On Sun, 05 Mar 2006 14:15:42 GMT, chuck wrote:

While opinions about accuracy vary, no numbers have been posted. Come
Spring, it would be great to see a couple of sailors with RDF's take
them out of hiding and check their accuracy against their GPS.

I used to rely on an RDF back in the 70s and have a good feel for both
accuracy and the source of errors. I'm not talking about aircraft
type VHF/VOR, but rather the traditional marine HF radio type with
rotatating antenna sensor.

RDF works by providing a bearing to a known object, in this case the
radio tower which is plotted on a chart. The bearing provided by the
RDF was typically relative to the heading of the boat except for the
hand held RDFs with built in bearing compass. Either way, the
accuracy of the bearing was only as accurate as the compass used,
typically plus or minus 2 or 3 degrees.

A single bearing to a radio tower provides only a single line of
position, i.e., your boat is located somewhere along that line but you
don't know exactly where without a second LOP from another radio
station. Ideally the second bearing should cross the first at 90
degrees but in real life that hardly ever happens. The narrower the
crossing angle, the more that any bearing error is magnified, and the
more uncertainty that is introduced in your position plot.

Back to accuracy. An error of plus/minus 3 degrees at 10 nautical
miles translates to an uncertainty of plus/minus 3/10ths of a mile or
about 3600 feet in total. Taken with a 90 degree crossing angle of
the same error magnitude, you now have an uncertainty box about
6/10ths of a mile on each side. With less than ideal crossing angles
the uncertainty box elongates to as much as one or two miles in real
world situations.

Compare that to LORAN-Cs 100 yard accuracy, or GPS/WAAS at 10 feet and
make your own decision regarding RDF accuracy.

On 5 Mar 2006 08:11:31 -0800, "Da Kine"
wrote:

If someone goes off shore with the hopes that GPS is going
to do it all for them then sooner or later, they will lose their boat
and maybe their life.

That is nonsense.

There is no viable alternative offshore other than celestial, and even
the Navy has stopped teaching it.

A couple of spare GPS handhelds with some extra batteries and you are
as secure as it gets these days. You DO need to know how to use them
of course.

The USCG requires it - today - for higher licensing. It's not
nonsense. A sextant is a great tool used to measure angles and a tool I
don't go off shore without. You can get an extremely accurate fix
with a sextant and a compass with only 1 known point on the horizon.

Just because you don't see a need for something doesn't mean
anything other then you don't know how to use it. Offshore, I can
think of more then a dozen ways to navigate using nothing more then
cloud formation, wind direction and change thereof, wave height and
shape and crossing, Wave interval between crests and OH so many more.

GPS is great right up until you get a lighting strike and everything
blows out. Those that say "that'll never happen to me" have never
been sailing in the tropics. My boat was hit along with 2 other boats
with the same lighting strike. My boat was spared because I was
navigating at the time with NO POWER ON and everything disconnected at
the breakers and I had no water in my bilge. Another boat with me lost
about 20K worth of electronics and was done for the season but suffered
no damage other then electronic and the third boat was unplugged but
somehow got a smoldering spark that later that night burned the boat to
the water line. There is not much you can do about fire but the lazy
ass friend of mine that wouldn't hand steer and thought GPS was the
only answer found himself on a boat that he couldn't do a thing with
after one little strike!

Those that advocate that GPS is the second coming are fooling
themselves. I hope they stay close to some of the not so lazy mariners
so that WHEN they step in it they'll have someone to pull them out.

Maybe I am misreading - GPS is not the only tool you need and it seems
that you are saying it is. Also, that little lightning detector that's
being sold is nothing more then an RDF with no other features.

I'm not tryiing to start a brawl here, I just see people thinking to
highly of GPS. The funniest thing is that most of the charts for other
parts of the world are not nearly accurate enough to even think GPS.
There are island groups on Caribbean charts with notes like "last
survey by captain so and so of the English admiralty 1841 - No chart
datum can fix that chart. Este sud Este is about 4 miles off of the
chart position and none of the little islands are in the same place or
the same shape as on the chart. Half moon key (cayo media luna)
isn't even there anymore, there is an atoll where it use to be (but
it is still on the charts.

