Accuracy of sextants?
jean somerhausen
>
> You use a sextant because the GPS may fail. You use GPS because the weather may be
> too bad for a sight. You use both to test your skills.
>
> Robert Bethune <reply.in....@thank.you> wrote in message
> news:01befb0d$4e381f00$0200a8c0@lakesuperior...
> > Every time I read something like this about high-accuracy sextant
> > positions, I always want to know the answer to one simple question: how did
> > they know where they were so accurately that that they could determine that
> > the accuracy of the sextant position was within a certain margin of error?
> >
> > If you can tell me that a sextant position is within 1/10 of a mile of your
> > true position, then you already know your true position--so why are you
> > taking a sextant position, unless it's for the fun of it?
> >
> > Glenn Ashmore <gash...@mindspring.com> wrote in article
> > <37D7DEFC...@mindspring.com>...
> > >
> > >
> > > Igor wrote:
> > >
> > > > If I understand it correctly, one can use a sextant to determine one's
> > > > position. How accurate would such a determination be? What do you need
> > > > for that, sun? Or stars?
> > >
> > > I am no expert and I only get a chance to do a half dozen real fixes a
> > year, but
> > > I usually get within a mile or two of the GPS. That is good enough for
> > open
> > > ocean work.
> > >
> > > The guy that taught me is a retired Chief Bosun's Mate. He can come
> > within 1/10
> > > mile almost every sight. Of course, he did it three times a day for 20
> > years.
> > >
> > > --
> > > Glenn Ashmore
> > >
> > > I'm building a 45' cutter in strip/composite. Watch my progress (or lack
> > there
> > > of) at: http://www.mindspring.com/~gashmore
> > >
> > >
> > >
I first practised with a sextant on a passenger liner in 1969, using the
French "Dieumegard" tables (which require only a few additons and no
high maths). When I later tried to use the sextant on my 36 ft sailboat,
I gfound that it was tantamount to shooting pigeons with a rifle from
the back of a galloping horse....
john
jean somerhausen
Fernando de Noronha, not to check the accuracy of their sextant, but
that of their chronometers. They took a sight whilst in view of the
island and the error found in their position was that of the timepiece
john
Igor wrote:
>
> Robert Bethune <reply.in....@thank.you> wrote:
> * Every time I read something like this about high-accuracy sextant
> * positions, I always want to know the answer to one simple question: how did
> * they know where they were so accurately that that they could determine that
> * the accuracy of the sextant position was within a certain margin of error?
> *
> * If you can tell me that a sextant position is within 1/10 of a mile of your
> * true position, then you already know your true position--so why are you
> * taking a sextant position, unless it's for the fun of it?
>
> Navigational aids, such as bouys, allow you to determine your position very
> precisely.
>
> Why would you want to fix your position with a sextant near a bouy?
>
> Well, the first thing that comes to mind is to test or practice with a
> sextant.
>
> igor
>
> * Glenn Ashmore <gash...@mindspring.com> wrote in article
> * <37D7DEFC...@mindspring.com>...
> * >
> * >
> * > Igor wrote:
> * >
> * > > If I understand it correctly, one can use a sextant to determine one's
> * > > position. How accurate would such a determination be? What do you need
> * > > for that, sun? Or stars?
> * >
> * > I am no expert and I only get a chance to do a half dozen real fixes a
> * year, but
> * > I usually get within a mile or two of the GPS. That is good enough for
> * open
> * > ocean work.
> * >
> * > The guy that taught me is a retired Chief Bosun's Mate. He can come
> * within 1/10
> * > mile almost every sight. Of course, he did it three times a day for 20
> * years.
> * >
> * > --
> * > Glenn Ashmore
> * >
> * > I'm building a 45' cutter in strip/composite. Watch my progress (or lack
> * there
> * > of) at: http://www.mindspring.com/~gashmore
> * >
> * >
> * >
>
> --
> ----------------------------------------------------------------------------
> char*p="char*p=%c%s%c;main(){printf(p,34,p,34);}";main(){printf(p,34,p,34);}
> http://www.algebra.com/~ichudov
jlrogers
the cosine-haversine formula and on the accuracy of his instruments, including his
timepiece, sextant, and estimate of height above sea level. If you're good, you're
better than GPS. If you're bad, you're probably sunk. There are computers and text
books of tables to eliminate the trigonometry involved.
Igor <ig...@Algebra.Com> wrote in message
news:slrn7tfm3...@manifold.algebra.com...
> If I understand it correctly, one can use a sextant to determine one's
> position. How accurate would such a determination be? What do you need
> for that, sun? Or stars?
>
Glenn Ashmore
Igor wrote:
> If I understand it correctly, one can use a sextant to determine one's
> position. How accurate would such a determination be? What do you need
> for that, sun? Or stars?
I am no expert and I only get a chance to do a half dozen real fixes a year, but
I usually get within a mile or two of the GPS. That is good enough for open
ocean work.
The guy that taught me is a retired Chief Bosun's Mate. He can come within 1/10
mile almost every sight. Of course, he did it three times a day for 20 years.
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack there
of) at: http://www.mindspring.com/~gashmore
Steven Shelikoff
>
> You can use sun or stars. Accuracy depends on the observer's technique for applying
> the cosine-haversine formula and on the accuracy of his instruments, including his
> timepiece, sextant, and estimate of height above sea level. If you're good, you're
> better than GPS. If you're bad, you're probably sunk. There are computers and text
> books of tables to eliminate the trigonometry involved.
I'd love to meet the person who can figure his position better with a
sextant than with GPS. Even on solid ground from a tall building with
the full horizon in sight, you won't do as good as GPS. On a boat, good
luck getting better than a mile or two. Now, if the GPS unit is broken
you might have a chance. Then again, if it's cloudy ...
Steve
--
/ / /
\ \ \ mailto:shel...@averstar.com
/ / /
Phoenix
of sight of your target. I think that is all anyone expected from a
sextant.
--
Jim
1994 Regal 256 for sale - see ad at
http://www.classifieds2000.com/cgi-cls/ad.exe?P1+C189+R1187463
Igor <ig...@Algebra.Com> wrote in message
news:slrn7tfq7...@manifold.algebra.com...
> Is it hard to use a sextant when the boat rolls in waves?
> --------------------------------------------------------------------------
Mr. GPS Answer Man
>If I understand it correctly, one can use a sextant to determine one's
>position.
>
Only if you know what time it is. GPS receivers are great for this
because they have very accurate atomic clocks built right in.
>How accurate would such a determination be?
>
Exactly right, plus or minus a hundred miles or so.
>What do you need for that, sun? Or stars?
>
Ideally, both. The moon and planets wouldn't hurt either. And don't
forget that GPS receiver for the time.
Brian Grant
the time and their location. The receiver calculates your position and
the correct time from this information.
Thomas Hood
"Mr. GPS Answer Man" lied:
<snip>
> Only if you know what time it is. GPS receivers are great for this
> because they have very accurate atomic clocks built right in.
Sheesh!! Where do these people get their information??? No atomic
clock in any GPS on earth.
>
> >How accurate would such a determination be?
> >
> Exactly right, plus or minus a hundred miles or so.
You must suck at math or have Cerebral Palsy...
>
> >What do you need for that, sun? Or stars?
> >
> Ideally, both. The moon and planets wouldn't hurt either. And don't
> forget that GPS receiver for the time.
I'll use my Timex thanks...
Thomas Hood
th...@ifn.com
Andy Bogle
latitude only. You will need a chronometer to determine longitude and
obtain a position. You can also use a sextant to measure your distance off
an object such as a lighthouse, provided that you have charts showing the
object height. Taking several such measurements you can obtain a fix on
your position. Using the sextant to obtain vertical or horizontal angles
can get you better than GPS position. But I agree that you would be hard
pushed to get within 1/2 nm using celestial observations.
Andy Bogle
Igor <ig...@Algebra.Com> wrote in message
news:slrn7tfm3...@manifold.algebra.com...
> If I understand it correctly, one can use a sextant to determine one's
> position. How accurate would such a determination be? What do you need
> for that, sun? Or stars?
>
> --
Glenn Ashmore
> Is it hard to use a sextant when the boat rolls in waves?
I practiced on a beach for weeks. Got to where I could shoot 4 or 5 stars in 10
minutes and the triangle of LOPs was no more than 3 miles on a side.
Then I went sailing. I was lucky to shoot 3 stars before I lost the horizon and
the triangle got as wide as 20 miles. I've gotten better over the years but it
takes a lot of practice. That picture of the noble seaman at the helm with
sextant to his eye is a fantasy. In reality, you wedge yourself in the
companion way and hope that D&^n cloud moves before you get seasick..
If you want to practice ashore. Find a teeter-totter and a couple of
hyperactive kids. Stand in the middle and take your sights.<g>
an_ex-qm
<gash...@mindspring.com> wrote:
>The guy that taught me is a retired Chief Bosun's Mate.
>
What navy let's a deck ape work a sextant? Maybe he was the recorder
for a Chief Quartermaster? I understand some boats actually know how
to write.
>He can come within 1/10 mile almost every sight.
>
Sure he can.
>Of course, he did it three times a day for 20 years.
