Nov 01 2009

Don’t throw your sextant away

Published by at 16:56 under navigation,safety,safety at sea,Sailing,tax

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Actually I never owned one. I did use one – poorly – when I was a midshipman. I could reliably find our continent, might even be able to hit something in California, but I wouldn’t want to trust my best work when it comes to navigating shallow waters, reefs and the like. On the other hand, if you’re going out for an extended period way off shore, you might want to consider some of the bad things that might happen to the GPS satellites while you’re gone.

sextant

An article I read recently talked about all the possibilities for GPS disaster, including that the Indians or Pakistanis my take the satellites out to deny each other targeting info. They also cited some more likely issues notably that the signal from the satellite is very weak and prone to physical interference. There’s also some fear that terrorists might jam the signals to disrupt shipping.

Frankly, I’m not nearly as concerned with any of that as I am with the errors you can see everyday. One of the annoying GPS errors is manifested in false anchor alarms. Not that I’d trust any device 100% to keep me off the rocks, but isn’t the device supposed to be accurate enough to tell if I’m dragging anchor?

Well, no.

In fact, I was able to see GPS error quite clearly on my SPOT. I thought I’d turned it off when I put it in my nav bag. I learned that it was on when one of my friends called to ask if I’d run aground, noting that SPOT showed my Jeanneau in my garage.

When I went to the map display two days later, I saw that my garage had made several jumps of 100 yards or more in the course of 48 hours … or the GPS position was occasionally out of whack. And now I know why the anchor alarm goes off randomly.

The article that started this discussion voiced the opinion that we shouldn’t throw our sextants away. Well, mine is as rusty as my skills. I doubt that I’ll clean the one or brush up the other. That is unless I head to open ocean. In the absence of other entertainment, you can find an engaging activity (and for me frustrating) at sunset, sunrise and noon.

In any event, if you’re not in home waters, it makes sense – has always made sense – to keep a DR track going.

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One response so far

One Response to “Don’t throw your sextant away”

  1. # Randy Bon 03 Nov 2009 at 10:16

    Ahhh, so that explains it. A month or two ago, my neighbor ran into the side of his garage coming home after a late-night party. His wife swears he was drunk, but now, thanks to Capt. Dan, we know that the garage must have been moving…

    Provocative write-up though. Middle Eastern skirmishes and terrorist attacks notwithstanding, the U.S. GPS satellite system may be in trouble. Antiquated hardware, coupled with budgetary shortfalls, could very well affect GPS accuracy and reliability as early as 2010. You can read all of details in a report released by the GAO in April of this year: http://www.gao.gov/products/GAO-09-325

    Operated by the U.S. Air force, the current system is the only one in operation, and is used all over the world. But that soon will change. The Europeans are slated to have their new and enhanced Galileo system on line by 2013-2014. This system could augment the U.S. system, or there could be potential problems with signal clutter and orbital interference. Stay tuned.

    In the meantime, I think DR plots are a great idea, as Capt. Dan suggests. I also keep a “cheat sheet” of compass headings for many points in the SB channel.

    I’m also keeping a copy of Dan’s article handy, just in case MY garage should decide to do some late-night moving around…

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