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    Re: As we debate GPS reliability
    From: Frank Reed
    Date: 2009 Nov 13, 21:38 -0800

    Lu, you wrote:
    "Although it's not exactly clear what the fourth dimension is (Jeppsen says 
    it's time, but then doesn't give an example of exactly how time figures in)"
    4D navigation is all the rage in air navigation today. For air traffic, it 
    makes a lot of sense since it's about saving fuel. A traditional flight from 
    one city involves a climb at some fixed rate to cruising altitude, a cruise 
    at that altitude, and then a descent to the destination airport. Supposedly, 
    this 4D type of navigation creates continuous contours in space and time 
    taking advantage of winds and the exact behavior of the aircraft's engines 
    and aerodynamics that can significantly increase fuel efficiency. Heck, 
    you'll arrive at the gate with just one drop of fuel left in the tanks (!!). 
    Marine navigation can be highly optimized to take advantage of currents and 
    avoid bad weather in order to reduce transit time as well as fuel 
    consumption. Some of this is simple, and there's already a lot of software to 
    do this. It's not so different from a road mapping program that finds the 
    quickest path to your destination by putting you on the highway just to get 
    across town even though your "instincts" might prefer a straighter path. 
    I do await the day when a dozen boats all running the same software converge 
    on the same offshore point simultaneously. I realize that software designers 
    think of these things and write code with such issues in mind, but then there 
    are those inevitable things called "bugs". I'm also waiting for a boat to 
    turn up in Portland, Oregon instead of Portland, Maine.
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