I carried my trusty Tamaya for many thousands of miles but only used it for diversion (the chess analogy is apt). Still the backup aspect is very real. I have only done two ocean crossings; on both my sextant had an LCD watch on the handle, set to UTC before departure, along with the
marine tables plus the appropriate almanac pages. Of course it was not likely that either the
GPS system would drop or my multiple
GPS units would fail, including the AA
battery powered handheld, just while I was out in the middle of the ocean. It is comforting nonetheless.
I have spent far more time coastwise and island hopping, and GPS has indeed failed me at critical moments. I am not about to forget the week number rollover problem that struck when I was on the Markermeer heading to
Amsterdam - the marina that evening was full of sailors trying to get their GPSs to
work. I have also enjoyed local blackouts courtesy of
Navy tests of their jamming
equipment (announced on Navtex and the local Notices to Mariners, which everyone is following, right?). The prudent mariner never relies on any one system - even one as good as GPS.
My first line of defense (after multiple, independent GPS receivers) is my other electronic
gear:
radar,
depth sounder,
knotmeter, heading
sensor, etc. I have known vessels to lose all electronic
gear, including GPS, when struck or nearly struck by
lightning. So my second line of defense is an adjusted (swung)
compass, a trailing log, lead line, and my sextant. Unfortunately the
compass is not immune to error caused by the field of a
lightning strike but there are no guarantees in life. Finally, being a little paranoid and recording position data in potentially dodgy situations (fog, rain, gales, out of sight of land, etc) doesn't hurt.
I would recommend carrying a sextant as a backup, but only after a lot of other backup and
safety equipment has been acquired first. Unless you are someone who just wants to go back to the old ways, in which case more power to you.
And for the
record, a lot of
marine publications that are published in both the US and the UK (including the Nautical Almanac) are actually a collaboration - an error in one will be in the other as well. Conspiracy theorists can have fun with that.