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    Re: US. Navy fix accuracy and Interval.
    From: Frank Reed
    Date: 2013 Mar 26, 08:25 -0700

    Jeremy, you wrote:
    "We tend to talk meters, cables (1 cable = 0.1 nm), and NM's."

    Cables? I had no idea "cables" were a commonly used unit. Do you really hear that often?

    And you wrote:
    "With the introduction of "L2" receivers using both GPS and GLONASS, accuracy is within a meter. So with properly set up and equipped ECDIS systems, you can know your position within a shipwidth at all times."

    What's the rate of receiver turn-over on large vessels? Do you expect that most large vessels currently have an L2 receiver? New smartphones mostly became dual GPS/GLONASS a little over a year ago, and with typical smartphone replacement rates, the majority (above 50%) in actual use should be GLONASS-capable sometime this year, if not already. Coincidentally, I was showing a GPS-skeptic the GLONASS sats on my phone just last night. And also, do you carry handheld GPS units or other "pocket" backups for the main sat nav system? Do you count a smartphone as a navigation backup?

    "Quite frankly, these days knowing where I am isn't the issue. I basically know where I am at all times."

    Yep. Navigation is a "solved problem". It's only those very rare emergency navigation situations and some details around the edges where there are still interesting issues. Apart from our amusement, of course.

    You also mentioned yachts getting in your way at inopportune times. This is a huge issue here near Newport in the summer. There are several 600-foot commercial ships running up or down the East Passage every day and hundreds of small boats in every imaginable class from pico-yachts bobbing along under a single sail to mega-yachts capable of high-speed operations and equipped with all the best electronics. How do you professional mariners deal with all that yachting flotsam? Do you have lots of cameras these days, to provide the equivalent of eyes looking right over the side and over the bow?

    You concluded:
    "My biggest issue is that the charts aren't accurate or detailed enough. Truely, the weakest link in navigation these days is the hydrographic data available to me."

    It's amazing when you stop and consider that the surface of the planet Mars has been mapped in vastly greater detail than 70% of the Earth's surface. If you want to map the ocean floor, you have to sail over it and get an echo off the bottom. Modern technology has barely changed this. There are numerous digital products (like Google Earth, for example) that display hydrographic data and underwater "topography" and create the illusion that this data is accurate and complete on a much finer scale than it really is. But much of this "data" has been derived from indirect sources and represents little more than some algorithm's "best guess". Naturally in shallow waters, where it matters more, the charts are usually based on real grid surveys, but they could be decades old, and you never know whether they were competently prepared. I presume the electronic chart systems usually indicate the age of the hydrographic survey data. Do they prominently indicate the source, too? Or do you have to dig down into the menus for that information?


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