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Subject: Re: Relative plotting vs Geographical plotting
Smith_Peter@XXX.XXX
Date: Fri Jan 11 2002 - 15:24:20 EST
Pierre Boucher [mailto:pboucher@XXX.XXX] said:
> Here is the problem (Radar) - from Ocean Navigator
>
> Own course and speed:
> 150d 12.0 kt
>
> On the radar screen (relative bearing mode):
> at 0448 a "spot" at 7.0 nm 322d on the screen
> at 0554 a "spot" at 5.4 nm 322d on the screen
>
> Request :
> Other ship's course
> Other ship's speed
...
> The reason for my posting is:
>
> Solving with the maneuvering board (relative mouvement NIMA Pub 217
method)
> give answers that somehow do not agree with the geographical plot solution
???
I don't have pub 217 handy, but if it gives a similar example to that
in the on-line Radar Navigation and Maneuvering Board Manual
(http://pollux.nss.nima.mil/pubs/pubs_j_show_sections.html?dpath=RNM&ptid=10
&rid=244)
you have to be careful. The example in the on-line manual is worked from
a PPI display -- i.e., north-is-up, as opposed to ship's-head-is-up. For
the vector addition to work, all vectors must be relative to the same
reference, either North or ship's heading.
To solve the problem on a maneuvering board:
-- Mark the two target positions: angle 322d, distance 7.0 and
angle 322d, distance 5.4. Draw a vector from the first to the
second. This is the target's relative motion vector. By my
reckoning it is 142d, distance 1.6 .
-- Draw your own motion vector from the origin: the direction of
the vector is 000d (your own heading relative to your own heading),
length 13.2 (12 knots for 1h 06m).
-- Transfer the target's vector (142d/1.6) to the end of the vector
just drawn, giving the vector sum of the two.
-- Draw a vector from the origin to the end of vector sum. This will
be the target's course and distance run RELATIVE TO YOUR OWN COURSE.
By my reckoning, it is 004.7d, distance=12.0 (which over 1h 06m
equals a speed of 10.9 knots)
-- To convert the relative course to true, add 150d (your course) and
wah la! Target's course is 154.7d, speed 10.9 .
-- Peter (who also get the wrong answer the first time by applying
the example in the manual)
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