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RE: Selective Availability (S/A)GPS Signal Degradation

joe@jre.com

Mon, Oct 12, 1998 6:36 PM

Hi Ron:

I can only repeat what we observed and continue to observe. With the
Magellan ColorTrak GPS NMEA interfaced to the NobelTec Visual Navigation
Suite computer on our boat, without any differential capabilities, we
are seeing phenomenal accuracy. I attributed this to the averaging
functions built into the ColorTrak because nothing else is special in
our setup. Maybe that isn't the reason. Maybe the DOD has a secret SA
test going on?

We think, and this is highly subjective, we are seeing 5 - 10 foot
repeatable accuracy on fixed objects such as bridges or fixed markers.
As I mentioned, on 2 recent occasions we have noted that we were shown
on the electronic chart, against our mooring float that is about 5 feet
wide. The image did not move about. We were shown facing the wrong
direction and on the wrong side of the float but at the right position
along the float.

So maybe we're wildly inaccurate on our estimates positions, maybe we're
30 feet off. That's still amazing. I don't know why but I would like
to hear from others with the latest generation of GPS receivers to see
if this is some anomaly or ??

I have no scientific explanation yet and I'm sure not betting my life on
this accuracy. If it reverts back to what we usually experience with
our older Garmin, I'll report that too.

Winter is coming here and I don't know how much more messing about we'll
be doing. Maybe I'll switch select back and forth between the GPS
receivers and see if that narrows it down.

Joe & Debbie Engel
Marine Computer Services & JRE Consulting, Inc.
MV Freda Fly - 40' Tollycraft Tri-cabin
Portland, OR

-----Original Message-----
From: Ronald C. Rogers [mailto:rcrogers@annapolis.net]
Sent: Monday, October 12, 1998 11:06 AM
To: trawler-world-list@samurai.com
Subject: Selective Availability (S/A)GPS Signal Degradation

<SNIP>

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 graphic display long enough or have a
charting device with a tracking feature you will see the 5% excursions
as I have.

Ron Rogers

[SNIP]

Hi Ron: I can only repeat what we observed and continue to observe. With the Magellan ColorTrak GPS NMEA interfaced to the NobelTec Visual Navigation Suite computer on our boat, without any differential capabilities, we are seeing phenomenal accuracy. I attributed this to the averaging functions built into the ColorTrak because nothing else is special in our setup. Maybe that isn't the reason. Maybe the DOD has a secret SA test going on? We think, and this is highly subjective, we are seeing 5 - 10 foot repeatable accuracy on fixed objects such as bridges or fixed markers. As I mentioned, on 2 recent occasions we have noted that we were shown on the electronic chart, against our mooring float that is about 5 feet wide. The image did not move about. We were shown facing the wrong direction and on the wrong side of the float but at the right position along the float. So maybe we're wildly inaccurate on our estimates positions, maybe we're 30 feet off. That's still amazing. I don't know why but I would like to hear from others with the latest generation of GPS receivers to see if this is some anomaly or ?? I have no scientific explanation yet and I'm sure not betting my life on this accuracy. If it reverts back to what we usually experience with our older Garmin, I'll report that too. Winter is coming here and I don't know how much more messing about we'll be doing. Maybe I'll switch select back and forth between the GPS receivers and see if that narrows it down. Joe & Debbie Engel Marine Computer Services & JRE Consulting, Inc. MV Freda Fly - 40' Tollycraft Tri-cabin Portland, OR -----Original Message----- From: Ronald C. Rogers [mailto:rcrogers@annapolis.net] Sent: Monday, October 12, 1998 11:06 AM To: trawler-world-list@samurai.com Subject: Selective Availability (S/A)GPS Signal Degradation <SNIP> 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 graphic display long enough or have a charting device with a tracking feature you will see the 5% excursions as I have. Ron Rogers [SNIP]