Thorny subject: Sextant and GPS era

[Adelie]

Originally Posted by Dockhead

It doesn't matter.

We need precise position data for PILOTAGE, not so much for navigation.

I would not freak out in the middle of the Atlantic if I lost all position data.

I would just do dead reckoning, keep a good hourly log of speed and course, and you'll at least hit the right piece of land. When you get in sight of land, then you just do normal non-electronic pilotage. You can get a very precise position in pilotage waters with a normal three point fix.

It wouldn't really be a big deal.

Losing GPS would be way down on the list of dangerous gear failures. I'd much rather lose GPS than the autopilot, for example. I might even prefer losing the GPS, to losing my microwave


And BTW I DO have a sextant on board. But it's for sale. Now that GLONASS is fully operational, and with every phone and tablet on board receiving GLONASS plus GPS, losing satellite position data is almost inconceivable.

What do you have and how much do you want?

[Dockhead]

Originally Posted by Adelie

What do you have and how much do you want?

It's a beautiful instrument, a Soviet SNO-M, considered to be one of the finest sextants ever made. My father used it before GPS and handed it down to me when he swallowed the anchor.

It's listed here:

SNO-M Sextant in Waterproof Case with Artificial Horizon

Almost free to a good home.

[Flagman101]

Originally Posted by Jammer

There are plausible scenarios that would affect both, such as unusual sunspot activity leading to electrical noise and a loss of radio reception, or hostile acts between nations.

Interesting thread, though. I think the loss of pilotage skill (to GPS and complacancy) is perhaps a more grave danger then the loss of celestial navigation skills.

Hostile acts between nations. Most military can jam GPS signals. This is local not global. Unless global conflict erupts. If local, this will not happen overnight Conflict usually builds up. Then I would argue, what the hell are you doing near that place anyways?? :-)
Solar flares. Yeah but would it permanently kill GPS?
If yes, modern aviation needs a better plan.

I do agree that celestial navigation is probably one of the last thing you need to learn before going sailing. Coastal navigation, Proper GPS navigation,weather forecasting, routing, first aid and survival at sea are way on top of my list before celestial. I left out the other technical stuff like electronics, mechanical, electric training out because this is a navigational thread.

[Adelie]

Originally Posted by Flagman101

Please keep in mind that we are not saying that DR is enough for all situations. The main subject is. Ever ear a story were someone was lost after GPS failure.

And I do mean at sea. Not on coastal navigation.



I do have questions about sextant. I do not know how to use one.

What is the best accuracy one can hope to achieve using sextant?

How long to come up with the most accurate position?.

What happens if it's completely overcast and raining for 2 days?

How many times a day do you or should you get a fix on your position?

Best accuracy is about 1 nm. Regardless of how good your technique is and which sight reduction method you use, the view of the horizon can be resolved to about 1 arc-min which limits precision to about 1nm.

With practice you can shoot a round of sights and reduce to a fix in about 30-60min.

If it’s overcast you update your position using DR until you get another fix. If the sun breaks thru for a couple minutes you need to be ready to shoot it immediately. The single line of position you get is not a fix but it will improve you DR. If you get another one later and update the DR to that it’s essentially a running fix.

Offshore once a day.

[dbaoter]

Navigation was more fun before GPS. Carried a sextant for years, one day opened a cabinet, there was the sextant. Had to give serious thought about the future. Removed the sextant from the boat. As for GPS failure, has not happened yet. Good navigational skills will get you thru a problem. Lost electronic charts one time, had paper backup, and will ALWAYS carry the paper.

[Adelie]

Originally Posted by Dockhead

It's a beautiful instrument, a Soviet SNO-M, considered to be one of the finest sextants ever made. My father used it before GPS and handed it down to me when he swallowed the anchor.

It's listed here:

SNO-M Sextant in Waterproof Case with Artificial Horizon

Almost free to a good home.

If you want I’ll post to the NavList. That’s the CelNav folks.

[jtsailjt]

Originally Posted by Flagman101

Hello, last week-end I got unwillingly into a thorny discussion with an other boater about Sextant and GPS.

So anyways this post is not to argue the necessity or not of knowing how to use a Sextant.

What I'm looking for is actual accounts of sailors, getting lost or in trouble because they're GPSs broke down and did not know how to navigate without a sextant.
Actual bleue water sailors.
I have never heard a single story about that.

You've probably never heard of it happening because broken down GPS's are pretty rare and if one does break down, you'd probably just refer to one of your many other GPS's that you have onboard. On my boat I've got the usual chartplotter at the helm, a USB GPS antenna that can be plugged into my laptop (running OpenCPN) if the info coming from the chartplotter is somehow lost, an iPad running Navionics as well as a couple of iPhones, also running Navionics, a handheld radio as well as my primary vhf radio that can all tell me precisely what my lat/long is. On the other hand, if it happens to be a cloudy couple of days or seas are up so the deck is pitching/rolling badly, you're completely out of luck if depending on a sextant to determine your position. So, I think it can be fairly argued that GPS nav is both much more accurate AND much more reliable than celestial nav is. Of course, both of them only tell you your position on earth, not where all the things you might bump into are. They both are reliant of the accuracy of the information on available charts and how the user interprets that information.

[fatherchronica]

We learned celestial navigation because a friend offered to teach us, and we both like math. What I would be afraid of in my old age dotage on technology is my GPS anchor alarm In bad weather when you wake up constantly to check your bearings at anchor it would be really hard to use celestial to find your position to within a few miles or so. We don't anchor out far enough for that kind of accuracy.

[CFS Klopas]

Wouldn't the easiest way to lose GPS be an electrical failure on your boat?

