ATM Modernization

Precision Approaches

By By Callan James | November 1, 2011
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During the course of the last 30 years, precision approach landing systems have undergone a series of name changes, technological advancements and system setbacks. However, the concept of using a dedicated network of precision GPS monitor receivers to guide aircraft into an airport remains a priority for air navigation services providers worldwide.

It’s often useful, as we look at where we are today, to recall where we were before we got here. And that’s certainly true with GPS Local Area Augmentation System (LAAS).

The late-1980s arrival of GPS into the civil aviation marketplace brought with it unprecedented navigation accuracy and, with it, increasing interest in its potential application to precision approaches. The thought that the satellite system could one day replace not only easy targets like VORs and DMEs, but ILS as well, was pretty heady stuff. So the search was on for the GPS technology that would do it, with the one impediment at the time being Defense Department’s Selective Availability (SA) function that added a random 100-meter accuracy “wander” to the civil GPS signals to — theoretically — deter adversaries from using them to guide missiles at us. But development commenced, on the assumption that Department of Defense would eventually turn off SA.

That finally happened in 2000, after which the development of precision approach guidance techniques rapidly accelerated. FAA guided those efforts along two separate tracks: one to provide improved en route performance, eventually leading to ILS-equivalent GPS Category 1 (Cat 1) precision approach capability, and the other to create a satellite-based replacement for the agency’s Cat II and III instrument landing systems. The en route/Cat 1 project became the Wide Area Augmentation System (WAAS) and the Cat II/III project became the LAAS. Generally, Cat 1 permits descents to 200 feet above ground (AGL), Cat II allows descent to 100 feet AGL and Cat III broadly covers descents to “below 100 feet,” depending on several conditions. Today, most current large airline aircraft are Cat III capable.

Similar, But Different

In very broad terms, the basic concepts of the two systems are similar — each uses a dedicated network of precision GPS monitor receivers dispersed at separate, accurately surveyed locations. In both cases each individual monitor’s incoming “raw” GPS signals are compared with their precisely known positions to determine the actual errors and, ultimately, the correction that should be applied to a user’s receiver at a given location. Only then do the similarities between LAAS and WAAS diverge, due to the way the corrections reach the user’s receivers. In WAAS, a large number of monitors across North America pass their data to a satellite ground station that then continuously loft the corrections up to a geostationary satellite. In turn, the satellite re-transmits them down to all receivers within view, which automatically applies them to their own raw GPS data. In LAAS, which usually employs just four local monitors, the corrections are computed locally and transmitted over a VHF data link to aircraft in the local airspace where, again, the corrections are applied automatically. In both cases, however, the two systems offer the unique capability — assuming obstacle clearance and other airport conditions are met — of being able to provide precision approach service to all an airport’s runways. Furthermore, the LAAS ground station and its supporting monitoring receivers and antennas can be located well away from the runway areas, while WAAS needs neither airport ground equipment nor special avionics.

Both WAAS and LAAS went on to successful certification, with WAAS subsequently achieving major marketing penetration. But LAAS, which met much more demanding certification standards in 2009, has had a harder struggle to win market acceptance. Both also had name changes, following FAA’s adoption of ICAO’s terminology for the two systems. WAAS became the Satellite Based Augmentation System (SBAS), and LAAS became the Ground Based Augmentation System (GBAS).

Well before that, however, GBAS had won strong support from a diverse group of users including Qantas, FedEx and Air Berlin, all of which either had unique approach needs, or anticipated having them in the future. All three built up their GBAS business cases from extensive flight evaluation programs using, in FAA terms, “demonstrably safe” development ground systems from Honeywell, located in Sydney; Memphis, Tenn.; Bremen, Germany; and Malaga, Spain, along with a test systems for Boeing at Moses Lake, Wash., and another at the FAA Technical Center at Atlantic City, N.J. In 2008, the Port of New York and New Jersey purchased and installed a Honeywell ground system at Newark Liberty Airport initially for Continental Airlines. To further evaluate airline “city pair” GBAS applications, the Memphis installation is being moved to Continental’s hub at Houston. Further plans call for evaluation systems to be installed at Rio de Janiero, Brazil and Seoul, South Korea.

On the avionics side, most installations have used “demonstrably safe” Rockwell Collins GBAS receiver modules embedded in multi-mode receivers (MMR), plus VHF data link units, with the complete GBAS package referred to as a GNSS Landing System (GLS). Recently, however, a FAA development contract for future production airborne and ground equipment was awarded to Honeywell under a competitive procurement, while Boeing has announced that all production 737s will carry provisioning for GBAS. Qantas, too, has equipped its 737 aircraft with GLS, and plans the same for its Airbus A380.

