Airport interest in satellite-based precision approaches is growing, as the potential benefits become evident.
Graham Warwick/ATLANTA
IN 1995, THE INTERNATIONAL aviation community, granted a stay of execution, to the venerable instrument-landing-system (ILS), while paving the way for its eventual replacement, by the global-positioning system (GPS). Since that decision, two things appear to have happened: the urgency to replace all-weather Category III ILS with a GPS-based landing system has lessened; and interest shown in the use of differential GPS (DGPS) for reduced-visibility Cat 1 operations has increased.
Airports are interested because local-area DGPS promises at least Cat I capability for considerably less cost than that of today's ILS. Whereas an ILS serves only one runway end, one DGPS ground station, for one-third of the cost, can provide precision-approach capability at every runway within a 40-60km (20-30nm) radius.
If some airports cannot afford an ILS, others cannot install the system, because its guidance signals are affected by terrain, buildings or, increasingly, interference from local radio stations. A DGPS ground station is simpler to install, although care must be taken to eliminate multipath effects caused by satellite signals reflecting off buildings and terrain.
To promote the use of the GPS for precision approach, performance standards for a Special Category I (SCAT-1) local-area DGPS were published by the US Radio Technical Commission for Aeronautics (RTCA) in August 1993. RTCA document DO-217 sets out the design requirements for the DGPS ground station, as well as defining the datalink to be used and the avionics required.
Whereas Cat 1 ground and airborne equipment can be certificated separately, and are guaranteed to be interoperable, the SCAT-I requires each combination of ground station and avionics to be certificated. The "private-use" limitation on the SCAT-1 restricts its use to aircraft operators, which have gained approval for their avionics and procedures when combined with the particular DGPS ground station installed by the airport.
The DO-217 describes a fail-passive DGPS ground station with separate master and integrity channels. Vertical accuracy is required to be 1-2m (3-6ft) and the system must be able to detect and warn of an integrity failure within 3s. GPS error corrections are transmitted to the aircraft via a datalink operating in the frequency band used by the existing VHF omni-range (VOR) navigation aid.
The RTCA standards allow for use of the datalink, to transmit way-points, defining the approach path. The US Federal Aviation Administration's SCAT-1 specification, although based on the DO-217 document, expressly excludes use of the datalink to up-link approach way-points, however. This requires aircraft to carry a database of "pseudo" approaches.
Industry is still tackling the issue of how to create and store the safety-critical approach database. Development of avionics compatible with the SCAT-1 was running an estimated six months behind that of the ground stations, but delays in certificating the ground equipment may have closed that gap.
HIGH-PROFILE HONEYWELL
Honeywell has been the most aggressive so far in promoting local-area DGPS, and has claimed some high-profile successes. The company is installing a SCAT-1 system at Minneapolis/St Paul International Airport in Minnesota. A similar system is to be installed at New York's Newark International Airport. Internationally, Honeywell has sold systems in Australia and Canada, and has a memorandum of understanding to supply equipment to the Philippines.
Honeywell's Business and Commuter Aviation Systems division teamed with Canada's Pelorus Navigation Systems in 1994 to develop the Satellite Landing System (SLS). Two systems have been launched, the fail-passive SLS-1000 and the fail-operational SLS-2000. Both are SCAT-1 systems with growth capability to Cat II. Honeywell's established GPS-receiver supplier, Canadian Marconi, is developing the SCAT-1 ground and airborne DGPS receivers.
The Minneapolis project is the "pathfinder" for Honeywell's DGPS programme, says Hall Pierce, manager of business development for navigation systems. The airport authority has purchased a fully redundant SLS-2000. This consists of three remote GPS receivers and a reference station with four DGPS processors and two datalink receiver/processors.
The remote receivers, located up to 100m (330ft) away from the reference station, provide three independent GPS measurements, to provide redundancy and to detect and eliminate multi-path effects, says Pierce. At Minneapolis, the SLS-2000 will be located in a car park, with the reference station on top of an elevator shaft and the GPS receivers mast-mounted on the parking deck.
Certification of the system was scheduled for January, but installation of the equipment is now expected to begin in April, he says. The latest target date for certification is June. Once operational, the system will provide SCAT-1 precision-approach capability at Minneapolis' three runways - and at the St Paul general-aviation airport 15km away.