The point is, if you don't KNOW how to navigate you're likely to
trust the wrong thing or not trust the right thing. If you know how to
really navigate - read waves, understand currents, read clouds, know
weather, do dead reckoning, keep a log, triangulate, use a sextant -
You'll need the old tools to do time honored navigation. GPS is a great
tool as long as it works and as long as the charts are accurate (which
they aren't and which get worse as you get further away from places
people frequent - like where most people like to cruise). GPS is
great if it doesn't run you aground on the reef that was suppose to
be 4 miles away!

As for RDF - I think anyone sailing in a tropical area should have
one for the one time they need to see the direction of a lightning
storm. I also think everyone there needs a sextant and knowledge of how
to do triangulation so they can keep track of a distant storms movement
with bass height (which requires weather knowledge). I think that
anyone stuck in a problem concerning those things that has not learned
what they should know and has left sight of land without the right
tools is asking for trouble.

waynes said ...
There is no viable alternative offshore other than celestial,
and even the Navy has stopped teaching it.

waynes i'm guessing that you mean that the navy doesn't teach RDF, not
that they don't teach celestial. only reason i say that is because it
seems like the military would need backups like celestial because i
understood that the EMP from a nuclear weapons blast could take out
electronics such as GPS systems. is that wrong ? i'm surprised that
someone in the navy who was going to navigate such important ships
would be able to get out of their training without knowing absolutely
every way there was in the world to navigate. it seems like a navy
sailor would have to know everything there is to know about navigation
before they could make decisions that affect the ship.

da kine said ...
GPS is great right up until you get a lighting strike and
everything blows out.

thanks da kine and everyone for the great posts. i want to know as
much about navigation as i can and really enjoy learning about all
these alternatives. you mentioned some others including cloud
formation, wind direction, wave intervals, etc, and i hope things like
that get mentioned in the two fat navigation books i'm currently
reading.

i wonder what other situations could lead to GPS not working ? i'm
with everyone else who thinks that GPS with some spares is really
important. i agree relying on just a GPS wired into the boat is a bad
idea because what happens if it stops working, or if your boat
batteries die, or you just forget to bring enough fuses or something,
or your power generation methods fail. i can't really think of much
that would stop a handheld unit from working except for
losing/destroying them, not having batteries, etc. i wonder what
situations outside of a sailors control could make GPS fail ? i mean,
i assume because so many people rely on GPS that the military wouldn't
just shut it off unless there was some really drastic reason for doing
so. i've read that they might make it more inaccurate or shut it off
in the event of war, would they ? and i assume too that they take
really good care of it since they and everyone relies on it so much ...
has it ever failed ?

As I understand it, they actually run off of a gyro system that is
better then GPS. Also, I have a good friend that captains a navy sub
that told me celestial navigation was required.

In your reading, pick up a red/orange colored paperback called
“Emergency Navigation�. Its available all over and you should be
able to find it at http://www.amazon.com for cheap. You won’t find
much of the good stuff in most books.

As for something else making GPS not work, there are a few things I
have come across and here they are.

On 9/11 GPS was scrabbled in many areas. For anyone in a critical
position in one of the scrabbled areas, OOPS!

In Mexico (and I love this one) I ran across and “electrical
engineer� that had a broken GPS and his spare didn’t work either.
This was a while back but … Turned out to be a bad antenna that I
fixed for him – the engineer☺

That’s the only way gps failures other then lighting that I know of.

As for a spare GPS, the rumor is that if you wrap a handheld in tinfoil
and get hit by lightning, your spare will still work. I don’t know
anyone that has put it to the test but I keep my spare wrapped just the
same.

GPS signals use to be scrabbled all the time. In about 1991 or so the
military started allowing public access of the gps signal. In times of
heightened defense alerts, the gps signal does get less accurate but
then there is also a differential signal that I don’t know much about
but is used to help the gps become more accurate again. All in all,
I’m more worried about lightning, having been through enough of it,
then anything else.

Da Kine wrote:
[snip]
In your reading, pick up a red/orange colored paperback called
"Emergency Navigation". Its available all over and you should be
able to find it at http://www.amazon.com for cheap. You won't find
much of the good stuff in most books.

good stuff, i will check into it ...

