A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Noon sun fix example
From: Jeremy C
Date: 2009 Sep 30, 16:33 EDT
From: Jeremy C
Date: 2009 Sep 30, 16:33 EDT
That's a hard question to answer fairly. We have constant position fixing and pretty much constant engine speed. Usually we DR one hour ahead, and perhaps to the end of the watch (4 hours ahead). The DR is pretty meaningless because we are constantly monitoring out position along the track line and periodically adjusting the heading to keep relatively close to the track line.
To answer the question, if the current changes and the mate is too lazy to change course during the watch, you can be several miles off of the DR. If your are slowed by current or weather, then you can certainly fall behind, but be on the track line. Other times, you can be basically right on the DR.
I calculate SMG and CMG every hour by sailing so I have a very good idea where we are going, and if the current doesn't change, I can be very close to my DR, even by changing the course once in the hour, or not at all. If you use an "instantaneous" GPS course and speed for SMG/CMG at the top of the hour for your DR, the error can increase.
Despite all of this, unless you are in pilotage waters or making landfall, there isn't a lot of concern these days with exact navigation in the deep water over the course of several thousand miles. I tend to be very close as a matter of professional pride.
In a message dated 9/24/2009 12:12:32 P.M. Eastern Daylight Time, firstname.lastname@example.org writes:
OK, Jeremy. While we're on the subject of ship navigation, I've often wondered how closely a ship's DR squares up with GPS. -Hewitt
On Wed, Sep 23, 2009 at 6:40 PM, <Anabasis75@aol.com> wrote:
Not a problem there Hewitt. I know that small boat sailors don't have many advantages with their shooting as I do, but I was wondering why the results of the reductions were different with the same data. This explains it.JeremyIn a message dated 9/22/2009 12:39:28 P.M. Eastern Daylight Time, email@example.com writes:Hi, Jeremy -
Doing celestial from a ship is obviously a lot different from doing it
aboard a sailboat. A fair number of small-boat sailors - me for one -
would consider a 4' difference between a GPS position and a celestial
fix pretty much OK - 4' being just a little beyond the horizon at an
eye height of 9 feet.
Anyway, the reason for the difference is that I have an aversion to
altering other people's data. So, I took your first and last sextant
shots as equal altitudes, even though they weren't quite. I did the
same thing in the 'Noon Sight Shootout' a few months ago. Just took
George's data as presented.
Attached is a work sheet where your last Hs is backed up to the time
when it would have equalled your first Hs.
That done, my longitude now comes out 0.2' West of yours.
Thanks again for the real-world data. -Hewitt
On 9/21/09, Anabasis75@aol.com <Anabasis75@aol.com> wrote:
> Hi Hewitt,
> Thanks for taking the time to do this via the table method. I have to say
> that I am pretty disappointed with the longitude determination from this
> method. 4.3' is a pretty big error in my book and would send me looking for
> math or sight errors if it happened out here. I find it curious why there
> is such a difference between this method and the ones Peter and I used to
> get much more accurate fixes.
> Being unfamiliar with the book that you used, I am wondering if there is
> some explanation as to why the results differ so greatly.
> In a message dated 9/19/2009 10:06:35 A.M. Eastern Daylight Time,
> firstname.lastname@example.org writes:
> Hi Jeremy -
> Here's a paper-and-paper method of finding Longitude at Noon based on
> the motion correction table published in Latitude and Longitude by the
> Noon Sight.
> My work sheet is the .doc attachment.
> The table scan is the .jpg attachment.
> As you'll see, this method differs from your 1300 GPS by .4' in Lat
> and 4.3' in Lon.
> Thanks for providing data from actual combat conditions. :-)
NavList message boards: www.fer3.com/arc
Or post by email to: NavList@fer3.com
To unsubscribe, email NavListemail@example.com
Join NavListYou can also join by posting. Your first on-topic post automatically makes you a member.
Posting CodeEnter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.