>
Man, not much shore leave for him, huh?
Jim Hollenback
: The guy that taught me is a retired Chief Bosun's Mate. He can come within 1/10
I don't know what navy you were in, but in the U.S. Navy a Chief Bosun's
Mate probably could show you a few knots you didn't know, but I would be
far more comfortable with the fix done by a Chief Quartermaster.
--
Jim Hollenback
jho...@cup.hp.com
Bob Richardson
Igor wrote in message ...
>Well, I assume that I can use a programmable calculator and an accurate
>watch (is the accuracy of the clock an issue?). That's not a problem. The
>question is in their inherent accuracy. Also, how hard is it to acquire
>the skills needed to be reasonably accurate? Let's say, I spend 8 hours
>practicing during various days. How well will I be able to do my job?
>And how much a "decent": sextant, however defined, could cost?
I used a CelesteComp programmable calculator and leaned how to find my
position after a few easy weeks of reading and practicing. I would get
three fixes which would give me three LOPs (lines of position). A good fix
was when the resulting triangle from the three lines (the "cocked hat")
showed that your estimated position was withing a small area - perhaps
within a triangle that was 2-3 miles on a side. I used an Astral IIIb
sextant which cost $300-400. The time pieces weren't very special, but I
checked them often with the WWV on the ham radio so I felt that my time was
never off more than 1-2 seconds. The hardest part of course is getting a
good fix when the waves get over 5 feet.
Unless you never get seasick, and are extremely good with math, I think the
programmable calculator is the way to go.
Glenn Ashmore
Jim Hollenback wrote:
> I don't know what navy you were in, but in the U.S. Navy a Chief Bosun's
> Mate probably could show you a few knots you didn't know, but I would be
> far more comfortable with the fix done by a Chief Quartermaster.
You are probably right. I was in the Army and never could figure out the Navy's
pecking order. It has been 15 years sence I last saw that guy. I remembered he was
a "Chief" sometning and took a SWAG at Bosun.<g>
Rod McInnis
"Mr. GPS Answer Man" wrote:
> Only if you know what time it is. GPS receivers are great for this
> because they have very accurate atomic clocks built right in.
Actually, the receivers have rather poor clocks. They don't need a
precise clock (which would be very expensive and burn batteries at an
incredible rate) because they can get the time from the satellite.
The GPS receiver uses the data from the satellite to solve for 4
unknowns: Latitude, longitude, altitude and time. To properly solve
all four, the receiver needs to get information from 4 different
satellites. If four are not available, it can generally estimate a
position with only three if it makes some assumptions, such as you
aren't likely to be much below sea level or at an extreme altitude, you
are in the same general area as you were last time it had an accurate
fix, etc.
Once the receiver has an accurate time solution, it can reset its
internal clock and then it can get by with only three satellites for
quite a while before its clock drift introduced a significant error.
Rod
Mark Crispin
> If you're good, you're better than GPS.
Just how did you arrive at that remarkable conclusion? GPS at its worse
(which you probably won't encounter on open ocean) is at least twice as
good as a reading taken by a highly-skilled individual with a high quality
sextant. When GPS is at its worst (e.g. in forest or urban) a sextant
isn't particularly useful.
If you're really good with a sextant, you can get a sextant reading to
about 1/10 of a minute, but that's still 6 seconds or about 185 meters.
Most people can get it down to minutes, but not to fractional minutes.
You also need to be damn good at recognizing noon down to 1/15th of a
second, since each time second represents 15 seconds of longitude. Once
again, recognizing noon to +/- 4 time seconds is a minute of longitude.
Quite frankly, these guys claiming that their sextants are more accurate
than GPS are blowing hot air. Sextants may appeal as an intellectual
challenge, or as something that doesn't depend upon batteries (although
you can carry a lot of spare batteries and a spare GPS or two for the
weight and space consumed by your sight reduction tables and almanac!!).
Sextant fixes are certainly good enough for open ocean.
However, sextants are less accurate than GPS, not particularly useful on
land (the artificial horizon being another thing to lug around), and much
more hassle to use. And for those of you who talk about batteries running
out, what about the chronometer? Mechanical, you say? But you forgot to
wind it, oh dear oh my...
There will always be a place for sextants, along with Morse code and
phonographs. Some people will even hold them in affection. But affection
for the technology of the past should not blind us to the superiority of
contemporary technology, not to mention the reasons why the past
technology died.
-- Mark --
* RCW 19.190 notice: This email address is located in Washington State. *
* Unsolicited commercial email may be billed $500 per message. *
Science does not emerge from voting, party politics, or public debate.
alan
Skipper
>
> Igor wrote:
>
> > Is it hard to use a sextant when the boat rolls in waves?
>
> I practiced on a beach for weeks. Got to where I could shoot 4 or 5 stars in 10
> minutes and the triangle of LOPs was no more than 3 miles on a side.
>
> Then I went sailing. I was lucky to shoot 3 stars before I lost the horizon and
> the triangle got as wide as 20 miles. I've gotten better over the years but it
> takes a lot of practice. That picture of the noble seaman at the helm with
> sextant to his eye is a fantasy. In reality, you wedge yourself in the
> companion way and hope that D&^n cloud moves before you get seasick..
>
> If you want to practice ashore. Find a teeter-totter and a couple of
> hyperactive kids. Stand in the middle and take your sights.<g>
ROFLOL!!!
--
Skipper
jlrogers
"better." I find that not depending on the U. S. Government to tell me where I am is
"better."
better, as in more advantageous or favorable.
Mark Crispin <m...@CAC.Washington.EDU> wrote in message
news:Pine.NXT.4.20.99090...@Tomobiki-Cho.CAC.Washington.EDU...
Robert Bethune
If you can tell me that a sextant position is within 1/10 of a mile of your
true position, then you already know your true position--so why are you
taking a sextant position, unless it's for the fun of it?
Glenn Ashmore <gash...@mindspring.com> wrote in article
<37D7DEFC...@mindspring.com>...
>
>
> Igor wrote:
>
> > If I understand it correctly, one can use a sextant to determine one's
> > position. How accurate would such a determination be? What do you need
> > for that, sun? Or stars?
>
> I am no expert and I only get a chance to do a half dozen real fixes a
year, but
> I usually get within a mile or two of the GPS. That is good enough for
open
> ocean work.
>
> The guy that taught me is a retired Chief Bosun's Mate. He can come
within 1/10
> mile almost every sight. Of course, he did it three times a day for 20
years.
>
jlrogers
too bad for a sight. You use both to test your skills.
Robert Bethune <reply.in....@thank.you> wrote in message
news:01befb0d$4e381f00$0200a8c0@lakesuperior...
jlrogers
pier. There were many days we couldn't get good sights, but there were more when
the Loran was not working (1962-1966). Rough seas nearly always stopped the Loran.
(shook to pieces - built by lowest bidder)
jean somerhausen <diplo...@worldnet.att.net> wrote in message
news:37D70D...@worldnet.att.net...
Phoenix
No.
Sincerely,
--
Jim
1994 Regal 256 for sale - see ad at
http://www.classifieds2000.com/cgi-cls/ad.exe?P1+C189+R1187463
Igor <ig...@Algebra.Com> wrote in message
news:slrn7tg5i...@manifold.algebra.com...
> Out of curiosity... Are there such things as electronic sextants? That
> is, sextants with a built in calculator and such. You would point it at
> sun or moon or whatever, press the moon button, or something, and then
> it would give you your position. They'd have a biult in clock and so on.
Mundo
>
> jean somerhausen <diplo...@worldnet.att.net> wrote in message
> news:37D70D...@worldnet.att.net...
> > I first practised with a sextant on a passenger liner in 1969, using the
> > French "Dieumegard" tables (which require only a few additons and no
> > high maths). When I later tried to use the sextant on my 36 ft sailboat,
> > I gfound that it was tantamount to shooting pigeons with a rifle from
> > the back of a galloping horse....
> > john
> >
>
>
reading. This is something I have done with fair success. 5 sights added
up and divided by 5. On a good day, clear sky, light seas I am happy with
3-5 miles. Heavyer weather I use running fixes with dawn, noon, and dusk
sight when available. I have yet to get a good star sight. Larger targets
are easier for me. My time piece is a stop watch set to time tone with
corrections added later and a rolex. Of course with gps I rarely miss by
more than 100'. I only have 1 sextant but keep 3 gps on board. Life has
become simpler.
--
Mundo
"The captain who is a bully and an ass"
Jim Scott
astro-inertial navigation system (ans). the great thing about the unit was
that is could "see" the stars in the daytime (yes they are still out- you
just can't see them) giving the aircraft a position within approx .3 miles.
it used two stars but selected them to provide perpendicular line of
positions. now if you could just find a surplus sr-71 somewhere, you would
have one hell of a navigation system, and a pretty fast ride also....
rdc
I also generally used the multiple sight averaging method while at sea. I
agree with your statement about being happy with 3-5 miles accuracy. That
is a realistic expectation when shooting astro from a small boat in
moderate seas. The running fix method is more than adequate when offshore.
