1. Subject Name: Microwave and Radar Subject Code: 10EC54
Subject Name: Microwave and Radar – Unit 8
Subject Code: 10EC54
Prepared By: Lakshmi C R, Dharani K G
Department: ECE
Date:
Subject Name: Microwave and Radar
Subject Code: 10EC54
Prepared By: Lakshmi C R
Department: ECE
Date:
22/8/14

2. MTI AND PULSE DOPPLER RADAR
UNIT 8
MTI AND PULSE DOPPLER RADAR

3. Topic details : Introduction to Doppler Effect MTI Radar
Delay Line Cancellers
Moving Target Detector
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4. The Doppler effect
Light from moving objects will appear to have different wavelengths depending on the relative motion of the source and the observer.
Observers looking at an object that is moving away from them see light that has a longer wavelength than it had when it was emitted (a redshift), while observers looking at an approaching source see light that is shifted to shorter wavelength (a blueshift).
Consider a stationary ground based radar observing an approaching aircraft.
•As the aircraft approaches each radar pulse travels a shorter and shorter
distance, consequently the phase of the signal is constantly changing with
each pulse or target position.
•The faster the aircraft approaches the radar the faster the rate of change of
the phase of the reflected signal.
•Thus the rate of change of the measured phase to the approaching aircraft is
relative to the velocity of the aircraft.
The phase represented by the two-way path from radar to target is
Ф = 2Π x 2 r / λ
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5. MTI RADAR
The purpose of moving-target indication (MTI) radar is to reject signals from fixed or slow-moving unwanted targets, such as buildings, hills, trees, sea, and rain, and retain for detection or display signals from moving targets such as air-crafts.
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6. MTI SYSTEM BlOCK DIAGRAM
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7. The transmitter shown employs a magnetron
The transmitter shown employs a magnetron. magnetron is a pulsedoscillator that has no phase coherence between consecutive pulses, a phase reference must be established for each transmitted pulse.
This is done by taking a sample of the transmitted pulse at a directional coupler, mixing this pulse withthe stalo (stabilized local oscillator) and then using this pulse to phase-lock thecoho (coherent oscillator). The coho then becomes the reference oscillator for ther eceived signals.
The lock-pulse amplifier is gated off just before the end of the transmitted pulse because a magnetron emits a certain amount of noise during the fall of the high-voltage pulse applied to it, and this noise can prevent perfect locking of the coho.
The received signals are mixed with the stalo and amplified in a linear-limiting
amplifier.
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8. The received signals are then compared in phase with the coho in a phase deector. The output of the phase detector is a function of the relative phase of the signal and the coho, and it is also a function of the amplitude of the signal.
At the output of the phase detector, the signal phase and amplitude information has beenconverted into bipolar video. If the point target is moving and if there is also a moving target in the region of strong clutter return, the superimposed bipolar video from several transmitted pulses.
The bipolar video is converted to digital words in an analog-to-digital (A/D) converter. The A/D output is stored in a PRI memory and also subtracted from the memorized AID output from the previous transmitted pulse
The output of the subtracter is a digital bipolar signal that contains moving targets, system noise, and a small amount of clutter residue if clutter cancellation is not perfect.
The absolute value of the signal is then converted to analog video in a digital-to-analog (DIA) converter for display on a PPI. The digital signal may also be sent to automatic target detection circuitry. The dynamic range (peak signal to rms noise) is limited to about 20 dB for a PPI display.
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9. Delay Line Canceller
MTI systems attempt to maximize signal to clutter ratios and are consequently dependent of the correlation of the clutter. The delay-line MTI subtracts an echo from that of the preceding pulse.
For moving targets the received signal will change from pulse to pulse, so the output from the delay-line MTI will be non-zero.
Echoes from stationary clutter will be constant and thus suppressed.
MTI systems can use phase, amplitude or phase and amplitude to improve the signal to clutter ratio. Systems that use only phase or amplitude do not match the performance of systems using both.
IN phase processing MTI the system distinguishes moving targets by virtue of the targets Doppler frequency. Consequently phase coherence within the radar system must be held in close tolerance. This coherence is provided by a Stable Local Oscillator (STALO). If there are to be two down conversion stages then a Coherent Oscillator (COHO) is used that establishes an intermediate frequency.
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10. The STALO translates the signal from Radio Frequency (RF) to anIntermediate Frequency (IF). The COHO provides a reference signals that effectively ‘remembers’ the phase of each transmitted and received pulse. In the simplest processor this phase detected signal is processed in a delay line canceller that forms the difference of two signals separated in time corresponding to an inter-pulse period.
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11. Moving Target Detection
The Moving Target Detector (MTD) is an advance on the simpler MTI system.
  We could consider a cascaded MTI system as an N point FFT performed on a time sequence of range bins. The zero Doppler bin is then removed prior to detection processing.
  However, we know that clutter doesn‘t always fall in the zero Doppler bin and that targets themselves can have inconsistent Doppler histories.
  An MTD enables full advantage to be taken in both the Doppler and range domains.
  A ‘clutter map’ can be formed in range-Doppler space that is a time averaged representation of the clutter in each range bin and in each Doppler bin.
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12. Block Diagram - Moving Target Detection
In the moving-target detector (MTD) the basic MTI principle, as described above, is enhanced by increasing the linear dynamic range of the signal processor, using a number of parallel doppler filters followed by constant-false-alarm-rate (CFAR) processing, and adding one or more high-resolution clutter maps to suppress point clutter residues.
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13. The MTD radar transmits a group of N pulses at a constant pulse repetitionfrequency (PRF) and at a fixed radar frequency. This set of pulses is usually referred to as the coherent processing interval (CPI) or pulse batch.
Sometimes one or two additional fill pulses are added to the CPI in order to suppress range ambiguous clutter returns, as might occur during periods of anomalous propagation.
The returns received during one CPI are processed in the bank of TV-pulse finite-impulse-response (FIR) filters. Then the radar may change PRF and/or RF frequency and transmit another CPI of N pulses.
Since most search radars are ambiguous in doppler, the use of different PRFs on successive coherent dwells will cause the target response to fall at different frequencies of the filter passband on the successive opportunities during the time on target, thus eliminating blind speeds.
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