[Fwd: [TowerTalk] GPS receivers]

[Pete Smith]

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 
called Selective Availability, which limited civilian GPS systems to tens 
of meters accuracy, and appears to be what your friends are telling you 
about.  That practice was discontinued then, and civilian systems, 
unaugmented, should have about the same accuracy as anyone else's, with a 
given set of satellite data.

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 
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.

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.



73, Pete N4ZR
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