Bob
an_ex-qm
<reply.in....@thank.you> wrote:
>If you can tell me that a sextant position is within 1/10 of a mile of your
>true position, then you already know your true position--so why are you
>taking a sextant position, unless it's for the fun of it?
>
"The prudent mariner will not rely solely on any single aid to
navigation . . ."
When all the power fails it's a little too late to start practicing
with the sextant.
Chuck Tribolet
Jim Scott wrote:
> now if you could just find a surplus sr-71 somewhere, you would
> have one hell of a navigation system, and a pretty fast ride also....
--
Chuck Tribolet
Internet: tri...@almaden.ibm.com (IBM business)
tri...@garlic.com (personal)
http://www.almaden.ibm.com/cs/people/triblet
Silicon Valley: Best day job in the world.
Glenn Ashmore
Robert Bethune wrote:
> Every time I read something like this about high-accuracy sextant
> positions, I always want to know the answer to one simple question: how did
> they know where they were so accurately that that they could determine that
> the accuracy of the sextant position was within a certain margin of error?
>
> If you can tell me that a sextant position is within 1/10 of a mile of your
> true position, then you already know your true position--so why are you
> taking a sextant position, unless it's for the fun of it?
I met my teacher on the beach at St.Simons Island when I was just starting to
learn. He offered to work with me and we met every morning and evening for two
weeks. If it was clear, he would call out 3 stars for me to shoot and then he
would shoot 3 more. Then I would work up fixes from both sets with him
supervising. My LOPs were all over the place but the corners of his triangles
were always within walking distance. .
This guy served on a carrier. He said that a sight was taken morning, noon and
evening when ever the cloud cover permitted. It was reduced and compared to
the inertial system, the Loran if available and the satalite fix. (He retired
before GPS) If the average variance of the 4 fixes was more than something
like a half mile, it had to be written up and reported to the captain.
I would imagine that taking a site from the bridge of an aircraft carrier would
be a bit easier than from a 45' sailboat.
jean somerhausen
>
> I live in Holland (flat as a CD) but how often do we have a clear
> view to the stars, also keeping in mind that a big part of NW Europe
> is not really dark at night, so you have a problem seeing the stars.
>
> You don.t really need a sextant if you learn to navigate properly
> Grx Rolphe
Maybe you don't but it can come in handy some time, as when we were
sailing a few miles North of Schiermonnikoog or Amerland (can't exactly
remember which) with a nice NE'ly on our quarter. I couldn't tell
exactly what distance we were from the island (it isn't easy just by
eye) so I took a sextant angle on the lighthouse and realized we were
about 1 mile too close and there was a sandbank ahead...
john
Charles Gallo
You just need a bigger SHIP. Talked with a guy who said it's
REAL easy from the deck of a ULCC (Ultra Large Crude Carrier). Says
he used to do it to stay in practice. I guess a 900 ft ship doesn't
bounce much
Charlie
-- PGP Key on Request
For the Children RKBA!
OTNMBRD
>If I understand it correctly, one can use a sextant to determine one's
>position. How accurate would such a determination be? What do you need
>for that, sun? Or stars?
First off, a sextant is for Ocean, not Coastal navigation, so the necessity to
know your position, within feet, Centimeters, etc, is mute, if not ridiculous,
unless you are some scientist, etc., searching for an needle in a haystack,
when in, let's say, the middle of the Pacific Ocean.
You can use either stars, sun, moon, or planets, or an overlap of all.
Have matched my sunlines to radar fixes.....my sunlines crossed thru the radar
fix......ie, close enough.......for most navigation, this GPS induced
necessity, to know your position to within inches, for normal navigation
purposes...note, I said NORMAL........is a bunch of elistist, technocrap,
bullcrap, generally pushed down people's throats by those, unfamiliar, or
uncomfortable, with navigation at sea, in general.
There......I got that off my chest........the accuracy you need, is relative to
the position you are in, and the method you are using to determine this
position, should be based on the accuracy you need.....if you have a good GPS,
and aren't worried about it crapping out, and you are able to use other methods
to determine your position, then you don't need celestial, radar, radio, decca,
loran, piloting, fathometer, capabilities........ or abilities........<BG>
Peter Bennett
>Andy Bogle <bo...@concentric.net> wrote:
>* Using a sextant to observe the sun or stars will allow you to calculate your
>* latitude only. You will need a chronometer to determine longitude and
You can determine latitude using a noon sight on the sun, and no knowledge
of the time. Any other form of sight requires accurate time.
>
>I assume that a decent electronic watch is good enough as a chronometer.
>
>Am I correct?
Yes - modern digital watches are probably more accurate than traditional
chronometers. However, you must check your watch against radio time
signals or other reference every few days, and keep a record of its drift.
>
>But, say, a 1nm acuracy is not all that unobtainable. Is that right?
>
For Canadian Power Squadrons' celestial navigation classes, sights taken
from shore positions must produce lines of position within 3 miles of your
known position for the sight to be acceptable, if I recall correctly. It
would be very difficult to get this accuracy from a small vessel.
A sight on any one body (sun, moon, planet, or star) will only give you a
line of position - to get a fix, you will need sights on two, or preferably
three, different bodies within a few minutes.
Star and planet sights can only be taken for a few minutes around sunset or
sunrise.
--
Peter Bennett, VE7CEI
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver-webpages.com/van-ps
TK
>
>What navy let's a deck ape work a sextant? Maybe he was the recorder
>for a Chief Quartermaster? I understand some boats actually know how
>to write.
>
>>He can come within 1/10 mile almost every sight.
>>
>Sure he can.
>
>>Of course, he did it three times a day for 20 years.
>>
>Man, not much shore leave for him, huh?
I'm LOL.... I was the Navigator on a cruiser (the kind with guns and
missiles) from 89-91 - we had a $35,000 GPS (which we purchased
instead of 7 toilet seats... :) and without the interference of SA, I
plotted a fix inside one of the locks of the Panama Canal which was
our exact position - proving to me that this new (at the time) GPS
stuff really worked.
The quartermasters who worked for me used to shoot stars, weather
permitting, just about every day at sea. Despite having quite a bit
of skill and excellent equipment (Weems & Plath), the best they could
get was within a quarter mile or so. This was pretty good however
compared to their young Lieutenant (a.k.a yours truly) who was
typically accurate to within only a few miles. I don't think anyone -
no matter how good, could ever do better than GPS.
The Bosun's were good guys without much hair on their knuckles - never
saw one with a sextant (although I did know one capable of using the F
word as a noun, adjective, verb, adverb and every other part of speech
all in the same sentence).
Couldn't tell you how much a top of the line sextant goes for, but my
guess would be in the low to mid 4 figures. Not sure how you would
ever use one on a rocking sailboat, but I guess there are those who
do.
TK
thru18
> I said better, not more accurate. Perhaps you find that the more
>accurate GPS is "better." I find that not depending on the U. S.
>Government to tell me where I am is "better."
Fair enough Mark, but you'd better check to see who prints the chart
you plot your position on.
Russ
Sent via Deja.com http://www.deja.com/
Share what you know. Learn what you don't.
WB
talking about sextant accuracy. if you acheive a nautical mile accuracy you
should be happy, but like i say it depends on firstly the stability of the
platform,, then we can start talking about sextant accuracy, sextant fro
backup, use a gps.
Igor <ig...@Algebra.Com> wrote in message
> If I understand it correctly, one can use a sextant to determine one's
> position. How accurate would such a determination be? What do you need
> for that, sun? Or stars?
>
Paul Kamen
>Using a sextant to observe the sun or stars will allow you to
>calculate your latitude only. ...
Not strictly true. There's a simple version of the "lunar distance method"
that can be used to get longitude without having an accurate time source
or any special tables or techniques. But the error is about 30 times as
large as for latitude.
All you do is take three sights, one of which is the moon. Plot the three
lines of position the usual way, as if you had accurate time info. Now
adjust the time 'till all three lines cross in one spot, and you have your
position.
Some details left out, but you get the idea. take advantage of the fact
that the moon moves fast enough relative to all other celestial bodies to
be treated as an independent variable. Workds great if you're not fussy
about a hundred miles this way or that.
--
fish...@netcom.com
http://www.well.com/~pk/fishmeal.html
-"Call me Fishmeal"-
Tim Jacobs
Tim
---
Tim Jacobs timothy...@gecm.com
These are my views, not GEC's.
Chuck Tribolet <tri...@garlic.com> wrote in article
<37D86213...@garlic.com>...
Sam Storm van Leeuwen
On a relatively merchant vessel with a clear sky you can reach a position
accuracy in the order of 0.3 nm or 500 m.
GPS is accurate to 100 m 95 %, but can be as inaccurate as many km's
in the remaining 95% !
Sam (ex-2nd mate)
Phoenix
--
Jim
1994 Regal 256 for sale - see ad at
http://www.classifieds2000.com/cgi-cls/ad.exe?P1+C189+R1187463
Peter Bennett <pet...@interchange.ubc.ca> wrote in message
news:37e66a0b....@news.supernews.com...
