Thorny subject: Sextant and GPS era

[Flagman101]

Originally Posted by CFS Klopas

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

Of course this could affect your main units. But best practice would be for you to have a batteries powered unit with fresh spare batteries.

[ArmyDaveNY]

Interestingly, the US Navy is teaching celestial navigation again.

[Flagman101]

Originally Posted by ArmyDaveNY

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 improperly is more helpful than a sextant but when used improperly is far more dangerous than a sextant.

Of course. And this is one of my main pitch. Learn proper coastal navigation skills. Learn and use them. Keep track of your route on charts. And do learn what a GPS does and how it does it.

I once was told by a guy ( Speedboat guy ) My GPS knows where the rocks are so I steer around them......
I only said wow cool and left.
No point waisting energy on him.

[Adelie]

Originally Posted by Davidhoy

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 ;-)

WAAS is not irrelevant, it’s still running and provides better accuracy than GPS could provide alone.

Where did you learn that the new satellites can’t turn on SA?

[captmikem]

Originally Posted by double u

"... charts, dead reckoning, and a compass..."
I wonder where that would land one after 1/2 Atlantic-crossing...

I have DR'd across the Atlantic when I could get no sights, really not a problem just keeping accurate data and applying it.

I took the first civilian GPS across the Atlantic in 1986, Trimble. I would get a "window of Fixes" six or seven times per day. I was continually checking the position with celestial fixes advanced to DR. I finally started using my GPS position as my AP, if the intercept was less than a mile I would plot the GPS as my fix.

GPS offshore is easy to get used to, but aboard ships I always keep the DR going and plot a GPS fix every 4 hours and compare it to the DR. I think nowadays having DR plotting skills (which are easy to learn and easy to maintain) I believe are more valuable for the average sailor than celestial navigation skills because celestial navigation is an art as well as a skill and needs to be practiced often to keep it to where you can rely on it.

Far to many people on boats these days can not even plot a position on a chart. That is the major problem, IMHO.

M

[roverhi]

GPS does screw up!!!!!!! On a recent delivery from SF to Oceanside had to duck into Monterey because of rigging issues. GPS plotter showed me 15 miles offshore. Turned inshore and motored for 2 hours but no land on the overcast horizon. Weird, so I got out the back up GPS which only gave lat/long position. We weren't 15 miles offshore but more like 50 miles. DR'd into Monterey from the back up GPS position. The original GPS plotter was locked on a position just off Monterey.

Bought a new GPS plotter in Monterey and finished the delivery without incident. When I got to Oceanside fired up the old Plotter and it was once again functioning properly. No idea why it had gone whacko off Monterey.

We did SoPac prior to GPS. Keeping a running DR and the sextant got us everywhere but it was time consuming and not all that accurate. Definitely not accurate enough to sail onto a shore in fog or limited visibility. The sextant wouldn't have been useful at all in the above situation because of overcast. Navigating with the Magenta line of the GPS has made me a bit lazy. Kind of taught me a lesson on the need to keep a DR plot going.

[Flagman101]

Originally Posted by captmikem

Far to many people on boats these days can not even plot a position on a chart. That is the major problem, IMHO.

M

100% agree.

[ArmyDaveNY]

For those discussing a loss of GPS signal, there is an additional, and much more dangerous issue. GPS is a very weak signal and there are many cases of GPS signals being manipulated or jammed. I am guessing, but not sure, that Galileo and GLONASS have similar signals. This is one reason why the aviation industry is considering bringing back an enhanced version of Loran called e-Loran.

Still, the odds of not having a satellite signal are small. Having said that, I still plot on paper and use the GPS for confirmation of what I am plotting. I like having the extra situational awareness and also knowing that in the unlikely, but still possible, event of losing my electronics for any reason, that I still have my data and still know where I am.

[Jammer]

Since there is widespread reliance on electronics (and, indirectly, GPS) for time, one might need a chronometer of some sort, as well as a sextant, to be prepared for scenarios where all onboard electronics fail.

[Adelie]

Originally Posted by tmccaffery

.....

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.

Celestial is still dependent on electronic or mechanical equipment, specifically the time pieces.

My thoughts on best practices here is to get a metal ammo box ($30 for 2 @ Costco). Keep the mechanical watch & 5 cheap waterproof quartz watches in it.
Daily wind the mechanical watch.
Weekly check the error of each watch against an outside source and DOCUMENT that error. Reset the watches before starting the passage but never on passage. You need to know the rate at which each watch it losing or gaining time.

If you lose accurate time there are a couple of ways to get it back. If anyone is interested I can explain.

Also going into my ammo box is a handheld GPS with charts, a plain GPS, extra Ni-MH batteries for each plus a solar charger to keep those topped up.

[jtsailjt]

Originally Posted by 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.

You do realize that a sextant is a mechanical device? Also, I disagree that you are likely to run out of electricity on a sailboat. You only need a tiny amount of electricity to make a GPS work and since so many sailboat have solar panels and/or wind generators onboard, as well as a genset and engine alternator, I think it's probably a lot more likely that someone might drop a sextant or fall against it or otherwise impact its accuracy or make a math error (no electronic calculator with the total electrical failure you envision) than they will run out of enough electricity to power at least one of the many GPS's found onboard a typical cruising sailboat.

[captmikem]

Originally Posted by Adelie

Celestial is still dependent on electronic or mechanical equipment, specifically the time pieces.


If you lose accurate time there are a couple of ways to get it back. If anyone is interested I can explain.

WWV (and WWVH) Maybe a "backup" HF receiver. Cheap insurance.

M

[ArmyDaveNY]

Originally Posted by jtsailjt

You do realize that a sextant is a mechanical device? Also, I disagree that you are likely to run out of electricity on a sailboat. You only need a tiny amount of electricity to make a GPS work and since so many sailboat have solar panels and/or wind generators onboard, as well as a genset and engine alternator, I think it's probably a lot more likely that someone might drop a sextant or fall against it or otherwise impact its accuracy or make a math error (no electronic calculator with the total electrical failure you envision) than they will run out of enough electricity to power at least one of the many GPS's found onboard a typical cruising sailboat.

I think that he was referring to a loss of electricity due to something like a lightening strike or an electrical system failure such as wiring, short, etc.

[TrentePieds]

Quote: "If you lose accurate time there are a couple of ways to get it back. If anyone is interested I can explain. "

Yes, please :-)!

TP

[Jammer]

Originally Posted by Adelie

If you lose accurate time there are a couple of ways to get it back. If anyone is interested I can explain.

Lunar distance is the obvious one. Or WWV. Are there others?