Monday, February 7, 2011

Sextants and GPS Take II

After all The Iphone stupidity I now have my Big 800w inverter dodgily clipped onto the batteries . Its happily running my computer, But like a vampire it is slurping the life blood from my old pair of geriatric batteries (I can tell it's enjoying the feast because the fan is buzzing away happily). The voltage is rapidly dropping from 12.2v down toward the 12.0v mark... But if I type real fast I may just get this post finished before the interior lights begin to fade.  The joys of being on a mooring.

Now back to the Sextant issue, I now finally have a photo to add.
see I told you it was a Beauty, A Freiburger, never been used. It's a very close relative to the Russian SNO-T sextant (The T stands for tropical apparently). I know this because My friend also lent me a very interesting book called "The Nautical Sextant" that gives in-depth details of the inner workings of every sextant since Sir Isaac Newton first thought of the concept. It will keep me awake again tonight if the batteries hold out long enough...

We discussed my sextant today at work over lunch (I worked as a builders labourer today) and we compared it to a GPS, Most of my fellow builders reckoned they wouldn't cross an ocean without a sextant and almanac, but aside from the rather remote possibility that all the satellites might fall out of the sky.. Or a lightning bolt might fry all 4 GPS units we would have replacing the sextant. No one could come up with a realistic scenario that didn't involve some sort of doomsday apocalyptic situation requiring the sextant.

That being said, I wouldn't cross a reasonable chunk of water without a sextant, maybe for superstitious reasons as much as anything else, I mean it is really tempting mr fate isn't it. There is also more to the sextant than just a backup, somehow carrying one makes you feel more independent, you are not relying on high tech satellites to get you from A to B. You could do it on your own if you wanted to. The satellites become just a convenience, not a necessity. For me at least that is one of the things I like about being at sea, a feeling of self reliance.

The other thing about carrying a sextant is that it links you to the seaman past, to Cook, to Columbus, and even in some ways to the amazing Polynesian navigators, they all had to navigate using the stars, sun and moon. You are a part of a tradition, and it feels right.

The last 1000 NM of the trip from Tasmania to Chile was done using Celestial Navigation only (except Dave who had the odd secret GPS peak for safety). The landfall was the most exciting ever, knowing we had found our position and navigated by the sun and stars. This is something every sailor should feel at least once.

Anyway the vampire has sucked the life blood down to 11.8 volts so if I don't stop soon I will kill the batteries for good. Since you've missed a few photo's I will give you two today, I have finally found a use for Snotty the sextant, It makes a great mouse table, and every thing on a boat needs two uses ...



Ps If you can think of a plausible reason for 4 waterproof, isolated GPS units (including one in a Faraday cage) to simultaneously die, for more than a couple of days, I would very impressed. (note this is a hypothetical situation)

Edit 15/2/11 After discussion with Chris at Brilliant star see here for more details and also the comments on this post. It seems we have our answer, there is a remote possibility a large enough solar flare could compromise the GPS, Galileo, glonass and irridium satellites and radio for long enough to make carrying a sextant across an ocean a good idea?

From the website they suggested that normal flares would only reduce the accuracy by 50 meters or so, not a big deal if you are navigating defensively. And they mentioned an 11 year solar flare peak 2013…

However the chances of an large enough solar event to wipeout these systems for any significant period seems remote, And it would also cause chaos ashore, so it falls into my doomsday apocalyptic situation. By coincidence today has the largest event this solar cycle according to (see I am turning into a journalist..) these guys

BUT I will still carry a sextant... And I will also keep an eye on the space weather and also be aware that a strong aurora and poor radio propagation may also indicate reductions in gps accuracy.

A Big thanks to Chris onboard Brilliant Star for the Info

for further info see

The 500 year flare.. NOAA
NOAA flare scales


  1. I guess The Americans could push the Selective Denial button and disable civilian GPS in an area near you, to maybe confuse some pirates or something, or wage a war.. but I think I would prefer to keep well clear of those waters anyway, I also suspect they would turn it on and off as they did in the first gulf war with Selective Availability rather than leave it off completely, This should enable you to get a fix every day or so. They may also degrade the signals or put in an error, but you should still be able to find land.

  2. Hi Ben,

    I have, and still carry, exactly the same sextant. It's really is a great instrument and has helped me make many a landfall. Now if I could just remember how to use it!

  3. Hi John, Good to hear it is a good sextant and thanks for your reply. It is interesting how GPS has taken over our lives, I remember a second trip Navigation Cadet not knowing how to put a radar position on the chart, we had just cleared the hamburg fairway bouy and he wanted to put a gps plot down...(but then the old timers would say why are you using radar, use the compass...) another one put a gps fix down in the middle of the suez canal, but he got a good ribbing for that. I guess the old timers have disapproved of things like this since the compass was invented.. or maybe even before.. (that fire thing you light at night will make you soft)


  4. Ben, thanks for the comment on Piracy. Joshua would need bigger tacks these days.

    Can you send me the date and time (UTC) of your GPS (non) event and a rough lat lon. I may have some thoughts for you.

    Ive been involved with GPS since 1973.
    Brilliant Star

  5. Hi Chris, Thanks for the interest, In fact I have never had an outage like that.. the most I have ever seen is a couple of hours of erratic behavour (this was many years ago), and occasional one off jumps out of position by a mile or so for about 30 seconds (mostly around high land or Ice).

    I assumed the first was due to a rogue satelite and the second due to reflected signals, but would be very interested in your thoughts on these.

    Of course I have had individual units and ariels die, but getting out the spare soon fixed that.

    I guess I was wondering if you had any viable reason that would justify carrying a sextant rather than a whole batch of spare GPS units. Other



  6. The reply from chris and follow on comment from me can be seen at

    Thanks again Chris