Navigation
1. GNSS Augmentation Systems
Global Navigation Satellite System (GNSS) augmentation systems are designed to facilitate the use of GNSS for navigation in all phases of flight. The use of GNSS and augmentation is designed to facilitate precision navigation without the extensive ground-based navigation infrastructure that is usually required. GNSS by itself may not provide the accuracy required for some critical navigation procedures due to issues of clock drift, ephemeris, and ionospheric delay.
Augmentation systems, also known as differential GNSS, are designed to calculate the errors present and broadcast correctional information to aircraft navigation systems thus allowing the aircraft to navigate accurately enough to be able to either fly on GNSS based route structures or carry out GNSS based approach or departure procedures. Augmentation also provides information that allows for greater reliability and availability of GNSS signals.
Augmentation systems can be divided into three categories:
a. SBAS: Satellite-Based Augmentation Systems
b. GBAS/GRAS: Ground-Based Augmentation Systems/Ground-Based Regional
Augmentation Systems.
c. ABAS: Airborne-Based Augmentation Systems
Augmentation systems offers the following advantages:
a. Improved landing minima (Cat 1 for SBAS and CAT I or better for GBAS) at
aerodromes currently equipped with only Non-Precision Approach (NPA) procedures.
b. Better service reliability with greater GNSS signal availability and integrity.
c. Increased consistency and predictability in aircraft tracking through guided approach
and departure procedures and greater accuracy of aircraft navigation.
d. Optimized airways, arrival, and departure procedures that improve overall flight service
efficiency, shorten flight time, consider environmental impacts and reduce airline
operations costs.
e. Reduced implementation and maintenance costs due to the reduced number of ground
navigation facilities required for instrument flight procedures.
2. APEC GNSS Implementation Team, GIT
APEC’s Transportation Working Group formed the GIT in 2002, tasking it to investigate multimodal uses of GNSS that would provide operational and financial benefits in the APEC region. The GIT seeks to identify measures to facilitate regional GNSS implementation as well as provide a public/industry forum to address GNSS-related issues in support of safe, secure, efficient, and financially sound multimodal transportation systems.
The CAA is a member of the GIT (since its establishment in 2002) and actively participates in the group meetings and research programs. The CAA participation in the APEC GIT is a prime method of exchanging information and assessing regional progress in GNSS development. The APEC GNSS Testbed (GTB) is under the APEC GNSS Implementation Team. The APEC GTB computes SBAS correction signals and transmits using VHF digital data transmission instead of via SBAS synchronous geostationary satellites (GEO). The testbed platform is not an operational system as it is without backup and operational hardware. The system consists of the following components:
a. Testbed Reference Station, TRS:
A distributed network of seven TRSs sited in Indonesia, Malaysia, Philippines, Thailand, Vietnam, Australia, and R.O.C. to collect data and translate the GPS signal to the format which is recognized by the Testbed Master Station (TMS) for wide-area collection of GPS data.
b. Testbed Master Station, TMS:
A TMS located in Bangkok for determination of the SBAS correction and integrity data and generation of SBAS messages.
c. Testbed VHF Station, TVS:
A TVS, also located in Bangkok, for broadcasting the SBAS messages over VHF.
d. Testbed User Platform, TUP:
A TUP that performs the VHF reception and SBAS message processing functions of user equipment.
e. Satellite Operation Center:
An operations center, which monitors and displays in real time the navigation capability and the sub-system operating condition.
It is planned to continue transmitting TRS-TAN data to the TMS in Bangkok, and to expand the Phase 1 testbed with a second TRS and a new Testbed Master Station (TMS) in Taiwan. This would complete a Taiwan GNSS Testbed capability without relying on the TMS of AEROTHAI. These efforts are conducted with the assistance of the Institute of Civil Aviation at National Cheng Kung University (NCKU). Due to re-prioritization, the test bed enhancement program was not implemented and was put on hold indefinitely.
3. Satellite-Based Augmentation System, SBAS
SBAS is a wide area augmentation system that uses a network of ground stations to calculate the various errors particular to the region in which the ground station is located. This information is then fed to a central processing station that collates and processes the network data and, through a broadcast station transmits the data to a GEO satellite that re-transmits the error correction information over a wide area for suitably equipped aircraft to receive and use to update their GNSS position accuracy and integrity. SBAS operation is depicted in Figure 1
Figure 1. SBAS Operation
Several implementations of SBAS are currently being developed globally. The MTSAT Satellite-Based Augmentation System (MSAS) was developed by Japan and MSAS coverage are Asia-Pacific, New Zealand, Australia, and West Coast in the USA. The Wide Area Augmentation System (WAAS) was developed by USA and WAAS coverage is whole America Continent. The European Geostationary Navigation Overlay System (EGNOS) and EGNOS coverage are Europe, Asia, and Africa. Those that have the most potential for use within the Taipei FIR are the MSAS in Japan, the Indian GPS Aided Geo-Augmented Navigation (GAGAN), and the proposed Chinese Satellite Navigation Augmentation System (SNAS). The future SBAS system coverage around the world is shown in Figure 2.
