My experiences this past summer are very different then those in CaptnWil's
post. Let me first state that I do not work for Garmin and that I do not
have a differential beacon.

The first page on the Garmin 175 shows what satillites are being used in
fixing the current position AND the amount of error (in feet) that SA has
introduced into the system. This amount is constantly being updated by the
GPS unit but has never exceeded 100 feet.

of

Storm

good

location

in

I have done this and both the GPS display and when outputing data to my
mapping program on a laptop have always showed my vessel exactly at the
correct position at the dock. The vessel was always stationary, not moving
about.

the

I have set the anchor alarm to 10 feet without any false alarms going off.

anchor

a

go

I do not have an autopilot (yet) so I can not comment on this.

business.

autopilot

Overall, I have been extremely pleased with the performace of my GPS.
Perhaps different models and manufacturers have built in different
algorithms to deal with the SA problem and that is why my experience is
very different than CaptnWil's.

Ned

Company:  RePlay Technologies, Inc.
E-mail:      Ned Robinson nedr@replayinc.com
Web:        http://www.replayinc.com
Voice:        516-385-1398      Fax:  516-385-1398

This information has been culled from several U.S. government sites and
two private GPS sites. Any comments that I make are enclosed in
<bold>[brackets]</bold>.

"The Department of Defense <bold>[Air Force]</bold> plans to provide, on
a daily basis at any position worldwide, horizontal positioning accuracy
within 100 meters (2 drms) <bold>[95% of the time]</bold> and 300 meters
<bold>[5% of the time] </bold>with 99.99 percent probability.

<bold>Selective Availability (SA)</bold>

SA is the intentional degradation of the SPS signals by a time varying
bias. SA is controlled by the DOD to limit accuracy for non-U. S.
military and government users. The potential accuracy of the C/A code of
around 30 meters <bold>[real potential civilian accuracy is closer to 1
meter] </bold>is reduced to 100 meters (two standard deviations). The SA
bias on each satellite signal is different, and so the resulting position
solution is a function of the combined SA bias from each SV used in the
navigation solution. Because SA is a changing bias with low frequency
terms in excess of a few hours, position solutions or individual SV
pseudo-ranges cannot be effectively averaged over periods shorter than a
few hours. Differential corrections must be updated at a rate less than
the correlation time of SA (and other bias errors).

<bold>Standard Positioning Service (SPS)

Civil users worldwide use the SPS without charge or restrictions. Most
receivers are capable of receiving and using the SPS signal. The SPS
accuracy is intentionally degraded by the DOD by the use of Selective
Availability.

SPS Predictable Accuracy 

    100 meter horizontal accuracy 

    156 meter vertical accuracy 

    340 nanoseconds time accuracy 

These GPS accuracy figures are from the 1994 Federal Radionavigation
Plan. The figures are 95% accuracies, and express the value of two
standard deviations of radial error from the actual antenna position to
an ensemble of position estimates made under specified satellite
elevation angle (five degrees) and PDOP (less than six) conditions.

For horizontal accuracy figures 95% is the equivalent of 2drms
(two-distance root-mean-squared), or twice the radial error standard
deviation. For vertical and time errors 95% is the value of two-standard
deviations of vertical error or time error.

Receiver manufacturers may use other accuracy measures. Root-mean-square
(RMS) error is the value of one standard deviation (68%) of the error in
one, two or three dimensions. Circular Error Probable (CEP) is the value
of the radius of a circle, centered at the actual position that contains
50% of the position estimates. Spherical Error Probable (SEP) is the
spherical equivalent of CEP, that is the radius of a sphere, centered at
the actual position, that contains 50% of the three dimension position
estimates. As opposed to 2drms, drms, or RMS figures, CEP and SEP are not
affected by large blunder errors making them an overly optimistic
accuracy measure Some receiver specification sheets list horizontal
accuracy in RMS or CEP and without Selective Availability, making

those receivers appear more accurate than those specified by more
responsible vendors using more conservative error measures.

Selective Availability (SA) is the intentional degrading of GPS accuracy.
The full accuracy is still available, but only to selected users.
<bold>The original (Block I) satellites did not have this feature, but
they are no longer in use.</bold> The US

government is going to evaluate turning off or replacing SA, but that is
still some time in the future. Meanwhile, SA affects most GPS users.
Since it was turned on 1990, there has only been one day of SA-free
operation <bold>[technically correct, BUT during the Gulf War they
"turned S/A down so that GPS had essentually zero error]</bold>.

Very little is known about SA except the specifications on how large it
is allowed to be. How it is actually implemented is secret. Nevertheless,
there seems to be agreement that it can take two forms: dithering the
satellite clock (delta-SA) and reducing the accuracy of the ephemerides
(epsilon-SA). The GPS uses the measured distance to the known position of
the satellite to determine its own position, so errors in either will
affect the accuracy of the GPS position. The first because the distance
is measured inaccurately. The second because the satellite is not where
it says it is. In this case, only the errors toward or away from the GPS
receiver are important.

Variations in clock dither SA can be plus or minus 50 meters, with
periods of several minutes. The epsilon-SA variations can be 50 to 150
meters, with periods of several hours.

Each satellite implements SA independently, as far as we know. With DGPS,
each satellite signal is corrected independently. With consumer GPS
receivers, the user is only shown the final position, which is usually
off at least a few meters due to SA and possibly much more. Although it
is easy to refer to this error in position as the 'SA error', it

is not the SA error. There is no one SA error. Rather, the error you see
is a combination of all the SA errors from each of the satellites your
GPS is using. The 'SA Error' you see will be different if it uses
different satellites. The error you see will also depend on other
factors, such as DOP, and will also include other errors which are not
due to SA."

<bold>The bottom line is that no one with civilian GPS can get better
accuracy than under 100m 95% of the time and 100 to 200m 5% of the time.
This changes with DGPS. I am sure that some receivers are better than
others - but that is a relative term and all receivers receive the same
degraded signal. If you watch your grpahic display long enough or have a
charting device with a tracking feature you will see the 5% excursions as
I have.

Ron Rogers

<bold>[SNIP]</bold>At 08:46 AM 10/12/98 -0400, you wrote:

CaptnWil's