[AJ_n_Audrey]

I cannot remember which famous bluewater sailor said this, but while interviewing him about his adventures, an earnest young journalist asked the old salt about celestial nav. The sailor said yeah, I used to take sights every day, but I stopped carrying a sextant years ago. "But...what would you do if your chartplotter failed?" "I'd use my secondary GPS". "But.....what if the batteries went out on the secondary?" "I'd use the tertiary" And so on.

Realistically, if a global war takes out GPS, we're screwed anyway, and if a solar event does it, well, we'll just figure out how to limp into port with charts, parallels, depthfinder, and compass. And I say all this as a guy who learned how to use sun sites as a surveyor in the Army to find direction in the 80's, and at the time, I thought the process was cool and very intellectually interesting, and I enjoyed it. But there are only 24 hours in a day, and celestial nav does not make the cut in terms of a claim on my time in prepping for cruising.

Maybe one day when I'm hanging on the anchor with nothing to do I'll learn. A local old salt gave me his sextant, so I could do so if the spirit moved me.

[Adelie]

Originally Posted by Pete7

Wasn't there a Japanese bloke who sailed around the world with nothing in a small boat, just pointed in a direction and went for it.



I have been looking at plastic sextants of late, but they seem to command serious money on e bay. I sold one previously making a small profit. It will now cost me twice that to buy another :-(

What do you want to do with the sextant? Use it regularly, use it for emergencies or ...?

If it’s just for emergencies, get a Davis-3 for $45. I don’t have the link handy but I read a good essay on why to choose those over the “better” plastic sextants.

[ArmyDaveNY]

Originally Posted by Flagman101

What I'm looking for is actual accounts of sailors, getting lost or in trouble because they're GPSs broke down and did not know how to navigate without a sextant.

Bluewater sailors tend to be more competent than the general boating population. In the general boating population I would argue that way too many don't know how to navigate with GPS either. Instead, they follow the arrow with little to no situational awareness.

Both the sextant and the GPS are tools, and thus neither good nor bad. Both when used properly are valuable and when used improperly are dangerous. I would say that the GPS when used properly is more helpful than a sextant but when used improperly is far more dangerous than a sextant.

[stewie]

In 82 we made our first Atlantic crossing and our satnav antenna amplifier was taken out by lightening a few day south of the Canaries. Fortunately we had bought a Davis plastic sextant and navigation tables before departing from Gibraltar. True our fixes on a small yacht we a bit zig zaggy on the way over but we picked up Barbados MW radio station a couple of hundred miles offshore with a transistor radio and homed in on the direction of the null. I have heard that trainee merchant officers are once again learning how to use a sextant as part of their training as the sun never fails. On the next crossing we had bought a good sextant.....

[Davidhoy]

Originally Posted by Dockhead

In those days, which seem like a million years ago, GPS was primarily a military system, which they U.S. military ALLOWED us to use, somewhat grudgingly, as they liked. They could and did switch it off from time to time.

The original GPS implementation included a feature called "selective availability", which the military controlled. It degraded the signal received by commercial units by encrypting the time signal, and the un-encrypted time was "dithered" to induce a greater error circle.

For accurate pilotage (as Dockhead pointed out, close coastal navigation), a system called DGPS (differential GPS) and later WAAS (wide area augmentation system) were created that broadcast time corrections over FM sub-carriers over a relatively small area to allow for much more precise positioning. When selective availability was turned off in 2000 on Clinton's orders, DGPS and WAAS became irrelevant.

During Desert Storm, GPS was never turned off, as suggested by a earlier post. Rather SA was turned OFF, making full accuracy signal available to all. This was because so many of the troops deployed to Kuwait were using commercial handhelds, and milspec units were in short supply.

As of 2007, new GPS satellites being launched no longer even have SA as an option, so the military can't turn it back on, even if they wanted to. They do have GPS jamming technology they can use in a local theater, if they want to deny enemy usage.

To the question of GPS and/or Glonass being out of service, the only possible scenario I can think of is an huge world-wide conflict where multiple powers are taking out each other's satellite constellations. In that eventuality, I think I'd prefer to be lost at sea...

Like many here, I have a sextant (two actually, a beautiful Hezzanith passed on to me when my dad died, and a "plastic fantastic" Davis Mk25). I cannot recall when I last used either for actual navigation. I did use the Mk25, with it's sun filters, to view the eclipse last year ;-)

[tmccaffery]

Electrical failure -- precisely the issue.

All electronics are 100% dependent on a reliable supply of electricity. No electricity, no electronics. For some, no electronics no charts. Not likely to happen on a large ocean going merchant or naval vessel, but very likely to happen aboard a sailboat, especially if the problem is fuel related. Mechanical damage is also an issue (e.g. your main GPS is damaged in an accident and then something happens to your backup).

In answer to the OP's post. Years ago in the SatNAV days I had a captain who was an amateur carpenter who wanted to make some book shelves for the chartroom. He used the SatNav unit as a saw horse to cut the boards and as a "work bench". After two days or so this the unit stopped working. I pulled out my sextant and started navigating without interruption or trauma although the younger officers aboard had thought that my sextant was "quaint". We navigated by sextant for the next week until we arrived, as scheduled, in Miami without any fuss.

The bottom line is that anything that is mechanical or dependent upon electricity can and will fail, usually at the worst possible time. Celestial navigation and paper charts are neither mechanical nor dependent upon electricity and are thus your ultimate backup. Finally, for coastal piloting, the sextant can be valuable in taking visual ranges of objects with known heights (light houses) using vertical sextant angles. You can also take bearings using horizontal sextant angles between two landmarks. With three landmarks you have two lines of position to make a fix.