In the worst visibility conditions, the operation of an ILS (Instrument Landing System) requires the utmost care to guarantee the safety of the provided service. This induces important air traffic capacity limitations. GIFT project has studied Ground Based Augmentation Systems (GBAS) configurations, considered as one of the more promising solutions to overcome ILS limitations.
The objectives of the GIFT project were the assessment of high level requirements and the conceptual design, simulation and evaluation of a combined GBAS/ABAS configuration for the achievement of CAT II/III landing operations.
For GBAS CAT I, all performance requirements have been defined by analogy with CATI ILS and agreed at ICAO level. For CAT III, GIFT has assessed that the derivation of the GBAS requirements using the same ILS-LOOK alike methodology as for CAT I, leads to a situation in which the performance can not be reached with sufficient availability, if a GPS L1 satellite navigation GBAS configuration is considered.
For long term, GIFT project has assessed that a GBAS configuration using GALILEO will provide a solution fixing these two issues:
- Continuity risk due to unexpected loss of the GBAS Signal in Space (SIS). This risks leads to both geometrical availability restrictions and to very stringent constraint on the GBAS hardware subsystem design
- Integrity risk not detected by the GBAS ground subsystem in charge of SIS monitoring that could be experienced in abnormal Ionospheric situation.
For shorter term, GIFT has investigated the benefits of the use of Inertial combined with a GPS L1 GBAS configuration, implementing the concept on the current airborne and ground architectures with minimum modifications .
Two different types of hybridisation algorithms using corrected code-range measurement (BGA : Basic GIFT Algorithm) or both code and carrier range corrected measurements (AGA : advanced GIFT algorithm) have been studied. It has been assessed that both algorithms can fix the continuity issue, and that the AGA could also fix the integrity issue. For CAT III high level performance requirements derived from ILS analogy, availability of GIFT could be around 99.9%, instead of 98.5% availability for a reasonably feasible standalone GBAS configuration.
Statistical evaluation of Touch Down position for a Single Aisle family Airbus aircraft using a GIFT navigation system simulator has been demonstrated through simulation including normal and limit tests cases defined in accordance with JAA Autoland certification methodology.
If the GIFT concept answers technically to the limitations of GBAS, economical and institutional limitations have been identified:
- the certification demonstration effort for the aircraft manufacturer will cost more as soon as some of the guidance in the most critical landing condition may rely only on the aircraft INS performance;
- service should be provided only to the aircraft equipped with INS and equivalent algorithms;
- in some conditions, the service provider is no longer responsible for the signals used for guidance during the approach and landing phase.
This transfer of responsibility from the ATC to the airborne users raises institutional questions.