JRC JMA-993-SA Instruction Manual page 217

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AbnormalPropagation
The maximum radar detection range depends upon the height of the scanner and the height of a
target as described in the section of "The Horizon for Radar Beam Radiation". lf a so-called
"duct" occurs on the sea surface due to a certain weather condition, however, the radar beam may
propagate to an abnormally long distance, at which a target may be detected by the radar.
For instance, assuming that the radar range is 6 NM (on the repetition frequency of 1100 Hz), the
first pulse is reflected from a iarget at about 76 NM or more and received during the next pulse
repetition time. In this case, a false echo appears at a position that is about 76 NM shorter than
the actual distance. lfthe false echo appears at 5 NM on the radar display, the true distance of
the target is 5 + 76 = 81 NM. On the radar range scale of 1.5 NM (on the repetition frequency of
1900 Hz), a false echo may appear at a position that is about 43 NM shorter than the actual
dislance.
This type of false echo can be discriminated by changing over the range scale (the repetition
frequency), because the distance of the target changes accordingly.
tvll Radar Interface
When another radar equipment using the same frequency band as that on own ship is near own
ship, a radar interference pattern may appear on the radar display. This interference pattern
consists of a number of spots, which appear, in various forms. These spots do not always
appear at the same places, so that they can be discriminated from the target echoes. (See Fig.
6.6)
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