Thorny subject: Sextant and GPS era

[Adelie]

Originally Posted by Davidhoy

SA is/was the civilian part of GPS. The military's only choice in the event of a future conflict will be to turn the entire GPS system off. Extremely unlikely, as their own weapons systems heavily rely on GPS. But, if the proverbial crap hits the fan, and the military needs to turn off GPS, I personally would like to be "lost" somewhere very far from any hostilities.

SA was not the civilian side, it was the process of degrading the results on the civilian side.

With SA off accuracy was on the order of 15m rms. SA on it was 50-100m.

The military side was on the order of 10m.

The DoD always had and still has the ability to turn off the civilian side without turning off the military side.

During the Gulf War in 1990 they left SA off because they did not have enough military receivers for their needs, so that had to use civilian models.

That is no longer the case. If they see a need to turn off the civilian side denying themselves use of GPS is not a consideration.

[sailpower]

Originally Posted by adlib2

Interesting to note that the US Naval Academy does not teach CELNAV any more,

The USNA went back to teaching celestial navigation a couple of years ago.

[Adelie]

Originally Posted by adlib2

Interesting to note that the US Naval Academy does not teach CELNAV any more, the Brits do, they are more traditional. Warships carry numerous GPS systems so less chance of them losing a position.
I carry a German Frieberger; however, my cruising days are over if anyone is interested in purchasing.

The USNA has started teaching it again. The enlisted ranks never stopped being taught it and on larger vessels it was still practiced some. The USNA started teaching it again because they realized it was not the minor activity they thought and the officers need to know how to do it in order to effectively manage the enlisted folks actually doing the work.

The Merchant Marine Academy which never stopped training this subject is helping the the USNA restart their program after the 10yr hiatus.

[turtleofdart]

The trouble with GPS is that we become dependent on a single source of information and that always makes me feel uneasy. Correct me if I'm wrong but GPS is operated by the US military who are able to turn it off at will. having a sextant on board for determining latitude through meridian passage observation is, in my opinion, a sensible thing to do on ocean voyages. a lot depends on the nature of the coast one is approaching. To rely on dead reckoning or estimated position (the latter takes into account tidal streams) when approaching small, low lying islands in the midst of a vast ocean can be fatal. If however one sails across the atlantic from east to west one is unlikely to miss the americas. another very useful thing we did before GPS.... (yes, I am that old)....is to radio passing ships and ask them for their position....and weather forecasts.... in any case I would warn against a blaze/ reliance on single sources of information...Polynesian and indian ocean navigators are renowned for their skill and observation. I can recommend Bary Blewett's excellent little book 'Celestial Navigation for Yachtsmen"

[LouK]

Rarely used Davis sextant in plastic case for sale $100+ shipping. Lou

[Dockhead]

Originally Posted by turtleofdart

The trouble with GPS is that we become dependent on a single source of information and that always makes me feel uneasy. Correct me if I'm wrong but GPS is operated by the US military who are able to turn it off at will. . . "

The U.S. military can't turn GLONASS off!

[Jammer]

Originally Posted by Dockhead

The U.S. military can't turn GLONASS off!

How do you know? Perhaps they can.

I seem to recall that they were able to turn off some centrifuges that they were not supposed to be able to turn off.

[waterman46]

Originally Posted by Flagman101

Irrelevant. If your are navigating with GPS you will get to safe harbour. Also if your using sextant.
One could probably make landfall using only compass, charts and dead reckoning but how close to target?? No idea.
Ever ear a story where someone lost GPS system and ended up lost?
That is my question.

I suspect we will never hear any stories from lost sailors who never found their way back to tell us.

An earlier post mentioned five things you need for celestial but forgot to mention you must know the GMT within a few seconds error or you are SOL except for a noon sight for latitude (then you only need to know the date - sometimes a tough thing to know if you didn't keep a log).

[Adelie]

Originally Posted by Dockhead

The U.S. military can't turn GLONASS off!

Any country with anti-satellite weapons can turn any or all systems off.

[Dockhead]

Originally Posted by Jammer

How do you know? Perhaps they can.

I seem to recall that they were able to turn off some centrifuges that they were not supposed to be able to turn off.

Ha, ha. You're thinking about the Iranians, I think. A backdoor spread through some networked printers we sold them or something like that.

The Russians are pretty good with IT security, maybe better than we are.

[Dockhead]

Originally Posted by Adelie

Any country with anti-satellite weapons can turn any or all systems off.

