Navigation
1. Satellite-Based Augmentation System (SBAS):
Relying on a network of reference stations placed at different locations, the reference stations measure Global Positioning System (GPS) errors and ionospheric delays and the measured errors are transmitted to a master control station for processing before the processed signal is uplinked to a geosynchronous earth orbit (GEO) satellite. The GEO satellite retransmits the correction signals on the same frequency as the GPS satellites. Since the system is dependent on corrections broadcast by the GEO satellites, the SBAS improvement in navigational accuracy is realized within the GEO satellite footprint. Several implementations of SBAS are currently being developed globally, including the Wide Area Augmentation System (WAAS) in the U.S., the European Geostationary Navigation Overlay System (EGNOS) in the European Union, the MTSAT Satellite-Based Augmentation System (MSAS) in Japan, and the GPS Aided Geo-Augmented Navigation (GAGAN) in India. WAAS, MSAS, and GAGAN have similar capabilities because both Japan’s and India’s SBAS programs are adaptations of the U.S. SBAS technology. The WAAS service area covers North America and parts of South America; the EGNOS service area covers the European area; and the MSAS service area covers most of Asia and the West Pacific. The MSAS and GAGAN systems have the most potential to provide coverage in the Asia and West Pacific region in the coming years. The future SBAS system coverage around the world is shown in Figure 1. SBAS operation is depicted in Figure 2.
Instrument Landing System (ILS) cannot be installed at several airports in the Taipei FIR due to terrain restrictions and obstacles. SBAS requires fewer ground facilities, and can reduce maintenance costs associated with traditional ground based navigation facilities. Using Global Navigation Satellite System (GNSS) and instrument landing procedures can achieve the following advantages:
a. Lower landing minima
b. Better service reliability
c. Safer straight-in approaches at some aerodromes
d. Optimized procedures at aerodromes and airways that improve overall flight service
efficiency, shorten flight time, and reduce airline operations costs
e. Reduced implementation cost and maintenance cost
f. Seamless flights resulting from use of GNSS for en-route, terminal, and approach
phases of flight.
Currently, the available SBAS system in the Asia Pacific area is the MSAS owned by Japan. Japan successfully launched MTSAT-1R on February 26, 2005. The Master Control Stations (MCS), Ground Monitor Stations (GMS), and Monitor and Ranging Stations (MRS) are completely installed, and are currently in normal operation. Japan also successfully launched MTSAT -2 on February 18, 2006, but it has been set to standby mode. MTSAT-2 will be put into normal operational service when the MTSAT-1R stops functioning. The estimated date when MTSAT-2 is expected to go into normal operation is 2010.
Figure 1 The coverage of future SBAS systems around the world
Figure 2 SBAS Operation
2. APEC GNSS Testbed ( APEC GTB )
APEC GTB system concept is shown in Figure 3. The objectives of this program are as follows:
a. Assessing the service capabilities of SBAS in the Asia Pacific Region.
b. Training participating economies to analyze and test GNSS performance capabilities.
c. Collecting GNSS data to analyze the effect of the atmosphere on GNSS in this Region.
d. Training participating economies to understand the maintenance and operational
concepts of future GNSS facilities.
The overall length of the implementation schedule is eight years, beginning in 2004 with evaluation of current ground-based NAVAIDs and landing systems. In October 2005, CAA joined the APEC GTB program. In 2007, after a test report that confirmed the benefits of SBAS, the CAA decided to add a RAIM Prediction System (RPS) to supplement the Navigation Sub-plan. Considering adjacent FIRs’ CNS/ATM implementation plans, the CAA would make a decision on whether to complete the implementation of SBAS in 2011. The CAA is cooperating with APEC and is working with the Institute of Civil Aviation at National Cheng Kung University (NCKU) to implement a set of SBAS TRS for the evaluation of the benefits of implementing SBAS in the future.
Figure 3 APEC GNSS Testbed
3. Ground Based Augmentation System (GBAS):
Conventional ILS systems are designed for only straight-in approaches. There are a number of aerodromes within Taipei FIR that have topographic restrictions that limit the installation of an Instrument Landing System. To improve those aerodromes’ service capability, and to overcome terrain problems, the CAA installed Microwave Landing Systems (MLS) at Taichung and Hualien aerodromes. However, MLS equipment is costly and airlines’ resistance to the purchase of MLS avionics makes it unreasonable to deploy additional MLS facilities. GBAS’s rely on a minimal number of ground facilities, monitor satellite signals and transmit differential signals via data link to aircraft in flight. GBAS reduces its equipment and service maintenance cost below that of traditional NAVAIDs and has the following approach procedure capabilities:
a. Currently supports Category I with migration to future Category II/III navigation services.
b. Advantages of GBAS over ILS include:
● Aerodromes with topographic restrictions on the installation of ILS can use GBAS
instead to provide Category l instrument approach service.
● GBAS uses the same frequency band as ILS.
● GBAS can be used under Instrument Meteorological Conditions (IMC), in critical
areas where ILS is prohibited.
● When the more stringent requirements of ILS category II and III may not be met,
GBAS can be used instead.
c. Enhances the following approach procedure parameters: variable glide path, variable
positions of the Runway Threshold, and abilities of curved approaches.
d. Meets the special requirements and terminal procedures of different aerodromes, such
as curved, offset, multi-segment approaches, multi-segment missed approaches, and
multi-segment departure procedures.
e. Provides guidance in use of PVC/FMS RNAV, which can simplify complicated
maneuvering procedures.
f. Supports ADS-B to provide aerodrome surface surveillance capability.
g. Provides coverage for multiple runways from a single GBAS, reducing the cost of
installation.
h. Reduces equipment and service maintenance costs of ground NAVAIDs.
i. Meets RNP 0.1 capability.
One or more ground-based GBAS will be able to provide ground reference stations 25-30 nautical mile range of service. GBAS operation is depicted in Figure 4. At present there is no mature operational GBAS system in the world. The developing GBAS systems are being developed in America as Local Area Augmentation System (LAAS) and in Australia as Ground Based Regional Augmentation System (GRAS). ROC will monitor the international development of relevant navigation feasibility studies closely and in coordination with the SBAS plan. The CAA has postponed the GBAS implementation in the Taipei FIR to 2011.
Figure 4 GBAS Operation
4. The Ground-Based Regional Augmentation System (GRAS- Alternative):
The GRAS is a new GPS enhancement system currently under development. GRAS differs from SBAS in that GRAS uses VHF Datalink (VDL) to transmit the correction data, while SBAS uses GEO satellites. Australia has successfully completed installation of GRAS Testbed, and GRAS service capability has been extended from en-route to APV approaches. In the Taipei FIR CNS/ATM program, the CAA is considering implementation of GRAS as an alternative to SBAS.
Because the Asia-Pacific region is located near the magnetic equatorial regions, GPS signals are subject to a higher level of ionospheric scintillation than less equatorial latitudes, which causes higher levels of signal transmission delay variations. The SBAS can only be used in APV approaches, where the decision height is 400 to 600 feet. GRAS is expected to achieve APV-II capability, where the decision height is 250 to 300 feet. GRAS can also achieve CAT-I capability, where the decision height is 200 feet. Therefore, although GRAS development is behind that of SBAS, its service capabilities are superior to those of SBAS.
GRAS is currently in the development stage. ICAO SARPs and RTCA Minimum Operational Performance Standards (MOPS) are still under development. The U.S. Company Honeywell has been awarded the software development contract with Australia for GRAS in June 2005, and the system is expected to be commissioned in 2008.
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