Adaptive Moving Target Indicator CHAPTER 1 INTRODUCTION Since conception of radar just prior to Second World War many different types of radar have been developed. Although the original radars were developed to extract information about the position of aircraft, the realization soon came that by transmitting different waveform and applying different forms of processing to the received waveform, much more detailed information could be obtained. It was realized that not only could more information about targets be obtained, but that information could also be gained about operating environment of radar. Example is weather radar, and sideways looking airborne radars. Radar (radio detection and ranging) is designed to detect and locate targets within the specified range. Targets are in general defied as planes and other aircraft. The radar emits an electromagnetic pulse, and each object that becomes illuminated by the pulse reflects a small replica of the pulse back to the radar. From this echo signal the radar then attempts to detect and locate the reflecting object. The radar pulse is reflected by targets, but may also be reflected by other objects, for instance the ground, high mountains and rain. Such reflections are called clutter, and they are regarded as noise since they reduce the radars ability to detect and locate targets. Targets are concentrated spatially, whereas clutter tends to occupy a larger Dept of TCE, BNMIT Page 1
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Adaptive Moving Target Indicator
CHAPTER 1
INTRODUCTIONSince conception of radar just prior to Second World War many different types of radar have
been developed. Although the original radars were developed to extract information about the
position of aircraft, the realization soon came that by transmitting different waveform and
applying different forms of processing to the received waveform, much more detailed
information could be obtained. It was realized that not only could more information about targets
be obtained, but that information could also be gained about operating environment of radar.
Example is weather radar, and sideways looking airborne radars.
Radar (radio detection and ranging) is designed to detect and locate targets within the specified
range. Targets are in general defied as planes and other aircraft. The radar emits an
electromagnetic pulse, and each object that becomes illuminated by the pulse reflects a small
replica of the pulse back to the radar. From this echo signal the radar then attempts to detect and
locate the reflecting object. The radar pulse is reflected by targets, but may also be reflected by
other objects, for instance the ground, high mountains and rain. Such reflections are called
clutter, and they are regarded as noise since they reduce the radars ability to detect and locate
targets. Targets are concentrated spatially, whereas clutter tends to occupy a larger area. The
random clutter process is therefore characterized by its spatial and temporal statistic.
Moving target indication (MTI) is a mode of operation of radar to discriminate a target against
clutter. The most common approach takes advantage of the Doppler Effect. For a given sequence
of radar pulses, the moving target will change its distance from the radar system. Therefore the
phase of the radar reflection that returns from the target will be different for successive pulses.
This differs from a stationary target (or clutter) which will cause the reflected pulses to arrive at
the same phase shift.
Clutter variation requires the use of adaptive cancellers that sense the clutter characteristics and
adjust their weights accordingly. Weights are the complex, than real value and there by allow the
nulls to be steered in Doppler frequency to cancel clutter as appropriate. Adaptive filters have
been proposed as an answer to the problem of clutter suppression in spatially and time varying
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clutter environments. The performance of general adaptive filters is reduced when the statistical
properties of the clutter process vary with radial distance, as the filter weights are estimated from
neighboring range bins. An alternative strategy is to make the MTI filter adaptive.
Radar can be broken down in 2 main categories those which can be transmit and receive
continuously called CW (continuous wave radar) and those that transmit for a short period of
time, and then received whilst the transmitter is turned off, called pulse radar. CW radar is quite
severely restricted in amount of information that they can provide hence much more attention is
given to pulse radar. The difficulty of isolating a power full transmitter from a sensitive receiver
tends to limit amount power transmittable and hence limits useful range of radar.
Whatever the type of radar that is employed certain principle always applies. One of these
overriding principles is that in absence of interference the delectability of targets increases as the
energy transmitted increases. This implies that the only limitation on the detection process is the
presence of thermal noise in front end of receiver. Thus from a delectability point of view the
optimum situation is to transmit as much power as possible for as long as possible.
Unfortunately, this conflict with requirement of resolution: there would be little use in defining
the presence of target, if its whereabouts couldn’t be accurately established. Its well known in
Fourier transform theory that in order to resolve something accurately in time, the bandwidth of
transmitted signal has to be large, that to resolve something accurately in frequency, the time
duration of signal has to be large. This is simply a consequence of time frequency duality.
