UNIT - 3por it is about radars and it's types and sizes and shapes and colours and it's working

It is about radars and it's types

1. MTI RADAR
•Radars can be classified into the following two types based on
the type of signal with which Radar can be operated.
1. Pulse Radar
2. Continuous Wave Radar
•Pulse Radar
The Radar, which operates with pulse signal is called
the Pulse Radar.
Pulse Radars can be classified into the following two types
based on the type of the Pulse repetition frequency .
a) Basic Pulse Radar (Pulsed doppler radar ) – High PRF
b) Moving Target Indication Radar – LOW PRF
•Pulse Doppler Radar (PRF is HIGH )
The Radar, which operates with pulse signal for detecting
stationary targets, is called the  Pulsed doppler Radar 
Ambiguity - In Range
Unambiguious – In Doppler Measurement

2. •Moving Target Indication Radar (PRF is Low)
The Radar, which operates with pulse signal for
detecting non-stationary targets, is called Moving Target
Indication Radar or simply, MTI Radar
•MTI Radar uses the principle of Doppler effect for
distinguishing the non-stationary targets from stationary
objects.
Ambiguity - In Doppler Measurement
Unambiguious – In Range Measurement
•Continuous Wave Radar
•The Radar, which operates with continuous signal or
wave is called Continuous Wave Radar . They use
Doppler Effect for detecting non-stationary targets.
Continuous Wave Radars can be classified into the
following two types.
a) Unmodulated Continuous Wave Radar
b) Frequency Modulated Continuous Wave Radar

3. •If the Radar is used for detecting the movable target,
then the Radar should receive only the echo signal due
to that movable target. This echo signal is the desired
one. However, in practical applications, Radar receives
the echo signals due to stationary objects in addition
to the echo signal due to that movable target.
•The echo signals due to stationary objects (places)
such as land and sea are called clutters because
these are unwanted signals. Therefore, we have to
choose the Radar in such a way that it considers only
the echo signal due to movable target but not the
clutters.
•For this purpose, Radar uses the principle of Doppler
Effect for distinguishing the non-stationary targets
from stationary objects. This type of Radar is called
Moving Target Indicator Radar or simply, MTI Radar.

4. •MTI is a necessity in high quality airsurveillance
radars that operate in the presence of clutter.
its design is more challenging than that of a
simple pulse radar or a simple CW radar.
•The basic MTI concepts were introduced
during World War II. and most of the signal
processing theory on which MTI radar depends
was formulated during the mid-1950s.
•lt took almost twenty years for the full
capabilities offered by MTI signal-processing
theory to be converted into practical and
economical radar equipment. The chief factor
that made this possible was the
introduction or reliable, small, and
inexpensive digital processing hardware .

5. •Description of operation :-
•A simple CW radar such as was described in Sec. 3.2 is shown in
Fig. 4. 1a. It consists of a transmitter, receiver, indicator, and the
necessary antennas. In principle,
• the CW radar may be converted into a pulse radar as shown in
Fig. 4.1b by providing a power amplifier and a modulator to turn
the amplifier on and off for the purpose of generating pulses.

6. •The chief difference between the pulse radar
of Fig. 4. lb and the one described in Chap. 1 is
that a small portion of the CW oscillator
power that generates the transmitted
pulses is diverted to the receiver to take
the place of the local oscillator. However,
this CW signal does more than function as a
replacement for the local oscillator.
•It acts as the coherent reference needed to
detect the doppler frequency shift. By coherent
it is meant that the phase of the transmitted
signal is preserved in the reference signal. The
reference signal is the distinguishing
feature of coherent MTI radar.

7. •

8. •The difference frequency is equal to the doppler
frequency.fd
•However, when the target is in motion relative to
the radar. .f has a value other than zero
•An example of the output from the mixer when the
doppler frequency fd is large compared with the
reciprocal of the pulse width is shown in Fig. 4.2b.
• The doppler signal may be readily discerned from the
information contained in a single pulse. while the
waveform of Fig. 4.2b might be more applicable to a
radar whose primary function is the detection of
extraterrestrial targets such as ballistic missiles or
satellites.

