A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2012 Aug 25, 17:27 -0700
Cmdr Weems was brought out of retirement to derive a new system of navigation to the moon. His reference survey is occasionally available, which combines orbital mechanics and celestial navigation via the nadir of your position to derive your position in 3 space.
Cmdr Weems, of the Weems System of Navigation, derived a new system of space navigation. Mr. Armstrong had access to that system
Paul Dolkas, you wrote:
" But there was also a rather sophisticated sextant system that they could use to navigate all the way back home in the event communications were lost and they couldn't make use of the tracking system on earth. As such, it was never used (not even during Apollo 13), except once as a test on the first cis-lunar flight, Apollo 8."
That sums it up nicely.
One of my two presentations at the "After Longitude" conference at the NMM in Greenwich, UK back in March was on "Lunars in the Space Age" and a major part of it was on the Apollo sextant system. They practiced with it on ever lunar mission, but it was understood by the crews that it generally was only a backup and a backup that they probably would never use. Only Jim Lovell on Apollo 8 completed enough observations to qualify as "real" celestial navigation in space. On Apollo 11, Mike Collins was the Command Module pilot and it was part of his duties to take sextant sights, but his training didn't prepare him for what he could actually see through the instrument (there was a lot of glare) and after several muddled attempts on the first day out to the Moon, Mission Control eventually told him to 'move on to other tasks'.
The Apollo sextant was a true sextant with an interesting feature. The "horizon sight line" was fixed to the axis of the spacecraft. So to take a sight, the entire spacecraft had to be turned. This was actually quite efficient since the computer and the navigation system were so tightly integrated. The sights that they would have taken for actual navigation were literally lunars: angles between the Moon's limb and bright stars. Unlike historical lunars, these were not used to determine GMT but rather to determine a cone of position. Today, as I have described over the years in quite a few NavList posts, similar cones of position can be used to get a position fix on the Earth using lunars even when no horizon is available for a standard celestial navigation fix.
The Apollo sextant did have one important navigational use that was not mere backup. It was used as an "astrocompass" to align the inertial navigation system on a regular basis. In this case only the reflected line of sight was used and the angles measured were relative to the spacecraft's axes. It's interesting to note that the computer would automatically calculate the star-to-star angles during this process and report the difference between observed and pre-calculated angles to the astronauts so that they could detect bad observations and reject them.
The Apollo sextant also served as a computer driven telescope. When the computer was given orbital elements for itself (the Apollo CSM) and another spacecraft, usually the Apollo LM, it could automatically point the sextant at the other spacecraft and track it in real time, which was a pretty good trick and convenient for rendezvous.
On later Skylab missions, the astronauts again experimented with lunars using a handheld sextant but this was all "make work" experimentation. The manned flyby of Venus that might have taken advantage of such navigation skills was already long cancelled.
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