look on another type of radar called “MTI(moving target indicator)
radar”,major differences between CW and MTI radar is CW radar operates with
continuous wave and the MTI radar operates with pulse wave,so MTI radar is also
called as pulse doppler radar.The principle remains the same but there is
slight differences between MTI and pulse radar.MTI radar operates with ambiguous
doppler measurements but with inambiguous range measurements,whereas the pulse
doppler radar operates exactly opposite.
The moving targets produce,with time ,a “butterfly” effect on the A-scope,but
for the display on the PPI(plane position indicator),we need delay line
canceller,which acts as a filter to eliminate the dc component of fixed targets
and to pass as components of moving targets.
*Another type of radar we have to know is “Tracking radar” which measures
the coordinates of target and provides data about the position of target in
future.
Appliations of Radar:
We have many applications of radar as it can see through those conditions
impervious to normal human,such as darkness,dnow,fog and rain.
In ATC(Air Traffic Control),radar is employed for purpose of safely
controlling air traffic en route and in the vicinity of airports.
In Aircraft navigation to avoid terror attacks.
In ship safety also radar is employed.In titanic movie the ship hit the
iceberg and collapsed because the human was visualising as the human eye cannot
see through snow he couldn’t inform it before.now in the ships radar is
employed to signal the operator before
he founds it.
In space also radar is employed to detect the asteriods and smash it when
found it is approaching towards earth.
In military also radar is employed to detect the enemy moves.
In highways police use it to detect the speed of the vehicles.
In sports like cricket it is used to measure the cricket ball speed.
There are many,these are only few.
.jpg)
.jpg)
.jpg)
