NavList:

" 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."

During the early years of the Apollo project, the ground-based tracking system did not exist. At the beginning of the program, the primary purpose of the sextant system was to periodically correct for the drift of the onboard inertial navigation system, used by the guidance computer to control the spacecraft. An analogy would be the use of lunars aboard a ship to correct the rate of a chronometer.

When ground-based tracking was developed and proved to be a better means of correcting the inertial navigation instruments, the sextant was relegated to a backup role.

More on the system:
http://www.ion.org/museum/item_view.cfm?cid=6&scid=5&iid=293

Don Seltzer
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