It might be thought that at first that the approximate time-frequency relationship given for simple
pulses would prevent the accurate simultaneous resolution of both time and Doppler frequency (
i.e. velocity) of radar target , but this need not to be so. It is possible to transmit pulses of a fairly
complicated structure that can resolve well in both time and frequency. This technique is
commonly referred to as pulse compression technique. the basic idea of any pulse radar is to
repetitively transmit a pulse, which need not necessarily be the same pulse each time, the time
interval between pulses needed not be same either. Usually it’s convenient to transmit same
pulse each time, and it is also convenient to make the inter pulse period either same each time or
to make periods related to each other by some simple ratio. A little thoughts shows that
transmitting a pulse train, instead of longer more complicated pulse, as a signal duration
increases, the bandwidth doesn’t decrease hence pulse repetition can be viewed as convenient
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way of increasing signal duration, without a proportionate decrease in bandwidth. This enables
pulse train to have good resolution properties in time in frequency. Using pulse trains, it’s
possible to design signals of extreme complexity and yet still use simple equipment. In particular
if the transmitted pulse stream is coherent that is burst of carrier maintains the correct phase
relationship with the last burst, then it is possible to get even higher resolution. This means that
coherent pulse radar can give a good performance even in dense target environment. This way
Problems of ambiguity are encounter both in frequency and time processing.
1.1 General Block diagram
Input from the receiver is series of pulses which is nothing but the reflected echoes from
different objects which is in the range of the radar. These echoes consists various information
about the object like distance, density, temperature. Each radar is designed to perform specific
function that it could detect either weather conditions or distance etc and any other information
obtained is considered as clutter to that radar.
Pulse compression is a signal processing technique mainly used in radar, sonar and echography
to increase the range resolution as well as the signal to noise ratio. This is achieved by
modulating the transmitted pulse and then correlating the received signal with the transmitted
pulse. The simplest signal pulse radar can transmit is a sinusoidal pulse of amplitude, A and
carrier frequency, f0, this pulse is transmitted periodically.
An adaptive system performs the processing by using an architecture having time-varying
parameters on the received signals which accompanies with clutters. An adaptive moving target
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INPUT FROM RECEIVER
DISPLAY DETECTION
PULSE COMPRESSION
ADAPTIVE MTI CFAR
Adaptive Moving Target Indicator
detector has been designed to meet the challenges of target detection amidst various levels of
clutter environments. The approach has been used that is able to overcome the inherent
limitations of conventional systems (e.g. Moving Target Indicator, Fast Fourier Transform etc.).
Constant false alarm rate (CFAR) detection refers to a common form of adaptive algorithm used
in radar systems to detect target returns against a background of noise, clutter and interference.
In the radar receiver the returning echoes are typically received by the antenna, amplified, down-
converted and then passed through detector circuitry that extracts the envelope of the signal. ,
unwanted clutter and interference sources mean that the noise level changes both spatially and
temporally. In this case, a changing threshold can be used, where the threshold level is raised and
lowered to maintain a constant probability of false alarm. This is known as constant false alarm
rate (CFAR) detection.
The combination of this group of trolleys and big swells present an excellent opportunity to
practice with our radar. In the image above, sea and rain clutter are turned off. Gain is turned up
to 50 - where we leave it offshore (this is too high for inshore work where there are close targets
surrounding us). Notice the almost solid return from the sea clutter within quarter of a mile of the
boat. ARPA vectors are set to true here, to show us the actual courses of these targets.
The output y (m) of a non-recursive filter is a function only of the input signal x(m). The
response of such a filter to an impulse consists of a finite sequence of M+ 1 sample, where M is
the filter order. Hence, the filter is known as a Finite-Duration Impulse Response (FIR) filters.
Other names for a non-recursive filter include all-zero filter, feed-forward filter or moving
average (MA) filter a term usually used in statistical signal processing literature.
4.1.1 FIR filter types which can be used to remove clutters are:
a) Low pass filterLow pass filters can be used for many applications. One area in which these filters can be
used is on the output of digital to analogue converters where they are able to remove the
high frequency alias components. However they can be used in many other areas where it
is necessary to pass the low frequency components of the signal, but remove the
unwanted high frequency elements. Active low pass filters are capable of providing a
relatively high level of performance for a small number of components.
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Fig 4.2: Low pass filter basic response curve
The shape of the curve is of importance with features like the cut-off frequency and roll off being
key to the operation. The cut-off frequency is normally taken as the point where the response has
fallen by 3dB as shown. Another important feature is the final slope of the roll off. This is
generally governed by the number of 'poles' in the filter. Normally there is one pole for each
capacitor inductor in a filter. When plotted on a logarithmic scale the ultimate roll-off becomes a
straight line, with the response falling at the ultimate roll off rate. This is 6dB per pole within the
filter.
Consider the design of a low-pass linear-phase digital FIR filter operating at a sampling rate of
Fs Hz and with a cutoff frequency of Fc Hz. The frequency response of the filter is given by
The impulse response of this filter is obtained via the inverse Fourier integral as
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b) High pass filterAs the name implies, a high pass filter is a filter that passes the higher frequencies and rejects
those at lower frequencies. In other words, high-frequency signals go through much easier
and low-frequency signals have a much harder getting through, which is why it's a high pass
filter. This can be used in many instances, for example when needing to reject low frequency
noise, hum, etc. from signals. This may be useful in some audio applications to remove low
frequency hum, or within RF to remove low frequency signals that are not required.
