Thorny subject: Sextant and GPS era

[sailorboy1]

Simple compass doesn’t work inside a metal submarine with electrical cabling all over the place. Not normally much call on a sub to take star positions.

But it was “ interesting”

[Davidhoy]

Originally Posted by Adelie

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

Quote from the Wikipedia page on "Selective Availability" - https://en.wikipedia.org/wiki/Error_...e_availability

On 19 September 2007, the United States Department of Defense announced that future GPS III satellites will not be capable of implementing SA,[12] eventually making the policy permanent.[13]

And from the official government GPS website, https://www.gps.gov/systems/gps/modernization/sa/

Selective Availability (SA) was an intentional degradation of public GPS signals implemented for national security reasons.

In May 2000, at the direction of President Bill Clinton, the U.S government discontinued its use of Selective Availability in order to make GPS more responsive to civil and commercial users worldwide.

The United States has no intent to ever use Selective Availability again.
In September 2007, the U.S. government announced its decision to procure the future generation of GPS satellites, known as GPS III, without the SA feature. Doing this will make the policy decision of 2000 permanent and eliminate a source of uncertainty in GPS performance that had been of concern to civil GPS users worldwide.

[Flagman101]

Originally Posted by Siberian Sea

In truth I believe many if not most of these sailors are indeed lost. Let’s just face the facts....... the GPS system has allowed way to many incompetent people to buy a boat and with little or no navigational skills at all just look at a laptop screen and watch the marker move across the digital chart. Backup paper charts?, understanding of meteorology?, even the proper calibration check (swing) the compass is beyond so many. The understanding and skilled use of a sextant sadly has gone the way of real shipwrights whom could spiral a Douglas fir plank in a wonderful wooden schooner.

What lost sailors?

Any facts here?

[Adelie]

Originally Posted by 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 mentioned the Lunar distance method for which you would want a high end sextant and the HO-229 reduction tables which are 6 volumes. That’s beyond me for cost, storage volume and level of required technique. The NavList folks regularly discuss Lunars if you are still interested.

A lightning strike that wipes out the backup GPSs in the ammo box will put paid to the Shortwave radio too so WWV et al are out.

Getting accurate time still involves having a mechanical timepiece and knowing it’s error rate. The error rate is how many seconds the piece gains or loses per day or or week and requires checking the error at least weekly over a month or so. Once you lose outside references you update the correction you apply to watch time based on how long it’s been since you last determined error from an outside source. If the correction at last check was -18s, the error rate is 2s gained per week and it’s been 49d (7wk) since last checked (7wk x -2s/wk = -14s change in correction. -18a + -14s = -32s for the current error.

The 2 techniques I would use would be:

A. Shoot a celestial body significantly east or west from a known position. Reduce the sight using your estimated time. Plot the line of position (LoP). Measure east or west from you position to the line. (Do not measure to the nearest point). Lines of position move west at 4 s/1nm. If the LoP is 40 nm west that’s 40nm X 4s/nm = 160s error. LoP west of actual position means you subtract to get actual time. If LoP is east add correction watch time to get actual time.
Then recompute with corrected time to be sure. This should get you within 10-20sec of correct time.
After that take 5-10 sights of same body averaging readings and times applying correction from first round to determine a better correction this should get you correction to 5sec or so of correct.

You now know the correction to watch time to get actual time. Going forward you use the error rate to update the correction.

B. There is a simplified Lunar method that can be used. When the moon is significantly east or west of you shoot the moon and a round of stars. Plot the LoPs for all the bodies. Probably you will get a reasonable cocked hat for all the stars with the moon line significantly east or west of this fix. The moon line moves east at about 2 min per Nm relative to the stars. Based on distance east or west from the cocked hat you can determine an approximate correction.
Now recalc and replot all the sights using the correction. And repeat until the moon line runs thru the middle of the stars’ cocked hat.

The correction you finally determine should be accurate in the range of 15-30 which means the LoPs will be accurate in the range of 4-8nm.

Yes this method is long and tedious but it is the difference between knowing your position to the nearest 5-10nm and knowing it to the nearest 100-150nm at the end of a passage. On passage it doesn’t matter whether you are off by 5 or 50 nm. As I approach land I’d rather know where I was within 10nm than within 50nm.


Both of these methods were developed by John Letcher JR and you can read about them in his book on using HO-208 which you can buy on Amazon for $20-30 currently

[thinwater]

I lost GPS once (water damage), back when it was new and having a back-up was very unusual. I had been logging position, speed, and course every two hours. after dead reconing for ~ 36 hours my landfall error was about 5%, and from there on it was coast pilotage. Honestly, since I was headed for a large coast, I didn't sweat the details of cross current, and I intentionally held off-shore until daylight.

No big deal in the mid-Atlantic, but it could be creepy in areas with coral atolls and small islands.