Well, sure. If someone starts shooting down satellites, then all bets are off. But in that case, no satellite position data will be the least of our problems, and we might not even want to find land at all.

[Jammer]

Originally Posted by Dockhead

The Russians are pretty good with IT security, maybe better than we are.

Well, then they can turn GLONASS off, at least for us, and perhaps GPS.

Seriously, though, anyone who thinks that these systems are 100% reliable and could never fail lacks either imagination or a sufficient background in engineering and history.

But the most interesting part of this thread, to me, is the considerable faith placed in dead reckoning by experienced sailors. I've never much trusted it on the water or in the air, usually relying more on pilotage or various radio aids to navigation (as my navigation experience predates the GPS era).

[double u]

on this morning's news: positioning devices in the eastern Med may be malfunctioning due to possible cruise missile attacs...

but apart from this I think we are worrying the subject (like the proverbial terrier...) from the wrong side altogether.
it is not about the possibility of "asking fishermen" in the Red Sea - a proposition of somebody who's never been outside the anglophone world. Even if he'd himself would speak Arabic it would have to be Sudanese-arabic, as simple fishermen are highly unlikely to understand high-arabic, probably not even the widespread egyptian dialect - they'd have to have TV sets...
it is not about the likelihood or otherwise of GPS, electricity & what not failing
about the accuracy of DR (btw: this was always surprisingly high given our not inconsiderable experience, but the nagging doubts, the nagging doubts on a windy night...)
it is about the meaning of blue water cruising, about the romance of crossing an ocean by harnessing the often so adverse powers of nature , the wind, the currents to our advantage
about NOT choosing the sensible way (& flying), about the satisfaction of having reached an exotic destination "under your own steam", about self reliance, about "getting away from it all"
the title of Randy Thomas' articles in Multihulls magazine, ages ago & worlds away summed it up so aptly: "out there doing it"...
for me determining my position on some vast Ocean that looks the same everywhere with a basically archaic instrument & books with endless columns of numbers & have the horizon going "fuzzy" as the tips of the palm trees of Salomon Atoll break the horizon after 3 weeks at sea - that was for me a very important part of the essence of "blue water" sailing (& still makes my eyes go misty sitting here in my office in the middle of the continent, even though it's been 26 years).
much as I would like to, I can't turn the clock back, & on our 3rd rtw we had a little GPS too, which took a lot of anxiety & trepidation away, but a lot of sense of achievement too. but I could still do one thing (call it "star-w...ing if you want...): keep up my skills & do a round of starsights if at all possible & at least have the confidence if bad comes to worst...
(& btw: of course I kept a time-log where all on board-watches were recorded, so even without SW-time signals I would have been ok for at least 2 months

"...& a star to steer her by..."

[billgewater]

Follow that other boat.

[Dockhead]

Originally Posted by Jammer

Well, then they can turn GLONASS off, at least for us, and perhaps GPS.

Seriously, though, anyone who thinks that these systems are 100% reliable and could never fail lacks either imagination or a sufficient background in engineering and history.

But the most interesting part of this thread, to me, is the considerable faith placed in dead reckoning by experienced sailors. I've never much trusted it on the water or in the air, usually relying more on pilotage or various radio aids to navigation (as my navigation experience predates the GPS era).

Well, I don't think anyone was arguing for DR alone. On the contrary, I think everyone was talking about the importance of non-electronic pilotage.

I don't think anyone is wrong here. Yes, a long-term outage of GNSS is extremely unlikely. Yes, it is possible. Yes, you can get by quite OK with DR and non-electronic pilotage. Yes, a sextant or RDF would be nice sometimes.


Bit of thread drift, but that thing which is most "make or break", in my opinion, for getting safely into an unfamiliar port, is a chart. If you have a chart, you can figure out the rest. Once upon a time, due to a grave error in passage planning on my part, I had to find my way into a complicated and unfamiliar port (Gedser in Denmark), without any chart, electronic or paper, ON A DARK OVERCAST NIGHT. That was some chit. There were buoys, but many were unlit. There were sand banks and shoals all over the place. I probably should not have entered -- probably should have kept sailing until morning.

There was a boat a few miles ahead of me, going into the same anchorage, and what saved me was that he was broadcasting AIS, so I was able to track him and drop waypoints along his track. Even so, there were some real brown shorts moments as we tried to follow the twisting channel, poorly marked with unlit buoys, crewman on the bow with a spotlight, with full astern used a few times when the depth came up to 0 under the keel . . .