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    Re: Role of CN at sea, was Re: Averaging sights ...
    From: Frank Reed CT
    Date: 2004 Oct 13, 02:35 EDT
    Herbert Prinz wrote:
    "In my admittedly very limited experience of ten thousand off shore miles over the last
    ten years I have not ONCE been in a situation where GPS didn't work, but cel nav
    would. In fact, I have not once been in a situation at sea where GPS didn't work.
    Full stop. But I have REPEATEDLY been in situations where celestial was
    unavailable for several days in a row and GPS was the only position finding tool
    available. Conclusion: Celestial is not even a backup!"

    When asked  the question "What's the best backup for GPS?", probably the best answer is "Another GPS!".

    There is an ambiguity in the above Q&A that may be worth pointing out. The obvious interpretation is that a sailor should carry a spare GPS receiver onboard in addition to the main GPS system. This spare (and maybe a spare for the spare) should be stowed securely in an emergency kit and checked occasionally just as you would check all other emergency gear. But there's another interpretation. What's the best backup for the GPS system itself? And the answer there, too, is another GPS --the whole system. The Europeans, if I remember correctly, are gearing up to spend a small fortune launching an independent GPS-like system. There are plenty of reasons for this, but the most basic is that ANY system should have a backup. And I should note that there is already a similar Russian system, but it seems to be less relevant for practical applications.

    I think there's a strong analogy in this current situation to the final days of lunars in the latter half of the nineteenth century. Back in 1850 the question would have been "What's the best backup for a ship's chronometer?" And the answer most navigators had settled on by that date was "Another chronometer!". Some small number of navigators continued to count lunars as their emergency backup, but in truth, were they shooting lunars for the same reasons that we shoot lunars today? For fun and challenge and a connection with the past.

    I think there's something poetic about GPS in the context of the history of the quest for longitude. In the late eighteenth century, the problem of navigation was seen as a great contest between machine and nature, between chronometers and the Moon. Machines won out, of course. GPS rules the world. But a GPS satellite is little more than a an amazingly accurate chronometer, an atomic clock, contained within a tiny artificial moon, a satellite. It's a bit of an unholy union, and yet lunars and chronometers are united in those orbiting clocks.

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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