> On Thu, 9 Sep 1999 15:00:58 -0500, ig...@Algebra.Com (Igor) wrote:
>
> >Andy Bogle <bo...@concentric.net> wrote:
> >* Using a sextant to observe the sun or stars will allow you to calculate
your
George Bynum
<jfga...@spamfree.mediaone.net> wrote:
>Peter, how can you do a noon sighting without any knowledge of the time?
hey, I'm not Peter, but LAN (Local Apparent Noon) is determined by the
time when the sun changes from rising to setting .. which the sextant
allows determining by continuous monitoring of the angle.
George
Steven Shelikoff
>
> On Thu, 9 Sep 1999 15:00:58 -0500, ig...@Algebra.Com (Igor) wrote:
>
> >Andy Bogle <bo...@concentric.net> wrote:
> >* Using a sextant to observe the sun or stars will allow you to calculate your
> >* latitude only. You will need a chronometer to determine longitude and
>
> You can determine latitude using a noon sight on the sun, and no knowledge
> of the time. Any other form of sight requires accurate time.
Actually, you can determine your longitude without time as well. See
the paper "Longitude Without Time" by J. W. Luce, in the Journal of the
Institute of Navigation, summer 1977.
Steve
--
/ / /
\ \ \ mailto:shel...@averstar.com
/ / /
Steven Shelikoff
>
> > I said better, not more accurate. Perhaps you find that the more
> >accurate GPS is "better." I find that not depending on the U. S.
> >Government to tell me where I am is "better."
>
> Fair enough Mark, but you'd better check to see who prints the chart
> you plot your position on.
And who prints the tables.
Steven Shelikoff
Actually, I should ammend this by saying that you do need a watch. It
just doesn't have to be set to the correct time. The method outlined in
the paper allows you to calculate the unknown watch error from celestial
observations.
Steven Shelikoff
>
> Andy Bogle <bo...@concentric.net> wrote:
>
> >Using a sextant to observe the sun or stars will allow you to
>
> Not strictly true. There's a simple version of the "lunar distance method"
> that can be used to get longitude without having an accurate time source
> or any special tables or techniques. But the error is about 30 times as
> large as for latitude.
>
> All you do is take three sights, one of which is the moon. Plot the three
> lines of position the usual way, as if you had accurate time info. Now
> adjust the time 'till all three lines cross in one spot, and you have your
> position.
>
> Some details left out, but you get the idea. take advantage of the fact
> that the moon moves fast enough relative to all other celestial bodies to
> be treated as an independent variable. Workds great if you're not fussy
> about a hundred miles this way or that.
Yup. That's the method described in the paper I cited in another post.
The paper gives all the details and procedures for calculating the
longitude.
If the moon bears due east or west, the error is magnified 21 times if v
is small, 35 times if v is large. (v is the increment in GHA of the
moon, from the almanac.) If the moon is 37 degrees away from east or
west, the error is 25% larger, or 26 and 44 times respectively. The
error I'm talking about is comparing the longitude obtained with this
method to what you would get if you knew accurate time.
Assuming that you are really good at taking your sights and you can get
within 1 mile of longitude with accurate GMT, then you could get
anywhere from 21 to 44 miles of your actual longitude without accurate
GMT, depending on the geometry.
Mark Kempisty
I think I'll replace the GPS in my APRS tracker with a sextant. Now as long
as I don't plow into the back of that tractor trailer while shooting a sight
and keying the location into the APRS modem ;-)
-- Mark
Paul Kamen <fish...@netcom.com> wrote in message
news:7rabn5$c...@dfw-ixnews17.ix.netcom.com...
> Andy Bogle <bo...@concentric.net> wrote:
>
> >Using a sextant to observe the sun or stars will allow you to
> >calculate your latitude only. ...
>
> Not strictly true. There's a simple version of the "lunar distance method"
> that can be used to get longitude without having an accurate time source
> or any special tables or techniques. But the error is about 30 times as
> large as for latitude.
>
> All you do is take three sights, one of which is the moon. Plot the three
> lines of position the usual way, as if you had accurate time info. Now
> adjust the time 'till all three lines cross in one spot, and you have your
> position.
>
> Some details left out, but you get the idea. take advantage of the fact
> that the moon moves fast enough relative to all other celestial bodies to
> be treated as an independent variable. Workds great if you're not fussy
> about a hundred miles this way or that.
Christian G. Taranti
be turned off because the US bombed Afeganistan and is afraid of the
terrorists...
Steven Shelikoff wrote:
> thru18 wrote:
> >
> > > I said better, not more accurate. Perhaps you find that the more
> > >accurate GPS is "better." I find that not depending on the U. S.
> > >Government to tell me where I am is "better."
> >
> > Fair enough Mark, but you'd better check to see who prints the chart
> > you plot your position on.
>
> And who prints the tables.
>
Anders Svensson
Th electronic sextant uses it's own special stars, and measures the distance to
them, not the angles (but they are closer and at well defined places). Then it
uses the inbuilt electronics and determines the position automatically.
I's called GPS.
Seriously:
There are many tools that can be used to make celestial navigation "easier", but
the basic weakness is that it ultimately relies on the operators dexterity, his
eyesight and his experience and training.
If you are generally interested in celestial navigation, go ahead and learn it -
it could be very rewarding. But as a navigation method, it is a little bit
outdated. It is generally considered as a backup system these days, not the main
navigation method. It is ofcourse a alternative to GPS, but so is deduced
reckonning (keeping track of course, speed and applying currents and drift) -
both simple in theory but hard to practice correctly.
To value the usefulness and safety, I recommend counting the single "points of
failure" the respective systems have. The fans of astro navigation may then be a
little be surprised.
Those using programmed calculators and have the tables in them are especially
vulnerable, IMO.
Anders
Igor skrev:
> Out of curiosity... Are there such things as electronic sextants? That
> is, sextants with a built in calculator and such. You would point it at
> sun or moon or whatever, press the moon button, or something, and then
> it would give you your position. They'd have a biult in clock and so on.
>
> ----------------------------------------------------------------------------
> char*p="char*p=%c%s%c;main(){printf(p,34,p,34);}";main(){printf(p,34,p,34);}
> http://www.algebra.com/~ichudov
--
-----------------------------------------------------------
Anders Svensson
Anders.-.Ei...@swipnet.se
-----------------------------------------------------------
Geoff Blake
: Only if you know what time it is. GPS receivers are great for this
: because they have very accurate atomic clocks built right in.
Erm, GPS satellites have the atomic clock, (run of the mill) GPS receivers
do not. They merely report the time as transmitted by the satellites.
Geoff
--
Geoff Blake geoff (at) palaemon . demon . co . uk linux 2.0.36
Chelmsford g8gnz @ g8gnz . ampr . org sparc - i586
Please, only use the .ampr.org address if you know what you are doing
Intel create faster processors - Microsoft create slower processes
Geoff Blake
: Here we go again...
: "Mr. GPS Answer Man" lied:
: <snip>
: > Only if you know what time it is. GPS receivers are great for this
: > because they have very accurate atomic clocks built right in.
: Sheesh!! Where do these people get their information??? No atomic
: clock in any GPS on earth.
Erm, there are atomic clock systems that use GPS receivers to synchronise
them to UT2 - weither this constitutes a GPS receiver may be open to debate.
vdg...@netland.nl
Outnumbered
Phoenix wrote:
> Peter, how can you do a noon sighting without any knowledge of the time?
The noon sighting is (as celestial is done today) a latitude LOP, based on when
the sun reaches it's highest angle for the day, at your specific
position.......time, does not designate LAN, the sun's angle designate LAN and
what the time of LAN is.
>
>
> --
> Jim
>
> 1994 Regal 256 for sale - see ad at
> http://www.classifieds2000.com/cgi-cls/ad.exe?P1+C189+R1187463
>
> Peter Bennett <pet...@interchange.ubc.ca> wrote in message
> news:37e66a0b....@news.supernews.com...
> > On Thu, 9 Sep 1999 15:00:58 -0500, ig...@Algebra.Com (Igor) wrote:
> >
> > >Andy Bogle <bo...@concentric.net> wrote:
> > >* Using a sextant to observe the sun or stars will allow you to calculate
> your
> > >* latitude only. You will need a chronometer to determine longitude and
Yes and no..........when taking stars, you cross at least two and preferably at
least three to give yourself an accurate fix, both of Lat AND
Long..............Sunlines, give you a LOP (Line of Position) which when crossed
with the noon latitude, will give you a Lat/Long position.
>
> >
> > You can determine latitude using a noon sight on the sun, and no knowledge
> > of the time. Any other form of sight requires accurate time.
> >
> > >
> > >I assume that a decent electronic watch is good enough as a chronometer.
> > >
> > >Am I correct?
> >
> > Yes - modern digital watches are probably more accurate than traditional
> > chronometers. However, you must check your watch against radio time
> > signals or other reference every few days, and keep a record of its drift.
> >
> > >
> > >But, say, a 1nm acuracy is not all that unobtainable. Is that right?
> > >
> >
> > For Canadian Power Squadrons' celestial navigation classes, sights taken
> > from shore positions must produce lines of position within 3 miles of your
> > known position for the sight to be acceptable, if I recall correctly. It
> > would be very difficult to get this accuracy from a small vessel.
