Offshore Navigation Committee Advisories

The following items have been published on the ONCom web pages as topics of which both Junior Navigation and Navigation students and instructors should be aware.

Use of Electronic Spreadsheets for Exams and Sight Folders

The Offshore Navigation Committee investigated whether the use of electronic spreadsheets and other computer-generated aids should be allowed for any part of the exams in the Offshore Navigation courses.  EDN-88 was generated as a result of this study, which defines that all worksheets and plots submitted with open book exams must be prepared by hand by the student, and that computer generated worksheets and plots would not be accepted.  The Offshore Navigation Committee later discussed the use of computer-generated spreadsheets and computer-generated Sight Reduction forms, and came to a similar conclusion — computer-generated aids would not be accepted for a Sight Folder. For Sight Folders submitted for grading, each sight reduction included in the Folder must be accomplished on a USPS SR96 Form with all values entered in the applicable blocks on the form by hand; the associated plots for these sights must also be prepared by hand by the student.

GPS Time vs. UTC

Some students have observed differences between the time their GPS receiver displays and Coordinated Universal Time (UTC) that they get from sources such as the US Naval Observatory or WWV.  Check out the GPS Time page to get some insight into possible causes. (Updated 13 Feb 06)

Windows XP and Plot Tools

Some users have reported a problem running Plot Tools on Windows XP systems.  If you encounter this problem, be sure you are running the latest version (1.5) of Plot Tools dated 19 January 2000.  If you are running an earlier version, download version 1.5.  Be sure to change the file name to "winplot.exe" when you save the file on your computer, then execute (double click) "winplot.exe" to install it.  If you are already running version 1.5, then try the following work around:

• Locate the PlotTools directory (usually installed under C:\PLOTTOOL) and right click on CLSPLOT.exe.
• Select "Properties".
• There should be two or three tabs, including "General" and "Compatability"; select "Compatibility"
• Click on "256 colors"
• Also click on "640x480"
• Click on "Apply"

Now, run Plot Tools again.    (16 Oct 05)

Changes in 2004 and Later Nautical Almanac Correction Tables

U. S. Naval Observatory has confirmed to the Offshore Navigation Committee that the values for Altitude Corrections in the 2004 Nautical Almanac have been slightly revised from earlier years. The changes are due to a recalculation of refraction and affect the corrections for all bodies. The Almanac pages affected are A2, A3, A4, xxxiv, and xxxv. Except for the additional corrections for planets on page A2, these pages have been considered permanent pages that do not change from year to year. With this recalculation they are still considered permanent and are not expected to change again in the foreseeable future.

The changes on page A2 for the Sun, Stars and Planets are limited to 0.1’ and because of the way the tables are arranged will be noticed in only a limited number of cases. Page A2 for altitudes 0º-10º has changes of up to 0.7’ at 0º altitude but only spotty changes of 0.1’ above 2º altitude. Page A4 for non-standard conditions has been redrawn and another column added to the table of corrections. This is where the majority of differences will be found. The changes to the moon corrections on pages xxxiv and xxxv are limited to altitudes below 4 degrees, and are 0.7’ at 0º altitude. These changes have an insignificant effect on practical navigation, but may cause the values obtained to be slightly different than if the previous almanacs were used.

The effects on the JN and N courses are described below. Please make sure this gets to all instructors and students.

The course material including homework and exams were developed prior to these changes, and are based on the pre 2004 Altitude Corrections. The pre 2004 Altitude Correction pages, except for page A4, are included in the “Excerpts from the 199X Almanac” in both the JN and N course material. For those not having access to a pre 2004 Almanac, a PDF copy of page A4 (58KB) is available. These pages should be used to obtain values for Altitude Corrections for our courses.

Sight Folders should use the Altitude Corrections from the Nautical Almanac for the year in which the sight was taken.   (16 Oct 05)

On-Line Nautical Almanac Notice

The  Educational Department has the following announcement regarding Nautical Almanac data:

There are several versions of the Nautical Almanac on the Internet.  The Offshore Navigation Committee has looked at these and found that the data varies slightly from that found in the commercially printed Nautical Almanac sold by retailers and USPS Headquarters.  More importantly, most of these on-line Nautical Almanacs do not provide all the intermediate values required on the SR96 Form (such as SHA, ‘d’ and ‘v’ correction), nor do they include the ‘increments and corrections’ section.  Sight folders will be graded using the data from the printed version of the Nautical Almanac, and discrepancies will be considered errors.

Please pass this information to celestial navigation students, sight proctors, and instructors.   (16 Oct 05)