Since the 1960s, airports around Europe have relied on Instrument Landing Systems (ILS) for approach and landing guidance. The ILS is effectively two sub-systems - a localizer to provide lateral guidance and a glide slope antenna for vertical guidance.

Although ILS has proven itself as a reliable and functional system for many years, the availability of advanced satellite navigation systems provides the possibility to implement a system that overcomes some of ILS’ challenges, and to meet the more demanding needs of the future. For example:

• Runway capacity continues to be limiting factor in bad weather conditions due to the need to protect ILS ground stations to prevent signal perturbation
• Multi-runway hubs installation and maintenance costs can be high
• The lack of radio frequency spectrum availability means that ILS installation has limitations in some areas
• Terrain can also have an adverse impact – if the terrain is sloping or uneven, reflections can create an uneven glide slope.

To overcome these challenges, SESAR is validating the use of Augmentation Systems (Satellite based-SBAS as well as Ground based-GBAS for precision approach) to increase the accessibility in airports, as an alternative to ILS or where ILS is not a viable or economical solution.

GBAS augments Global Satellite Navigation Systems (GNSS) for use in precision approach and landing guidance. Infrastructure typically includes up to four GNSS reference receivers, a GBAS computing facility and a VHF data broadcast transmitter on the airfield, alongside GBAS avionics installed on the aircraft. GBAS ground systems receive positioning data from GNSS satellites, compute error corrections and satellite heath information and broadcast the necessary data out to all GPS Landing System (GLS)-equipped aircraft transitioning from en-route to terminal airspace. With a single GBAS able to manage complex simultaneous approaches across multiple runways, cost savings, reduced delays, increased airport capacity, greater ATC flexibility and improved safety can all be realised.

Under the umbrella of the SESAR Programme, two ground manufacturers, Thales and INDRA-NAVIA, and two airborne manufacturers, Thales Avionics and Honeywell, are developing GBAS CAT II/III ground and airborne equipment. Against this background, the Ground equipment was installed at two airports -Toulouse and Frankfurt - for flight testing and system validation to CAT II/III (GAST D).

In September 2013, SESAR’s partners (HONEYWELL, INDRA-NAVIA, AIRBUS, THALES, DFS, DSNA and EUROCONTROL) achieved an important milestone by carrying out flight tests at Frankfurt and Toulouse Airports. The flight tests, which were carried out by Honeywell with a Falcon 900EX, demonstrated that Cat II/III performances are achievable through the GASTD system based on a unique GNSS constellation and mono frequency signal.

The key objective of these validation exercises was not simply to see whether the landing was accurate enough but was also designed to test the interoperability of each component. Over the past 10 months the project partners have been working together to ensure that the CAT III avionics receiver prototypes integrate seamlessly with the prototypes for ground stations and airport air traffic control infrastructure. A range of different conditions and scenarios were tested during the trials, including multipath evaluations, full scale deviations and CATIII to II regression testing and back, in order prove the validity of GBAS.

The interoperability of the airborne receiver performed exactly as it was designed to, seamlessly integrating with the different ground stations at Frankfurt and Toulouse. While this initial test was conducted using a business jet, other flight tests are also being prepared using commercial aircraft, making this an important leap towards the end goal of revenue-generating GBAS flights in Europe.

The full validation cycle of the system and procedures is not yet completed and further testing of the current trial systems at Frankfurt and Toulouse is scheduled for mid-2014. SESAR is also exploring the potential for a future extension of the project for multi-constellation multi-frequency GBAS, which would lead to an even more robust system suitable for use virtually anywhere in the world as a replacement for existing ILS. It is also expected to extend activities to validate enhanced functions supporting curved approaches and displaced runway threshold capabilities.

Despite the work still left to do, the results of these initial flight tests have proved that GBAS is a viable alternative to ILS, bringing additional benefits including lower maintenance costs, easier installation paths, reduced tarmac and approach congestion and a greater range of complex approaches to European airports.