NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2023 Aug 12, 07:20 -0700
Eric
More on the Seven Shot Two Star Sandwich Fix.
I finished this a couple of days ago, but I’ve only just had time to check it. As I said earlier, ‘Astro Fiddlers’ are invariably caught out, because despite the occasional small error over + & - signs etc (back plotting being about twice as hard as normal plotting), the fix always ends up where the ‘fiddler’ wants it to be.
My working, which had to begin with ‘First make your chart’, is attached. For my values, I used the US Air Almanac 2023, and the US HO249 (2020 Epoch) adjusted for 2023 P&N. You will note an absence of precision in places compared to some of the members’ postings. That’s because if you want to keep ahead of the aircraft when flying at 8 miles a minute, you can’t waste time estimating to the closest 0.1 of a degree or minute of arc.
By using a sandwich fix mid-time, you can manage with just one assumed position and one move for Coriolis effect and p&n. By correcting your Ho s for acceleration errors before plotting intercepts, you don’t need to move the PLs again.
As there was no cloud above you in the tropopause, you didn’t need opportune shots, so you could nearly always pre-compute for a 10minute Air Almanac time. If you did have a snag getting the sextant up on time, you simply took all the shots late and moved the assumed position west. As I said, you mustn’t let yourself get behind the aircraft.
I can’t vouch for my assumed typical heading and speed changes because I rarely saw that bit. I was too busy taking the shots.
We would normally place an MPP along the line between the GPI(DR) position and the fix position, but I’ve left it out, because my chart is already very complicated. I’ve included the instructions for using the Mears Slide. Next post, I’ll send the instructions for how to correct the PLs for acceleration error without using the Mears Slide.
Sun, Moon, and planet shots required other parts of the form to be filled in, but perhaps we can leave that until later. DaveP
