7-9 Section 7 True and False Echoes on Display
7.4.5 Second time echoes
The maximum radar detection range depends upon the height of the radar antenna and the height of a
target as described in the section "7.1 Radar Wave with the Horizon". If 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 transmitter pulse length is MP3 (on the repetition frequency of 1400
Hz), the primary pulse is reflected about 58 NM or more away from a target and received during the
next pulse repetition time. In this case, a target image appears as a false echo (second time echo) at a
position that is about 58 NM less than the actual range on the radar screen.
If the false echo appears at 5 NM on the radar display, the true range of the target is 5 + 58 = 63 NM.
On the transmitter pulse length is SP1 (on the repetition frequency of 2250 Hz), a false echo may
appear at a position that is about 36 NM less than the actual range.
This type of false echo can be discriminated by checking the change of the target range by switching
the transmitted pulse length (the repetition frequency).
False echoes can be suppressed by specifying [Economy] after selecting [Setting] – [TXRX] – [PRF] or
by setting [Stagger Trigger] to On after selecting [Settings] – [TXRX] - [Stagger Trigger]. (For the
details, refer to "13.7 Setting Radar Antennas".)
7.4.6 Radar interference
When another radar equipment using the same frequency band 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. In many cases, these spots do not always appear at the same places, so that
they can be discriminated from the target echoes.
If radar equipment causing an interference pattern and this equipment are of the same model, their
transmitting repetition frequency is nearly the same. As a result, interference patterns may be
displayed concentrically.
In this case, since the interference patterns cannot be rejected by using only the interference rejection
function, fine-tune the transmitting repetition frequency. (Refer to "13.7 Setting Radar Antennas")
By applying different transmitting repetition frequencies, the interference rejection effect can be
improved.
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