Avionics News May 2020 - 41

Getting ADS-B coverage off the ground
Let's hear it for RADAR, as we've known the
technology, which the U.S. Navy named in 1940 as an
acronym for RAdio Detection And Ranging. Through
various incarnations, from analog to digital, RADAR
helped air-traffic controllers manage air-traffic flow.
While the technology improved with those various
evolutionary steps, limitations remained - limitations
rooted in the radar technology itself. For example, radar
accuracy suffers as the distance to targets increases.
The ability of altitude encoders to accurately report
an aircraft's height above the ground also suffers from
limitations.
RADAR's limitations played into air-traffic procedures
designed to allow for the known inaccuracies of the
system to help provide safe separation. As air traffic
increased, those limitations effectively limited how much
traffic the network could accommodate.
To accommodate more traffic, planes need to fly closer
together, in both the en route and terminal environments.
The world's aviation community already adopted RVSM
- Reduced Vertical Separation Minimums - to add new
altitudes for high-flying traffic. Instead of 2,000-foot
vertical separation above Flight Level 290, RVSM
allowed for separations of 1,000 feet at altitudes up to
Flight Level 410.
While certainly a big help for those operators using
those altitudes, RVSM did nothing to improve horizontal
separation, which can be miles apart over land - and
more over water.
ADS-B, conversely, delivers extremely accurate
reporting of both position and altitude thanks to its use
of position and altitude data generated by a GPS receiver
enhanced by the wide-area augmentation system, or
WAAS.
WAAS corrects for GPS signal errors caused by
ionospheric disturbances, timing and satellite orbit
errors. WAAS also provides vital integrity information
regarding the health of each GPS satellite.
How WAAS works its magic
WAAS employs a network of precisely located ground
reference stations that constantly monitor GPS signals
from the constellation of GPS satellites.
WAAS hardware consists of 38 ground reference
stations, three master stations, three geostationary
satellites with navigation transponders on board, six

uplink stations, two operational control centers, and the
WAAS terrestrial communications network.
The FAA located the ground-reference stations
throughout the continental U.S., Alaska, Hawaii, Puerto
Rico, Canada and Mexico. The stations collect and
process GPS information and send the information to
WAAS master stations.
The WAAS master stations develop a WAAS
correction message that is sent to user receivers via a
'GPS-like' signal from navigation transponders on board
geostationary satellites.
The WAAS message improves the accuracy,
availability and integrity of GPS-derived position
information. With the WAAS corrections, GPS signal
accuracy improves from about 65 feet (20 meters) to
approximately 5 feet to 6.5 feet (1.5 to 2 meters) in both
the horizontal and vertical dimensions, according to the
FAA.
WAAS can also signal to a navigator when accuracy
falls below its normal range. And WAAS GPS provides
the accuracy needed for new GPS-only instrument
approaches.
For the past 60 years, the Category-1 instrument
landing system has been the gold-standard instrument
approach at airports throughout the national airspace
system. With ground-based guidance systems, the ILS
guides aircraft to as low as 200 feet above the runway.
Today, WAAS provides similar capability, but at many
more runway ends - and without airport infrastructure
requirements. These WAAS GPS approach procedures
are called localizer performance with vertical guidance.
WAAS provides the accuracy needed to support
precision GPS instrument approaches and enabled a
new localizer performance approach that provides the
same lateral accuracy as LPV but without the vertical
guidance.
The accuracy of airborne WAAS GPS navigators ends
when an aircraft leaves airspace covered by the WAAS
network or comparable systems in other countries. But
the latent accuracy of non-WAAS GPS remains good
enough to provide closer traffic separation in oceanic
environments, particularly if served by the Aireon
satellite-based ADS-B network. That accuracy level also
provides a safer traffic environment compared to oceanic
and other areas lacking any air-traffic radar coverage.
Continued on following page

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Avionics News May 2020

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