NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Deviation Card with GPS
From: Bill B
Date: 2006 Jul 26, 01:25 -0500
Fred wrote:
> The orientation of the GPS antenna doesn't affect the GPS readings.
I fully agree. And at 3+ nm a masthead rolling through 30d won't much
affect a bearing. Given display in 1d increments, below the noise level.
Set anchor with the wind holding the bow in one direction and old swells
coming from another point, and try use a GPS as a compass. Fruitless.
Almost all the motion from a masthead antenna will be lateral.
In our situation, we were making 3.5 kt forward (nominally 6 ft per second),
and maybe rolling 3-5d from side to side. Assuming a 50' stick, the
masthead antenna is moving laterally at 1-2 ft per second given swell
frequency on the the southern end of of Lake Michigan. If one is attempting
to obtain course (more properly COG--not bearing to an object) from the GPS,
what one observes is a rapid change from the mean to 5d to 10d on either
side of the mean.
To compound the situation, the Catalina 350's mast is not keel-stepped. It
is forward of the keel. The hand-held unit in the cockpit (aft) was subject
to the whim of the helmsman but closer to the water line and well aft of the
keel. As a result the mast antenna was moving (with yaw) in the opposite
direction of the hand-held unit, the hand-held unit at greater velocity due
to its distance from the pivot point. Yadda yadda yadda.
As implied, a poorly set up trial and in the end a fool's errand. Getting
the GPS units to agree on course on reaches was closer to chance than design
(given "reach" is debatable choice of words under power, but rather reflects
the wave action relative to the lubberline).
To paraphrase Thomas Edison, "I know one more way not to box a compass."
<G>
Bill
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From: Bill B
Date: 2006 Jul 26, 01:25 -0500
Fred wrote:
> The orientation of the GPS antenna doesn't affect the GPS readings.
I fully agree. And at 3+ nm a masthead rolling through 30d won't much
affect a bearing. Given display in 1d increments, below the noise level.
Set anchor with the wind holding the bow in one direction and old swells
coming from another point, and try use a GPS as a compass. Fruitless.
Almost all the motion from a masthead antenna will be lateral.
In our situation, we were making 3.5 kt forward (nominally 6 ft per second),
and maybe rolling 3-5d from side to side. Assuming a 50' stick, the
masthead antenna is moving laterally at 1-2 ft per second given swell
frequency on the the southern end of of Lake Michigan. If one is attempting
to obtain course (more properly COG--not bearing to an object) from the GPS,
what one observes is a rapid change from the mean to 5d to 10d on either
side of the mean.
To compound the situation, the Catalina 350's mast is not keel-stepped. It
is forward of the keel. The hand-held unit in the cockpit (aft) was subject
to the whim of the helmsman but closer to the water line and well aft of the
keel. As a result the mast antenna was moving (with yaw) in the opposite
direction of the hand-held unit, the hand-held unit at greater velocity due
to its distance from the pivot point. Yadda yadda yadda.
As implied, a poorly set up trial and in the end a fool's errand. Getting
the GPS units to agree on course on reaches was closer to chance than design
(given "reach" is debatable choice of words under power, but rather reflects
the wave action relative to the lubberline).
To paraphrase Thomas Edison, "I know one more way not to box a compass."
<G>
Bill
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