9. •If, on the other hand, fd is small
compared with the reciprocal of the
pulse duration, the pulses will be
modulated with an amplitude given
by Eq . (4.3 ) (Fig. 4.2c) and many
pulses will be needed to extract the
doppler information. The case
illustrated in Fig. 4.2c is more typical
of aircraft-detection radar.

10. •Moving targets may be distinguished from stationary targets
by observing the video output on an A-scope (amplitude vs.
range). A single sweep on an A-scope might appear as in Fig.
4.3a.
•This sweep shows several fixed targets and two moving
targets indicated by the two arrows .
On the basis of a single sweep, moving targets cannot
be distinguished from fixed targets. Successive A-scope
sweeps (pulse-repetition intervals) are shown in Fig. 4.3b to e.
•Echoes from fixed targets remain constant throughout ,
•but echoes from moving targets vary in amplitude from sweep
to sweep at a rate corresponding to the doppler frequency.

11. Echoes from fixed targets remain constant throughout ,
but echoes from moving targets vary in amplitude from sweep to sweep at a
rate corresponding to the doppler frequency. The super position of the
successive A-Scope sweeps is shown in Fig 4.3 f . The moving targets produce
with time a “butterfly” effect on A- Scope

12. •We can classify the MTI Radars into the
following two types based on the type of
transmitter that has been used.
•MTI Radar with Power Amplifier
Transmitter
•MTI Radar with Power Oscillator
Transmitter
MTI RADAR with POWER – AMPLIFIER
TRANSMITTER
MTI Radar uses single Antenna for both
transmission and reception of signals with
the help of Duplexer. The block
diagram of MTI Radar with power
amplifier transmitter is shown in the
following figure.

13. • Pulse Modulator − It
produces a pulse
modulated signal and it is
applied to Power Amplifier.
•Power Amplifier − It
amplifies the power levels
of the pulse modulated
signal.
•Local Oscillator − It
produces a signal having
stable frequency fL.
Hence, it is also called
stable Local Oscillator. The
output of Local Oscillator
is applied to both Mixer-I
and Mixer-II.

14. •Coherent Oscillator − It produces
a signal having an Intermediate
Frequency, fc. This signal is used as
the reference signal. The output of
Coherent Oscillator is applied to both
Mixer-I and Phase Detector.
•Mixer-I − Mixer can produce either
sum or difference of the frequencies
that are applied to it. The signals
having frequencies of  fL l and  fc  are
applied to Mixer-I. Here, the Mixer-I is
used for producing the output, which
is having the frequency  fL+ fc.
•Duplexer − It is a microwave switch,
which connects the Antenna to either
the transmitter section or the receiver
section based on the requirement.
Antenna transmits the signal having
frequency fL+ fc when the duplexer
connects the Antenna to power
amplifier. Similarly, Antenna
receives the signal having
frequency of fl+fc±fd when the
duplexer connects the Antenna to
Mixer-II.

15. •Mixer-II − Mixer can produce
either sum or difference of the
frequencies that are applied to it.
The signals having frequencies 
fL + fc ±fd  and  fL  are
applied to Mixer-II. Here, the
Mixer-II is used for producing the
output, which is having the
frequency  fc ±fd
•IF Amplifier − IF amplifier
amplifies the Intermediate
Frequency (IF) signal. The IF
amplifier shown in the figure
amplifies the signal having
frequency fc ±fd . This amplified
signal is applied as an input to
Phase detector.
•Phase Detector − It is used to
produce the output signal having
frequency  fd  from the applied
two input signals, which are
having the frequencies of  fc ±fd
 and fd  . The output of phase
detector can be connected to
Delay line canceller.

16. MTI Radar with Power Oscillator
Transmitter
•The block diagram of MTI Radar with
power oscillator transmitter looks similar
to the block diagram of MTI Radar with
power amplifier transmitter. The blocks
corresponding to the receiver section will
be same in both the block diagrams.
Whereas, the blocks corresponding to the
transmitter section may differ in both the
block diagrams.
•Before the development of the
klystron amplifier, the only high-
power transmitter available at
microwave frequencies for radar
application was the magnetron
oscillator. In an oscillator the phase of
the RF bears no relationship from
pulse to pulse. For this reason the
reference signal cannot be generated
by a continuously running oscillator.
•However, a coherent reference signal may
be readily obtained with the power
oscillator by readjusting the phase of the
coho at the beginning of each sweep
according to the phase of the transmitted
pulse. The phase of the coho is locked to
the phase of the transmitted pulse each
time a pulse is generated.