In today's age, with back-up (probably several) GPS standard, I wouldn't give it a second thought.

[Flagman101]

Originally Posted by sailorboy1

Simple compass doesn’t work inside a metal submarine with electrical cabling all over the place. Not normally much call on a sub to take star positions.

But it was “ interesting”

Why do they have those compass on periscopes then?
Mind you, I have never used one, but about all the movies I have seen with subs have a periscope with an integrated compass and they call bearing and headings of targets.

[ZULU40]

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?

depends on the instrument and the user, otherwise about 1 NM

Originally Posted by Flagman101

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

shouldnt take much longer than a few seconds

Originally Posted by Flagman101

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

then you use dead reckoning

Originally Posted by Flagman101

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

the easiest and most crucial are the primary sights, noon, sunrise and sunset

[Flagman101]

Originally Posted by thinwater

I lost GPS once (water damage), back when it was new and having a back-up was very unusual. I had been logging position, speed, and course every two hours. after dead reconing for ~ 36 hours my landfall error was about 5%, and from there on it was coast pilotage. Honestly, since I was headed for a large coast, I didn't sweat the details of cross current, and I intentionally held off-shore until daylight.

No big deal in the mid-Atlantic, but it could be creepy in areas with coral atolls and small islands.

Would anyone, even experts, recommend navigating coral atolls by sextant?
I was in the Grenadines in march and i did not even trust the GPS to round a reef. I aimed for it and when I was in visual contact I corrected course to round it, never looking back at the gps. For me it's charts and DR complemented by the GPS.

[Adelie]

Originally Posted by Flagman101

You do know that the cheapest quartz watch will be more precise then your most expensive mechanical watch?

I'd rather rely on quartz then mechanical watches.



And I do own a nice Citizen Skyhawk eco drive that is radio controlled.

I have owned it for 9 years now and it never needs batteries as it's solar powered and always very precise because overnight it tunes to an antenna and just accordingly. I have never had to just time on it.

I think it would probably get the signal very very far from shore. I live in Montreal Canada and the nearest antenna is in Denver Colorado. There is on in Japan and one in Europe.

IT's titanium and 200mt water resistant.

Any lightning strike that fries you backup electronics in the ammo case is probably going to dry all the quartz watches too.

The mechanical watch is just the ultimate backup timepiece. I got a Molinja for $35 off eBay.

[ZULU40]

Originally Posted by Flagman101

Why do they have those compass on periscopes then?
Mind you, I have never used one, but about all the movies I have seen with subs have a periscope with an integrated compass and they call bearing and headings of targets.

its not a compass, its called a pelorus
It is a simple gradated mark from the head of the boat

most especially for the USN who use bearings by degree, as opposed to red and green in my part of the world

the real compass in days when they were popular , had an instrument parked on the upperworks of the boat and the read compass was a simple repeater

all the same, my Steinar binocs have an internal compass, and they were navy issue

[Flagman101]

Originally Posted by ZULU40

its not a compass, its called a pelorus
It is a simple gradated mark from the head of the boat

most especially for the USN who use bearings by degree, as opposed to red and green in my part of the world

the real compass in days when they were popular , had an instrument parked on the upperworks of the boat and the read compass was a simple repeater

all the same, my Steinar binocs have an internal compass, and they were navy issue

Can you get your heading with a Pelorus?

[sailorboy1]

[QUOTE=Flagman101;2613537]Why do they have those compass on periscopes then?
Mind you, I have never used one, but about all the movies I have seen with subs have a periscope with an integrated compass and they call bearing and headings of targets.[/QUOTE

Because it gets ifo from the gyro, which is unless other than bearing if the gyro no longer knows where you are.

[Adelie]

Originally Posted by Flagman101

Wow scary that such a weapon does not even have a simple compass.

Also confusing north from south..suprising, NW to NE maybe.

But I don't know much about nuclear subs.

Magnetic compasses don’t work in subs, the hull completely surrounds the compass and prevents the compass from giving an accurate reading.

Subs use gyrocompasses which are complex electro-mechanical devices that have a different set of problems to deal with.

[sailorboy1]

I’m going to unscribe to the thread. But I’ll leave this parting thought:

I wouldn’t cross an ocean now days without a backup to my main chart plotter. But a backup gps is cheap (I have a plug-in for my phone that was $99 that is normally accurate to 15’) so I have no interest in a sextant.

[ZULU40]

Originally Posted by Flagman101

Can you get your heading with a Pelorus?

steering and combat systems are geared toward the heading, which the pelorus shows. Everything is relative to the boats heading, but it wont give you an actual compass heading in itself.

Submarines will also mount a pelorus on their bridge for surface attacks and shooting bearings. It used to be that they would demount them when they dive as they were part of the attack computations.

Ships have them too on the bridge and on the flybridge because they are simple to use to shoot bearings to ships and landfalls