Platform_yaw_angle as defined per Bruno/CF makes more sense to me, but
it appears to be incorrect with respect to the current definitions you
can easily find for yaw angle. (hard to find something definitive
here) Attack angle is around the horizontal left-right axis -- it is
the difference e.g. between the airfoil angle and the line of movement
of the airplane through air.
I wish platform_orientation (which is 3D to me) could be
platform_pointing_direction, by the way, since it can't be platform
heading.
But that aside, the difference in the original definitions is that the
first (platform orientation) is measurement of aircraft orientation
with respect to a fixed external reference (true north or magnetic
north being the most common), so it's a number between 0 and 360;
while the second (platform yaw angle) is the difference between the
platform orientation (which way it's pointing) and the actual
direction of travel (track made good over the ground), so it's
typically a number between -10 and 10 for airplanes, and highly
varying numbers for ships. (A slow ship in high currents may have to
point almost totally upstream to go horizontally, which would make for
a yaw angle near 90 degrees.)
john
On Oct 23, 2008, at 4:08 PM, Godin, Michael wrote:
> Bruno,
>
> Like all decisions made by committee, the decisions on these names
> seem rather odd now. I think I had originally proposed only
> "platform_heading" to indicate the "forward looking" direction of a
> platform, and Roy Lowry wisely pointed out the ambiguity between
> longitudinal axis and direction of travel.
>
> I countered with "platform_forward_looking_direction".
>
> Nan Galbraith noted that on "ships, true_heading is the gyro
> reading, corrected for magnetic declination. Course is the direction
> of the ship's movement, generally acquired from gps."
>
> Chris Webster noted that for aircraft "track_angle is our direction
> of travel."
>
> After a discussion of true vs magnetic heading, Nan asked "Is there
> anything wrong with using platform_heading, platform_heading_true
> (or platform_heading_corrected), and platform_course?"
>
> Jonathan Gregory noted and asked "The problem with "heading" is that
> it means direction of travel as well as the direction you're
> pointing in. That's what I understood it to mean when Michael
> proposed it, and why I asked if it was the same thing as course.
> Looking on Google, I find that half the definitions are direction of
> travel.
> Hence "heading" is too confusing, I'd say. How about
>
> platform_orientation (degree) for the direction the platform is
> pointing in platform_course (degree) for the direction it's
> travelling in?"
>
> And then (quite clearly out of my mind), I noted: "The one case I
> can think of a yaw variable being useful and measurable is in the
> case of towfish, in which yaw is typically defined as the difference
> between orientation and course. I imagine there may be other
> similar useful situations, so I'd support adding the variable:
> platform_yaw ('difference between orientation and course')(degree)"
>
> I'm not sure why I inexplicably used the word "yaw" when I should
> have used "attack angle".
>
> Anyway, everyone was sick of the discussion at that point in time,
> and the variables were added with no further discussion as:
>
> platform_orientation ("pointing direction") (degree)
> platform_course ("travel direction") (degree)
> platform_yaw ("difference between orientation and course")(degree)
>
> However, the documentation for platform_yaw (wisely?) disagrees with
> the above.
>
> If I were sane and looking at this today, I'd 1) declare
> platform_yaw to be "travel direction", 2) make platform_course an
> alias for platform_yaw, and 3) add a new standard name
> platform_attack_angle -- which is a variable I'm going to need soon.
>
> Mike
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John
--------------
John Graybeal <mailto:graybeal at mbari.org> -- 831-775-1956
Monterey Bay Aquarium Research Institute
Marine Metadata Interoperability Project:
http://marinemetadata.org
Received on Thu Oct 23 2008 - 17:55:24 BST
