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    Re: What precision is required in cel nav?
    From: Frank Reed
    Date: 2015 Aug 1, 23:01 -0700

    Jeremy,

    Welcome back. How was the ocean this time? :)

    You wrote:
    "As to the original post, the saying is to use ALL means to determine position. So if you have a sextant and a GNSS, then you should be using one to corrobrate the other in the open ocean."

    Yes. I agree that the best reason to continue using celestial is simply because "it is there" and the best philosophy of navigation, or the "saying" as you put it, is to use everything available. It's really unlikely that this would ever correct a GNSS position, but spoofing is a real, albeit remote, possibility, and there's always the chance of some serious human error in reading or using a GNSS position which might then render the celestial information relevant.

    Of course this reason for employing celestial implies also that all tools are on the table. We don't have to turn back the clock to an earlier era. And in particular we can increase our frequency of sight-taking by using electronic means to generate useful navigation data from the observations. Spending fifteen minutes looking up refraction in a table, calculating LHA, and then looking up or calculating Hc and Zn, and finally plotting it all with traditional paper plotting charts by hand... well that may be good for the soul, but it isn't necessarily practical navigation in the year 2015.

    You wrote:
    "When near land, you use the radar and perhaps some visual bearings as well."

    This reminds me of another coastal approach tool that's potentially available to ocean navigators today, a bit further out to sea. The AIS system turns every container ship, every cruise ship into a lighthouse. Assuming that the AIS receiver is a separate system, which does often seem to be the case today (or is that just on smaller vessels?), it's providing us exact positions of objects (other ships) which are frequently visible to a navigator a hundred miles from land. Treating these as "lighthouses" and using the usual coastal navigation tricks then provides another, quite accurate test of our ship's position. Yes, it's still GNSS/GPS-based since the positions provided by the other vessels come from their GPS systems, but it does not necessarily depend on our own on-board GNSS navigation systems. I mentioned this as a "theoretical" idea to a friend of mine who captains a commercial fishing boat, and he said that he does this, in a casual way, on a regular basis (and he never thought of it as navigation per se). He likes the feeling of confirmation that he gets from seeing that the configuration of other shipping traffic displayed by the AIS, matches what he sees visually, even vessels that are many miles away just peeking over the horizon. The AIS data is like air traffic control data broadcast far and wide for anyone to pick up and plot. 

    Frank Reed

       
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