T
-900 series Chapter 7 Principle of radar system
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7-4
The microwave beam radiated from the antenna has a side lobe in different direction than that of the
main beam. Since this side lobe is lower than that of main beam, the effect is negligible for targets at a
long range, but a close, strong reflecting target may cause false image in a circular arc shape.
If a large target such as land exists near your ship, a circular arc shaped
image may appear.
3
1
HU
Side lobe
Main lobe
Antenna
Virtual image
by sidelobe
Real image
Figure 7-7 False image by sidelobe
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False image of a distant target caused by “duct”
Depending on a weather conditions, a duct occurs by the temperature inversion, etc.
In this case, the unusual propagation of the radio wave may reach distant targets out of radar range.
A target at more than the maximum range may appear as an image, and may be displayed as a false
image closer than an actual distance. This phenomenon is a result of the long range echo delay time
exceeds the transmission period, and is displayed as the echo in the next cycle.
When changing the range of a distance, if the distance of the target changes, it can be judged as a
false image.
7.3
Radar interference
When the radar with the same frequency band is used nearby, interference noise appears on a display.
Although appearance of interference is not constant, the shape is almost always swirling or radial. This
device features an interference rejection facility to reduce interference.
