Avionics News March 2018 - 53

its great potential, proving its utility and accuracy in tests at the
Wallops Island test site operated by the National Aeronautics and
Space Administration, at what was then called Washington National
Airport, and at other sites around the country and the world.
But in the end, reality killed MLS.
First, MLS lacked a choice of affordable navigation receivers
suitable for mass installation. Second, the high costs of existing
MLS receivers discouraged adoption by airlines and businessaircraft operators.
Finally, MLS was not a viable option for high-accuracy en route
navigation, a latent need for improving airspace-system capacity.
Thus, MLS slipped quietly into history, a success that failed.
In failing, the baton of MLS' promise passed to GPS as the viable
option in the decades-long quest for a navigation option to replace
the increasingly expensive gold standard of precision-approach
technology, the Instrument Landing System, or ILS.
In between MLS and WAAS, aviation benefited from its
early tastes of area-navigation through early IFR-approved GPS
navigators. Beginning with FAA TSO-C129, Class A1 standard and
following through successively higher levels of technical standard
orders, avionics makers delivered a number of GPS navigators
approvable for en route and terminal IFR flight and as guidance
systems from non-precision instrument approaches.
Garmin offered the first IFR GPS approved under FAA
TSO-C129, Class A1 in the groundbreaking Garmin GPS 155.
Other companies followed, and IFR GPS won greater appeal among
pilots thanks to some creative thinking.
Developers drew many of those IFR GPS approaches to replace
less-accurate non-precision approaches overlaid on previously
available - and dependent on - VOR stations, DMEs and nondirectional beacons.
Thousands of operators embraced these navigators for their
ability to guide aircraft on filed-direct cross-country flights; the nonprecision approaches available to new runway ends and the higher
accuracy of overlay approaches contributed to that appeal.
But these progressive advancements still fell far short of meeting
the goal of the Federal Aviation Administration - and in reality,
virtually every other aviation regulator - for a system with accuracy
to meet goals for precision en route and terminal operations.
In other words, something more accurate than the ILS, than radar
or anything else ever developed.
The key to meeting those goals: A new generation of morepowerful, more-sophisticated GPS satellites, other satellites
dedicated to correcting inaccuracies in GPS signals from space,
and a widespread ground network of precisely sited ground stations
serving as the position reference needed to calculate and broadcast a
correction to GPS navigators.
Years of work went into the preparation, network construction,
satellite launches and testing the network. The Wide Area
Augmentation System became the result.

Avidyne IFD510
WAAS Defined
What exactly makes up the Wide Area Augmentation System?
First, WAAS consists of two master stations located on both
coasts talking to two geostationary satellites hovering near the
equator.
Also, part of the WAAS network, approximately 25 ground
reference stations are positioned across the United States. The
ground reference stations receive the same satellite signals as GPS
receivers and forward a correction message to the master stations.
Correction messages reduce error by correcting for satellite orbit
and clock drift and signal delays from the satellites. The corrected
message is then broadcast through one of two geostationary
satellites (the ones located over the equator) to WAAS-enabled
receivers on a standard GPS signal. Currently, WAAS is only
available in North America. A WAAS GPS receiver operating
outside North America can use a WAAS-enabled GPS, but the data
have not been corrected to deliver WAAS-level accuracy.
At Age 15, WAAS is a proven winner
The FAA commissioned the WAAS network for aviation use on
July 10, 2003. That date marks the beginning of aviators being able
to navigate in instrument conditions solely by the guidance of an
approved WAAS GPS navigator.
The goal of the FAA for WAAS was to provide reliable signals
with an accuracy of 7 meters, both horizontally and vertically, a full
95 percent of the time a WAAS signal is being received. Current
testing by manufacturers of handheld GPS receivers have shown
that actual accuracy levels typically hover around 2 to 3 meters - 6.5
feet to 9.8 feet.
In service, accuracy tests even higher. Original GPS provided an
accuracy level of about 100 meters - 330 feet - in part due to the
Defense Department's use of Selective Availability, a technology
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