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    GPS versus sextants ?
    From: Peter Fogg
    Date: 2002 Feb 5, 10:37 +1100

    The fragility of GPS is that it is the last link of two chains, of which each 
    link represents a complex piece of technology. One chain leads to the 
    satellites being up there and, we trust, continuing to do their job 
    perfectly. The other starts on board, with the availablity of clean fuel, 
    then the motor and/or the solar panels, progresses through the alternator 
    then the batteries via the fuses and the boat's electrical
    system. If any link of these chains fails then at best we are relying on batteries with finite lives.
    Yes sextants are fragile too, but they are a link in a chain that does not 
    depend (once the almanac has been published! - although there are 
    alternatives - extended almanacs are available) on complex technologies.
    But that's not the whole answer. Navigation is an art as well as a science. 
    Its just our approach that is scientific. The Polynesians had their own 
    approach, one that relied on taking a great interest in everything that was 
    around them. Every sailor does this to some extent too, for example 
    monitoring the direction of the swell and the wind as primary navigational 
    guides, independant of all instruments. The Polynesians
    developed this approach to such a level that they could detect land over the 
    horizon by the minute difference it would make to the swell, or the tinge of 
    green reflected from a cloud, as just one small example.
    One of the many horror stories I have heard about GPS involved a 'Sports 
    Cruiser' that went offshore and, hopefully, had a lovely time, zooming about. 
    To go home they invoked the waypoint for their port and at 20 knots, running 
    with flawless accuracy towards it, hit a reef.
    These are examples of a difference in attitude that has got nothing to do with 
    one technology over another.
    > Dan-
    >  Obviously I have some interest in celestial but when I hear  on a boat in mid-ocean with only it and no backup?> that arguement against 
    GPS holds no water. One can break a sextant quite easily, in fact I defy 
    anyone to drop a sextant 4' to a hard deck and then use it again. Most GPSes 
    can tolerate a similar drop, or at least a good fumble bouncing on one's 
    feet. Most sextants can't.
    >  If you have a sextant you need a backup, figure $500 for the first sextant 
    and $125 for the cheap Davis backup. That's enough money for six GPSes or 
    four GPSes plus two solar chargers, four sets of rechargeable batteries, and 
    a gross or so or regular batteries as well.
    >  Sorry, but "the dog ate my navigational instrument" doesn't have any 
    bearing on the choice of instrument, in fact it makes the GPS more "reliable" 
    since you can carry 4-6 of them for the price of a minimal sextant with 
    backup. As for the entire GPS constellation going down...if THAT happens the 
    odds are that WW3 has broken out or aliens have arrived with an attitude. In 
    either case navigation will be the least problem.

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