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9 Compensation of Signals from Stationary Objects
• significant excess of interference power over the receiver’s own noise power
(dynamic range of passive interference, i.e., the ratio of the passive interference
power to the noise power can reach 90 dB);
• the difference of passive interference from signals, reflected from moving targets
due to different dynamic characteristics of reflecting objects (radial velocity, acceleration, etc.) or statistical characteristics (correlation function or power spectral
density) of the interference itself.
The organization of suppression process of passive interference can be carried out
in different functional parts of the radio-engineering system.
Transmitting path. The protection against interference is possible here due to
the correct choice of frequency, duration and repetition period of sounding pulse,
changing of polarization law, etc. As is known, the contrast between signals, reflected
from a target and the underlying surface, increases with decreasing long pulse.
Increasing the wavelength is also an effective method for suppression of some types
of passive interference (flocks of birds, insects, moisture targets).
Antenna path. Here, the following methods of interference countering are possible:
reducing the width of directional pattern and the level of its side lobes; upward
deviation of the antenna directional pattern DP (to reduce the reflection level from
the underlying surface); use of dual-beam antennas.
Intermediate-frequency path. Temporary automatic gain control, pulse width
discriminator (interfering reflections can have a much longer duration than the
sounding signal).
Block of primary processing of radar information: notch filters, a set of narrowband filters, discrimination of moving targets.
Block of secondary processing of radar information: formation of a noise map,
rejection of false target blips at the stage of constructing a target trajectory.
At the moment, the main trend in the further development of passive interference countering methods is focused on improving the algorithms for primary and
secondary processing.
The existing detection algorithms against the background of interference,
depending on the principle of operation, use either static differences between targets
and passive interference, or spectral.
Countering passive interference requires, first of all, the weakening of power of
interfering reflections received by the radar antenna and narrowing of the dynamic
range of interference to prevent overloading the receiving path. The first of mentioned
tasks is the most common when ground-based radar needs to detect aerial targets,
for example, in air traffic control systems.
To reduce the intensity of signals, reflected from objects located on the ground
surface, two main methods are used. The first one consists in deflecting the antenna
beam (DP) of the ground radar upward and makes it possible to improve the power
ratio of the useful and reflected signals from ground objects by 15–20 dB. However,
with wide antenna DP, the lower edge of the pattern still irradiates the ground surface,
and the antenna receives signals reflected from it.
9 Compensation of Signals from Stationary Objects
• significant excess of interference power over the receiver’s own noise power
(dynamic range of passive interference, i.e., the ratio of the passive interference
power to the noise power can reach 90 dB);
• the difference of passive interference from signals, reflected from moving targets
due to different dynamic characteristics of reflecting objects (radial velocity, acceleration, etc.) or statistical characteristics (correlation function or power spectral
density) of the interference itself.
The organization of suppression process of passive interference can be carried out
in different functional parts of the radio-engineering system.
Transmitting path. The protection against interference is possible here due to
the correct choice of frequency, duration and repetition period of sounding pulse,
changing of polarization law, etc. As is known, the contrast between signals, reflected
from a target and the underlying surface, increases with decreasing long pulse.
Increasing the wavelength is also an effective method for suppression of some types
of passive interference (flocks of birds, insects, moisture targets).
Antenna path. Here, the following methods of interference countering are possible:
reducing the width of directional pattern and the level of its side lobes; upward
deviation of the antenna directional pattern DP (to reduce the reflection level from
the underlying surface); use of dual-beam antennas.
Intermediate-frequency path. Temporary automatic gain control, pulse width
discriminator (interfering reflections can have a much longer duration than the
sounding signal).
Block of primary processing of radar information: notch filters, a set of narrowband filters, discrimination of moving targets.
Block of secondary processing of radar information: formation of a noise map,
rejection of false target blips at the stage of constructing a target trajectory.
At the moment, the main trend in the further development of passive interference countering methods is focused on improving the algorithms for primary and
secondary processing.
The existing detection algorithms against the background of interference,
depending on the principle of operation, use either static differences between targets
and passive interference, or spectral.
Countering passive interference requires, first of all, the weakening of power of
interfering reflections received by the radar antenna and narrowing of the dynamic
range of interference to prevent overloading the receiving path. The first of mentioned
tasks is the most common when ground-based radar needs to detect aerial targets,
for example, in air traffic control systems.
To reduce the intensity of signals, reflected from objects located on the ground
surface, two main methods are used. The first one consists in deflecting the antenna
beam (DP) of the ground radar upward and makes it possible to improve the power
ratio of the useful and reflected signals from ground objects by 15–20 dB. However,
with wide antenna DP, the lower edge of the pattern still irradiates the ground surface,
and the antenna receives signals reflected from it.