AOPA Turbine Pilot Magazine - October 2018 - 22

WAYPOINTS

The WAAS factor

BY THOMAS B. HAINES
Editor in Chief

Making ADS-B work-and driving up the cost

Editor in Chief
TOM HAINES

invested in ADS-B
Out in 2013 to get
traffic information. At that time,
traditional trafficalerting systems
cost two or three
times as much.

22 | AOPA PILOT October 2018

THE TRANSITION TO ADS-B has been underway for well
more than a decade now, yet questions abound. Automatic
Dependent Surveillance-Broadcast forms the backbone of
the FAA's next generation air transportation system. No
longer will controllers be entirely reliant on ground-based
radar to manage air traffic. Instead, aircraft will broadcast
their positions to ATC and to each other. In order to provide the most accurate position information practical, the
FAA insists that aircraft have a GPS receiver that incorporates a WAAS correction factor.
The Wide Area Augmentation System itself is a
marvel of engineering. Thirty-eight reference stations
around the country, plus two masters and a backup,
sit on precisely measured sites. Sensors there know
exactly where they are, and they compare that position to the ones they receive from the constellation
of GPS satellites-then account for small errors that
may result from atmospheric or other interference.
The ground stations send the corrected information
to the master stations, which send the corrections up
to three geostationary satellites. Those satellites broadcast the corrections on a frequency near the one used
by the GPS satellites themselves. Your WAAS-enabled
receiver uses the correction factor to further refine
your position information, providing a more precise
position than the raw GPS signal.
That position can then be broadcast by your ADS-B
Out system to let ATC and others know where you are.
That same WAAS precision allows us to shoot
localizer performance with vertical guidance (LPV)
approaches to Category I minimums at hundreds of airports that in the past did not have any approaches, or
perhaps none with minimums that low. According to the
FAA, there are 3,400 LPV approaches available. A great
advantage of LPV approaches is that, aside from some
approach lighting systems, no ground infrastructure
is necessary at the airports. The signals are all spacebased, saving installation and maintenance costs for
the FAA. Some 80,000 aviation users take advantage of
WAAS, which has been in service since 2003.
While the system all works great, it's also expensive for the end user. The FAA insists that the GPS
receivers used as an ADS-B position source and for
instrument procedures be certified to a very high level
and that they have integrity monitoring to alert the
pilots within six seconds if the position accuracy is
in doubt. All of that adds to the cost of development

and manufacturing. Meanwhile, we're all carrying
around in our phones, cars, tablet computers, and
other devices non-certified WAAS receivers that do
a marvelous job of letting us know where we are and
helping us get around-on the ground.
The FAA says those positions are not good enough
for approaches and for separating aircraft. And they're
probably right regarding the approaches. When you're
approaching the ground to within 200 feet while in the
clouds, you want a reliable and highly accurate position
source that alerts you to any errors.
The debate becomes whether that level of accuracy
is necessary for separating aircraft that typically are no
closer than five miles apart in flight when on an IFR
flight plan. The FAA says accuracy is necessary because
in the future, it plans to fly aircraft closer together,
improving the efficiency of the burgeoning airspace system. In addition, ADS-B is also meant to separate aircraft
on the ground where airliner wing tips may pass within
a few feet of each other on a foggy day at LaGuardia.
Despite the debate, the agency has not budged on
its insistence that approved ADS-B systems use a certified WAAS receiver or its equivalent. So if you've
wondered why ADS-B Out solutions cost as much as
they do, that's part of the reason.
The good news is that with the encouragement of
AOPA, the FAA and manufacturers over the past four
years have worked collaboratively to introduce products that meet the standard but do so at prices a fraction
of what they were five years ago. Instead of starting
at $5,000 just for the equipment, today you can buy it
starting at $1,800. The complexity of the installations
has come down as well. And, if you invest another few
hundred dollars, you can get an ADS-B In receiver that
gives you weather and traffic in the cockpit, further
improving the value equation.
Keep in mind that if you don't fly in airspace
where a transponder is required today, you don't
need to equip with ADS-B to meet the 2020 mandate.
However, to be detected by other nearby aircraft and
to get reliable traffic information yourself-a great
safety aid-consider making the investment. More
than 44,000 GA airplanes were equipped with ADS-B
Out as of August 1. I have talked to no aircraft owners
who regretted the investment.
AOPA
EMAIL thomas.haines@aopa.org

AOPA Turbine Pilot Magazine - October 2018

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