Furuno FR-1500 - Manual (Page 78)

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Indirect echoes
A target at close range is usually picked up directly, but it can also be received as reflection from a
large, flat surface. This will result in the radar presenting two or more echoes on the display, each
at a different range. The ARPA or ATA can acquire and track the false echo if it is detected by five
consecutive scans. Reduction in radar GAIN can eliminate the multiple echoing but care should be
taken as range detection also will be reduced.
Radar interference
If interference is extreme due to another radar operating at close range, spiral "dotting" and/or false
targets may appear momentarily. The interference rejector can clear the display.
To receive radar beacon or SART signals, turn off the radar interference rejection and echo
average which operate on the correlation technique.