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rgleason · 02-08-2014, 06:31

Inside the Plugin there is and Information Html documentation I know we are not writing a How to here, but perhaps NAV could review it?
Following are some of the possible inclusions in that documentation.(Celestial Navigation_pi > Celestial Navigation Sights menu > Information.

Also, separately, we should probably be updating Cnav Plugin documentation on the Opencpn.org website... There does not seem to be documentation for using Cel Nav on the Plugins page here...
Plugins | Official OpenCPN Homepage - Cel Nav does not appear
Celestial Navigation | Official OpenCPN Homepage -here a result of search, which is a begining.
Celestial Navigation PlugIn | Official OpenCPN Homepage - Old note
http://opencpn.org/ocpn/node/143 - Supplimentary Software
PlugIns | Official OpenCPN Homepage - Download page
I will look at this in the next couple of days. NAV if you can help with any suggestions, write-ups, links or editing. Please advise. (You can register yourself to be an editor, and then login and edit. If you want to..)User account | Official OpenCPN Homepage I will pull some of this together into a Cel Nav Plugin page and get it started..

Quote:

Standard practice in navy and merchant navy with regard to celestial positioning is as follows (assuming no artificial horizon is available on the sextant):

During morning nautical twilight (about 15 minutes): if possible take 6 star/planet observations.
In the morning at about 09:00LT: take a sun's observation (later to be shifted to noon-time).
At local noon: take a sun's observation (traditionally only direct latitude calculation from sun at meridian passage). Construct a fix for noon from both sun observations.
During evening nautical twilight (about 15 minutes): if possible take 6 star/planet observations.
Use moon for an observation in above practice when situation permits.

Quote:

Digital Nautical Almanac on the web. It's from 1994. Link: http://reednavigation.com/files/Naut...manac-1994.pdf

rgleason · 02-08-2014, 07:13

I've reconnected the orginal page and made some prelim edits to be more accurate. Will be getting screenshots and better documentation together. Any help, writeups, procedures would be most appreciated! If NAV could review this page and advise on inaccuracies and improvements it would be great.
Celestial Navigation PlugIn | Official OpenCPN Homepage
I will try to incorporate what Sean has under the Information Tab as well.

Thanks.

NAV · 03-08-2014, 00:34

Quote:

Originally Posted by rgleason

NAV if you can help with any suggestions, write-ups, links or editing. Please advise.

I suggest to base the general theory on the document found here (say chapters 1 to 3): http://www.celnav.de/page2.htm . And perhaps partly on information from Bowditch and the Nautical Almanac.

The actual theory and computation methods to calculate a fix from reduced measurements used in the plugin should initially be written by Sean I think. It's an innovative method of which only he knows the background en details. When I look at the code (which I understand only marginally, I still have some questions; I'll write these down later.)

NAV · 03-08-2014, 01:08

Two more documents with test results. Another one for a comparison of altitude reduction, and one for a comparison between the direct computation method for a fix as explained in the Nautical Almanac (1994, pages 283 para 12 and page 283), and the fix results of the plugin.

rgleason · 03-08-2014, 06:13

Nautical Almanac 2004
Looks like H apparent is good. Is there some difference in the way H Observed is calculated? --Something to do with Refraction, Parallex, Par.in Alt. or SD?

Direct Comp
I'll let Sean respond. I don't know enough yet.
Thank you for the good links on General Theory.

seandepagnier · 03-08-2014, 11:16

Quote:

Originally Posted by rgleason

Nautical Almanac 2004
Looks like H apparent is good. Is there some difference in the way H Observed is calculated? --Something to do with Refraction, Parallex, Par.in Alt. or SD?

Refraction varies with temperature and pressure.

I would like to compare the values the plugin uses for refraction, parallex, and limb correction to the almanac test data.

Also would like to compare the geographical position. So far from the data NAV has posted, the differences in Ho are much smaller than the measurement error of reading a sextant.

Also consider my models are more recent and probably more accurate.

Quote:

Direct Comp
I'll let Sean respond. I don't know enough yet.
Thank you for the good links on General Theory.

Maybe a separate dialog for this one? It would be good to offer the current methods as well as the more traditional LOP methods. The simplest is by using linear least squares for each celestial body. This method can be iterated several times to converge on the fix. It would be good to figure out how to display computations, and even plot the actual lines as a learning tool for teaching traditional celestial navigation.

rgleason · 03-08-2014, 17:45

I've started adding to Celestial Nav Plugin Docs here, others are welcome to edit and add. I have plans to add NAV's links and the original sightings he listed, for reference.
Also I will format it better and try to complete the process more completely.
Celestial Navigation PlugIn | Official OpenCPN Homepage

Right now I am having trouble getting this page to show up in the documentation.
Why?

rgleason · 04-08-2014, 07:23

Documentation: The page is now showing in Plugins secton. I had to go down to the bottom while editing the page and select "Book Outline" and set the priority "-13" and pick the parent directory "Plugins". Now it shows in the Plugins list! Plugins | Official OpenCPN Homepage
I had not placed it in the Contents Index.

NAV · 04-08-2014, 09:18

Quote:

Originally Posted by boat_alexandra

I would like to compare the values the plugin uses for refraction, parallex, and limb correction to the almanac test data.
Also would like to compare the geographical position. So far from the data NAV has posted, the differences in Ho are much smaller than the measurement error of reading a sextant.
Also consider my models are more recent and probably more accurate.

Accuracy of data in the printed Nautical Almanacs:
The largest error that can occur in GHA and declination of any body other than the Sun or Moon is less than 0.2'; it may reach 0.25' for the Sun and 0.3' for that of the Moon. In practice it may be expected that only one third of the values of GHA and declination will have errors larger than 0.05', and less than one tenth will have errors larger than 0.1'.

The errors in the altitude corrections are nominally in the same order (but the actual values of dip and refraction at low altitudes may differ considerably in extreme atmospheric conditions).

Depending on the type of sextant, the reading accuracy of the sextant can be 0.2', 0.1' or 10". Measurement and reading errors made by the navigator will be larger ...

rgleason · 04-08-2014, 09:55

Ok the Cel Nav plugin Documentation has been updated.
http://opencpn.org/ocpn/celestial_navigation_plugin

Please review and advise on any problems, errors inaccuracies, bad links.

Also just a question, out of curiosity is NAV Henning Umland?

rgleason · 04-08-2014, 10:38

From celnav.de Page 13-3 http://www.celnav.de/page2.htm

Quote:

For great circle navigation, the gnomonic projection offers the advantage that any great circle appears as a straight
line. Rhumb lines, however, are curved. A gnomonic chart is obtained by projecting each point on the earth’s surface
from the earth’s center to a plane tangent to the surface.

Are gnomonic charts able to be created from spheric on a computer?
Maybe this chart would help with time offset, if it is easy to make and use this type of chart?

NAV · 04-08-2014, 10:50

Quote:

Originally Posted by rgleason

Also just a question, out of curiosity is NAV Henning Umland?

No, certainly not :-). Henning Umland is a scientist. I'm only an ex-navigator (merchant navy, navy, naval air department).

NAV · 04-08-2014, 11:40

Attached descriptions from Andres Ruiz of modern calculation methods in astronomical positioning may also be interesting. ( https://sites.google.com/site/navigationalalgorithms/papersnavigation)