17. •portion of the transmitted
signal is mixed with the stalo
output to produce an IF beat
signal whose phase is
directly related to the phase
of the transmitter.
•This IF pulse is applied to the
coho and causes the phase of
the coho CW oscillation to
"lock" in step with the phase
of the IF reference pulse.
• The phase of the coho is then
related to the phase of the
transmitted pulse and may be
used as the reference signal for
echoes received from that
particular transmitted pulse.
•Upon the next transmission
another IF locking pulse is
generated to relock the
phase of the CW coho until
the next locking pulse comes
along.

18. DELAY-LINE CANCELERS
•butterfly effect is suitable for recognizing moving targets on an
A-scope, it is not appropriate for display on the PPI.
•One method commonly employed to extract doppler information
in a form suitable for display on the PPI scope is with a delay-
line canceler .
•The delay-line canceler acts as a filter to eliminate the d-c
component of fixed targets and to pass the a-c components of
moving targets.
•The video portion of the receiver is divided into two channels.
1. is a normal video channel.
2.the video signal experiences a time delay equal to one
pulse-repetition period (equal to the reciprocal of the pulse-
repetition frequency).

19. The outputs from the two
channels are subtracted from
one another. The fixed
targets with unchanging
amplitudes from pulse to
pulse are canceled on
subtraction.
However, the amplitudes of
the moving-target echoes
are not constant from pulse
to pulse, and subtraction
results in an uncanceled
residue.

20. •For typical ground-based air-surveillance radars this might be several
milliseconds.
•Delay times of this magnitude cannot be achieved with practical
electromagnetic transmission lines. By converting the electromagnetic
signal to an 'acoustic signal it is possible to utilize delay lines of a
reasonable physical length since the velocity of
propagation of acoustic waves is about 10
-5 that of
electromagnetic waves.
•After the necessary delay is introduced by the acoustic
line, the signal is converted back to an electromagnetic
signal for further processing.
•The early acoustic delay lines developed during World
War 2 used liquid delay lines filled with either water or
mercury. Liquid delay lines were large and inconvenient
to use.
• They were replaced in the mid-1950s by the solid fused-
quartz delay line that used multiple internal reflections to
obtain a compact device

21. •Filter characteristics of the delay-
line canceler :-
The delay-line canceler acts as a filter
which rejects the d-c component of
clutter

22. •The output from the canceler [Eq. (4.6)]
consists of a cosine wave at the doppler
frequency fd with an amplitude 2k sin πfdTfdT:
Thus the amplitude of the canceled video
output is a function of the doppler
frequency shift and the pulse-repetition
interval, or prf. The magnitude of the relative
frequency-response of the delay-line canceler

23. •Blind speeds :-
The delay line canceler not only eliminates the d-c
component caused by clutter , but unfortunately it
also rejects any moving target whose doppler
frequency happens to he the same as the prf or a
multiple thereof. Those relative target velocities which
result in zero MTI response are called blind speeds

24. •The blind speeds are one of the limitations of pulse MTI radar
which do not occur with CW radar.
•They are present in pulse radar because doppler is measured by
discrete samples (pulses) at the prf rather than continuously.
• If the first blind speed is to be greater than the maximum radial
velocity expected from the target, the product λfp fp must be large.
•Thus the MTI radar must operate at long wavelengths
(low frequencies) or with high pulse repetition
frequencies, or both.
•Low radar frequencies have the disadvantage that
antenna beam-widths, for a given-size antenna, are wider
than at the higher frequencies and would not be
satisfactory in applications where angular accuracy or
angular resolution is important
•Unfortunately, there are usually constraints other than blind
speeds which determine the wavelength and the pulse repetition
frequency. Therefore blind speeds might not be easy to avoid.
•The pulse repetition frequency cannot always be varied over wide
limits since it is primarily determined by the unambiguous range
requirement.