While Minneapolis has ILS on all its runways, the airport sees several potential benefits of DGPS, Pierce says. Parallel DGPS approaches will be possible to two runways, 1,040m apart, which are too close together for simultaneous ILS approaches, because of signal overlap. The airport also wants to introduce 4° approaches, rather than the standard 3° ILS glide-slope, to reduce noise. At St Paul, local-area DGPS would replace a planned ILS, while one runway, not suitable for an ILS because of the location of an antenna farm, would receive a DGPS-based curved approach, he says.
Initially, SCAT-1 approaches at Minneapolis will overlay existing ILS procedures, but new approaches are now being designed, says Pierce. Northwest Airlines is expected to announce soon plans to equip an aircraft to use the Minneapolis DGPS. As Northwest serves both Newark and Regina, Saskatchewan - where an SLS-1000 is being installed - the airline will provide testing "at both ends", according to Pierce.
Continental Airlines, meanwhile, plans to equip a McDonnell Douglas MD-83 with Honeywell's FMZ-2000 flight-management system, VL-2000 VHF datalink receiver and dual DGPS receivers for flight trials at Newark. The SLS-2000 installation on top of Terminal C at Newark, is expected to be certificated by the end of the second quarter.
Internationally, installations are under way at Regina in Canada and at Armidale in New South Wales, Australia. Saskatchewan purchased five systems in 1995 for $1.5 million, for installation at Regina and four other airports in the province. Pierce says that an SLS-2000 will be installed at Moscow's Zhukovsky Airfield in August for use in a Russian programme to establish certification and operational procedures for local-area DGPS.
The memorandum of understanding with the Philippines, announced at the Asian Aerospace '96 show in Singapore earlier in February, covers the installation of SLS-2000 ground-stations at three airports. Installation of the first system is scheduled for the third quarter, the company says.
MORE PLAYERS
At least four other companies are developing DGPS ground stations, although without Honeywell's high profile. Most expect to certificate their SCAT-1 systems during the third quarter of 1996.
ILS manufacturer Airport Systems International has teamed with GPS-receiver producer Interstate Electronics to pursue local-area DGPS. Airport Systems has a contract to install its Model 8000 SCAT-1 ground station at Lugano, Switzerland, for flight tests involving a Crossair Saab 2000 equipped with an Interstate IEC-9001 GPS navigation and landing system.
Interstate says, that the new Saab 2000, is scheduled to be delivered to Crossair in late March, enabling the delayed flight trials to get under way. Earlier flights revealed that GPS signals were being jammed unintentionally by a transmitter over the border in Italy. Interstate says it has proposed a solution to the jamming, which does not affect either the DGPS ground station or the VHF datalink.
The Lugano tests are intended to collect data on DGPS performance for comparison with earlier microwave-landing-system trials. They could lead to Swiss certification of a SCAT-1 ground-station at the Alpine airport, which is unsuitable for ILS installations because of the terrain.
Collins DASA Avionics Systems is a joint venture between Rockwell-Collins and Daimler-Benz Aerospace (DASA), formed to pursue the market for, among other things, local-area DGPS. The venture plans to have a SCAT-1 ground station available in the third quarter as an interim step towards a Cat II/III local-area augmentation system.
A prototype ground station has been installed at Munich Airport in Germany for a programme to gather data on DGPS approaches. The system is integrated with air-traffic control, but is not used for landings. Instead, aircraft are flown on ILS approaches and DGPS data are gathered for comparative purposes. Other ground-station prototypes are being used in various German and European DGPS test programmes, Collins says.
A major demonstration is planned in Xi'an, China, in mid-March, in which two Cessna Citations equipped with Collins AvSat satellite-based avionics will make precision approaches using a DASA ground station.
E-Systems' Montek division expects to certificate its DIAS-3000 SCAT-1 ground station in August, says director of marketing Jeff Brabender. The company has funded production of an initial seven units, two of which have been sold to the US military for an unspecified purpose. The company has an agreement with ARINC on joint development and marketing of SCAT-1 local-area DGPS and the team plans to install a ground station at Alliance Airport in Fort Worth, Texas, in March/April. This may be offered as a test-bed for interoperability testing, he says.
