[Fwd: [TowerTalk] GPS receivers]

[Tower (K8RI)]

I'm having to add quotes as the message must have been in HTML originally.


At 08:52 AM 9/20/2004, Jim Brown wrote:
>My friends who claim to be educated in matters of GPS say that
>because of the manner in which the system is modulated for security
>reasons, consumer units are limited to accuracy on the order of several
>meters in plan and tens of meters of elevation. A search on google for
>GPS accuracy a year or so ago appeared to confirm that my friends
>were correct.


"I think this is a bit confusing.  Before 2003, there was something in place

You have to be careful with the FAA's use of terms.

"called Selective Availability, which limited civilian GPS systems to tens
"of meters accuracy, and appears to be what your friends are telling you

I'm quoting the Garmin site, not my firends or my use of GPS which I have in 
my plane.

"about.  That practice was discontinued then, and civilian systems,

Selectve availability added varriable errors.  Back then I could take my 
aviation GPS, sit on the couch and watch the altitude wander as much as a 
100 feet.

"unaugmented, should have about the same accuracy as anyone else's, with a
"given set of satellite data.

Pretty much

"The following, from the FAA's web site, may be of general interest:


"The Wide Area Augmentation System (WAAS) is a GPS-based navigation and
"landing system that provides precision guidance to aircraft at thousands of
"airports and airstrips where there is currently no precision landing
"capability. Systems such as WAAS are known as satellite-based augmentation

Even WAAS is available for non precision approaches and point to point 
navigation only.
WAAS is not a precision landing capable system.  Yes, it is far better than 
a VOR, but has a ways to go before it can match the latter part of an ILS. 
We are not permitted to use it for precison approaches.

"systems (SBAS). WAAS is designed to improve the accuracy and ensure the
"integrity of information coming from GPS satellites. The FAA is using WAAS
"to provide a Lateral Navigation/Vertical Navigation (LNAV/VNAV) capability
"with commissioning in 2003. Concurrently, the FAA will evaluate the
"approach to achieve Global Navigation Satellite System (GNSS) Landing
"System (GLS) capability in later years. WAAS testing in September 2002
"confirmed accuracy performance of 1 - 2 meters horizontal and 2 -3 meters
"vertical throughout the majority of the continental U.S. and portions of
"Alaska.

"Presumably (me again) the current accuracy of non-WAAS GPS receivers would
"be somewhere between the two extremes.

Going back to the pratical implementation of GPS the Garmin site quotes WAAS 
enabled GPS as  "less than" 3 meters while current non WAAS as +/- 15 meters 
average and that is with a 12 channel receiver.

"To get back on the original topic, I don't think anyone associated with
"HFTA would argue that positioning errors on the order of tens of meters
"would have a significant effect on the accuracy of HFTA predictions, except
"in some fairly special cases where significant near-field obstacles are
"quite close to the antenna system.

IF you can find a nearby survey point AND get ahold of a ground based 
augmentation transmitter it is possible to measure within centimeters. 
These are used in geologic surveys.
There are a LOT of survey markers around, they are just difficult to find.

http://www.garmin.com/aboutGPS/applications.html briefly talks about GPS and 
geologic surveys and centimeter accuracy.

Probably one of the easiest locators is to find a local topigraphical map 
and pinpoint your location on that.  Those come in pretty high resolution 
contours. As I recall the one I have for Midland county is in a 5 foot 
resolution. OTOH around here there is less than 60 feet difference between 
the highest and lowest elevations in the county.

Roger Halstead (K8RI, EN73 & ARRL Life Member)
N833R, World's Oldest Debonair (S# CD-2)
www.rogerhalstead.com


73, Pete N4ZR
The World HF Contest Station Database
has just been updated
2853 contest stations at
www.pvrc.org/WCSD/WCSDsearch.htm

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See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless 
Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any 
questions and ask for Sherman, W2FLA.

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