25. •the first blind speed v1 is plotted as a function of the maximum
unambiguous range (Runamb = /2), with radar frequency as the
parameter. If the first blind speed were 600 knots, the maximum
unambiguous range would be 130 nautical miles at a frequency of
300 MHz (UHF), 13 nautical miles at 3000 MHz (S band), and 4
nautical miles at 10,000 MHz (X band). Since commercial jet aircraft
have speeds of the order of 600 knots, and military aircraft even
higher, blind speeds in the MTI radar can be a serious limitation.

26. •The presence of blind speeds within the
doppler-frequency band reduces the detection
capabilities of the radar.
•Blind speeds can sometimes be traded for
ambiguous range, so that in systems applications
which require good MTI performance, the first
blind speed might be placed outside the
range of expected doppler frequencies if
ambiguous range can be tolerated . (Pulse-
doppler radars usually operate in this manner).
•the effect of blind speeds can be significantly
reduced, without incurring range ambiguities,
by operating with more than one pulse
repetition frequency. This is called a staggered-
prf MTI. Operating at more than one RF frequency
can also reduce the effect of blind speeds.

27. MULTIPLE OR STAGGERED, PULSE
REPETITION FREQUENCIES
•The use of more than one pulse repetition frequency offers
additional flexibility in the design of MTI doppler filters.
• It reduces the effect of the blind speeds
•The blind speeds of two independent radars operating at the same
frequency. if one radar were “ blind ” to moving targets, it would
be unlikely that the other radar would be “ blind " also.
•The blind speeds of two independent radars operating at different
frequency. Then if one radar were “ blind ” to moving targets, the
other radar would not be “ blind " for same target .
•Instead of rising two separate radars, the same result can
be obtained with one radar which time-shares its pulse
repetition frequency between two or more different values
(multiple prf’s).
•The pulse repetition frequency might be switched every other
scan or every time the antenna is scanned a half beam width, or
the period might be alternated on every other pulse. When the
switching is pulse to pulse, it is known as a staggered prf.

28. •An example of the
composite (average)
response of an MTI
radar operating with
two separate pulse
repetition frequencies
on a time-shared basis
is shown in Fig. 4.16.
The pulse repetition
frequencies are in the
ratio of 5 : 4.
•Zero response
occurs only when
the blind speeds of
each prf coincide. In
the example of Fig.
4.16, the blind speeds
are coincident for
4/T1 = 5/T2 .

29. •The closer the ratio T1 :
T2 approaches unity, the
greater will be the value
of the first blind speed.
•T1 / T2 is a
compromise between
the value of the first
blind speed and the
depth of the nulls
within the filter pass
band.
•The depth of the nulls
can be reduced and
the first blind speed
increased by operating
with more than two
interpulse periods.

30. •Figure 4.17 shows the response of a five-pulse
stagger (four periods). In this example the
periods are in the ratio 25 : 30 : 27 : 31 and
the first blind speed is 28.25 times that of a
constant prf waveform with the same average
period.

32. •A disadvantage of the staggered prf is its
inability to cancel second-time-around
clutter echoes. Such clutter does not appear at
the same range from pulse to pulse and thus
produces uncanceled residue.
•Second-time-around clutter echoes can be
removed by use of a constant prf , providing
there is pulse-to-pulse coherence as in the power
amplifier form of MTI.
•The constant prf might be employed only over
those angular sectors where second-time-around
clutter is expected (as in the ARSR-3 of Sec. 14.3),
or by changing the prf each time the antenna
scans half-a-heamwidth (as in the MTD of Sec.
4.7), or by changing the prf every scan period
(rotation of the antenna).

33. Double cancellation.
The frequency response of a single-delay-
line canceler does not always have as
broad a clutter-rejection null as might be
desired in the vicinity of d-c.
The clutter-rejection notches may be
widened by passing the output of the
delay-line canceler through a second
delay-line canceler as shown in Fig. 4.9a.
Thus the frequency

35. Transversal Filters
•The three-pulse canceler shown in Fig. 4.9b is an
example of a transversal filter. Its general form
with N pulses and N — 1 delay lines is shown in
Fig.4.11.
• It is also sometimes known as a feedforward
fitter, a nonrecursive filter, a finite memory filter
or a tapped delay-line filter.
•The weights w, for a three-pulse canceler utilizing
two delay lines arranged as a transversal filter
are 1, - 2, 1. The frequency response function
is proportional to sin2 rift T. A transversal filter
with three delay lines whose weights are 1, — 3,
3, — I gives a sin' off T response. This is a four-
pulse canceler.

