7-11 Section 7 True and False Echoes on Display
7.4.4 False echo by multiple reflection
When there is a large structure or ship with a high vertical surface near own ship as shown in the
following Figure, multiple refection may appear on the radar display. These echoes appear in the same
intervals, of which the nearest echo is the true echo of the target.
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 setting [Economy] to [TXRX] - [TXRX] - [PRF] on the Settings
menu or setting ON in [TXRX] - [TXRX] - [Stagger Trigger]. (For the details, refer to "18.6 Setting up
Radar Antennas".)
False Eco by Multiple Reflection
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