External GPS antenna and WAAS?

[Steve Alexanderson]

I just received my new Garmin Map76s. I haven't had it on the boat yet. I
took it to the beach yesterday (Oregon coast) and pointed it to the south
west, slightly elevated. After a few minutes, it found satellite 47, and
after a few minutes more it indicated differential GPS. It didn't take very
long in the car traveling home before it lost the differential indication.
My question is: After mounting the bracket on my boat, will I only get
differential GPS when traveling SW, and would an external antenna be the
cure? Thanks.

 

[Steve Alexanderson]

The internal antenna is clearly directional, as signal strength varies
greatly as it is moved about. That's why I wondered if travel direction (or
mounting angle) was important when in a fixed mount. Maybe once it locks on,
though, a lower signal strength will keep WAAS active?? Guess I'll know
better once I have it in the boat.

 

[Rodney Myrvaagnes]

I just received my new Garmin Map76s. I haven't had it on the boat yet. I
took it to the beach yesterday (Oregon coast) and pointed it to the south
west, slightly elevated. After a few minutes, it found satellite 47, and
after a few minutes more it indicated differential GPS. It didn't take very
long in the car traveling home before it lost the differential indication.
My question is: After mounting the bracket on my boat, will I only get
differential GPS when traveling SW, and would an external antenna be the
cure? Thanks.

My hand-held Garmin 48 works at the nav table inside the cabin. The
fixed mount one at the helm has an internal antenna and works fine. It
is also a Garmin, but I forget the model.


Unless you have a metal roof over it you should be ok. The deck over
the nav table is balsa cored FG, about 1 1/4 inch thick. I suppose if
the core were wet I might have difficulty.


Rodney Myrvaagnes Opionated old geezer

Faith-based economics: It's deja voodoo all over again

 

[Short Wave Sportfishing]

On the west coast, the WAAS birds are only about 15 degrees above
the horizon, so it's really easy to loose them on land. The good news
is that they stay locked up well on the ocean, at least in the Monterey
area. As long as your GPS has a clear view to the southwest, it
should work fine.

Travel direction isn't relevant, except as it might make part of the
boat hide the bird.

You learn something new everyday - I didn't know that.

Interesting - thanks.

Later,

Tom
S. Woodstock, CT

 

[Short Wave Sportfishing]

To find the angle of the geostationary WAAS satellites from your area,
go to:
http://www.directv.com/DTVAPP/DishPointerAction.do
at Direct TV and put your zip code into the box on the page. DTV will
tell you how to set the elevation on your DTV dish and that angle is
the angle the WAAS birds are from your latitude. All geostationary
satellites occupy the same 22,800 mile band around the equator.

For Charleston, SC, the data comes back:

For your zip Code 29418 the azimuth and elevation are displayed below:


DIRECTV Dish Pointer

Azimuth 220.9

Elevation 45.6

The Azimuth is irrelevant to WAAS birds. The elevation above the
horizon is the same. This is also the elevation if you have a 137 Mhz
GOES weather satellite receiver or any other geostationary satellite
use. The higher, the better, of course. The direct satellite radios
like XM and Sirius work better in the South, too...(c;

Now that's two new things I learned today.

I keep this up, I might get smart or something.

Later,

Tom
S. Woodstock, CT

 

[Chuck Tribolet]

Larry, this is one of the rare occasions when you are wrong. Different
geostationary birds will have different elevations. The one straight
south of you will have an elevation of about 90 minus latitude degrees
and will be the highest. One 180 degrees around the world will
have a negative elevation.

--
Chuck Tribolet
[email protected]
http://www.almaden.ibm.com/cs/people/triblet

Silicon Valley: STILL the best day job in the world.

 

[Short Wave Sportfishing]

On a more serious note, I don't believe the GPS has to have CONSTANT
WAAS data to make it accurate. The drift you get corrected for is
VERY slow moving, so if it had a lock on WAAS data every few minutes
that should be good enough for good correction, shouldn't it? I'd
think the manufacturers would tell the receiver to use the LAST
available data until new data arrives.

It is my understanding that the software in WAAS enabled receivers do
not check constantly - that is my understanding.

Curiously enough, I have a Magellon 315 and 330M and the 315 is a tad
more accurate (will show zero error for instance) than the 330 (I
never get zero error) which is WAAS enabled.

When I first noticed that, I thought that the WAAS receiver would be
more accurate.

Later,

Tom
S. Woodstock, CT

 

[Chuck Tribolet]

That "error" calculation may not include the accuracy added by WAAS.
And it is a SWAG based on satellite geometry at the moment, and the
two receivers may use different SWAGs.

--
Chuck Tribolet
[email protected]
http://www.almaden.ibm.com/cs/people/triblet

Silicon Valley: STILL the best day job in the world.

 

[Short Wave Sportfishing]

That "error" calculation may not include the accuracy added by WAAS.
And it is a SWAG based on satellite geometry at the moment, and the
two receivers may use different SWAGs.

Good point.

Later,

Tom
S. Woodstock, CT

 

[Short Wave Sportfishing]

Aboard Lionheart, leaving the Raymarine Raystar 120 Seatalk WAAS-GPS
and its RL70CRC Plus radar display/chartplotter on for long periods of
time tied to E-dock, I plot the Raystar 120's position centered around
its position on the hard top varying from about 30' off the stern to
near the bow to about halfway across the boats next door to port and
starboard, easily within the 3 meter accuracy circle that's specified.
The boat's in Charleston, SC, so has good WAAS view at about 45
degrees elevation.

It's not quite good enough for ILS approach, but damned close!

Clicking the waypoints on top of a bouy symbol is NOT a good idea....

I could tell you a story, but it would take to much bandwidth. :>)

I tried the same thing by comparing the Furno to the Megellon
handhelds by leaving them all on at a dock one day while I was helping
my brother with his boat.

They were all pretty accurate within their specifications.

Later,

Tom
S. Woodstock, CT

 

[Chuck Tribolet]

The DirecTV test is wrong because the DirecTV bird is much closer to due south
of the western US than either of the WAAS birds. I dug up the orbital elements
for the WAAS birds and plugged them into a satellite position calculator I had.
It's about 15 degrees.

--
Chuck Tribolet
[email protected]
http://www.almaden.ibm.com/cs/people/triblet

Silicon Valley: STILL the best day job in the world.