TWL: GPS & PC Displays
As I continue my search for a package of upper helm electronics, I found a
second marine supplier for commercial fishermen here in Victoria. This one
specializes in all things electrical and claims to offer one of the most
complete hands-on displays on the West Coast. I was impressed, but then, I
still consider myself quite ignorant. (I didn't find them earlier because
their advertising is not up to their merchandising.)
In any event, they have many GPS systems up and running as well as the most
popular PC-based cartography packages. I'll go back go back for a few more
looks, but my initial impressions are that a colour display seems a great
deal easier to read and feels a lot "bigger" than the same unit with a B&W
screen. When I commented that neither made me feel confident about using
them to navigate through the hundreds of islets around here, the rep - a
former commercial fisherman, himself - merely said, "Boats that don't try
to go where they shouldn't be don't get into trouble".
When I noted that I have a decent laptop to spare, both commercial supply
stores strongly recommended PC-based systems over the small GPS displays,
and said they can integrate the PC into a system with a simple GPS, radar,
and autopilot. However, I was somehow disappointed in the level of detail
on the CRT display for the PC-based charts. Perhaps my expectations were
unrealistic, so I'll take a paper chart in and compare it to the screen.
Final comment for this post, I mentioned that there is a Loran-C unit at
the lower helm, and thought to keep it there, and put the new GPS up top.
He said,"Sure, if you want positional accuracy only to half a mile." I
didn't want to argue a subject in which I have zero competence, but I was
under the impression that Loran-C is actually more, not less, accurate than
straight GPS. False?
Cheers, Garrett
At 02:27 PM 07/08/2000 -0400, Jack wrote:
There's a superb article in the latest (July 2000) issue of Southern
Boating magazine. It's on page 50 and is entitled "GPS MINUS SELECTIVE
AVAILABILITY PLUS WAAS...". It's well worth buying and reading.
Essentially, keep your differential receiver, but very soon.... even more
accuracy through the use of the FAA's Wide Area Augmentation System (WAAS).
Arild writes:
Let me play devil's advocate for a moment.
As a general issue, just how accurate do we really need to get in our
boating navigation?
Removal of SA was no doubt a big tangible benefit to all of us. However,
now that we are getting 3 - 10 meter accuracy with normal GPS and under
3 meter accuracy with DGPS what further benefit do we really get in
recreational boating?
Before I get blasted by every techie expert, let me ask this.
How many of you actually run software which has a scalable ships icon
which accurately represents your particular boats shape and which also
positions the GPS antenna to an accuracy of one decimeter ( that's 10
centimeters ) ?
Unless you do have a scalable ship icon and the GPS antenna location is
accurately shown on that ship icon then you are guessing as to the
positional accuracy on your chart display. Is your GPS antenna right in
the center of the boat, or forward or aft.? Maybe it is mounted on a pole
at the side of the boat, but which side? So, in other words your
electronic navigation display will have an error equal to the size of your
boat, which may now be greater than the accuracy of the GPS system
without SA.
Then you have the accuracy of the chart at the native scale to which it
was drawn. No matter how much you zoom in on a 1:40,000 scale chart
the objects positioned on that chart will still only have a certain
accuracy of 1 millimeter on the paper chart. That translates into 40,000
millimeters or 40 meters in the real world. Gee, 40 meters is a lot
more than the GPS accuracy.
So now you are trying to get 1- 3 meter accuracy in positioning your
boat while using a chart that is drawn to a plus/minus accuracy of 40
meters. Something isn't going to fit all the time.
And that doesn't take into account the accuracy of the datum shift.
Most hydrographic offices have charts which were originally surveyed to
NAD 27. When NAD 83 and WGS 84 came along they did what was called a
block shift to correct the datum. In many cases this was sufficient,
but I can tell you from personal experience that many areas contain
local anomalies which will not be detected or corrected until that area
is resurveyed using DGPS technology. That will be a long time coming.
Survey crews can only do so much each season. Given priorities
dictated by economics, the hydrographic office will first resurvey areas
used mostly by commercial shipping. Later they will survey the less
frequented wates. Which waters do you do most of your boating in? In
the middle of the commercial shipping lanes or in the quiet backwaters
well removed from it all?
