Thorny subject: Sextant and GPS era

[double u]

DH, I beg to differ:
1. there are unlit coasts full of danger that one would not see until the last moment - approaching the Tuamotus just on DR for the last 300 miles...
2. celestial was at times indispensable in the Red Sea where the coast is nearly completely featureless & the wind sometimes quite "up"
3. the only thing that counts with celestial is practice, practice practice! anybody finding the maths with HO 249 too difficult better abstain altogether...

[Flagman101]

Originally Posted by double u

DH, I beg to differ:
1. there are unlit coasts full of danger that one would not see until the last moment - approaching the Tuamotus just on DR for the last 300 miles...
2. celestial was at times indispensable in the Red Sea where the coast is nearly completely featureless & the wind sometimes quite "up"
3. the only thing that counts with celestial is practice, practice practice! anybody finding the maths with HO 249 too difficult better abstain altogether...

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?

[double u]

"...Ever hear a story were someone was lost after GPS failure...."
...they'd have to admit it first.....
I just recall the some of the crowd spending the SoPac cyclone season 90/91 in NZ were very nervous when GPS was offline for a few days because of some war or other...

[Cu Mor Glas]

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.

If worst came to worst i COULD use my sextant to get a celestial fix. but it's been years since i did that. what i use my sextant for the most is coastal piloting. set on it's side it is a great way to get very accurate angles between landmarks. the angle you get is a true angle and doesn't have any worries about how timid virgins make dull company. get that on three landmarks and you have a pretty darn good idea where you are.

[Dockhead]

Originally Posted by double u

DH, I beg to differ:
1. there are unlit coasts full of danger that one would not see until the last moment - approaching the Tuamotus just on DR for the last 300 miles...
2. celestial was at times indispensable in the Red Sea where the coast is nearly completely featureless & the wind sometimes quite "up"
3. the only thing that counts with celestial is practice, practice practice! anybody finding the maths with HO 249 too difficult better abstain altogether...

You are of course right about all of this, and I oversimplified. Thank you for the correction.

But the broad point is still correct -- with no GPS and no celestial, you are still not dead -- you are just more cautious, heave-to at night with a good margin of error if you are approaching an unlit coast, etc.

Red Sea, like other places with featureless empty coasts, you just stand off the coast and proceed until you see some landmarks. Like sailors have done for thousands of years before celestial was invented. But you are absolutely right that there are some situations where you would want more than DR; I should not have made such a sweeping statement.

[Flagman101]

Originally Posted by double u

"...Ever hear a story were someone was lost after GPS failure...."
...they'd have to admit it first.....
I just recall the some of the crowd spending the SoPac cyclone season 90/91 in NZ were very nervous when GPS was offline for a few days because of some war or other...

Please elaborate on the SoPac thing.
Also lets keeps this after 94 when Clinton basically opened up the possibilities of GPS by allowing more precise positioning

[Dockhead]

Originally Posted by Flagman101

Please elaborate on the SoPac thing.
Also lets keeps this after 94 when Clinton basically opened up the possibilities of GPS by allowing more precise positioning

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.

But those days are long gone. Maybe someone knows the policy on GPS today, but I would guess that they are no longer allowed to do it except in a case of grave crisis like a nuclear war.

Besides that, the U.S. military no longer even has the power to switch off satellite navigation, because the Russian GLONASS system is fully operational (and all of my GNSS receivers receive it) and working extremely well, and the European and Chinese systems are getting pretty close to being operational.

I think the chances of being without satellite nav are vanishingly small. That's why my beloved SNO-T sextant, handed down from my father, is, sadly, for sale -- I need the storage space for other things.

[double u]

pffff, standing off & keeping on can be trying in the RS...! along some stretches there are very well sheltered anchorages (& welcome they are if the headwinds are up...), but one sees them only if directly off the entrance & quite close inshore. There is a "bit of reef" close to the coast to in places so - on our first 2 rtw I took some sunshots to determine our position so as not to miss the right Marsa ...(of course third time around with GPS...)
given the right sort of conflict I suspect the men-in-black might diddle with GPS...
as to accuracy: in the pre-GPS days I mainly did starsights mornings & evenings -with "selected stars"- & I'd never count on an accuracy of better then 2.5 miles, even though most of the time it was much better. <0,5nm would be a lucky thing though & until GPS one never had absolute confirmation...

[Flagman101]

Originally Posted by Dockhead

That's why my beloved SNO-T sextant, handed down from my father, is, sadly, for sale -- I need the storage space for other things.

I'm sure it would look very good in your living room book case.

[double u]

just saw "SNO-T sextant" is the Freiberger of the GDR (mine is from GDR tiems too: 1979)

[Flagman101]

Originally Posted by double u

pffff, standing off & keeping on can be trying in the RS...! along some stretches there are very well sheltered anchorages (& welcome they are if the headwinds are up...), but one sees them only if directly off the entrance & quite close inshore. There is a "bit of reef" close to the coast to in places so - on our first 2 rtw I took some sunshots to determine our position so as not to miss the right Marsa ...(of course third time around with GPS...)
given the right sort of conflict I suspect the men-in-black might diddle with GPS...
as to accuracy: in the pre-GPS days I mainly did starsights mornings & evenings -with "selected stars"- & I'd never count on an accuracy of better then 2.5 miles, even though most of the time it was much better. <0,5nm would be a lucky thing though & until GPS one never had absolute confirmation...

Not sure I understand then. See in bold.

[double u]

5 or so sunsights in close succession just before local noon in the RS gave us a pretty good idea where to look for entrances...

[Flagman101]

Originally Posted by double u

5 or so sunsights in close succession just before local noon in the RS gave us a pretty good idea where to look for entrances...

So no other boats or fisherman around, no features at all, no port authority to call on VHF?

[Jammer]

Originally Posted by Dockhead

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.

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.

[Adelie]

Originally Posted by james247

I can use both. gps has never let me down. On a small boat say under 80ft the use of a sextant is never perfect the Cox hat puts you in about 500m of accuracy. Not an issue deep at sea shouldn't be an issue as you approach land albeit more cautiously than you might with the accuracy of gps.
.....

As somebody who participates in the celestial navigation list, 500m is not going to be typical accuracy, more like 2nm or so.

Also GPS is more precise than most maps, is some cases over 1nm so you want to be careful with that too. Some maps in the South Pacific are based on surveys from the late 1700s.