> >
> > A sight on any one body (sun, moon, planet, or star) will only give you a
> > line of position - to get a fix, you will need sights on two, or
> preferably
> > three, different bodies within a few minutes.
> >
> > Star and planet sights can only be taken for a few minutes around sunset
> or
> > sunrise.
Startime for sunset...........sunset til about 20 minutes after sunset; for
sunrise........20 minutes before, till sunrise. This time is variable, depending
upon the scope of the sextant, and eyesight and abilities of the user.
The accuracy of fixes, obtained by celestial navigation, depends on many
factors.....the sextant (its accuracy, corrections, optics) the operator
(his/her experience, abilities, known fudge factors) the platform ( stable, calm
seas rough seas.....the size of the vessel is immaterial and not to be used as
an excuse, though obviously a smaller vessel will tend to bounce around more)
accurate time(a good watch with known error and rate, frequently checked).
When I was using a sextant, on a normal basis, on a ship, I would frequently
test myself against a Radar, or visual fix, to check my accuracy.....if my LOP,
did not go through the visual/radar fix, my first check would be the
visual/radar fix, THEN I would check my own work and sextant......<BG>
otn
ea...@neosoft.com
>jlrogers wrote:
>>
>> You can use sun or stars. Accuracy depends on the observer's technique for applying
>> the cosine-haversine formula and on the accuracy of his instruments, including his
>> timepiece, sextant, and estimate of height above sea level. If you're good, you're
>> better than GPS. If you're bad, you're probably sunk. There are computers and text
>> books of tables to eliminate the trigonometry involved.
>
>I'd love to meet the person who can figure his position better with a
>sextant than with GPS. Even on solid ground from a tall building with
>the full horizon in sight, you won't do as good as GPS. On a boat, good
>luck getting better than a mile or two. Now, if the GPS unit is broken
>you might have a chance. Then again, if it's cloudy ...
>
We used to be able to get a 20 yd fix in open ocean with
a Type XI periscope, back in the days before GPS.
Comes close to the official accuracy of civilian GPS now.
(not differential or military)
(when you plot a 25 star fix and the result is a point instead of
a triangle or polylateral you know you have something special!!)
*********************
ea...@Neosoft.com
The meek shall inherit the Earth
the rest of us are going to the Stars!
*******************************
ea...@neosoft.com
>Every time I read something like this about high-accuracy sextant
>positions, I always want to know the answer to one simple question: how did
>they know where they were so accurately that that they could determine that
>the accuracy of the sextant position was within a certain margin of error?
>
>If you can tell me that a sextant position is within 1/10 of a mile of your
>true position, then you already know your true position--so why are you
>taking a sextant position, unless it's for the fun of it?
>
Position1 : tied up alongside submaine tender .
Reason : Calibration of periscope before getting underway for 70 days.
Sextant was backup for SINS ( inertial navigation system), the active
navigation system for ship position to be given to Polaris bird.
Position2 : at a secret location somewhere in the ocean, marked by
observing the bottom contours. (ah -- there is the 5 foot rock
on the 201 foot hill ! ) (As previously determined by special
oceanographic survey ships -- and previous voyages)
Reason2 : calibrating sextant again to see how much the electronic
systems have drifted in the last few weeks.
ea...@neosoft.com
>But once printed and checked by an independent source, tables and maps can not
>be turned off because the US bombed Afeganistan and is afraid of the
>terrorists...
>
>
But that checking by independant source is the kicker!!
Remember the little problem we had in early '60s when we discovered by
sat mapping that the USSR had fooled us into a 50 mile error in
location for all their cities. The Birds would have flown and blown up
the neighboring forests. (The bombers would have followed the
railroad tracks and found the target however)
R.W. Hutchinson
>>be turned off because the US bombed Afeganistan and is afraid of the
>>terrorists...
>>
>>
>
>But that checking by independant source is the kicker!!
Several countries have mapping agencies.
--------------------------------------------------------------
"I would predict that there are far greater mistakes waiting
to be made by someone with your obvious talent for it."
Orac to Vila. [City at the Edge of the World.]
-----------------------------------------------
R.W. Hutchinson. | rwh...@nr.infi.net
Sam Wormley
>
> If I understand it correctly, one can use a sextant to determine one's
> position. How accurate would such a determination be? What do you need
> for that, sun? Or stars?
>
The sextant in a skilled celestial navigator's hands can consistantly
measure angles with respect to the horizon of Sun, Moon, Planets, and
bright stars to better than one minute of arc (better than 1 nautical
mile), but requires good weather. The skilled navigator will record
observations from several celestial objects and incorporate these data
with Dead Reckoning and making calculations of estimated error.
GPS may be replacing the sextant for sea faring navigators, but I'll bet
the sextant is kept for a backup. Thomas R. Metcalf wrote an excellent
set of algorithms for the HP-48SX/GX (Sparcom's Celestial Navigation Pac)
and some enhancements and documentation is available on-line. I have
found this software to be particularly valuable for astronomical and
navigational calculations.
_________________________________________________________________________________
Sam Wormley - http://www.cnde.iastate.edu/staff/swormley/maps/maps_waypoints.html
Sam Wormley
Time consuming corrections must be applied to many sightings for variables
such as abberation, precession, nutation and refraction. Tabular tables
must be interpolated. Sightings must be time-taged. The celestial navigator
spends a lot of time getting the same information that a GPS receiver can
provide in minutes or seconds. GPS is reliable, real-time, more accurate,
works in any weather and sea conditions, and certainly easier to use.
Sam Wormley
>
> You can use sun or stars. Accuracy depends on the observer's technique
> for applying the cosine-haversine formula and on the accuracy of his
> instruments, including his timepiece, sextant, and estimate of height above
> sea level. If you're good, you're better than GPS. If you're bad, you're
> probably sunk. There are computers and text books of tables to eliminate
> the trigonometry involved.
>
At sea, I doubt that any celestial navigator can consistantly exceed a
a hundred meters under ideal conditions. Most GPS receivers do at least
that well without all the sight reduction calculations.
So I'll argue that a celestial navigator CANNOT do better than GPS.
Skipper
> Time consuming corrections must be applied to many sightings for variables
> such as abberation, precession, nutation and refraction. Tabular tables
> must be interpolated. Sightings must be time-taged. The celestial navigator
> spends a lot of time getting the same information that a GPS receiver can
> provide in minutes or seconds. GPS is reliable, real-time, more accurate,
> works in any weather and sea conditions, and certainly easier to use.
Funny, Harry has been strangely silent on the issue this go 'round.
Wonder if he's had a come to God. His foolish prior pos on this issue
will not stand the test of time. Sextants are going to be relegated to
the mantle for all but the die-hard ocean crossers.
--
Skipper
Skipper
> At sea, I doubt that any celestial navigator can consistantly exceed a
> a hundred meters under ideal conditions. Most GPS receivers do at least
> that well without all the sight reduction calculations.
>
> So I'll argue that a celestial navigator CANNOT do better than GPS.
On a rocking sea in a small boat with the sight being taken so very
close to the waterline, only a fool would prefer a sextant estimate over
a GPS fix. Overcast days just don't help this situation very much. I
only know of one contributor who still believes the sextant is primary
to the GPS. And he's *certified* in taking foolish positions.
--
Skipper
brian whatcott
>
... with the sight being taken so very
>close to the waterline, only a fool would prefer a sextant estimate over
>Skipper
Being close to the waterline is a condition for an accurate fix.
Height above, indicates a need for an error correction.
Skipper's point is somewhat overstated.
Brian Whatcott Altus OK
Skipper
>
> Skipper says...
> ... with the sight being taken so very
> >close to the waterline, only a fool would prefer a sextant estimate over
> >a GPS fix....
> Being close to the waterline is a condition for an accurate fix.
> Height above, indicates a need for an error correction.
> Skipper's point is somewhat overstated.
As you are measuring the angle of the horizon to a celestial body, being
elevated above the waterline make determination of the true horizon much
easier. That said, the observation that the GPS is quickly relegating
the sextant to the mantle for most recreational boaters was not
overstated.
--
Skipper
HarryKrause
Using a sextant properly is a mental exercise as much as it is a
navigational exercise. I wouldn't expect you to understand that, SkipKnot.
--
Harry Krause
- - - - - - - - - - - -
Concerto (n): a fight between a piano and a pianist.
Skipper
> Using a sextant properly is a mental exercise as much as it is a
> navigational exercise. I wouldn't expect you to understand that, SkipKnot.
I do know that a while back when you where asked to prioritize the most
useful 'boat goodies' you wrote:
> Skipper wrote:
> > How would you prioritize them?
> SSB/VHF/Cell Phone (If you are sinking, you need help FAST)
> Compass
> Chart
> Accurate Watch
> Sextant
> Depth Finder
> GPS
We discussed your prioritization of the GPS at the bottom of list. You
held that the Sextant was more important for position determination than
the GPS. I responded that the best place to store that sextant,
particularly when using your current boat, was on the mantle. Has your
position changed?
It might be most interesting to ask this question of the NG again as it
was originally posed in 1997:
The question came up the other day as to which of the following is the
*most* useful goody on a boat and how they should be prioritized:
Auto Pilot
VHF Radio
GPS
Compass
Radar
Hailer
Plotter
windlass
SSB
Depth Finder
Cell Phone
How would you prioritize them?
