Radar Observer · Operation — Guard Zones
A radar/ARPA guard zone or guard ring is used to:
- APhysically protect the antenna
- BSound an alarm when a target enters (or auto-acquire it within) a set range/bearing area, aiding the lookout✓ Correct
- CBlock targets from the display
- DSet the maximum range scale
Explanation
A guard zone is an operator-defined annulus or sector; a new target crossing it triggers an audible/visual alarm (and on ARPA can be auto-acquired). It is a useful backstop on a quiet watch but must not replace continuous lookout (Rule 5) — weak targets may not trip it.
Authority: IMO Performance Standards for ARPA
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