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    Invert it: GPS as backup to primary celestial navigation
    From: Frank Reed
    Date: 2018 Sep 26, 13:27 -0700

    Here's a philosophy to ponder for ocean-crossing navigation...

    Electronic systems fail when they are active and at a rate that is roughly proportional to time in use. If your GPS system is active for 100 days, and there is a 1-in-10,000 daily failure rate, then you have a 1% chance in those hundred days of seeing a failure in the system. So turn it off... Save it for a rainy day.

    As we all know, for nearly all mariners, knowing your exact position to the nearest few meters out in the middle of the ocean serves no purpose (except in crowded sea lanes where it is broadcast as part of the ever more important AIS). Celestial navigation is more than adequate for the task of fixing a vessel's position in good weather. We can get decent running fixes once every couple of hours during the day. Throw in a star fix or two, and what more do you need? When less than 100 miles from land, turn on the full GPS system. And if it's been cloudy for a day and shows no sign of improvement, set aside the sextant and switch to GPS. Save GPS for when you need it. And if you doubt your celestial at all, then try something that confirms your celestial via GPS (an inverse use of my GPS.anti.Spoof app for example).

    Consider this as yet another alternative way of "justifying" celestial navigation. It turns the usual logic of celestial as backup on its head. Treat celestial as the primary navigation system to be validated occasionally by GPS. It's a means of saving your GPS functionality for the circumstances where you really need it. Use your sextant to conserve your batteries and your electronics. Of course, when in doubt, under the slightest uncertainty about position, you use GPS and you back that up with more GPS/GNSS. That's king. But when the weather is fine, and the rocks are far away, celestial can serve...

    Frank Reed


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