Figure 2. The coverage of future SBAS systems around the world
MSAS and GAGAN are based on the U.S. Wide Area Augmentation System (WAAS). The CAA has been closely monitoring the progress of these SBAS systems, particularly MSAS. It is envisaged that in the future each of the different SBAS implementations will be harmonized with aircraft navigation systems to provide a seamless update network.
4. Ground-Based Augmentation System, GBAS
GBAS is an augmentation system designed to facilitate GNSS based precision approach/departure procedures and terminal area operations. GBAS focuses on the airport environment and it has an effective range of around 20-30 NM. GBAS is also known as Local Area Augmentation System (LAAS) in the U.S.
GBAS operates similar to SBAS with regards to GNSS receivers but differs in that the correction signal provided to the aircraft is sent by a ground transmitter station, i.e., there is no satellite used for correction signal broadcast. GBAS operation is depicted in Figure 3.
Figure 3. GBAS Operation
The intention of GBAS is to provide the required level of GNSS accuracy, availability, and integrity to allow an alternative to instrument or microwave landing systems (ILS/MLS). GBAS, along with GNSS, provides additional flexibility via allowing the creation of curved approach and departure procedures.
There are a number of aerodromes within Taipei FIR that have topographic restrictions that limit the installation of an ILS. The CAA has installed Microwave Landing Systems (MLS) at Taichung and Hualien as a landing aid solution at these aerodromes, however MLS equipment is costly and airlines’ resistance to the purchase of MLS avionics makes it unreasonable to deploy additional MLS facilities. To improve affected aerodrome service capability and to overcome terrain problems the CAA is considering the installation of GBAS.
To understand GNSS satellite signal availability, the CAA completed a GNSS Signal Test and Monitoring Report of 17 Aerodromes in Taipei FIR on 30 April 2004. The CAA’s implementation of GBAS will follow APEC GIT’s recommendations to improve the effectiveness of approach service, using GBAS for CAT-I, II, and III precision approach.
In accordance with the GBAS development schedule, the CAA may, implement GBAS CAT-I to provide precision approach. The CAA will co-operate with the modern dual frequency satellites (Galileo and GPS modernization plan) and install Aerodrome Pseudo-Lite (APL), in order to provide CAT-II and III precision approach capabilities in a step-by-step fashion.
GBAS provides greater accuracy and integrity than SBAS or GRAS. GBAS currently supports CAT-I GNSS precision approaches with research into future CAT-II/III navigation services planned. This gives the CAA the ability to provide CAT-1 or better GNSS based approaches at locations without ILS or CAT-II or III at locations with ILS CAT-I. Also aerodromes with topographic restrictions on the installation of ILS can use GBAS instead to provide CAT-I instrument approach services.
GBAS provides coverage for multiple runways from a single transmitter station. This can reduce the cost of NAVAID upgrade or initial installation (compared to alternatives) and can reduce equipment and service maintenance costs of ground NAVAIDs. GBAS has a limited range so can not provide a system wide augmentation system solution for the Taipei FIR. GBAS must be used with wide area augmentation system (SBAS or GRAS) to provide this solution.
GBAS facilitates the special requirements and terminal procedures of different aerodromes via allowing the provision of curve, offset, oblique angle and multi-segment approaches, multi-segment missed approaches and multi-segment departure procedures.
5. Ground-Based Regional Augmentation System, GRAS
GRAS is an alternative form of ground based augmentation designed for application over a wide area. GRAS is designed as an alternative to SBAS enroute and to improve approach services at NPA aerodromes that do not require upgrade to CAT-I or better. GRAS, like GBAS, uses VHF Datalink (VDL) to transmit GNSS correction data instead of GEO satellites. GRAS differs from GBAS though in using multiple transmitter stations for correction data thus providing a wider coverage area. Coverage is dependent on the number of ground stations.
GRAS provides potential benefits over SBAS, particularly for Taiwan, because the region is located near the magnetic equatorial regions. GNSS signals are subject to a higher level of ionospheric scintillation near the equator than at higher latitudes, which causes higher levels of signal transmission delay variations. 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.
GRAS consists of:
a. GRAS Reference Station, GRS:
The GRS collect and monitor GPS signals
b. GRAS Master Station, GMS:
The GMS (at least one station) which processes the signals from GRSs and produces GPS correction and integrity data.
c. GRAS VHF Satation, VGS: `
The VGS receive correction signals and integrity data from the GRAS Master Station, and broadcast the data to aircraft via VHF.
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