[gps-talkusers] What is WAAS?
- From: "Richard Myers" <dkmyers28@xxxxxxxxxxx>
- To: gps-talkusers@xxxxxxxxxxxxx
- Date: Sat, 24 Sep 2005 06:57:40 +0900
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.
15 meters:Typical GPS position accuracy without SA.
3-5 meters:Typical differential GPS (DGPS) position accuracy.
< 3 meters:Typical WAAS position accuracy.
(8):[(8)
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