The accuracy of the GBAS
service may support future safety enhancements such as a high-quality
electronic taxi map display for pilot use in bad weather. This could
help reduce runway incursion incidents and facilitate airport movements
in low visibility. The service also may support automated systems
for runway incursion detection or alerting.
As important as the accuracy
of the GBAS service is the integrity monitoring provided by the
GBAS facility. Any specific level of RNP operation within GBAS coverage
should be more available because the user receivers no longer will
require redundant satellites for satellite failure detection (e.g.,
Receiver Autonomous Integrity Monitoring).
Because the GBAS PS is
optional for ground stations under the ICAO standards, some ground
stations may only provide PT 1 service. The messages uplinked through
the VDB data link indicate whether or not the ground station supports
the GBAS PS and specify the level of service for each approach for
which a channel number has been assigned. When the GBAS PS is provided,
a specific five-digit channel number is assigned to allow selection
of a non-approach-specific GBAS PS from that station. Consequently,
the channel selection process allows different users to select different
approaches and levels of service.
The GBAS PS and the PT
1 service are not exclusive. If the ground station provides the
GBAS PS, selecting a channel number associated with any particular
approach automatically will enable the GBAS PS service. The receiver
provides corrected PVT information to intended airplane systems
as long as the GBAS PS is enabled. ILS-like deviations also are
available when the airplane is close enough to the selected approach
ICAO is continuing development
of a specification for service levels that would support Category
II and III approaches.
BENEFITS OF THE GLS
From the user perspective,
the GBAS service can offer significantly better performance than
an ILS. The guidance signal has much less noise because there are
no beam bends caused by reflective interference (from buildings
and vehicles). However, the real value of the GLS is the promise
of additional or improved capabilities that the ILS cannot provide.
For example, the GLS can
approach and takeoff guidance service to multiple runways through
a single GBAS facility.
- Optimize runway use
by reducing the size of critical protection areas for approach
and takeoff operations compared with those needed for ILS.
- Provide more flexible
taxiway or hold line placement choices.
- Simplify runway protection
- Provide more efficient
airplane separation or spacing standards for air traffic service
- Provide takeoff and
departure guidance with a single GBAS facility.
From the service provider
perspective, the GBAS can potentially provide several significant
advantages over the ILS. First, significant cost savings may be
realized because a single system may be able to support all runways
at an airport. With the ILS, each runway served requires an ILS
and a frequency assignment for that ILS, which can be difficult
because of the limited numbers of available frequencies. Operational
constraints often occur with the ILS when an Air Traffic Service
provider needs to switch a commonly used ILS frequency to serve
a different runway direction. This is not an issue with the GBAS
because ample channels are available for assignment to each approach.
In addition, because the GBAS serves all runway ends with a single
VHF frequency, the limited navigation frequency spectrum is used
much more efficiently. In fact, a GBAS may even be able to support
a significant level of instrument approach and departure operations
at other nearby airports.
The siting of GBAS ground
stations is considerably simpler than for the ILS because GBAS service
accuracy is not degraded by any radio frequency propagation effects
in the VHF band. Unlike the ILS, which requires level ground and
clear areas on the runway, the siting of a GBAS VHF transmitter
can be more flexible than ILS. The removal of the requirement to
provide a large flat area in front of the ILS glideslope alone can
represent a very significant savings in site preparation cost and
opens up many more locations for low-minima instrument approach
Although GBAS accuracy
can be affected by multipath interference, careful siting of GBAS
receivers can readily eliminate multipath concerns because GBAS
receivers do not need to be placed near a runway in a specific geometry,
as is the case with the ILS or MLS. Hence, this virtually eliminates
the requirements for critical protection areas or restricted areas
to protect against signal interference on runways and nearby taxiways.
Finally, the GBAS should
have less frequent and less costly flight inspection requirements
than the ILS because the role of flight inspection for GBAS is different.
Traditional flight inspection, if needed at all, primarily would
apply only during the initial installation and ground station commissioning.
This flight inspection would verify the suitability of the various
approach path (FAS) definitions and ensure that the GBAS-to-runway
geometry definitions are correct. Because verifying the coverage
of the VDB data link principally is a continuity of service issue
rather than an accuracy or integrity issue, it typically would not
require periodic inspection.
GBAS systems capable
of supporting Category II and III operations internationally are
envisioned. Airborne system elements that would be necessary for
the enhanced GLS capability (e.g., MMR and GLS automatic landing
provisions) already are well on the way to certification or operational
authorization. Airborne systems and flight deck displays eventually
will evolve to take full advantage of the linear characteristic
of the GLS over the angular aspects of the ILS.
To date, flight-test
and operational experience with the GLS has been excellent. Many
GLS-guided approaches and landings have been conducted successfully
at a variety of airports and under various runway conditions.
Both automatic landings
and landings using head-up displays have been accomplished safely
through landing rollout, in both routine and nonnormal conditions.
On the pilots flight
displays, the GLS has been unusually steady and smooth when compared
with the current ILS systems even when critical areas necessary
for the ILS approaches were unprotected during the GLS approaches.
The Boeing Technology
Demonstrator program has used a 737-900 to demonstrate successful
GLS operations to airline customers, airplane and avionics manufacturers,
airport authorities, Air Traffic Service providers, and regulatory
The GLS represents a
mature capability ready for widespread operational implementation.
When implemented, the GLS will improve safety, increase capacity,
and provide operational benefits to airlines, pilots, passengers,
airports, and Air Traffic Service providers. Boeing plans to certify
the airborne aspects of the GLS on the 737 by the end of 2003 to
support Category I operations, with other models to follow. Work
is continuing for the airborne certification of the GLS to support
Category II and III operations when suitable GBAS ground facilities
are specified and made available.