--
Skipper
Outnumbered
Sam Wormley wrote:
> jlrogers wrote:
> >
> > You can use sun or stars. Accuracy depends on the observer's technique
> > for applying the cosine-haversine formula and on the accuracy of his
> > instruments, including his timepiece, sextant, and estimate of height above
> > sea level. If you're good, you're better than GPS. If you're bad, you're
> > probably sunk. There are computers and text books of tables to eliminate
> > the trigonometry involved.
> >
>
> At sea, I doubt that any celestial navigator can consistantly exceed a
> a hundred meters under ideal conditions. Most GPS receivers do at least
> that well without all the sight reduction calculations.
<BG> In the days, when I was using a sextant, I'd have probably taken this
bet......and considering the scale on the charts we'd be using, been sure to use
either a very sharp or very dull pencil, when doing my plot, depending on how much
"fudge factor" I felt was needed, to fall within these parameters.
BTW, from sight to finish plot, was normally about 5 mins, max........on a slow day.
>
>
> So I'll argue that a celestial navigator CANNOT do better than GPS.
If the argument is about "purist" accuracy, you are probably correct......but....
considering the area of the ocean, where you would normally be using celestial
navigation, there's no real need to attain accuracy, to within "feet", or even a few
hundred "feet" (the normal exceptions to every rule applies).
At sea, GPS has ONE major advantage over celestial (assuming the heavenly bodies can
be seen, and the batteries don't crap out)......time..........time to enjoy, time to
watch, etc.
otn
jean somerhausen
>
> Marv Shirley wrote:
>
> > In this day and age, why would anyone want to use a sextant? I realize
> > GPS's can be dropped, you could run out of batteries, etc. But the
> > satellites are not going to fall out of the sky.You could buy 5
> > inexpensive GPS's for the price of a good sextant. They are not all
> > going to break or malfunction. Why do people still use them? It's like
> > going back to the horse and buggy era. I have 2 GPS's and would never go
> > back to a sextant. Engineers used to use sliderules before small
> > calculators were available. Sliderules are not even made anymore! I
> > apologies if I offended anyone on this list. Marv
>
> By the same token, in this day and age, why would anyone want to sail a
> small boat from one point to another when it's faster to fly and you don't
> need to buy the plane? There is a definite sense of satisfaction being
> able to do things without modern technology. (Aside from using a
> celestical calculator that costs twice the price of an entry level GPS, of
> course.) On a long ocean passage there isn't a whole lot else to do to
> break the manotony. I look forward to the daily routine of taking a sight
> to see if I can resolve it. GPS takes all that away. I'm sure the
> fascination will wear off of course, once I've done it enough times. I
> don't recall what ever happened to my old slide rule, but I'm sure it must
> hold some fascination for someone who hadn't used one before. Well, not
> everyone.
> --
> Al Gunther, Kingston, WA <---- 47° 48.1'N, 122° 30.0'W
> http://www.silverlink.net/~agunther/
I've used the sextant exclusively for a crossing in 1981 (for the simple
reason that GPS wasn't available to us common mortals) but I prefer
spending my time baking bread on board now....
john
jean somerhausen
afterwards so that one can correct GPS readings to an error of +/- 1 cm
if the GPS receiver has been kept in the same position for about 24
hours.
john
Jon V. wrote:
>
> On Mon, 13 Sep 1999, Steven Shelikoff wrote:
>
> <gps accuracy>
>
> > Just something I read somewhere. The receivers I'm talking about have
> > extremely stable clocks, they don't have SA, and they cancel the effects
> > of atmospheric disturbances. I can't get into too much detail. But if
> > you want to agree that they only have 17cm horizontally and about 30cm
> > vertically, that's fine with me as well. It's still a lot more accurate
> > than you can get with a sextant.
>
> For the subject line, I have no real idea what the theoretical limit is...
> but the practical limit is "good enough for navigation."
>
> Sextant vs GPS... there is no real argument. No sextant technology
> employable on any boat is capable of the accuracy GPS is capable of...
> though perhaps with SA you could get a more accurate sextant position now
> and again. Given the choice, you should have both, because ether
> technology may fail. You wouldn't give up your compass because you bought
> a GPS... I hope. Also, it should be remembered that SA is scheduled to be
> turned off.
>
> The research I did was before commercial GPS receivers were
> really available... No SA. Good clocks.
>
> Canceling atmospheric disturbances (mostly multipath) requires time to
> accumulate data. Same theory as SA being programmed patterns instead of
> random noise, because random noise can be factored out. Atmospheric (and
> tree/building clutter) are random, and for the most part can be averaged
> out.
>
> Currently the only people who are trying to get accurate position
> information from the GPS system are surveyors (land, property lines, that
> sort of surveyor), and SA or no, they most often do 12+ hour integrations
> with DGPS to get the accuracy they require. They may well be seeing within
> 1cm for all I know... they wouldn't accept much worse. 17cm wasn't flying
> with them at the time.
>
> There are some now good GIS/GPS books that have a *lot* of information...
> almost worth the $90-180 buck a pop... but not quite.
>
> -Jon
>
> -----------------------------------------------------------------------------
> "Consequences, Schmonsequences, as long as I'm rich."
> -- "Ali Baba Bunny" [1957, Chuck Jones]
jean somerhausen
the earth crust at the Etna, amongst other places.
john
Jon V. wrote:
>
> On Sun, 12 Sep 1999, jean somerhausen wrote:
>
> > Actually, the DoD publishes corrections some two or threeweeks
> > afterwards so that one can correct GPS readings to an error of +/- 1 cm
> > if the GPS receiver has been kept in the same position for about 24
> > hours.
>
> Not "actually". Our information agrees.
>
> All the DoD info really is is the SA offset tables... by merging them
> with recorded data from the GPS receiver you get the non-SA signal after
> the fact. You then integrate the long dataset (longer the better) to get
> rid of multipath etc. variation and achieve your maximum accuracy.
>
> 12+ was my figure, 24 is yours, either one is "a long time" to me...
> thousands of samples. It is a case of diminishing returns that dictate
> how long you collect. You are on an asymptotic approach to perfection.
>
> Using the same tables (or turning off SA) you can get 17cm x 30 cm without
> integrating a long data collection, which is where my numbers came from.
>
> Both statements make sense to me, it all jives with my research for the
> most part, so I'd call it right... the only "fact" I'm not sure of is the
> 1cm... but it isn't an unreasonable, just an unresearched, number.
>
> -Jon
>
> -----------------------------------------------------------------------------
> Whitehead's Law:
> The obvious answer is always overlooked.
Henri
pre Y2K bug, I was very happy to have a sextant :-).
Now about sighting while the yacht is rolling. There is a great difference
regarding the accuracy of your sight between a plastic sextant and a good
one. This is mainly due to the angle of sight and the coverage. Shooting the
sun with a placstic sextant in a formed sea is very difficult. With a
freiberger Yacht sextant it is somewhat easier and it will go smoothly on a
full size Cassen Plath. Personally I use a chinese sextant (Astra) which is
very sturdy and accurate. Now the question about the precision of a GPS
should be modulated. If your boat will roll a lot in a heavy sea, chances
are that your GPS will be inaccurate if your antenna was mounted high up,
because of the speed of the antennas erratic movements. But if you have a
one mile precision in the open Ocean, we can assume it's accurate enough.
Once you reach the coast you will probably use navigation elements, which I
am sure will be more accurate than GPS or sextant alone.
--
Henri
ne...@sacharter.com
http://www.sacharter.com
(do not press "reply" the sender's address is incorrect)
WB <will...@yahoo.com> wrote in message
news:mm0C3.10980$1E2....@ozemail.com.au...
> on small yacht that is yawing and pitching, there is very little point
> talking about sextant accuracy. if you acheive a nautical mile accuracy
you
> should be happy, but like i say it depends on firstly the stability of the
> platform,, then we can start talking about sextant accuracy, sextant fro
> backup, use a gps.
> Igor <ig...@Algebra.Com> wrote in message
> news:slrn7tfm3...@manifold.algebra.com...
> > If I understand it correctly, one can use a sextant to determine one's
> > position. How accurate would such a determination be? What do you need
> > for that, sun? Or stars?
> >
> > --
>
> --------------------------------------------------------------------------
fresh...@earthlink.net
>*most* useful goody on a boat and how they should be prioritized:
>
>Auto Pilot
>VHF Radio
>GPS
>Compass
>Radar
>Hailer
>Plotter
>windlass
>SSB
>Depth Finder
>Cell Phone
>
>How would you prioritize them?
----- All you need to pilot any coastal waters
0: Local Chart(s)
1: Compass
2: Depth Finder
----- Safety added in near shore areas
3: VHF
4: Hailer
5: Radar
----- Stuff to make your work easier
6: Autopilot
7: Windlass
----- Creature comforts
8: Plotter
9: SSB
10: Cell Phone
I added "charts" at #0. This is an interesting exercise. Kinda like
"what would you take to Mars," but far more useful :)
Karl Denninger
I pretty much agree, but SSB isn't a "creature comfort" more than 25 miles
or so out (pretty much the limit for VHF). If you don't have SSB out there
you're not using a radio for anything useful (like calling for help).
