04-08-2012, 04:29 PM
RADAR
RADARQNAUNITV.pdf (Size: 327.5 KB / Downloads: 93)
What is the principle of MTI radar?
The radar which uses the concept of Doppler frequency shift for distinguishing desired
moving targets from stationary objects i.e., clutter is called as MTI radar (Moving Target
Indicator).
The block diagram of MTI radar employing a power amplifier is shown in Fig. 5.1. The
significant difference between this MTI configuration and that of Pulse Doppler radar is the
manner in which the reference signal is generated. In Fig. 5.1, the coherent reference is supplied
by an oscillator called the coho, which stands for coherent oscillator. The coho is a stable
oscillator whose frequency is the same as the intermediate frequency used in the receiver.
In addition to providing the reference signal, the output of the coho fc is also mixed with
the local-oscillator frequency fl. The local oscillator must also be a stable oscillator and is called
stalo, for stable local oscillator. The RF echo signal is heterodyned with the stalo signal to
produce the IF signal, just as in the conventional superheterodyne receiver. The stalo, coho, and
the mixer in which they are combined plus any low-level amplification are called the receiverexciter
because of the dual role they serve in both the receiver and the transmitter.
Write about delay line canceller.
The simple MTI delay-line canceler shown in Fig. 5.3 is an example of a time-domain
filter. The capability of this device depends on the quality of the medium used is the delay line.
The Pulse modulator delay line must introduce a time delay equal to the pulse repetition
interval. 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.
Explain about Double delay line canceller and three pulse canceller..
The frequency response of a single-delay-line canceller 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 canceller through a second
delay-line canceller as shown in Fig. 5.6.1. The output of the two single-delay- line cancellers
in cascade is the square of that from a single canceller. Thus the frequency response is
4 sin2 πfdT. The configuration of Fig. 5.6.1 is called a double-delay-line canceller, or simply a
double canceller. The relative response of the double canceller compared with that of a singledelay-
line canceller is shown in Fig. 5.6.2. The finite width of the clutter spectrum is also
shown in this figure so as to illustrate the additional cancellation of clutter offered by the
double canceler.
Draw the block diagram of MTI radar using range gates and filters and explain.
The block diagram of the video of an MTI radar with multiple range gates followed
by clutter-rejection filters is shown in Fig. 5.8. 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.