SESAR members -AENA, Airbus, DFS, DSNA, Eurocontrol, Honeywell, Indra Navia and Thales - have recently completed the next stage of flight trials for the development and deployment of CAT II/III GBAS-controlled landings for business aircraft. In this article, Jolana Dvorska, ATM project manager at Honeywell Aerospace’s Advanced Technology Europe division and member of one of the SESAR Project teams which carried out the validation exercise, recaps the progress so far and outlines what’s left to do on the road to satellite-based precision approach and landings for Europe.

While Ground Based Augmentation System (GBAS) is rightly considered to be an advanced, “cutting edge” technology, in Europe we have in fact been flying commercial GBAS-enabled CAT I approaches since 2012. That is why SESAR is taking the system a step further by conducting flight trials and research into GBAS CAT II/III approaches that enable the aircraft to land in even lower visibility.

Before going into more detail on the projects themselves, let us remind ourselves what GBAS is and how it works. The most common way of flying a precision approach today is by using an Instrument Landing System, or ILS. This system is effectively made up of two main components - the glideslope, which provides vertical guidance, and the localiser, which provides lateral guidance. These components use separate antennae and together they project analogue radio beams that on board the aircraft can be used to present one fixed 3D approach path for one end of a single runway. Aircraft when suitably separated can simply line up, acquire the signal and fly the fixed approach.

ILS has been in mainstream use since the 1960s, and its design principles go back even further. Consequently it is little surprise that for many airports, especially those experiencing high traffic volumes, the limitations to operational capabilities when using an ILS are restricting growth, as well as cost and environmental savings. But it is not just airport and airspace capacity that is an issue. ILS is costly to maintain to CATII/III operations standards and has strict operational limitations as it is easily affected by interference from buildings, terrain and even taxiing aircraft near the runway. As a result, airports that are expanding their infrastructure, requiring taxiway capacity growth, or in need of greater landing assurance in all weathers are now looking for an alternative to ILS.

This is where GBAS comes in. GBAS takes Global Navigation Satellite System (GNSS) signals, augments them to make them very accurate with high integrity for precision approaches, and relays the data to approaching aircraft via a digital VHF signal. A GBAS is comprised of at least four GNSS reference receivers, GBAS computing equipment also located on the airfield and the VHF broadcast transmitter. Because the computing facility knows the precise location of the GBAS reference receivers, it can calculate corrections and integrity data then broadcast them back up to the approaching aircraft together with the approach information.

The benefits are numerous. Firstly because the GBAS broadcast is digital, it isn’t subject to the same interference as the ILS signal is, thereby providing a vastly greater level of information integrity and landing assurance. Secondly, because you only need one GBAS station per airfield, there are operational cost savings too. While a single ILS can provide approaches for one end of one runway, a GBAS can provide 26 or more separate approaches across the airfield within a certain distance. Finally, GBAS offers ATC greater flexibility too. To adjust an ILS so that it broadcasts a new approach, you have to manually re-align the hardware on the airfield and then fly a series of calibration approaches. With GBAS, each approach is simply a software load. This means ATC can publish and aircraft can fly complex approaches such as those utilising Performance-Based Navigation to fly around challenging terrain, as well as approaches allowing for noise reduction in highly populated areas surrounding airports.

For more than a decade, SESAR members such as Honeywell, Thales and Indra have been working on the development of GBAS technology, with systems already deployed at airports around the world. Combined with this, most modern aircraft rolling off production lines today now feature GPS Landing System (GLS) equipage, the avionics required for GBAS-controlled landings, either as an option or as standard. With both the airborne and  ground station technology now readily available, the next stage is to validate GBAS approaches at minima lower than the current certification of CATI -- a primary objective of SESAR.

Within SESAR, projects are validating GBAS for use in Europe’s future ATM system at CAT II/III minima. The aim of  CAT II/III is to provide a level of safety when landing in low visibility conditions, equivalent to that of 'normal' operating conditions. CAT II is a precision approach and landing with decision height lower than 60m (200ft) but not less than 30m (100ft), while a CAT III approach will have a decision height of between 30m (100ft) and 0. This decision height is the altitude at which the pilot must choose to abort the landing if he or she does not have visual contact with the runway, or complete the landing if the runway is in sight. CAT I on the other hand requires a minimum decision height of 200ft, the level to which GBAS is certified today.

On 25 September 2013, SESAR projects (9.12/15.3.6) reached a major milestone - a first CAT III approach enabled solely by GBAS. Since then we have been analysing the data gathered from this flight and the 55 other subsequent flights we made last year, and enhancing the airborne GLS receiver’s functionality as well as supporting baseline ICAO Standards and Recommended Practices (SARPS) validation.

June and July 2014 saw the team embark on their next round of successful validation flights, testing the Honeywell airborne VIDL-G for CAT II/III avionics receiver prototype with ground stations at Frankfurt (Indra), Toulouse (Thales), Bremen, Houston and Atlantic City (Honeywell). Over 90 flights were completed using a mix of satellite guidance and autopilot, focusing on further data collection with the business aircraft airborne receiver and ground stations in the final stages of prototype development for ICAO SARPS validation.

On 29 July Florian Guillermet, Executive Director of the SESAR Undertaking, joined the team at Frankfurt to observe the trial work taking place, and spent time reviewing the test programme status and examining their latest results. Commenting on the day, Mr Guillermet said; “SESAR is focused on developing and implementing better performing ATM systems across Europe, guaranteeing the highest degree of safety, efficiency, capacity and operation in all conditions, and this is exactly where satellite navigation comes in. From a passenger perspective it’s just like any other flight -- you would not notice the difference with GBAS. The benefit is that the flight becomes more predictable in all conditions, bringing substantial benefits to the airlines and to the operation of the air transport system in Europe.”

Following this latest round of trials, the team has paved the way for CATII/III GBAS operation. Flight test recordings will be used by Honeywell and Eurocontrol engineers to assess that the performance achieved by the GBAS system is compliant with the draft requirements. Similar results are expected from the mainline aircraft flight tests, as well as ground stations, and overall system validation. So far the results are positive and assuming that standardisation and regulation progress as planned, the entry into service of GBAS Cat II/III can now be expected in the 2018-2019 timeframe.