By Chris Baur
Ground Based
Augmentation Systems
What good is a technology if it only has one name? Ground based augmentation systems (GBAS) is one such technology that is part of the FAA’s NextGen roadmap. It resides within a category known as
LAAS (local area augmentation system).
Its purpose is to supply a differential correction or augmentation to the basic GPS
or GNSS, while providing a significant
improvement in accuracy. Instrument
approaches flown with GBAS are called
GLS approaches, otherwise known as
global navigation satellite landing system.
You may have also heard of WAAS (wide
area augmentation system) and want to
know what the distinction between them
is. Are you wondering why we even need
augmentation for GPS/GNSS?
A previous article discussed FAA’s Next-Gen roadmap relating to performance-based navigation (June 2009, page 60). We
looked at required navigation performance
(RNP) as a “game changer,” as it provides
greater access, increased safety, and reductions in both environmental impact and
fuel consumption. RNP has certain accuracy limitations that are based on “raw”
GPS/GNSS data from an aging constellation of satellites. Unlike a monitored
ILS navigation aid, GPS approaches have
no means of providing a real-time alarm,
flag or alert for signal loss, interference, or
satellite clock drift. This is where LAAS
and WAAS come in to provide the alerting
and correction for these potential sources
of error, and providing lower instrument
minimums than raw GPS alone.
Each sector of our industry incubates
and shares different technologies, techniques and procedures. While RNP began
with the airlines, it has quickly spread to
corporate aviation and the military. Like
RNP, GBAS also has its origins in the airlines, and now this technology is spreading.
The military version of GBAS is called joint
precision approach landing system (JPALS),
which will allow the military to use GPS/
GNSS augmentation to perform precision
landings anywhere and at any time. The U.S.
military requires the capability at airfields,
aboard ships or temporary ad-hoc locations.
Many former Army pilots can probably
recall the “TAC NDB” approaches. Imagine
possessing the all weather capability to operate Category I instrument approaches day or
night, even when surrounded by terrain.
While both LAAS and WAAS provide
signal augmentation, each system uses a
different method to accomplish this task,
and ultimately performs a different service
for the user. WAAS utilizes a network of
ground-based reference stations to detect
minute variations in the GPS/GNSS signal.
This information is routed to master stations and ultimately geostationary WAAS
satellites where the correction is broadcast
to WAAS-enabled receivers.
In contrast, LAAS is focused on providing signal augmentation at a single
location. GBAS provides the correction
with a sole VHF signal, broadcast on or
near the local airport site. In comparison to
WAAS, GBAS can provide an accuracy of
one meter or better and provides auto-land
capability for Category I/II/III. Combined
with RNP, GBAS can support curved path
multi mode approaches. These curved
path approaches can be flown in a fusion of
RNP and GBAS, with RNP supporting the
curved path portion of the approach while
GBAS provides the necessary accuracy to
support Category I/II/III minimums.
Pilots flying a GBAS approach will
require minimal training, since GLS
approaches appear the same as legacy ILS
approaches. Unlike a costly ILS system, one
GBAS station can support 26 runway independent approaches, supplied in a digital
broadcast on a single VHF frequency. Without RNP, GBAS can also provide curved
path approaches and missed approaches as
another feature called terminal area path, or
TAPs. Since this technology is runway independent, it can be installed above ground,
even on the roof of a building. GBAS stations
are installed and operational in Sydney, Australia, Malaga, Spain, Frankfurt and Bremen,
Germany, Guam, and later this year, Newark,
N.J. In Australia, GBAS-equipped operators
such as Capt. Alex Passerini are enthusiastic
about the results in their Boeing 737 and
Airbus A380 fleets, and see much potential
for GBAS helicopter operations as well. A
single GBAS station can provide a multitude
of approaches to heliports, offshore oil platforms, and independent precision helicopter
approaches at larger airports. This will
provide unprecedented all-weather access in
congested urban airspace, hospitals, conventional heliports and challenging locations
with terrain or obstacle constraints. While
the GBAS technology began with airlines,
the potential for helicopters is huge.
