JRC JMA-7710-6 Instruction Manual page 210

6.4
[V] Anomalous Propagation
The maximum radar range depends on the heights of the antenna and target as explained in section
"6.1 Radar Line-of-sight Range". If duct occurs on the sea surface in certain atmospheric conditions,
however, a transmitted beam of enery will be abnormally propagated and targets far away from the
own ship may be detected.
For example, when the radar has been set to the range of 6nm short pulse (with repetition frequency
of 1560Hz), the first pulse is reflected from a target about 52nm or more away and is received at the
time of next pulse repetition and its false echo is formed at a place about 52nm shorter than the actual
distance.
Should a false echo appears 5nm away on the screen, the true distance of the target is 5+52=57nm.
When the radar has been set to the range of 1.5 nm short pulse or medium pulse (with the repetition
pulse of 2080Hz), a false echo may appear at a place about 39nm shorter than the actual distance.
This kind of false echo can be judged in observing distance changes of echoes by changing the range
scale (the repetition frequency).
When those false echoes always appear on the screen, set "ECONOMY" mode of transmission
repetition frequency.
See subsection "8.5.3.6 Setting Transmission Repetition Furequency" or section "3.4 Using Function
Key [FUNC]".
[VI] Radar Interference
When a radar of the same frequency band is near the own ship, a radar interference pattern will
appear on the screen. This interference causes various patterns consisting of many spots. Since these
spots do not appear at same places, they can be distinguished from target echoes. (see Fig. 6.7).
Fig. 6.7
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