I put a handheld GPS up there with the first group; they're so cheap these
days and include the compass function (and one that won't get munged by
magnetic things in its vicinity) that I can't imagine going anywhere beyond
the local "chain of lakes" without it.
GPS + a chart = "where are you and what's near me". Add the depth sounder
when shoals can be a concern, although a depth sounder won't tell you how
deep the water is AHEAD of you, and they're not completely reliable (unable
to get a reliable lock) in some conditions, particularly if the water is
very murky (lots of suspended solids) and the bottom soft enough (ie: muck).
My favorite depth sounder joke is that the sounder will reliably tell you
that you just ran aground :-)
--
--
Karl Denninger (ka...@denninger.net) Web: http://childrens-justice.org
Isn't it time we started putting KIDS first? See the above URL for
a plan to do exactly that!
PrinceMyshkin
says...
>Once you reach the coast you will probably use navigation elements, which I
>am sure will be more accurate than GPS or sextant alone.
It's called LORAN. Where available.
Skipper
> >How would you prioritize them?
>
> ----- All you need to pilot any coastal waters
> 0: Local Chart(s)
> 1: Compass
> 2: Depth Finder
> ----- Safety added in near shore areas
> 3: VHF
> 4: Hailer
> 5: Radar
> ----- Stuff to make your work easier
> 6: Autopilot
> 7: Windlass
> ----- Creature comforts
> 8: Plotter
> 9: SSB
> 10: Cell Phone
>
> I added "charts" at #0. This is an interesting exercise. Kinda like
> "what would you take to Mars," but far more useful :)
Certainly like your list and priorities much better than the original. I
do have two questions:
1- You prioritize the compass over the depth finder. It seems that a
great many of us boat in waters containing shoal draft dangers and in
sight of landmarks. Would you care to share your thoughts on why you
consider the compass a higher priority?
2- Notice you do not have an inexpensive handheld GPS backup for the
plotter. Can you elaborate?
Again, nice list!!
--
Skipper
Skipper
> I pretty much agree, but SSB isn't a "creature comfort" more than 25 miles
> or so out (pretty much the limit for VHF). If you don't have SSB out there
> you're not using a radio for anything useful (like calling for help).
>
> I put a handheld GPS up there with the first group; they're so cheap these
> days and include the compass function (and one that won't get munged by
> magnetic things in its vicinity) that I can't imagine going anywhere beyond
> the local "chain of lakes" without it.
>
> GPS + a chart = "where are you and what's near me". Add the depth sounder
> when shoals can be a concern, although a depth sounder won't tell you how
> deep the water is AHEAD of you, and they're not completely reliable (unable
> to get a reliable lock) in some conditions, particularly if the water is
> very murky (lots of suspended solids) and the bottom soft enough (ie: muck).
>
> My favorite depth sounder joke is that the sounder will reliably tell you
> that you just ran aground :-)
Damn good comments Karl. Would you care to post your prioritized 'top
ten' items?
--
Skipper
Al Gunther
Karl Denninger
I thought I pretty much just did.
The "what's in my dockbag" list varies by where I'm boating. If I'm on
the big lake or an ocean I have an entirely different set of issues to deal
with (and items in the boat) than if I'm on the inland chain of lakes
around here, where there are both plenty of other craft around (a white
towel will draw help; no real need for a radio) but a SHITLOAD of shallow
water hazards and good ways to mash your prop and/or drive.
Funny thing is that in many parts of the chain you could sink but not
actually have the boat go completely under. It makes for some bad
hazards if you don't know what's where, and the charts are pretty much
useless - or worse, since they'll lead you to believe you have 2' more
water under your keel than you really do in a lot of places.
Last weekend I nearly ran aground *IN A MARKED CHANNEL*! Yes, you read
that right. I only draw ~20 inches trimmed the way I was (its shallow
in that channel, and is a no-wake, but not THAT shallow!) Good thing we
have a muck bottom there, because if it had been rocks or something firm
I would have both been hard aground and done some damage to the hull. As it
was I was astonished that I was not only blowing black shit out the back of
the boat (with the drive trimmed all the way up!) but ALSO had perceptable
drag on the hull itself. Needless to say I wasn't happy, but I got out
without having to unload the pax and push (literally; the water couldn't
have been deeper than my knees where I ran into trouble). With all the
traffic in the chain, shallow water and muck bottom you can pretty much
forget about stable depth sounder readings and being able to SEE the
bottom, even under those crappy conditions.
The only thing I can figure is that some yahoo came by just OUTSIDE the
buoys in a "go fast" at speed and redistributed two feet of the bottom
into the channel, probably destroying his lower unit(s) and/or prop(s)
in the process. Needless to say this weekend I stayed the hell away
from that particular place - in the channel or not, I didn't care for
a repeat performance.
fresh...@earthlink.net
>great many of us boat in waters containing shoal draft dangers and in
>sight of landmarks. Would you care to share your thoughts on why you
>consider the compass a higher priority?
Because I'm in Southern California and most of that "shoal" stuff is
apocryphal at best :) That and depth finders are great for showing
you what space is left under your boat, but not ahead.. Of course,
specific units have look ahead now but I'm thinking more along the
lines of a lead line.
>2- Notice you do not have an inexpensive handheld GPS backup for the
>plotter. Can you elaborate?
Oh, GPS wasn't on the original list I was working from and everything
else was. Just workin' with the building blocks I had in front of me.
Only exception I made was the charts, and I pointed that one out :)
HarryKrause
Not only that, all, but don't you just know on a long sailing voyage that
whatever electronics you carry will crap out? That and the intellectual
challenge were the reasons why I first learned to use a sextant.
We live in a society where, if it isn't easy enough for the least able to do
it, it isn't considered worth doing. Sad but I think very true.
Phoenix
least able to do
> it, it isn't considered worth doing. Sad but I think very true.
damn Harry, when you make a statement like that, and then go to the
Annapolis Boat Show on Thursday (VIP Day/Republican Day) ; ), instead of
with the masses, it is obvious that you are a closet Republican. Great that
you finally saw the light.
PS - I am sending this via email instead of posting it in the NG, because I
don't want to tell anyone else your secret.
--
Jim
1994 Regal 256 for sale - see ad at
http://www.classifieds2000.com/cgi-cls/ad.exe?P1+C189+R1187463
HarryKrause <hkr...@erols-nospam.com> wrote in message
news:37DCDC05...@erols.com...
HarryKrause
I used to be a Republican, until the party apparatus and philosophy was
taken over by religious bigots, extremists and the excessively selfish.
--
Harry Krause
- - - - - - - - - - - -
If opportunity doesn't knock .... build a door.
Eisboch
>
> damn Harry, when you make a statement like that, and then go to the
> Annapolis Boat Show on Thursday (VIP Day/Republican Day) ; ), instead of
> with the masses, it is obvious that you are a closet Republican.
Ahhh ... the old "Haves" and "Have Nots". Just shows that Harry can be
a republican when he wants to be. If he were a true democrat,
he'd get all of us in on VIP day.
Eisboch
HarryKrause
If it were truly a VIP day, it would have some snob appeal. But the show is
open on Thursday to anyone with $25. And if you are that tight, buy your
tickets in advance. Even the $12 general admission is discounted to $10.50
by pre-ordering. Hell, $1.50 is more than Ja'me has spent three years at the
local boat dealerships he wants everyone to patronize.
Try:
--
Harry Krause
- - - - - - - - - - - -
"A government is the only known vessel that leaks from the top." - James
Reston
Sam Wormley
>
> You use a sextant because the GPS may fail. You use GPS because the weather may be
> too bad for a sight. You use both to test your skills.
>
"You use both to test your skills". That is an excellent idea for land
navigation also. Not sextant, although that's not a bad idea either, but
map, compass and altimeter skills. One should get, within reason, the
same position information with GPS and other navigational tools.
-Sam
Sam Wormley
>
>
> <BG> In the days, when I was using a sextant, I'd have probably taken this
> bet......and considering the scale on the charts we'd be using, been sure to use
> either a very sharp or very dull pencil, when doing my plot, depending on how much
> "fudge factor" I felt was needed, to fall within these parameters.
> BTW, from sight to finish plot, was normally about 5 mins, max........on a slow day.
>
Actually--I'd be surprised if the capability has not at least been
thought about: shipboard systems that would have an all sky view,
inertial reference frame and computer to "fit the" computered generated
sky with the observed sky and spit out the position coordinates and
velocity information to an accuracy equivalent to GPS. The system
would, of course, have to know the date and time (no problem) and
wwould have to have some of the sky unobstructed by fog or clouds.
Such a system would tend to remove human error in the measurement
process.
-Sam
Marc Auslander
carried a backup sextant.
GPS receivers are cheap enough to carry many for the price of one
sextant.
And as has been pointed out many times, a sextant needs a time source.
By far the most reliable time source available is a GPS receiver.
If you want to play with a sextant, play. But plan enough redundancy
in your GPS system so you can count on it.
