On Fri, 15 Jul 2005 22:11:05 +0000, Gordon Wedman wrote:


"Anchor" wrote in message
news
On Wed, 13 Jul 2005 22:02:44 -0400, Larry wrote:

"FMac" wrote in
:


Have you gone through the Canal? If so, explain what mountains you
noticed. I didn't see any mountains, but I did see and experience a
"Cut". The cut was a bit narrow, but not for a medium sized
sailboat.
We spent an overnight in the big lake, caught a few fish, had a
great
dinner and pressed on the following morning. The canal is not a
navigational thing, it is nothing more than mere piloting. That
said, I'm aware the transit price has gone up considerably since my
transit in the mid "90's".




Never been through the canal. My post was from a news item I found on
a maritime website. The idea was the canyon it's in may cause poor
reception of WAAS satellite correction data.

Ideas are fine but modern science dictates one dismisses ideas and
rejects theories that do not apply.

In this case the idea that the Gatun Cut causes GPS satellite
visibility issues is nonsense and the idea must be rejected and deleted
from your knowledge base.

The cut is far from a canyon with steep walls. There is plenty of sky
for the GPS to see satellites.

WAAS has nothing to do with it.

Our primitive 1995 vintage Garmin 45 pseudo-tracker multiplexed GPS
worked just fine when we went through the Gatun Cut in January of 1998.

You don't have to be in a canyon to have WAAS problems. I live in
Nanaimo which is on the East side of Vancouver Island. We have high
land to the west and my Garmin GPSMap 182 cannot get a WAAS signal in my
marina. It receives GPS satellite signals without trouble but the WAAS
satellites are low in the south-west and apparently the hills shadow
this signal. Further away from shore I can pick up WAAS signals.

There seems to be some confusion between WAAS and DGPS.

WAAS is on on some GPS satellites, not all. If a GPS can see a WAAS
equipped satellite, it has WAAS information for that satellite. Clearly
if a satellite, WAAS or non WAAS, is in shadow the GPS cannot use that
satellite. A WAAS GPS will need 3 or more WAAS satellites to produce a
WAAS fix. Three or more satellites but less than 3 WAAS satellites in
view generates an ordinary fix. This has nothing to do with whether or
not you are in harbor.

Satellites are never in shadow in open water. Define open water for GPS
satellite viewing purposes to be "nothing on the horizon".

Parts of a vessel may shadow a GPS antenna if the antenna is mounted low
and below some large vessel component such as a smoke stack. It is clear
geometry. If an eye ball at the GPS antenna position can see the horizon,
so can the GPS antenna.

Some GPS manufactures refer to a WAAS fix as a differential fix. This is
a poor choice of nomenclature because a WAAS fix is a WAAS fix, not a
differential DGPS fix.

DGPS relies on land based LF transmitters to broadcast differential
corrections. Poor reception of these DGPS signals will result in loss of
DGPS capability. If your harbour is sheltered from DGPS reception, you
will not have DGPS capability in that harbor.

If the DGPS signal is receivable by AIS equipped vessel in the Gatun Cut,
all DGPS equipped vessel can receive it or there is something wrong with
the DGPS installation on board vessels that cannot receive it.