9 Compensation of Signals from Stationary Objects
173
Fig. 9.4 Timing diagrams of OPC circuit operation
output of the OPC unit. Hence, the effectiveness of OPC circuit is limited. Failure
to meet these conditions, as well as the presence in the background composition,
on which a moving target is detected, of moving elements (e.g., vegetation in the
wind) leads to the appearance of uncompensated residues at the output of overperiod subtraction (canceling) circuit. Some types of passive interference have a
regular velocity component. Among them are rain clouds which are moved by the
wind as a single object. If you do not take measures to consider the speed of such
formations, the video noise pulses will be modulated by the Doppler beat frequency,
and uncompensated noise residues will be present at the unit output. Therefore, in
some cases, a wind compensation unit is applied, by using of which the oscillation
frequency of coherent local oscillator is changed by a value proportional to the
wind speed. The interference phase relative to the oscillation phase of coherent local
oscillator becomes unchanged from period to period, the interference compensation
is improved.
Let us determine the frequency transfer characteristic of OPC circuit. For this, we
use the impulse transfer characteristic of this circuit:
h(t) = δ(t) − δ(t − T n ),
(9.8)
The voltage transfer characteristic is defined as the Fourier transform of the
function h(t):
173
Fig. 9.4 Timing diagrams of OPC circuit operation
output of the OPC unit. Hence, the effectiveness of OPC circuit is limited. Failure
to meet these conditions, as well as the presence in the background composition,
on which a moving target is detected, of moving elements (e.g., vegetation in the
wind) leads to the appearance of uncompensated residues at the output of overperiod subtraction (canceling) circuit. Some types of passive interference have a
regular velocity component. Among them are rain clouds which are moved by the
wind as a single object. If you do not take measures to consider the speed of such
formations, the video noise pulses will be modulated by the Doppler beat frequency,
and uncompensated noise residues will be present at the unit output. Therefore, in
some cases, a wind compensation unit is applied, by using of which the oscillation
frequency of coherent local oscillator is changed by a value proportional to the
wind speed. The interference phase relative to the oscillation phase of coherent local
oscillator becomes unchanged from period to period, the interference compensation
is improved.
Let us determine the frequency transfer characteristic of OPC circuit. For this, we
use the impulse transfer characteristic of this circuit:
h(t) = δ(t) − δ(t − T n ),
(9.8)
The voltage transfer characteristic is defined as the Fourier transform of the
function h(t):