Join Date: Aug 2006
Location: Arlington, VA
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GPS system failure is a distant possibility, excepting the potential for localized jamming or the USG deciding to once-again implement "selective availability".
However, the possibility of onboard GPS units failing is much greater. Power failures, battery depletion, unit malfunctions, and EMF due to nearby or direct lightning strikes are much more a possibility.
Celestial is a good backup, and it's very good exercise for the brain. I agree that one needn't "master" the spherical trigonometry involved. The noon sight is very easy to learn. So, too, is the noon sight for longitude, which involves taking equal altitude sights before and after meridian passage. Learn these, and if you're interested and so inclined, continue on from there.
Not sure about the plastic sextants. I like good equipment. Good sextants cost a bit, but they're worth it. Get a good used Plath or Tamaya or Freiburger or Simex or Aires sextant. For heaven's sake, don't waste your money on any of the many "replicas" available on the market....they are strictly for show, not serious use.