Unless you are using charts scaled at 1:10,000 or larger and which have
been resurveyed using DGPS in the past decade you will not realize the
full benefit of DGPS. Your position may vary in the real world from
your displayed position!
So back to the FAA's Wide Area Augmentation System (WAAS)
What will it actually do for you? Are you going to have an FAA
approved GPS receiver? If not, how will that affect your results?
Even when it works; do you really need such accuracy and can you actually
make use of it given the charts you use for that location?
The FAA's Wide Area Augmentation System (WAAS) was developed in response
to aviation requirements for a GPS approach system that could work
under IFR conditions. Does your boating need that same level of
technical sophistication?
Planes work in three dimensions, but a boat is limited to two. Planes
approach the landing strip at high velocity and a few feet of error
could mean a crash. How fast do you approach your dock? A plane
landing under IFR conditions has a decision point requiring split second
timing to continue or abort the landing. Do you cut your margin of
error that fine? If so, why??
What's the hurry and need for such a gamble?
While I do have a keen interest in all things technical, I always ask
if I really need this or is it just a nifty techie toy to play with.
Just my $0.02 worth
cheers
Arild
On Sat, 08 Jul 2000 09:27:12 -0700, you wrote:
E16 wrote:
Final comment for this post, I mentioned that there is a Loran-C unit at
the lower helm, and thought to keep it there, and put the new GPS up top.
He said,"Sure, if you want positional accuracy only to half a mile." I
didn't want to argue a subject in which I have zero competence, but I was
under the impression that Loran-C is actually more, not less, accurate than
straight GPS. False?
The conventional wisdom has been that Loran-C is more accurate in terms of
repeatability, but not in terms of accuracy of coordinates. That is, if you're
at a given location, and memorize that location on your loran, you should be
able to go back to with 50' (+/-) of that position. This assumes, of course,
good loran coverage and proper station geometry. However, this is not at all
the same thing as punching in coordinates of a waypoint or destination and
expecting the loran to take you there with the same accuracy. I don't know what
the official "wisdom" is on that, but my experience is that the estimate of 1/2
mile accuracy may be in the ballpark.
BUT... now that the GPS selective availability is off, I doubt that loran has an
accuracy advantage when it comes to repeatability. I still carry loran though,
because I like the redundancy of a second system of navigation.
--
Dave Goodman
M/V Dragonfly
Sarasota, FL
At 13:22 7/8/2000 , Dave Goodman wrote:
... snip ...
The conventional wisdom has been that Loran-C is more accurate in terms of
repeatability, but not in terms of accuracy of coordinates. That is, if
you're at a given location, and memorize that location on your loran, you
should be >able to go back to with 50' (+/-) of that position. This
assumes, >of course, good loran coverage and proper station geometry.
However, this is >not at all the same thing as punching in coordinates of a
waypoint or >destination and expecting the loran to take you there with the
same >accuracy. I don't know what the official "wisdom" is on that, but my
experience is that the estimate of 1/2 mile accuracy may be in the ballpark.
BUT... now that the GPS selective availability is off, I doubt that loran
has an accuracy advantage when it comes to repeatability. I still carry
loran though, because I like the redundancy of a second system of
navigation.
There's a superb article in the latest (July 2000) issue of Southern
Boating magazine. It's on page 50 and is entitled "GPS MINUS SELECTIVE
AVAILABILITY PLUS WAAS...". It's well worth buying and reading.
Essentially, keep your differential receiver, but very soon.... even more
accuracy through the use of the FAA's Wide Area Augmentation System (WAAS).
http://www.southernboating.com/
Jack Haring
WhoopeeWagen
"Removal of SA was no doubt a big tangible benefit to all of us. However,
now that we are getting 3 - 10 meter accuracy with normal GPS and under 3
meter accuracy with DGPS what further benefit do we really get in
recreational boating?"
I couldn't agree more, and in fact I posted my own rant about this to the
list back when the removal of SA was announced. Even for ticklish tasks
like repeatably putting divers on an off-shore wreck, these accuracies are
more than good enough.
Certainly it's potentially dangerous to blithely assume that your
navigational accuracy necessarily improves because your position fixing is
more precise. Navigation is a long chain with errors at every link. Let's
not cut things too fine, especially in bad weather.