--
Marc Auslander <ma...@watson.ibm.com> 914 945-4346 (Tieline 862 Fax x4425)
Jack Yeazel
>
> You didn't carry an astrolabe because your sextant might fail - you
> carried a backup sextant.
>
> GPS receivers are cheap enough to carry many for the price of one
> sextant.
Good point... Reminds me of Capt. Robert Fitzroy who took
along 22(!) chronometers (and the famous Mr. Darwin) to the
Galapagos Islands in order to do nautical surveying with the
Beagle...
> And as has been pointed out many times, a sextant needs a time source.
> By far the most reliable time source available is a GPS receiver.
Yeah, one GPS beats even 22 chronometers...
--
Jack
Get general GPS information at http://joe.mehaffey.com/
Del Cecchi
Sam Wormley <swor...@cnde.iastate.edu> writes:
snip
|> Actually--I'd be surprised if the capability has not at least been
|> thought about: shipboard systems that would have an all sky view,
|> inertial reference frame and computer to "fit the" computered generated
|> sky with the observed sky and spit out the position coordinates and
|> velocity information to an accuracy equivalent to GPS. The system
|> would, of course, have to know the date and time (no problem) and
|> wwould have to have some of the sky unobstructed by fog or clouds.
|>
|> Such a system would tend to remove human error in the measurement
|> process.
|>
|> -Sam
Why bother? If GPS is not good enough, use inertial navigation system.
Automating a sextant seems sort of pointless.
--
Del Cecchi
cecchi@rchland
Steven Shelikoff
>
> Recently, when I discovered that my GPS went out of order, because of some
> pre Y2K bug, I was very happy to have a sextant :-).
Interestingly enough, if you were using a sextant in 1800, you would
have been lost after March 1st. The almanac printed for that year had
incorrectly listed 1800 as a leap year when it wasn't. All of your
fixes after March 1st would have been off by quite a bit. That was a
Y1.8K bug.
Steve
--
/ / /
\ \ \ mailto:shel...@averstar.com
/ / /
Steven Shelikoff
Military GPS systems have 1cm accuracy without needing DGPS. There's no
need for the military to go back to sextants as long as GPS is working.
Jon V.
> Military GPS systems have 1cm accuracy without needing DGPS. There's no
> need for the military to go back to sextants as long as GPS is working.
Actually, unless the technology has changed remarkably, the optimum
non-differential accuracy is about 17cm horizontally and about 30cm
vertically.
Way back in the dim and distant, before handheld GPS was, I was hired to
write a proposal that involved embedding a GPS navigation system, so I
studied up... there was no info on the SA accuracy at that time, but after
about a week going through fiche indexes I was able to find some pretty
good numbers on the full-spec accuracy. I am sure the algorithms have
improved somewhat, but I'd be surprised if it was a 17x improvement.
Actually, that was not an accumulated position. What is the time base for
that 1CM accuracy? 12Hrs?
How's that for interesting, but off topic?
-Jon
-----------------------------------------------------------------------------
Do you want to know what's ahead for you, in your happiness at home,
your business success? Here's a telling test: Look in the mirror. Is
your skin smooth and lovely, your hair gleaming, your make-up glamorous?
Are you slender enough for your height? Do you stand erect, confident?
Yes? Then you are on your way to success as a woman.
-- Ladies' Home Journal, 1947 advertisement
Jordan Bigel
>Peter, how can you do a noon sighting without any knowledge of the time?
Watch the sun NEAR noon. Take sights as it raises. When it stops rising its
noon. When it starts falling its afternoon. No?
J.
Steven Shelikoff
>
> On Mon, 13 Sep 1999, Steven Shelikoff wrote:
>
> > Military GPS systems have 1cm accuracy without needing DGPS. There's no
> > need for the military to go back to sextants as long as GPS is working.
>
> Actually, unless the technology has changed remarkably, the optimum
> non-differential accuracy is about 17cm horizontally and about 30cm
> vertically.
>
> Way back in the dim and distant, before handheld GPS was, I was hired to
> write a proposal that involved embedding a GPS navigation system, so I
> studied up... there was no info on the SA accuracy at that time, but after
> about a week going through fiche indexes I was able to find some pretty
> good numbers on the full-spec accuracy. I am sure the algorithms have
> improved somewhat, but I'd be surprised if it was a 17x improvement.
>
> Actually, that was not an accumulated position. What is the time base for
> that 1CM accuracy? 12Hrs?
>
> How's that for interesting, but off topic?
Just something I read somewhere. The receivers I'm talking about have
extremely stable clocks, they don't have SA, and they cancel the effects
of atmospheric disturbances. I can't get into too much detail. But if
you want to agree that they only have 17cm horizontally and about 30cm
vertically, that's fine with me as well. It's still a lot more accurate
than you can get with a sextant.
Steve
Jon V.
<gps accuracy>
> Just something I read somewhere. The receivers I'm talking about have
> extremely stable clocks, they don't have SA, and they cancel the effects
> of atmospheric disturbances. I can't get into too much detail. But if
> you want to agree that they only have 17cm horizontally and about 30cm
> vertically, that's fine with me as well. It's still a lot more accurate
> than you can get with a sextant.
For the subject line, I have no real idea what the theoretical limit is...
but the practical limit is "good enough for navigation."
Sextant vs GPS... there is no real argument. No sextant technology
employable on any boat is capable of the accuracy GPS is capable of...
though perhaps with SA you could get a more accurate sextant position now
and again. Given the choice, you should have both, because ether
technology may fail. You wouldn't give up your compass because you bought
a GPS... I hope. Also, it should be remembered that SA is scheduled to be
turned off.
The research I did was before commercial GPS receivers were
really available... No SA. Good clocks.
Canceling atmospheric disturbances (mostly multipath) requires time to
accumulate data. Same theory as SA being programmed patterns instead of
random noise, because random noise can be factored out. Atmospheric (and
tree/building clutter) are random, and for the most part can be averaged
out.
Currently the only people who are trying to get accurate position
information from the GPS system are surveyors (land, property lines, that
sort of surveyor), and SA or no, they most often do 12+ hour integrations
with DGPS to get the accuracy they require. They may well be seeing within
1cm for all I know... they wouldn't accept much worse. 17cm wasn't flying
with them at the time.
There are some now good GIS/GPS books that have a *lot* of information...
almost worth the $90-180 buck a pop... but not quite.
-Jon
-----------------------------------------------------------------------------
"Consequences, Schmonsequences, as long as I'm rich."
-- "Ali Baba Bunny" [1957, Chuck Jones]
R.W. Hutchinson
That was also the first voyage ever to use the Beaufort Wind Scale in its
official ship's log. Truly an epoch making event.
--------------------------------------------------------------
"I would predict that there are far greater mistakes waiting
to be made by someone with your obvious talent for it."
Orac to Vila. [City at the Edge of the World.]
-----------------------------------------------
R.W. Hutchinson. | rwh...@nr.infi.net
Aharon
suspect you are approaching "high noon" you start taking frequent sun
sightings until the sun hits its max height and starts arching back
down.
Of course if you knew your exact longitude, had a good set of tables and
an accurately set watch you would be able to figure out exactly when
high noon should be. ;-)
-Aharon
Jon V.
> Actually, the DoD publishes corrections some two or threeweeks
> afterwards so that one can correct GPS readings to an error of +/- 1 cm
> if the GPS receiver has been kept in the same position for about 24
> hours.
Not "actually". Our information agrees.
All the DoD info really is is the SA offset tables... by merging them
with recorded data from the GPS receiver you get the non-SA signal after
the fact. You then integrate the long dataset (longer the better) to get
rid of multipath etc. variation and achieve your maximum accuracy.
12+ was my figure, 24 is yours, either one is "a long time" to me...
thousands of samples. It is a case of diminishing returns that dictate
how long you collect. You are on an asymptotic approach to perfection.
Using the same tables (or turning off SA) you can get 17cm x 30 cm without
integrating a long data collection, which is where my numbers came from.
Both statements make sense to me, it all jives with my research for the
most part, so I'd call it right... the only "fact" I'm not sure of is the
1cm... but it isn't an unreasonable, just an unresearched, number.
-Jon
-----------------------------------------------------------------------------
Whitehead's Law:
The obvious answer is always overlooked.
Allen Perrins
Just one small observation.
If the GPS system burps, all one hundred spare GPS's burp
If one sextant fails, they don't all fail, they don't require
power and the constellation doesn't fail. Even a poor
"clock" can give at least an estimate. Three is the minimum number
of "clocks" for redundancy. Latitude does not need time data, just
noon visibility.
Damned if I can find my sextant. It's stored somewhere in the attic :>)
Al
--
BARBCAT NS 30 C
R.W. Hutchinson <rwh...@nr.infi.net> wrote in message
news:7rjukk$kr7$1...@nw003t.infi.net...
Allen Perrins
Best way to do it is to take timed sites before, during and
after and plot so that the slope before and after can be drawn and
their intersection, local noon can be accurately pin pointed.
ANY timing device can be used to do this, including an hour-glass.
Carry a spare in case your primary gets trashed.!
Al BARBCAT NS 30 C
Jordan Bigel <jbi...@BIX.com> wrote in message
news:7rjh9e$i...@lotho.delphi.com...