IN FOCUS: Round two for Satnav to the ground

United Airlines, Honeywell and the US Federal Aviation Administration are plotting a slow, but deliberate, return-to-test of a GPS-based precision landing system that was foiled by unexpected intruders - GPS jammers - during a trial in late 2009. The plan then was to use the system for Continental Airlines operations at Newark international airport. Now, flight testing of a revamped Honeywell-built ground-based augmentation system (GBAS) - paid for by the Port Authority of New York and New Jersey - could begin using specially equipped United aircraft as soon as July.

A second GBAS is being moved from a test site in Memphis to Houston's George Bush Intercontinental airport, where United aircraft will be able to use the system for Category 1 instrument approaches in the spring. Once Newark is operational, United will be able to use the approaches on the Newark-Houston city pair. The work is part of a broader push by FAA officials to move away from traditional ground-based instrument landing systems (ILS) with their costly infrastructure, and towards satellite-based systems.

They will do this via a combination of vertical guidance precision (LPV) approaches, which use wide area augmentation system-enabled GPS airborne receivers and no ground infrastructure, aside from runway lighting systems, for Category 1-type precision approaches - 200ft (60m) decision height - to ground-based augmentation systems (GBAS) with multi-mode airborne receivers designed to provide Category 3-type guidance to pilots - 50ft decision height down to 0ft with autoland.

© AirTeamImages United Airlines is to conduct GBAS tests at Houston's George Bush Intercontinental

GBAS could be ready for Category 3 operations by 2016, says the FAA. While there are more than 2,700 LPV approaches in operation at domestic US airports, no GBAS approaches are up and running there despite equipment approvals going back almost a decade.

GUIDANCE SIGNALS

The beauty of GBAS, compared with ILS, is that one system of four ground-based receivers can service all runway ends at an airport, whereas an ILS is ideally targeted to one runway end. The system uses four antennas for redundancy and to reduce the impact of signal noise through averaging. GBAS transmits differential GPS guidance signals to the aircraft on the 108-118MHz frequency, the same band used by the ILS. In 2003, the FAA approved funding to purchase and roll out the approach systems, previously called local area augmentation systems, for Category 1 approaches as an alternative to the ILS. The introduction would be a stepping stone to achieving Category 3 performance with the system. However, technical issues with the integrity of the navigation solution, including the potential impact of space weather disturbances such as sunbursts, sent the system back to the lab.

© Air Team Images GPS-based precision landing systems lie ahead

Although technical issues had been solved for the Category 1 system, rolling out the system was not in the FAA's plans. Honeywell, however, decided to seek non-federal approval of its SLS-4000 GBAS ground system on its own, a hurdle the company vaulted in September 2009. The Port Authority bought a system from Honeywell for Newark, set to be the first operational GBAS in the USA when installation testing started in November 2009. Honeywell has also sold GBAS units to the FAA, Brazil, Spain, Australia and Germany.

After activating the newly approved SLS-4000 GBAS ground station at Newark in 2009, engineers quickly found the system flagged up a problem with the ambient GPS environment and shut down, derailing operational approval. While interference events were happening only a few times a day, an outage of even once a day "is totally unacceptable", says John Warburton, navigation branch manager at the FAA's advanced concepts and technology division in Atlantic City, New Jersey.

An investigation later found GPS jammers - AKA personal privacy devices (PPDs) - in cars and trucks on Interstate 95 just to the east of the airport were interfering with the GPS signals. The issue of illegal jammers aside, the installation at Newark was more vulnerable to the problem because limited real estate required the four ground-station receivers to be located near the highway.

A second Honeywell GBAS the FAA had been testing in Memphis did not suffer similar interference problems, partly because the airport was larger and the receiver quad farther away from any roads.

Solving the Newark interference problem involved looking at "the whole range of mitigations possible to make GBAS more resistant to the illegal radiation", says Warburton. From May to October 2010, the FAA led a team to develop a series of GBAS siting and software upgrades to provide a more robust system.

For the installation aspects, the FAA investigated spreading the receivers further apart to stop them all being affected by PPD interference at the same time. It also looked at raising or lowering the height of certain receivers and considered putting a "big block wall" between receivers and the highway. The agency has settled on a final configuration. This must be bought off by the Port Authority, which is going to have to pay for the changes.

ROBUSTNESS UPGRADE

Honeywell is now completing a new system design approval process for software modifications in the SLS-4000. A "Notices to Airmen" alert prohibits aircraft from using the GBAS approaches at Newark until a new interference "robustness upgrade" for the ground stations is complete, expected this year. The efforts may not be bulletproof. Warburton says operational outages could still occur with "very strong" PPDs. On the other hand, he points out, stronger PPDs will be easier to track down and be seized by authorities.

"Robustness" - regarding the upgraded systems - is a relative term. While vertical guidance accuracy would almost always be less than a metre (3.3ft) - to match the equivalent safety level of a traditional ILS - GBAS guidance errors of more than plus or minus 10m vertical or 40m lateral at the decision height must be shown to have a probability no more than once in 10 million approaches. For Category 3 approaches, the integrity-based error limits are 100 times tighter from 50ft down to the runway surface.

The FAA decided to move the Memphis GBAS to Houston, partly because FedEx shelved its fleet of Boeing 727s in the wake of high fuel costs. The carrier planned to equip the gas-guzzling trijets with GBAS-capable multi-mode receivers (MMRs), to test the GBAS approaches. At Houston, in addition to supporting some FAA closely-spaced parallel runway operations testing, the equipment could be used by the same portion of the United fleet that is equipped to receive the signals at Newark. GBAS-capable MMRs are built by Honeywell and Rockwell Collins.

SINGLE FREQUENCY

In terms of GBAS for Category 3, the FAA has largely scaled back its efforts to research mode, after determining last year that equipping airports with the system was "unaffordable" until at least 2014.

Ongoing contracts with Honeywell include efforts to upgrade the SLS-4000 to comply with international GBAS approach service types requirements for single-frequency GPS, for Category 3 approaches.

In addition to determining whether the Category 1 algorithms can be extended to Category 3 approaches, Honeywell will also test the equipment to see how it fares with Newark-like interference.

Meanwhile, the FAA spectrum office is working with the Federal Communications Commission, the Department of Homeland Security, and others to rid Newark's airport perimeter of PPDs, which is about all Warburton is willing to share. "I can't provide details, but the FAA has installed, and is testing, some prototype [radio frequency interference] detection equipment."

Source: Flight International