[GeoStL] Re: Perfection is hard to achieve
- From: "Gale R. Nie" <showme69@xxxxxxxx>
- To: <geocaching@xxxxxxxxxxxxx>
- Date: Wed, 2 May 2007 20:49:09 -0500
-
From the Garmin website:
You've heard the term WAAS, seen it on packaging and ads for Garmin®
products, and maybe even know it stands for Wide Area Augmentation System.
Okay, so what the heck is it? Basically, it's a system of satellites and
ground stations that provide GPS signal corrections, giving you even better
position accuracy. How much better? Try an average of up to five times
better. A WAAS-capable receiver can give you a position accuracy of better
than three meters 95 percent of the time. And you don't have to purchase
additional receiving equipment or pay service fees to utilize WAAS.
The origins of WAAS
The Federal Aviation Administration (FAA) and the Department of
Transportation (DOT) are developing the WAAS program for use in precision
flight approaches. Currently, GPS alone does not meet the FAA's navigation
requirements for accuracy, integrity, and availability. WAAS corrects for
GPS signal errors caused by ionospheric disturbances, timing, and satellite
orbit errors, and it provides vital integrity information regarding the
health of each GPS satellite.
How it Works
WAAS consists of approximately 25 ground reference stations positioned
across the United States that monitor GPS satellite data. Two master
stations, located on either coast, collect data from the reference stations
and create a GPS correction message. This correction accounts for GPS
satellite orbit and clock drift plus signal delays caused by the atmosphere
and ionosphere. The corrected differential message is then broadcast through
one of two geostationary satellites, or satellites with a fixed position
over the equator. The information is compatible with the basic GPS signal
structure, which means any WAAS-enabled GPS receiver can read the signal.
Who benefits from WAAS?
Currently, WAAS satellite coverage is only available in North America. There
are no ground reference stations in South America, so even though GPS users
there can receive WAAS, the signal has not been corrected and thus would not
improve the accuracy of their unit. For some users in the U.S., the position
of the satellites over the equator makes it difficult to receive the signals
when trees or mountains obstruct the view of the horizon. WAAS signal
reception is ideal for open land and marine applications. WAAS provides
extended coverage both inland and offshore compared to the land-based DGPS
(differential GPS) system. Another benefit of WAAS is that it does not
require additional receiving equipment, while DGPS does.
Other governments are developing similar satellite-based differential
systems. In Asia, it's the Japanese Multi-Functional Satellite Augmentation
System (MSAS), while Europe has the Euro Geostationary Navigation Overlay
Service (EGNOS). Eventually, GPS users around the world will have access to
precise position data using these and other compatible systems.
It just keeps getting better
100 meters: Accuracy of the original GPS system, which was subject to
accuracy degradation under the government-imposed Selective Availability
(SA) program.
15 meters: Typical GPS position accuracy without SA.
3-5 meters: Typical differential GPS (DGPS) position accuracy.
< 3 meters: Typical WAAS position accuracy.
----- Original Message -----
From: "Stephen Martin" <faery_generic@xxxxxxx>
To: <geocaching@xxxxxxxxxxxxx>
Sent: Wednesday, May 02, 2007 8:14 PM
Subject: [GeoStL] Re: Perfection is hard to achieve
> -
>
>
>>When taking coordinates is not having a WAAS fix the best way to go? Is
>>not
>>that better than averaging the waypoint?
>
> What is a WAAS fix?
>
>
>
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