37. •Filter is optimum in several senses , but it
may not have desirable char’s for MTI Radar
•Notches will increase , if we increase the
delay line cancelers
•Added delay line cancelers will increase
clutter attenuation , it may mask the targets
too
•So transversal filter needs a flat shape at
where the target is presented
•And need a stop band at where the clutter is
presented
•So it need a freedom to design the shape of
the filter

38. •

40. RANGE-GATED DOPPLER FILTERS
•The delay-line canceler, which can be considered
as a time-domain filter, has been widely used in
MTI radar as the means for separating moving targets
from stationary clutter.
• It is also possible to employ the more usual
frequency-domain bandpass filters of
conventional design in MTI radar to sort the doppler-
frequency-shifted targets.
•The filter configuration must be more complex,
however, than the single, narrow-bandpass filter.
• A narrowband filter with a passband designed
to pass the doppler frequency components of
moving targets will " ring " when excited by the
usual short radar pulse. This destroys the range
resolution.

41. •If more than one target is present they cannot be
resolved. Even if only one target were present, the noise
from the other range cells that do not contain the target
interfere with the desired target signal. The result is a
reduction in sensitivity due to a collapsing loss
•The loss of the range information and the collapsing loss
may be eliminated by first quantizing the range (time)
into small intervals. This process is called range gating.
•The width of the range gates depends upon the range
accuracy desired and the complexity which can be
tolerated
•Range resolution is established by gating . Once the
radar return is quantized into range intervals, the output
from each gate may be applied to a narrowband filter
since the pulse shape need no longer be preserved for
range resolution.
•A collapsing loss does not take place since noise
from the other range intervals is excluded.

42. •A block diagram of the MTI radar with
multiple range gates followed by clutter-
rejection filters is shown in Fig. 4.19.

43. •The output of the phase detector is sampled sequentially by the
range gates.
•Each range gate opens in sequence just long enough to sample the
voltage of the video waveform corresponding to a different range
interval in space.
•The range gate acts as a switch or a gate which opens and
closes at the proper time.
•The range gates are activated once each pulse-repetition
interval.
•The output for a stationary target is a series of pulses of constant
amplitude. An echo from a moving target produces a series of pulses
which vary in amplitude according to the doppler frequency.
•The output of the range gates is stretched in a circuit called
the boxcar generator, or sample-and-hold circuit.
•whose purpose is to aid in the filtering and detection process
by emphasizing the fundamental of the modulation frequency
and eliminating harmonics of the pulse repetition frequency .
• The clutter rejection filter is a band pass filter whose bandwidth
depends upon the extent of the expected clutter spectrum.

44. •Following the doppler filter is a full-
wave linear detector and an
integrator (a low-pass filter).
•The purpose of the detector is to
convert the bipolar video to unipolar
video.
•The output of the integrator is applied to
a threshold-detection circuit.
•Only those signals which cross the
threshold are reported as targets.
•Following the threshold detector, the
outputs from each of the range
channels must be properly combined
for display on the PPI or A-scope or
for any other appropriate indicating
or data-processing device.

45. •The bandpass filter can be designed with a variable
low-frequency cutoff that can be selected to
conform to the prevailing clutter conditions . The
selection of the lower cutoff might be at the option
of the operator or it can be done adaptively.
•A variable lower cutoff might be advantageous
when the width of the clutter spectrum changes
with time as when the radar receives unwanted
echoes from birds.
•A relatively wide notch at zero frequency is needed to
remove moving birds.
•If the notch were set wide enough to remove the birds, it
might be wider than necessary for ordinary clutter and
desired targets might be removed.
•Since the appearance of birds varies with the time
of day and the season, it is important that the
width of the notch be controlled according to the
local conditions.

47. LIMITATIONS TO MTI
PERFORMANCE
improvement in signal-to-clutter ratio of an
MTI is affected by following factors.
1. Instabilities of the transmitter and
receiver,
2. physical motions of the clutter,
3. the finite time on target (or scanning
modulation),
4. limiting in the receiver
Before discussing these effects, some
definitions will be stated.

