170
9 Compensation of Signals from Stationary Objects
with the emitted pulses in phase, is usually used. The detection of small target shift
in range is carried out using the phase methods, as the most sensitive.
At the output of phase detector of coherent-pulsed radar, there is a video voltage
of pulses burst, reflected from a moving target, modulated by the Doppler frequency.
Using the coherent technique, the voltage of pulse train from a stationary target is
synchronized relatively to reference voltage of phase detector, due to which there is
no burst modulation.
Under such conditions (phase of reflections is unknown), the optimal signal extraction from the noise background requires the presence of two quadrature phase detectors and maintaining a constant phase shift for all pulses in the burst. However, this
can only be done with stationary targets. This means that a circuit based on two
phase detectors with a phase shift of its reference voltages by 90° would give the
advantages of a stationary target over a moving one. Therefore, in moving target
discrimination systems, where the signals of stationary and low-moving targets are
interfering, they abandon the quadrature scheme, thereby leveling the possibilities
for target discrimination regardless of their velocity. Naturally, this leads to losses in
use of signal energy in comparison with noise, to an increase in signal discrimination
coefficient against the background of noise. In MTI systems, such losses are incurred,
because the main task for them is to suppress not noise, but passive interference. If
the usual method of accumulating a burst of pulses against a background of noise is
their summation after the detector, performed with a period equal to pulses repetition
period, then processing in the interests of eliminating interfering reflections consists
in over-periodic subtraction of video pulses.
Consider the particularities of moving targets detection by the phase method.
Figure 9.2 shows timing diagrams for several pulse repetition periods at receiving of
signals, reflected from a stationary and moving target. The reference signal is used
to determine the phase of the incoming signal relative to the emitted one. In Fig. 9.2,
we can see that the time delay of pulses from a stationary target does not change
from one period to another. The pulses of a moving target are shifted along the time
axis (in range). At the initial moment of time, the signal delay from a moving target
is equal to:
t mov =
2D 0
c
,
(9.1)
where
D 0 is the distance to a target at the initial moment of time
c
is the speed of light.
In the next receiving path, the signal will already have a delay:
t mov1 = t mov0 + t =
2D 0
c
+
2D
c
,
(9.2)
9 Compensation of Signals from Stationary Objects
with the emitted pulses in phase, is usually used. The detection of small target shift
in range is carried out using the phase methods, as the most sensitive.
At the output of phase detector of coherent-pulsed radar, there is a video voltage
of pulses burst, reflected from a moving target, modulated by the Doppler frequency.
Using the coherent technique, the voltage of pulse train from a stationary target is
synchronized relatively to reference voltage of phase detector, due to which there is
no burst modulation.
Under such conditions (phase of reflections is unknown), the optimal signal extraction from the noise background requires the presence of two quadrature phase detectors and maintaining a constant phase shift for all pulses in the burst. However, this
can only be done with stationary targets. This means that a circuit based on two
phase detectors with a phase shift of its reference voltages by 90° would give the
advantages of a stationary target over a moving one. Therefore, in moving target
discrimination systems, where the signals of stationary and low-moving targets are
interfering, they abandon the quadrature scheme, thereby leveling the possibilities
for target discrimination regardless of their velocity. Naturally, this leads to losses in
use of signal energy in comparison with noise, to an increase in signal discrimination
coefficient against the background of noise. In MTI systems, such losses are incurred,
because the main task for them is to suppress not noise, but passive interference. If
the usual method of accumulating a burst of pulses against a background of noise is
their summation after the detector, performed with a period equal to pulses repetition
period, then processing in the interests of eliminating interfering reflections consists
in over-periodic subtraction of video pulses.
Consider the particularities of moving targets detection by the phase method.
Figure 9.2 shows timing diagrams for several pulse repetition periods at receiving of
signals, reflected from a stationary and moving target. The reference signal is used
to determine the phase of the incoming signal relative to the emitted one. In Fig. 9.2,
we can see that the time delay of pulses from a stationary target does not change
from one period to another. The pulses of a moving target are shifted along the time
axis (in range). At the initial moment of time, the signal delay from a moving target
is equal to:
t mov =
2D 0
c
,
(9.1)
where
D 0 is the distance to a target at the initial moment of time
c
is the speed of light.
In the next receiving path, the signal will already have a delay:
t mov1 = t mov0 + t =
2D 0
c
+
2D
c
,
(9.2)