Fig 4.3: High pass filter basic response curve
The shape of the curve is of importance. One of the most important features is the cut-off
frequency. This is normally taken as the point where the response has fallen by 3dB.
Consider the design of a high-pass linear-phase digital FIR filter operating at a sampling rate of
Fs Hz and with a cutoff frequency of Fc Hz. The frequency response of the filter is given by
The impulse response of this filter is obtained via the inverse Fourier integral as
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c) Band pass filter As the name implies a band pass filter is one where only a given band of frequencies is allowed
through. All frequencies outside the required band are attenuated. There are two main areas of
interest in the response of the filter. These are the pass-band where filter passes signals and the
stop-band where signals are attenuated. As it is not possible to have an infinitely steep roll off,
there is an area of transition outside the pass-band where the response is falling but has not
reached the required out of band attenuation. Band pass filters are used in many areas of
electronics. They are particularly widely used for RF applications where tuned circuits are used.
However for lower frequencies, active band pass filters provide an effective means of making a
filter to pass only a given band if frequencies. For these filters the most widely used active
element is an operational amplifier, or op amp. These op amp band pass filters are easy to design
and construct, requiring only a minimum of components. In addition to this, these active band
pass filters provide a very effective level of performance.
Fig 4.4: frequency response of band pass filter
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Consider the design of a band-pass linear-phase digital FIR filter operating at a sampling rate of
Fs Hz and with a lower and higher cutoff frequencies of FL and FH Hz. The frequency response
of the filter is given by
The impulse response of this filter is obtained via the inverse Fourier integral as
d) Band stop filter A band stop filter is a circuit that ideally filters out signals with frequencies in a certain range.
This range can be quite large, depending on inherent characteristics of the circuit. The smaller
the range of frequencies the circuit filters, the higher the Q factor it is said to have. Band stop
filters with high Q Factors are also called notch filters. The band pass filter passes one set of
frequencies while rejecting all others. The band-stop filter does just the opposite. It rejects a band
of frequencies, while passing all others. This is also called a band-reject or band-elimination
filter. Like band pass filters, band-stop filters may also be classified as (i) wide-band and (ii)
narrow band reject filters. The narrow band reject filter is also called a notch filter. Because of its
higher Q, which exceeds 10, the bandwidth of the narrow band reject filter is much smaller than
that of a wide band reject filter.
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Fig 4.5: frequency response of band stop filter
Consider the design of a band-stop linear-phase digital FIR filter operating at a sampling rate of
Fs Hz and with a lower and higher cutoff frequencies of FL and FH Hz. The frequency response
of the filter is given by
The impulse response of this filter is obtained via the inverse Fourier integral as
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4.1.2 Filter using window techniques
The window method is most commonly used method for designing FIR filters. The simplicity of
design process makes this method very popular. A window is a finite array consisting of
coefficients selected to satisfy the desirable requirements. This provides a few methods for
estimating coefficients and basic characteristics of the window itself as well as the result filters
designed using these coefficients. The point is to find these coefficients denoted by w[n].
When designing digital FIR filters using window functions it is necessary to specify:
A window function to be used; and
The filter order according to the required specifications (selectivity and stop band
attenuation).
These two requirements are interrelated. Each function is a kind of compromise between the two
following requirements:
The higher the selectivity, i.e. the narrower the transition region; and
The higher suppression of undesirable spectrum, i.e. the higher the stop band attenuation.
The table 4.1 below gives the equations for different window types.
Window Type Weight Equation
Rectangular
Bartlett
Hanning
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Hamming
Blackman
Frequency response and weight values of different window types
Fig 4.6 frequency response and weight values
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4.2 Recursive or Infinite Impulse Response (IIR) FiltersA recursive filter has feedback from output to input, and in general its output is a function of the
previous output samples and the present and past input samples as described by the following
equation
y(m)=∑k =1
N
ak y (m−k )+∑k=0
M
bk x (m−k )
Fig 4.7: direct-form pole-zero IIR filter
Fig shows a direct form implementation of IIR. In theory, when a recursive filter is excited by an
impulse, the output persists forever. Thus a recursive filter is also known as an Infinite Duration
Impulse Response (IIR) filters. Other names for an IIR filter include feedback filters, pole-zero
filters and auto-regressive-moving-average (ARMA) filter a term usually used in statistical
signal processing literature.
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4.3 Comparison of FIR and IIR filters
FIR filter uses only current and past input digital samples to obtain a current output sample
value. It does not utilize past output samples. Simple FIR equation is mention below.