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    CPA
    From: Kev
    Date: 2010 Feb 18, 18:02 -0800

    Further to NavList 10169 re... pre-ARPA days etc.

    If the nearest approach of a target vessel is obtained by aligning the bearing cursor/grid with the target’s trail on the Plan Position Indicator (PPI) this will show the CPA by eye estimation relative to range rings or by variable range marker. If this target is detected at short range and the CPA needs to be increased a quick assessment for own vessels change of course is required.

    If the bearing cursor/grid is rotated from the target to align with the direction it is wished for the target’s relative trail to track on the PPI (allowing for the greater CPA) then this direction can be ‘read off’ and the difference from the targets present relative direction noted , (say x° ) to starboard.

    If both observer and target vessel are proceeding at the same speed then own vessel needs to alter course to starboard by 2 x° to achieve this wished CPA.
    If target vessel speed ‘v’ is slower than own speed ‘u’ then an alteration of less than twice will suffice and similarly if ‘v’> ‘u’ a larger alteration than twice will be needed.

    The course alteration factor C is suggested by deriving an empirical formula C= x (1 +v/u).

    From plots I have tried I suspect that it is only valid for v=u?

    If such a short table of multiplication factors could be found then as a quick rule of thumb they would be useful for short range encounters.

    ‘v’ would be obtained by eye estimation relative to ‘u’ or by AIS.

    Am I pursuing an erroneous method?

    Regards


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