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Subject: TWL: Chart/WAAS accuracy
From: Arild Jensen (elnav@XXX.XXX)
Date: Thu Jan 02 2003 - 21:50:16 EST
-----Original Message-----
I think the real reason for wanting GPS with "pinpoint" accuracy is not that
mentioned above, accurate chartplotting, but rather accurate "anchor
alarming". These are situations where we don't care to know precisely where
we are located, but we do want to know when we have truly drifted away from
that unknown position.
Jim
REPLY
This argument doesn't stand up to close scrutiny either.
With SA removed, the jitter or positioning error was reduced to almost nil.
However, a boat anchored in 20 feet of water with normal 5:1 or 7:1 scope
would have 100 - 140 feeet of anchor rode paid out.
Assuming the vessel is allowed to shift with the wind or current without the
anchor breaking out; it can still move wiithin the circle defined by the
anchor rode plus the lenght from bow chock to GPS antenna location.
That produces a circle almost 300 feet in diameter. Assuming the anchor
breaks out at the turn of the tide and resets itself in about 50 feet of
dragging you now increase the acceptable variation to about 320 - 350 feet.
The question is, are you going to set the alarm circle so tight that normal
movement swinging at anchor is going to trip the GPS alarm?
If you don't want constant false alarms how much more room will you allow?
Pretty soon you have a circle as great as what you normally had when SA was
still turned on.
The normal stability of regular GPS is such that WAAS simply isn't an issue.
WAAS was developed to refine positioning accuracy for aircraft making a
blind landing.
Cat III landing is much more demanding than any anchoring you are likely to
need.
And if you need that degree of accuracy on your boat, you should be looking
at a dynamic positioning system, not a single anchor and civilian GPS. Come
on now! Who is kidding who?
At the moment WAAS is undergoing trials and two satellites are in place.
Visibility may not be adequate in the higher latitudes such as the PNW; nor
will it be good if terrain blocks the line of sight.
Tucked into a sheltered cove surrounded by tall trees, or worse, high hills
may well mean the two WAAS satellites are not visible.
Now add to that the information that the authorities have acknowledged that
some of the older satellites are degrading and the process of geting
replacements installed will be delayed.
In other words you are simply kidding yourself that a WAAS equipped receive
will guarantee safer anchoring as compared to an orodinary GPS.
In fact if this is the only thing you rely on when anchoring out, please
don't invite me to stay onboard ovenight.
And if you are using something else as well why is this even being
discussed.
Cheers
Arild
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