A Happy Landing For LAAS?

By By William Reynish | January 1, 2007
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Starting in the early 1990s, FAA launched a development program to produce a GPS-based replacement for the aging Instrument Landing System (ILS). The goal — to provide routine and precise satellite guidance to aircraft landing in low-visibility conditions — eluded aviation experts for many years. It is only now being attained with the GPS Local Area Augmentation System (LAAS).

Why has this taken so long, when GPS reportedly provides centimeter accuracies? For such results, researchers record thousands of GPS positions at a known fixed point for lengthy periods of time and then process, analyze and average the data, removing various anomalies, such as less than optimum satellite "geometry," local GPS signal interference or other random effects. If the results aren’t quite what were hoped for, the experiment can be repeated.

Making a landing approach isn’t like that. The aircraft is moving quickly toward the runway, getting lower, and there is no option of postponing things until tomorrow. Pilots want to know that a guidance system will bring them safely and accurately to the runway with an extremely high degree of confidence. That confidence is usually described as the guidance system’s integrity. For the least demanding, Category I application, where an aircraft breaks out from the overcast at 200 feet above the ground, LAAS certification rules say the probability of guidance failure must be not greater than one in 10 million, or 1x10-7. What’s more, if there is a failure, the pilot must be warned within six seconds of its occurrence. For the more demanding Cat II and Cat III situations, when the ceiling and visibility gradually lower to almost zero, the integrity numbers get tighter and tighter, the reason GPS-guided approaches have been such a difficult nut to crack.

The basic concept behind LAAS was fairly straightforward. Several high precision GPS signal monitors are spaced on or around an airport at accurately surveyed positions, from which they each record the incoming satellite signals. Their data is continuously fed to a central processing unit, where the individual data sets are compared against the "ground truth" of their respective locations. The differences between the received data and the ground truth represent the GPS position error at that specific instant. The calculated composite position error derived from the monitors then is continuously transmitted by VHF data link to approaching aircraft, where it is automatically applied to the aircraft’s GPS receiver and produces very high accuracy guidance to the runway. The pilot’s steering guidance cues are identical to those from ILS and use the same cockpit display. As an additional benefit, the accuracy corrections can be broadcast from the ground via an omni directional data link antenna, which then provides equal quality precision guidance to both ends of all airport runways, unlike an ILS installation, which only serves one end of one runway.

And the system worked, often with exceptional results. But it was beset by the same random anomalies that scientists experienced in their work, the difference being that in aviation, the data couldn’t be taken back to the laboratory for detailed analysis and interpretation. The LAAS processor had to decide whether the data is good within three seconds of the six-second time window it had to alert aircraft. That was the key challenge for the several companies developing LAAS ground stations.

In 2001, FAA solicited bids from industry to transition its previous work into a phased development program leading to an initial Cat I LAAS production run. Honeywell won the contract, but was unable to achieve FAA’s specified integrity requirements. The contract was canceled in 2004, and the LAAS program was relegated to a research and development activity, supported at a much lower funding level.

At around the same time, an FAA official told an industry meeting "there was no business case" for LAAS Cat I, since the system’s benefits were insufficient to justify the costs of either an FAA or a user switch away from ILS.

While cancellation of the contract was a setback for Honeywell, the company believed the integrity problems could be overcome and continued development work with its own funding and R&D support from FAA. The project subsequently expanded to include cooperative activities with Australian, German and Spanish aviation authorities, plus delivery giant FedEx, which saw the system’s benefits at some of its less well-equipped overseas stations. (The International Civil Aviation Organization refers to LAAS as the Ground Based Augmentation System, or GBAS.)

Today, prototype Honeywell LAAS/GBAS installations are being tested in Sydney; Bremen, Germany; Malaga, Spain; and Memphis, Tenn. FAA funding is limited to the work at Memphis, FedEx’s hub.

"All integrity and performance issues associated with safe and reliable Cat I GBAS operations have been addressed. It’s now a matter of working through the system design approval process with the FAA," Bill Corwin, Honeywell’s LAAS/GBAS program manager told Avionics.

