Garmin GNC 420W Supplementary Manual page 5

Detailed Description of Feature Changes
1. TSO
TSO C-129/C-129a allowed the use of GPS for
supplemental navigation for non-precision approaches.
An installation meeting TSO-146a requirements allows
the use of your 400W/500W navigators as the primary
navigation device for precision approaches.
See your dealer for detailed information about how the
new TSO affects the operation of your aircraft.
2. WAAS Capable
WAAS (Wide Area Augmentation System) is a
system to provide improved integrity and accuracy of
GPS signals used for navigation. "Wide Area" refers to
a network of ground reference stations that cover the
entire U.S. and some of Canada and Mexico. Imple-
mented by the FAA (Federal Aviation Administration)
for aviation users, these reference stations are located
at precisely surveyed spots and compare GPS distance
measurements to known values. Each reference station is
linked to a master station, which puts together a correc-
tion message and broadcasts it via satellite. The corrected
differential message is then broadcast through one of two
geostationary satellites. The information is compatible
with the basic GPS signal structure, which means any
WAAS-enabled GPS receiver can read the signal. WAAS
capable receivers typically have accuracies of one meter
horizontally and 2 meters vertically.
Basically, it' s a system of satellites and ground sta-
tions 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
two meters 95 percent of the time. And you don't have to
purchase additional receiving equipment or pay service
fees to utilize WAAS. WAAS also provides the capabil-
ity of quickly determining when signals from a given
satellite are wrong and removing that satellite from the
navigation solution.
The Federal Aviation Administration (FAA) and the
Department of Transportation (DOT) are developing
the WAAS program for use in precision IFR approaches.
Without WAAS, 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 near real-time integrity infor-
mation regarding the health of each GPS satellite.
Currently, WAAS satellite coverage is only available
in North America. There are no ground reference sta-
tions 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 WAAS
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 aviation
applications. Your Garmin 400W/500W series system
provides for primary GPS navigation in areas where no
WAAS coverage is available using a Fault Detection and
Exclusion (FDE) RAIM algorithm.
Other governments are developing similar Satellite-
Based Augmentation Systems (SBAS). In Asia, it' s the
Japanese Multi-Functional Satellite Augmentation System
(MSAS), while Europe has the Euro Geostationary Navi-
gation Overlay Service (EGNOS). Once these systems
are fully tested and operational, your 400W/500W series
system will be capable of using these systems.