The Joint Precision Approach and Landing System (JPALS) is a variant of the civil GBAS for the U.S. military, primarily for U.S. Navy aircraft carriers and helicopter-equipped vessels. JPALS has a number of features, including advanced data link capabilities and specialized avionics, not offered on civil systems.

It will, however, be completely compatible with civil GLS avionics that will be carried by U.S. Air Force aircraft, such as the C-17, to ensure full interoperability with land-based JPALS installations. In 2008, a Raytheon team, including Rockwell Collins and Northrop Grumman, was awarded a JPALS ground station development contract.

SCAT-1: Special Category 1 (SCAT-1) was the nomenclature given to an early GPS precision landing guidance system, prior to the development of requirements for today’s GBAS. Yet while regarded as obsolete, and operationally incompatible with GBAS, SCAT-1 today illustrates a nice “back-to-the-future” scenario.

In 2007, facing the need for reliable operations into some of Norway’s challenging coastal airports that were ringed with high terrain, Wideroe Airlines and Northrop Grumman subsidiary and SCAT-1 developer Park Air cooperated in bringing a system back into service. With the approval of Norway’s regulators and Avinor, the country’s air navigator services provider, the first system was installed at the Bronnoysund airport, 500 miles north of Oslo, and the site of a fatal 1988 accident to one of Wideroe’s aircraft while performing a non-precision approach. Based on SCAT-1’s successful re-introduction at Bronnoysund, Avinor has subsequently installed SCAT-1 systems at 20 remote Norwegian airports with difficult approaches. But because of SCAT-1’s non-conformity, all Wideroe’s fleet of Dash-8s are equipped with specially modified Universal Avionics SCAT-1 receivers.

GBAS Market

Some U.S. observers have expressed doubt about the market for a Cat-1 GPS-based landing guidance system in a nation where more than 1,500 public airports already offer that capability with ILS. It’s a fair statement, except that outside the United States and Western Europe, no other area in the world offers the same density of ILS installations. As well, neither FAA nor the European ANSPs intend to replace ILS Cat-1 with GBAS equivalents, since current thinking suggests Cat-1 SBAS will gradually take over that role as ILS Cat 1 installations are progressively withdrawn. Essentially, GBAS Cat 1 is regarded primarily by FAA and by European authorities as the necessary foundation from which to develop Cat II and III systems. (While SBAS can also support Cat 1, it cannot be further developed to meet Cat II and III criteria.) Yet there is a real need of those higher categories within the next five to 10 years, particularly in Europe, where several of the Cat III ILS installations are reportedly facing increased levels of interference from other emitters breaking into the frequency band.

On the other hand, FAA is now facing two problems in moving ahead with its Cat II/III development. The first is money, where the current fiscal climate forced the agency’s planned acquisition and procurement activity to be placed on hold earlier this year. As a result, FAA’s Cat II/III program moved from Washington headquarters to the Atlantic City Technical Center where it continues as an R&D program, with the expectation of a four- or five-year delay in procurements.

But procurement funding is a familiar challenge in all government work, and one that has frequently delayed FAA plans in the past, calling for short-term work-around plans until things return to normal.

However, FAA’s second GBAS problem is a new, and more troubling, one. In 2010, the Newark system experienced occasional random shutdowns, for which technicians could find no obvious cause. One minute the system would be working perfectly, and the next minute it would shut itself down, with no evidence of a technical failure.

Intensive investigation during several months revealed the cause of the problem: external jamming of the GPS signals from a quite unexpected source. GPS jamming had always been considered a possible threat from terrorists or pranksters, but the culprits this time turned out to be truck drivers along the nearby New Jersey Turnpike. The reason for the truckers’ use of jammers came from the installation of automatic GPS tracking devices in tens of thousands of commercial vehicles to allow company dispatchers to monitor their locations for scheduling and security purposes. To many truckers, of course, this was unwanted eavesdropping and, albeit illegal. Small jammers powered from vehicle cigarette lighters, soon entered the marketplace. It was recently estimated that by the end of 2009, more than 100,000 of these devices had been sold in the United States.

As a result, while the Newark GBAS installation was expected to be declared operational for Cat 1 operations last year, it can now only be used for non-instrument approaches until a solution to possible jamming has been found. Honeywell advised Avionics Magazine in August that it had earlier submitted a solution to FAA, but was not prepared to expand on that information. Somewhat bizarrely, however, this starts to resemble the cat-and-mouse military measures, counter-measures and counter-counter measures, as each side progressively increases its ability to overcome its adversary’s moves. Unfortunately, GPS is extremely vulnerable to jamming, so it will be a continuing concern for aviation, perhaps already causing some officials to reconsider total reliance on satellites for the air traffic management systems of the future.

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