Honeywell also adjusted its marketing approach. Under the original FAA contract, LAAS units had to be built to strict agency specifications, which often put the equipment financially out of reach for most other customers. Honeywell’s continuing program aimed at meeting the less rigorous FAR Part 171 specification for non-federal navigation facilities. Both specifications call for the same quality of guidance signals to pilots, with the difference being a dramatic reduction in non-essential system refinements and "gold plated" manufacturing processes. Honeywell expects to receive FAA type acceptance of its Memphis prototype as a "provably safe design" in 2007, after which the company expects to complete its Part 171 qualification work using the Australian system, with a target date of December 2008. The Memphis, Bremen and Malaga systems will subsequently be Part 171 certified by FAA, followed by German and Spanish certifications, respectively.

With certification achieved, Honeywell intends to accelerate its already ongoing international GBAS sales program. Corwin sees a promising market for Cat I GBAS, particularly in developing nations now considering upgrades to their less well-equipped airports. He feels the continuing worldwide expansion of satellite positioning applications can only improve the market for GBAS.

Outside of North America, Honeywell has a marketing agreement with Airservices Australia, that country’s privatized air-traffic control provider. Airservices already has a large number of aviation support contracts in Asia and Africa, and has made significant investments in the Australian GBAS evaluation and certification program. Australia’s Qantas airline has nine of its newest Boeing 737NGs equipped to use the Sydney system, with more aircraft installations planned.

Keith McPherson, Airservices Global Navigation Satellite System (GNSS) manager, said GBAS will be essential at Sydney when Qantas starts operating the 20 Airbus A380s it has on order, since the guidance beams of the airport’s ILS installations risk being distorted when aircraft that large move around its confined layout. This effect was dramatically demonstrated several years ago at London’s Gatwick airport when, on a bright sunny day, a Swissair MD-80 crew decided to fly a practice, hands-off automatic landing. At around 200 feet on final approach, a 737 which had landed ahead of them turned off the runway and its vertical stabilizer deflected the ILS localizer beam. The MD-80’s autopilot immediately put the aircraft into a steep bank to regain the beam, and only quick action by the crew prevented a tragedy.

Interestingly, Airservices developed a complementary system to GBAS, which it calls the GNSS Regional Augmentation System (GRAS). Essentially, this can be thought of as a sort of upside down version of the Wide Area Augmentation System (WAAS). With WAAS, a widely spaced network of GPS ground monitors continually send their received data to a processing center, which then sends a composite error correction to a geostationary satellite over the equator. The satellite, in turn, rebroadcasts the corrections down to users over a wide area.

But Australia already had a nationwide GPS monitoring network, plus an equally extensive network of unmanned VHF transceiver stations. Consequently, in view of the high costs of the three geostationary satellites that would be required to cover the country, Airservices opted to process the data from its GPS monitor network and send GPS corrections to the ground-based VHF stations, which broadcast them to overflying aircraft, instead of having them transmitted down from a satellite. A valuable side benefit is that the dedicated VHF receiver used to receive the GRAS corrections and deliver them to the onboard GPS receiver can do the same with LAAS signals.

Competition from WAAS

With technical challenges behind it and certification nearing, LAAS would appear to have a promising future, needing only to show airport managers its benefits compared to ILS. But a competing landing guidance system appeared from an unlikely source. Last July, FAA said after extensive signal monitoring and flight tests in the United States and Europe, WAAS had demonstrated equivalent performance to a Cat I ILS, and, by extension, a Cat I LAAS. This was an unexpected turn of events, particularly as FAA had, on two previous occasions, announced its intention to evaluate WAAS for Cat I, but had not proceeded.

But LAAS/GBAS has two powerful proponents. Boeing and Airbus see the system as a natural complement to their Required Navigation Performance (RNP) initiatives, which employ basic, unaugmented GPS (to be supplemented by Europe’s Galileo system in the future) tightly coupled with advanced flight management and inertial systems. For somewhat unclear reasons, both airframers reject WAAS in their RNP packages. But both are anxious to see LAAS/GBAS developed further to meet the much more demanding Cat II and III requirements, enabling seamless transition to full auto land operations.

Honeywell also is interested in the further development of LAAS/GBAS, although it would involve substantial additional technical effort and a much more arduous certification process, along with significant additional investment. Corwin expressed confidence that if the company decided to aim for the more stringent requirements, they could be achieved.

Will LAAS return to FAA’s list of future programs? A government brochure published in early 2006 described LAAS as "FAA’s Future GPS Approach and Landing System." But the agency’s currently tight, cost-control environment, coupled with the financial imperatives of defining and creating the Next Generation Air Transportation System, offer little likelihood of additional LAAS development funds over the short term.

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