Re: HP41 Navigation Pac Perpetual Alminac Message #3 Posted by Dan Pfeiffer on 3 Sept 2013, 7:39 p.m., in response to message #2 by Dave Shaffer (Arizona)
See the PDF with the data here:
dan.pfeiffer.net/boat/hp41nav vs almanac.pdf
The tests were all at the same location so I don't know about that variability. The difference did not change in any trend I could see. I tested for the sun and the moon figuring the moon was more likely to show error.
I am not sure that using different longitudes would make a difference in what I was testing? I was not finding altitude/azimuth from a given location. Just the GP of the body by the almanac vs by the nav pac. Now that I think about it I should look at SHA/dec for the celestial coordinates and that would take out the error from the aries difference. I think.
Not sure about shape of the earth but the almanac routines do correct for precession, nutation, aberration and proper motion according to the manual. And the reference "Low-Precision Formulas for Planetary Positions" by Van Flandern and Pulkkinen who I believe were at the USNO back in the 70's. I don't know what time corrections are applied. My understanding is that the nav rom was largely the work of Kenneth Newcomer and I have a book he wrote with Henry Shufeldt called "The Calculator Afloat" and perhaps there are some insights to methodology employed in the nav pac. It is from 1980, right about the time they would have had development in high gear?
In general for navigation purposes getting a celestial fix that is good to within a mile is a job well done. If you're that close to your destination you have switched over to mark I eyeball for fixes and if the conditions are unsuited for that you're not likely to be doing any sextant work either.
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