ILS manufacturer Wilcox Electric has been less aggressive in promoting the SCAT-1 than the other manufacturers, placing its emphasis on developing Cat II/III local-area DGPS, but the company expects to certificate its DGLS-2000 ground-station. Wilcox continues to manufacture an ILS, and, in 1995, won a $475 million FAA contract to develop the wide-area augmentation system (WAAS). This is intended to improve the availability, integrity and accuracy of the GPS over the USA, with the aim of providing Cat 1 capability.
While SCAT-1 standards are established, specifications for Cat II/III local-area DGPS have yet to be defined. In 1995, under an FAA feasibility programme, both E-Systems and Wilcox demonstrated that Cat III accuracies could be achieved using different GPS-receiver technologies. Also in 1995, four DGPS teams participated in a Boeing Cat III automatic-landing evaluation - Collins/ DASA, Honeywell/Pelorus, Litton/Wilcox and Interstate/Airport Systems. Again, Cat III accuracies were achieved.
Boeing is continuing simulation work in an effort to develop certification standards for a Cat II/III GPS landing-system (GLS), while the FAA is expected to award contracts within the next two months to begin development of a Cat II/III local-area augmentation system.
ACTIVE MARKET
Only a year ago, the market for the SCAT-1 ground station seemed doubtful. Now, manufacturers report significant market activity, with a substantial number of requests for proposals "on the streets". E-Systems' Brabender predicts industry-wide sales of 12-15 systems this year, and expects the market "...to be on its feet" by 1998.
Most of the activity is international, despite some confusion within the industry over whether the International Civil Aviation Organisation recognises the SCAT-1 specification. The fact is that no other standard exists, and the number of sales, and pending sales, suggests that the SCAT-1 will become a de facto international standard.
Led by the Philippines, at least half a dozen countries in the Far East are actively pursuing local-area DGPS, plus at least three in the Middle East and Africa, and four in Latin America. Honeywell will be demonstrating its system at March's FIDAE '96 show in Santiago, Chile, says Pierce, who nevertheless expects the next few sales to be in the Asia-Pacific region.
There is interest in the USA, as evinced by Minneapolis and Newark, and it appears to be spurred by a growing recognition of the likely limitations of the WAAS. According to Wilcox, the WAAS is scheduled to become operational early 1998, at which time it will enable "primary-means" use of the GPS for non-precision approaches and "supplemental-means" use to back up ILS Cat 1 approaches. Approval of WAAS/GPS as the primary means of guidance for Cat 1 approaches is not scheduled until completion of the system in 2001. Even then, it is possible that Cat 1 WAAS/ GPS will not be available at some times, in some areas, because of terrain and satellite geometry.
International equivalents of the WAAS can be expected to become operational only some years after the US system, hence the growing worldwide interest in a near-term Cat 1 capability using local-area DGPS. Airbus, for one, plans to certificate a Cat 1 DGPS capability on its aircraft by mid-1997, using a multi-mode ILS/GLS receiver. Using an A340 and Thomson/Wilcox and portable Sextant ground stations, the manufacturer has conducted local-area DGPS demonstrations in France, China and Africa
SCAT-1's "private-use" limitation does not appear to be a major hindrance to the system's acceptance internationally, although the requirement to certificate both ground and airborne equipment jointly is likely to slow its implementation. The SCAT-1 provides the same landing minima as Cat 1 and the eventual approval of the ground stations for full Cat 1 use is expected.
CAT 1 TO COME
Pierce believes that Cat 1 approval will come once the FAA has gained sufficient experience with SCAT-1 systems. Brabender says that Cat 1 approval is likely to require the development of a certification standard for the avionics which will ensure that any airborne unit will work with any ground station. This will allow certification of the ground and air elements to be decoupled, as is now the case with the ILS.
The RTCA is presently revising document DO-217 to "tweak" the SCAT-1 standards to improve interoperability. According to Pierce, Honeywell invited all the equipment manufacturers to an interoperability conference on 20 February, in a bid to ensure that SCAT-1 ground and airborne systems now under development will end up being interoperable.
Brabender believes that the major barrier to SCAT-1 sales remains the lack of certificated hardware. Most manufacturers appear to have been overoptimistic in their predictions, expecting the first certifications by the end of 1995. Now, the third quarter of 1996 seems more likely. All companies report that they have begun the certification process with the FAA - but the regulatory authority, although accustomed to approving ILS, is breaking new ground in certificating the software-intensive local-area DGPS.
Source: Flight International