48. •MTI improvement factor. The signal-to-
clutter ratio at the output to signal-to-
clutter ratio at the input
•Clutter visibility factor. The signal-to-
clutter ratio, after cancellation or doppler
filtering, that provides stated probabilities
of detection and false alarm.
•Clutter attenuation. The ratio of clutter
power at the canceler input to the clutter
residue at the output,

49. •Equipment instabilities. Pulse-to-pulse changes in
the amplitude, frequency, or phase of the
transmitter signal, changes in the stalo or coho
oscillators in the receiver, jitter in the timing of the
pulse transmission, variations in the time delay
through the delay lines, and changes in the pulse
width can cause the apparent frequency spectrum of
clutter to broaden and thereby lower the
improvement factor of an MTI radar. The stability
of the equipment in an MTI radar must be
considerably better than that of an ordinary
radar
•Internal fluctuation of clutter. Although clutter
targets such as buildings, water towers, bare hills, or
mountains produce echo signals that are constant in
both phase and amplitude as a function of time,

50. •there are many types of clutter that cannot be
considered as absolutely stationary. Echoes from
trees, vegetation, sea, rain, and chaff fluctuate with
time, and these fluctuations can limit the
performance of MTI radar.
•The echo at the radar receiver is the vector
sum of the echo signals received from each of
the individual scatters; that is, the relative phase
as well as the amplitude from each scatterer
influences the resultant composite signal. If the
individual scatters remain fixed from pulse to
pulse, the resultant echo signal will also
remain fixed. But any motion of the scatterers
relative to the radar will result in different
phase relationships at the radar receiver . Hence
the phase and amplitude of the new resultant echo
signal will differ pulse to pulse.

52. •Limiting in MTI radar. A limiter is usually
employed in the IF amplifier just before the MTI
processor to prevent the residue from large
clutter echoes from saturating the display.
•However, when the MTI improvement factor is
not great enough to reduce the clutter
sufficiently,,the clutter residue will appear on the
display and prevent the detection of aircraft
targets
•This condition may be prevented by setting the
limit level L, relative to the noise N, equal to the
MTI improvement factor 1; or L/N = 1 If the limit
level relative to noise is set higher than the
improvement factor, clutter residue obscures part
of the display. If it is set too low there may be a
"black hole" effect on the display.

53. MTI versus PULSE DOPPLER RADAR
Both RADARS are designed for the identifying the moving
targets
A pulse radar that extracts the doppler frequency shift for the
purpose of detecting moving targets in the presence of
clutter is either an MTI radar or a pulse doppler radar.
In a pulse radar, ambiguities can arise in both the doppler
frequency (relative velocity) and the range (time
delay) measurements.
Range ambiguities are avoided with a low sampling rate
(low pulse repetition frequency),
Doppler frequency ambiguities are avoided with a high
sampling rate (high pulse repetition frequency),
However, in most radar applications .the sampling rate,
or pulse repetition frequency, cannot be selected to
avoid both types of measurement ambiguities.
.

55. Therefore a compromise must be made and
the nature of the compromise
generally determines whether the
radar is called an MTI or a pulse
doppler.
MTI usually refers to a radar in which the
pulse repetition frequency is chosen
low enough to avoid ambiguities in
range (no multiple-time-around echoes).
but with the consequence that the
frequency measurement is ambiguous
and results in blind speeds,
Pulse doppler radar, on the other hand,
has a high pulse repetition frequency
that avoids blind speeds, but it
experiences ambiguities in range

56. •The pulse doppler radar
•is more likely to use range-gated doppler filter-
banks than delay-line cancelers .
•A pulse doppler radar operates at a higher duty
cycle than does an MTI
•When the prf must be so high that the number
of range ambiguities is too large to be easily
resolved, the performance of the pulse-doppler
radar approaches that of the CW doppler radar .
•The pulse-doppler radar, like the CW radar, may be
limited in its ability to measure range under these
conditions. Even so, the pulse-doppler radar has an
advantage over the CW radar in that the detection
performance is not limited by transmitter leakage or
by signals reflected from nearby clutter or from the
radome.

57. •The pulse-doppler radar avoids this
difficulty since its receiver is turned
off during transmission, whereas the
CW radar receiver is always on .
•On the other hand, the detection
capability of the pulse-doppler radar
is reduced because of the blind spots
in range resulting from the high prf