[Az-Geocaching] How accurate is the GPS?

[Jim Krider]

Thanks to everyone for your help.  It does make since to just wait a
while for the GPS to settle down.  I was walking with the wife and
grandson so I only paused for a moment to make the waypoint in both
cases.  I have not redone the test but will soon.  

Brian's graphic is interesting.  The concept that your position is a
vertical column projected on a incline making an ellipse is helpful.  At
my age I avoid the cliffs though.

Bill's article about WAAS was helpful too.  When I was at ASU looking
for GCG059 Mat274 my technique was to get in the middle of tall
buildings and look at the screen that shows the GPS signal strength.
When I had a strong signal then I looked for the cache.  Now that I know
the WAAS satellites are on the equator perhaps moving as far north as
possible to expose as much southern sky as possible would be a better
way to locate the cache.  That way I might pick up the WAAS satellite.
I wonder if the satellite is higher than 35 degrees in AZ since we are
further south than Kentucky?

So all of your suggestions have altered my searching techniques. 

Thanks Brian, Bill, Trisha, TeamDaSh, Scott, Guy, and SquishyGeko.

Jim Krider (KNOWBODY)


-----Original Message-----
From: az-geocaching-admin@listserv.azgeocaching.com
[mailto:az-geocaching-admin@listserv.azgeocaching.com] On Behalf Of
Atherton, Bill (AZ15)
Sent: Friday, June 04, 2004 12:36 PM
To: listserv@azgeocaching.com
Subject: RE: [Az-Geocaching] How accurate is the GPS?

Here is a good writeup on accuracy and how WAAS helps.  
Bill - The Watcher

http://www.kentuckyhydrografx.com/gpserror.htm
GPS Accuracy
The good news is that the GPS units everyone purchased are now accurate
to
better than 10 meters 95% of the time. That's only about two boat
lengths.
I don't know about you, but I can return to individual stumps, brush
piles,
and points in no time if I know I am 30 feet or less from it. 

UPDATE!!!! 
Now what about the "WAAS Capable" thing I see in the ads for GPS??
All error has not been removed from the gps signals. Only the
"intentional"
error by the military has been removed. 
Other sources of error include the slight deflection of the satellite
signals as they come through a portion (ionosphere) of the atmosphere.
Local
satellite signal reflections (multi-path) occur around trees, buildings,
and
other stuctural features. Your own body can even reduce signal strength
and
quality when using a handheld gps.
The good news is that the ionospheric error can be corrected for by WAAS
or
DGPS enabled receivers. If a gps unit receives the timing adjustments
along
with the regular satellite signals, it can then use that information to
compute a more accurate position with less error. 
The WAAS signals are in the same frequency range of the GPS satellite
signals, which means your 12 channel receiver will use 1 of it's
channels to
receive the WAAS corrections. A DGPS receiver is a separate device that
has
to be purchased and hooked to your existing GPS.
WAAS is an error correction system designed for use by the aviation
industry
to reduce the atmospheric error potential in gps calculations. A series
of
land based stations calculate and provide timing corrections to
counteract
the atmospheric signal delays. These land based receivers relay their
information to satellites in geo-synchronous orbit (that means they are
in
the same place all the time relative to the earth -- unlike the moon and
unlike normal GPS satellites ). 
Land based DGPS is basically the same idea and has been around for a
long
time. DGPS corrections have been available through coast guard beacons
distributed near most major rivers and lakes where barge traffic is
common.
In our area, there are beacons in St. Louis, Memphis, and Louisville.
DGPS
is a low frequency radio signal that requires a separate receiver that
then
hooks to your gps to provide correction information. 
>From what I can tell, there is only 1 WAAS satellite that we can receive
the
corrections from. That's OK, since it only takes 1 for the information
we
need. The interesting thing is that the WAAS satellites are located
directly
above the equator and relatively low on the horizon. 
The WAAS satellite location relative to Kentucky is at an azimuth of
approximately 130 degrees (roughly southeast) and 35 degrees above the
horizon. That means if you are fishing a reservoir with fairly tall
ridges,
the WAAS signal may be unavailable since it is blocked by hills. The
signal
has to be "line of sight" or you won't get it. Folks in the western US
may
not benefit much at all since it will be only 10-15 degrees above the
horizon there. WAAS is still in the development stage and we will
hopefully
see additional satellites come online in the future. A satellite
directly
south of the US would increase the height above the horizon
significantly.
We should be fortunate here in western kentucky, since we are in rather
"rolling" terrain, the hills shouldn't be too much of a problem. 
GPS accuracy with WAAS corrections is specified to be < 3 meters 99% of
the
time. From some of the tests I have seen on consumer grade receivers,
the
specifications are accurate.
You will literally be within "spitting distance" of a spot when < 3
meters
away.
If you have noticed, commercial GPS manufacturers are really starting to
promote "marine" receivers. This is partly because of the WAAS
availability
plus they are adding moving map capabilities with lake information. They
will not likely push the WAAS thing as hard for gps units designed for
land
use since the WAAS signal is more likely to be obstructed by trees or a
variety of other structures. 
Just to summarize the accuracy you might expect to get with a common gps
receiver.
GPS with no correction capability -- <10 meters 95% of the time
GPS with DGPS receiver -- 3 - 5 meters 95% of the time
GPS with WAAS corrections -- < 3 meters 99% of the time

FYI there are survey-grade receivers and equipment available that can
get
accuracy in the sub-centimeter range. That kind of stuff usually sells
in
the neighborhood of 40 - 50,000 dollars.
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