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What
is WAAS?
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. |
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| 15
meters: |
Typical
GPS position accuracy without SA. |
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| 3-5
meters: |
Typical
differential GPS (DGPS) position accuracy. |
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| <
3 meters: |
Typical
WAAS position accuracy. |
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