As for something else making GPS not work, there are a few things I
have come across and here they are.

i spent a little while reading about this since my last post and found
some interesting things i didn't know, i'll post some of them here
since you are interested too! from some of the things i read it looks
like GPS does/has failed before, i didn't know that before now.
there's a quote in one of these documents that i found interest, the
document says the quote is from "Interagency GPS Executive Board. GPS
policy, applications, modernization, international cooperation.
February 01". here's what it says ...

"GPS provides many benefits to civilian users. It is vulnerable,
however, to interference and other disruptions that can have harmful
consequences. GPS users must ensure that adequate independent backup
systems or procedures can be used when needed."

then later in that same document it is talking about situations where
GPS doesn't work, and apparently it's pretty easy to make it not work!
you can build GPS jammers using plans on the net to jam GPS signals,
and there are even jammers sold for doing the same thing. one paper i
read said that these jammers are sold to militaries that want to be
able to selectively jam areas for security reasons, and to defend
themselves from attack, and that some of these jammers are huge like
radio stations.

this one paper i read had these real world situations where GPS had
failed ...

"Jamming in Mesa, AZ
13 - 18 Dec 01, GPS jammer caused GPS failures within 180nm of Mesa,
AZ
Boeing was preparing for upcoming test
Accidentally left Jammer on L1 frequency radiating at .8mW
Jammer operated continuously for 4.5 days
Impact to ATC operations
A/C lost GPS 45nm from PHX, performed 35° turn toward traffic
NOTAM was not issued until 2nd day
Numerous pilots reported loss of GPS NavAid
Reports of hand-held GPS receivers not working"

"Jamming in Moss Landing Harbor, CA
15 Apr 01 - 22 May 01, VHF/UHF television antenna with pre-amplifier
caused GPS failures to all of Moss Landing Harbor
Boat owner purchased TV antenna, which was equipped with pre-amp
From interior location Amp's emitter jammed all of Moss Harbor and
1km out to sea
No GPS in entire area = 37 days
Impact to Moss Harbor
Research vessels relied heavily on timing from GPS
Extreme difficulty going through harbor in foggy conditions
Notification to all vessels in area that GPS was down
Switched back to radar control for harbor entrances"

the source for those was a document from a search engine with the title
"Civilian GPS Systems and Potential Vulnerabilities"

i also read online that geomagnetic storms can cause GPS to fail,
especially in higher latitudes and during periods of intense solar
activity. it said that storms can introduce errors of 1km and more
depending on location and how active the ionosphere is. and then there
are other kinds of errors too that i read about, ones that seem well
known to people who are really into that kind of thing.

thinking back on it, i also remember another reference to GPS failure,
somewhere in the red sea or east coast of africa there was a harbor
where GPS doesn't work, but now i can't remember where i read it. i
think it may have been in jimmy cornell's "world cruising handbook" but
i glanced through it just now and couldn't find the reference.
apologies to the author if i am wrong about that!

i guess like anything else, GPS is just another aide to navigation.

These are great examples of why I keep up on my other nav skills and
why I make a point to bust the chops of the guys that try to preach gps
like they do.

As well intentioned as people are, they are blindly falling into a trap
and leading others with them when they set aside the old world skills
of navigation.

Good for you for being smart!

I meant that I really might have misread your post. The way I read it
said to me that you were saying gps is the way, and only way. Sorry
that you thought I was being a jerk - I didn't want to come off like
that.

chuck wrote:
A couple of comments.

While opinions about accuracy vary, no numbers have been posted. Come
Spring, it would be great to see a couple of sailors with RDF's take
them out of hiding and check their accuracy against their GPS.

No RDF is going to provide a position within a few meters except by
accident. but most of us work really hard to avoid needing that kind of
accuracy.

i'm going to try it.

i'm just going to get some of this RDF equipment and try it out on land
while using my handheld GPS and some maps to check against. or maybe
in the truck, i have an icom radio in the truck i can use with an
antenna, it has a strength meter built into it. i'll have to find a
good directional antenna to use, or make one. i'm going to check
around for some of the actual RDF equipment used in marine navigation
too though, i bet there is tons of this stuff in people's garages just
waiting for someone to find. when i was reading it looks like you can
also find old RDF/ADF equipment out of old airplanes too to play with,
and these have needles that turn to whatever transmitter is using the
frequency you tune too! fun stuff.