A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brian Walton
Date: 2017 Dec 30, 08:23 -0800
Solstice Sun Lines
Between 1991 and 2001 I was flying EFIS Boeings around the world. I was also an experienced ocean yachtmaster, well versed in CN techniques employed before the advent of scientific calculators and GPS.
For my own interest, I undertook astro fixes whilst in the cruise. By then, programmable calculators existed, and I used a Casio 1270 running Astro Nav Comp, and later a Psion 3a into which I could add sub-routines, for instance, Coriolis. This obviated carryingalmanacs, reduction tables, and plotting gear. I usually carried a A10 or Navy A5, both hand held bubble sextants, either ofwhich fit easily in a flight bag. I did over 150 Pacific crossings, somewhat less Atlantic crossings, and usually managed a midway astro fix on most. Let’s say, 100 fixes. Here is a brief resume of the procedure I used.
1. Set the aircraft up in auto-pilot, auto-throttle, great circle tracking between waypoints. Have Lat/long, GMT and Fix pages displayed. Hand over control.
2. Look for a bright body on the beam, and another near the nose, between about 40-69° high.. Estimate, or calculate their rough altitude.
3. Set the sextant to the body abeam, gather and refine. At a smoothe moment, take a single shot, note time, and Hs. No averaging.
4. Quickly reset the sextant to the body on the nose, gather, and shoot. This should take less than a minute. Having shot, press fix to freeze position and time, and note them down, with ground speed and heading.
5. Copy fix position (DR) groundspeed, body, time and Hs of first shot, into the calculator. This will give a PL. Copy second body shot into the calculator; this will give a second PL. My calculator would come up with a 2 PL Lat/long fix within seconds, and give bearing and distance to the (DR) fix waypoint, that is the accuracy of the astro fix.
I ignored auto-pilot wander, distortion near windscreen edges, different refraction at height, and cross track error caused by not doing a running fix. I was more interested in getting a fix within 2 to 4 minutes.
Most results were within 20 miles, many within 10, and some much luckier. Those greater than 20 or so were usually finger trouble. I was quite happy with an accuracy within 20 -30 miles, since from height, an aircraft can get ground based electronic information from over 100 miles, and possibly see mountains, at least on radar.
Many forum members may question this poor accuracy, but, in a yacht, I am happy with a 5 mile fix if I’m aiming for a lighthouse or mountain visible at 20 miles. There may be more important things to do.
The above technique gave me confidence following a double FMC (on-board computer) failure, and also whilst flying over the millenium change, when many mooted multiple computer crashes.