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Landing Aids: Zeroing in on GPS

2 September 2010




When Afriqiyah Flight 771 crashed short of the Tripoli runway in May, it killed all but one of the 104 people onboard. Dr Gareth Evans takes a closer look at the ensuing debate over existing landing aids and the wider potential of GPS.


The discussion in the wake of the tragic air disaster was largely inevitable; as initial thoughts turned to the possibility of human error or aircraft malfunction, speculation arose – and some of it from pilots familiar with the approach – that the airport's outmoded system might well have played a significant part in the tragedy.

Tripoli International Airport lacks an instrument landing system; runway 09 – the in-bound Flight 771's intended target – has non-precision VHF omnidirectional radio (VOR) and non-directional beacon (NDB) approaches available, but as the following notice to airmen (NoTAM) excerpt makes clear, their reliability is open to question. "Interference from signal being experienced during current airport development work (VOR should be used with caution)." Anecdotal evidence puts it even more prosaically; it seems instrument readings change every time the construction workers move their cranes.

Approaching decision height, the crew of the Afriqiyah Airlines Airbus A330-200, would have needed to make a visual check to make sure of their line – and at that time and on that heading, the rising sun would have been shining directly into their eyes.

Whatever the real cause of what happened next, only nine-year-old Ruben van Assouw – somehow miraculously flung clear by the ensuing explosion – survived.

"Instrument landing systems (ILS) fundamentally depend on an even older innovation – radar."

Changing times

Much of the navigational technology currently in use has a long history; VOR – VHF omnidirectional radio range – for example, is around 60 years old. It first arose from the earlier visual-aural range (VAR), and the development of solid state units in the early years of the 1960s saw it become established as the dominant radio navigation system by the end of the decade. Instrument landing systems (ILS) fundamentally depend on an even older innovation – radar – and it is said to have been first used in 1938 to allow a Pennsylvania Central Airlines Boeing 247-D to touch down safely in a Pittsburgh snowstorm.

26 years later, in March 1964, the first automatic ILS landing took place at the UK's Bedford Airport, leading to radar-based navigation and landing systems remaining as state-of-the-art for the next half century. All that, however, may be about to change with the first GPS-powered landing of an Airbus A380 – Qantas flagship Nancy-Bird Walton – at Sydney in January 2009.

Augmenting GPS

Since its inception, GPS has expanded exponentially to become centrally embedded across a wide range of technologies, devices and applications, but there is one key problem with extending satellite geospatial navigation to aircraft – and specifically to systems designed to land them. That is the inherent margin of error, which according to some estimates, can be 30ft or more for a rapidly descending aeroplane. Making GPS work under these circumstances calls for the input from the orbiting constellation of positioning satellites to be supplemented and enhanced from the ground.

SmartPath, the ground-based augmentation system (GBAS) from Honeywell Aerospace, has been designed to do exactly that, and the growing roll-call of airlines and airports taking up the technology seems to provide a pretty clear endorsement of its potential.

At the system's heart lie four GPS antennae, which are located around the airport site, and some impressive software. Taking a raw feed – with an intrinsic accuracy of 10m – proprietary algorithms collect, collate and correct the positional information from all the aircraft in a 20-mile envelope, producing a finished data set that is accurate to within 1m.

"The first GPS-powered landing of an Airbus A380 –Qantas flagship Nancy-Bird Walton – took place at Sydney in January 2009."

SmartPath then communicates via VHF with onboard multimode receivers, which are able to assimilate ILS radar and the aircraft's own GPS in addition to the GBAS data – providing the appropriate positional information for each individual plane, the system interaction also enabling continuous in-flight updates.

With a pilot-friendly interface, there are said to be few noticeable differences between the user's experience of SmartPath and conventional instrument landings, which means a relatively short training programme is required for proficiency.

The system may look and feel much the same, but there is, however, one very significant underlying difference.

Instead of the single approach provided by current ILS, GBAS offers airports and pilots 26 options – some, for example, to allow for fuel-saving continuous descents, while others help reduce noise by detouring around residential areas – and the benefits do not stop there.

Cutting waste

Estimates suggest that around 11% of the fuel a plane burns, between the inherent inefficiencies of conventional straight line routes and tarmac delays, is effectively wasted. As David Melcher, senior VP of ITT, arguably Honeywell's main competitor in the field, puts it, "there are numerous benefits to moving to a GPS-based system, including fuel savings, lower carbon-dioxide emissions, and better-controlled ascents and descents."

GBAS should allow taxiing aircraft to remain close to the end of their allocated runways, saving valuable minutes, and fuel, over the day and by allowing planes to fly closer together safely in all prevailing weathers, more efficient use can be made of the skies.

Perhaps more importantly still, GPS could enable airports themselves to run at their optimum: under current arrangements capacity can drop by as much as a quarter in bad weather.

By simply landing four additional flights an hour at Newark Liberty in adverse conditions, Continental Airlines calculate that ground delays could be halved, slashing up to 45 minutes off the time spent waiting at the gate.

"GPS could enable airports to run at their optimum."

Lighting the way

SmartPath became the first GBAS to gain system design approval from the Federal Aviation Administration in September 2009, and it is currently in operation in 25 airports around the globe, with the company forecasting a rise to 600 within ten years.

However, for the rest of the world's airports, at least until GBAS becomes universal runway lighting remains one of the most effective and versatile landing aids of all – though sadly even the best lights money can buy are of little use when the sun really is in your eyes.

The preliminary investigation into the Afriqiyah Airline's crash reported no evidence of a technical malfunction, medical emergency or fuel shortage, and no assistance was requested by the crew in the final minutes of their ill-fated flight. Until the matter is finally resolved, the speculation and debate over the cause will undoubtedly continue, and like the earlier Smolensk crash, which wiped out virtually all the Polish elite, and with which it shares some parallels, Flight 771 will feed a host of rival theories. Whether or not construction work and unreliable radio beacons are ultimately found to be to blame, one thing seems clear; it is getting harder and harder to resist the lure of GPS.

Afriqiyah Airlines Airbus A330-200 (5A-ONG); the aircraft was to crash as Flight 771 on final approach to Tripoli, leading some to speculate over the role played by the airport's reportedly unreliable VOR.
Qantas Airbus A380 (VH-OQA) – Nancy-Bird Walton – taking off from London Heathrow; in January 2009, this aircraft was the first Airbus to make a GPS-powered landing.
A 3D representation of the GPS constellation of satellites. Ground-based augmentation systems (GBAS) aim to enhance the raw feed to unprecedented levels of accuracy for air traffic use.
Qantas is one of a number of airlines actively pursuing the SmartPath GBAS route.
Image showing the wide area augmentation system, the Federal Aviation Authority's space-based augmentation system (SBAS). It shows the general location of the 38 reference stations, the two master stations and the four ground uplink stations.
Effective runway lighting remains an important aid; these high intensity lights at Palanga, Lithuania, were installed in 2007.