Radar Observer · Collision Avoidance — Assumptions from ARPA
ARPA-computed CPA, TCPA, and target course/speed should be treated as:
- AA substitute for the steering and sailing rules
- BAlways exact and beyond question
- CAids subject to error (sensor accuracy, tracking lag, target maneuvers) that must be checked against observation and good seamanship✓ Correct
- DValid only above 12 knots
Explanation
ARPA outputs carry errors from radar sensor accuracy, the tracking filter's lag (especially just after a target or own ship maneuvers), and gyro/log inputs; the data also takes time to settle. The officer must regard them as decision aids, verify with continued observation, allow for them being wrong, and still comply fully with the Rules.
Authority: IMO Performance Standards for ARPA; Navigation Rules Rule 7
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Related Radar Observer questions
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