Regards, Robert Bryett
Sydney, Australia.
mailto:rbryett@ibm.net
Passagemaker recently had a good artical on helm station desin/layout
Austin Gilbert
Colourmegone
----- Original Message -----
From: E16 E16@telus.net
To: trawler-world-list@samurai.com
Sent: Saturday, July 08, 2000 12:27
Subject: TWL: GPS & PC Displays
As I continue my search for a package of upper helm electronics, I found a
second marine supplier for commercial fishermen here in Victoria. This one
specializes in all things electrical and claims to offer one of the most
complete hands-on displays on the West Coast. I was impressed, but then, I
still consider myself quite ignorant. (I didn't find them earlier because
their advertising is not up to their merchandising.)
In any event, they have many GPS systems up and running as well as the
most
popular PC-based cartography packages. I'll go back go back for a few more
looks, but my initial impressions are that a colour display seems a great
deal easier to read and feels a lot "bigger" than the same unit with a B&W
screen. When I commented that neither made me feel confident about using
them to navigate through the hundreds of islets around here, the rep - a
former commercial fisherman, himself - merely said, "Boats that don't try
to go where they shouldn't be don't get into trouble".
When I noted that I have a decent laptop to spare, both commercial supply
stores strongly recommended PC-based systems over the small GPS displays,
and said they can integrate the PC into a system with a simple GPS, radar,
and autopilot. However, I was somehow disappointed in the level of detail
on the CRT display for the PC-based charts. Perhaps my expectations were
unrealistic, so I'll take a paper chart in and compare it to the screen.
Final comment for this post, I mentioned that there is a Loran-C unit at
the lower helm, and thought to keep it there, and put the new GPS up top.
He said,"Sure, if you want positional accuracy only to half a mile." I
didn't want to argue a subject in which I have zero competence, but I was
under the impression that Loran-C is actually more, not less, accurate
than
straight GPS. False?
Cheers, Garrett
Regarding having Loran in addition to GPS: The reason for having Loran on board
in addition to GPS is not that one is "better" than the other, but one backs up
and cross checks the other. They are two totally different systems using totally
different technology. Away from the shoreline you can use either one as input to
your autopilot with good results. Most of us, most of the time will use GPS. But
what if they turn it off? Or your GPS dies? Always good to have a backup, in my
opinion.
Russ
Arild Jensen wrote:
|----------------------------------------------------------------------|
| Russ and Donna Sherwin |
| "Four Seasons" |
| 1981 Marine Trader DC44 |
| Sunnyvale, Ca 94087 |
|----------------------------------------------------------------------|
Arild writes:
Would any of our UK list members care to comment on how the removal of SA
has affected the use of DGPS in the UK.
I am aware that over there you have to pay a subscription fee for the
differential service which is encrypted.
I wonder if the removal of SA has eliminated the need for DGPS by
recreational boaters to the extent that the service is no longer
economically viable.
I recall reading in a European boating magazine that some people were
trying to use french DGPS stations while boating in UK waters to avoid
the subscription fee.
regards
Arild Jensen
At 08:22 PM 07/10/2000 +1000, Robert Bryett wrote:
What is the future of the Loran chains nowadays? I read something about
their being gradually shut down as the US Department Of Defence withdrew
funding.
Regards, Robert Bryett
Sydney, Australia.
mailto:rbryett@ibm.net
Arild writes
That is only true of the US based Loran stations. I have read a number of
reports that several Lortan stations in Europe are being tested as
Differential beacon transmitters for GPS.
The Loran transmitters have better range than the beacon band, their
location is already precisely known and the Loran network already come
equipped with atomic clocks for timing synchronization. This givea
double check on the GPS satellite clocks.
Since each country is responsible for DGPS broadcasts covering their own
shores, each country has to fund their own efforts.
The established loran network is cheaper to maintain than a comparable
network of shorter range beacon band stations.
regards
Arild
"Regarding having Loran in addition to GPS: The reason for having Loran on
board in addition to GPS is not that one is "better" than the other, but
one backs up and cross checks the other."
What is the future of the Loran chains nowadays? I read something about
their being gradually shut down as the US Department Of Defence withdrew
funding.
Regards, Robert Bryett
Sydney, Australia.
mailto:rbryett@ibm.net