Business & GA, Commercial, Military

Multimode Receivers: More Demand, More Capability

By Charlotte Adams | April 1, 2002
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Multimode recievers (MMRs), the high-integrity, multifunction navigation units installed on many air transport-class airplanes today, are expanding their scope. Driving this expansion are military aviation needs and the evolution of the National Airspace System (NAS).

MMRs started with the instrument landing system (ILS) function and gradually incorporated GPS en-route functions. Rockwell Collins now is adding microwave landing system (MLS) MMRs to U.S. Air Force tanker/transports and is leading the way to GPS landing system (GLS) performance. (GLS, as embodied in the U.S. Local Area Augmentation System [LAAS] program, eventually will enable Cat II and Cat III landing services.) These functions can be packaged in a standard enclosure the size of pre-MMR, standalone ILS boxes.

MMR Evolution

MMRs evolved from the need to maintain Cat IIIB ILS yet capture the potential of sat nav and MLS in a high-integrity environment. Today, in the civil air transport world, MMRs are regarded as the aircraft’s primary position, velocity and time reference. While initial installations of en-route GPS were not part of the MMR, a consensus rapidly developed to integrate GPS into the MMR because of the multimode receiver’s flight-critical environment and connections to displays and flight controls.

As part of the Global Air Traffic Management (GATM) program, Collins will be adding MMRs with ILS, VOR, marker beacon, MLS and GPS/GLS for U.S. Air Force KC-135s. Collins’ MLS module is fully monitored to support Cat IIIB operations. The KC-135s will use the same Collins GLS technology that was certified on a FedEx Express Boeing 727-200 last year. Collins also is equipping RC-135s with VOR/ILS MMRs and C-5s with VOR, ILS, marker beacon and MLS functions.

A final FAA technical standard order (TSO) for airborne equipment is expected this summer. On the other hand, LAAS ground stations–the underpinning of U.S. civil air transport GLS performance–have yet to be commissioned. A Federal Aviation Administration (FAA) request for offer is expected this spring, leading to a contract by the end of 2002 for the initial purchase of 10 development systems and 56 production LAAS ground systems. FAA plans to commission these contracted, "federal" facilities, starting in December 2004. "Non-federal" systems will be procured by airports that are not scheduled to receive FAA units. The "non-federal" ground systems, for which demand has yet to be determined, will be approved in a separate "type acceptance" process While Collins is the acknowledged leader in GLS, Honeywell hopes to catch up, thanks to the lag time in TSO delivery and ground station commissioning. "A lot of [MMR] selections were based on a commitment to upgrade radios for DGPS [differential GPS], thus enabling GLS, as we get the changes," says Paul Healy, Honeywell’s manager of air transport MMR programs. The LAAS gaps "are going to work in our favor." The aerospace firm expects "red label," pre-production boxes by mid-2003 and plans to certify GLS-capable, production MMRs, starting in 2004. Honeywell and Thales are working on both the air and the ground segments of GLS.

Collins in the Lead

Collins provided MMRs for Boeing 757 aircraft delivered to Finnair in December 1997. They were perhaps the first MMRs in the civil air transport sector. Thales began selling its TLS755 MMR (with ILS and GPS) to launch customer, United Airlines, in roughly the same time period. All told, MMR sales break down as follows:

  • Collins, the leader, says it has sold approximately 10,000 MMRs, more than 7,000 of which have been delivered;

  • Honeywell says it has sold about 3,000 MMRs and delivered 1,600 units; and

  • Thales says it’s won more than 3,000 MMR orders from 40 airlines.

"Every airplane coming off the line at Airbus and Boeing has an MMR on it," says James Kendrick, Collins’ marketing manager for precision landing programs.

Military manufacturer BAE Systems also has delivered more than 300 MMRs for U.S. C-17, P-3C, A-10 and F-16 aircraft, as well as for Italian air force C-130Js.

Collins’ MMRs are certified on all Boeing and Airbus production aircraft except the B717, which is seller-furnished equipment, Kendrick says. Collins MMRs also are approved on the MD-80, A300-B2 and -B4, A300-600, A310, Boeing Business Jet and the Fokker 100. Thales is certified on all modern Airbus and Boeing jetliners, says Karine Perret, marketing manager for air transport avionics. Honeywell is certified on all current Boeing aircraft, with approval expected on A320-series airplanes and A330s/340s by October 2002.

But Thales may be the first to have ushered in MMRs. Rafale fighters began using Thales (then Sextant Avionique) multimode receivers in 1992, according to Perret. Sextant had presented the MMR concept to various standards bodies many years before that, she says.

BAE Systems claims, in 1995, to have demonstrated the first "true" multimode landing system receiver–with ILS, MLS and DGPS functions–to another military customer. This work led to the development of the AN/ARN-155 precision landing system receiver (PLSR), first installed on U.S. C-17s. BAE Systems offers MMRs with ILS only and with ILS, VOR and Mil-Std-1553B data bus interfaces. Other BAE variants include ILS/VOR/1553B plus MLS, or GPS, or both MLS and GPS. The company has demonstrated LAAS operations with both VHF and C-band data links.

Race to GLS

The next big step for mainstream MMRs will be precision landing capabilities using LAAS ground stations and GLS-enabled MMRs. A Collins MMR with GLS (GNLU-930) has been certified, under interim RTCA standards, on a FedEx Express 727-200 under the FAA government-industry partnership (GIP), in collaboration with Honeywell, the Memphis, Tenn., airport authority and FAA. The aircraft has been operated under special operational approvals, and planned improvements include migration to final RTCA and ICAO (International Civil Aviation Organization) standards for Cat I, II and III, according to FedEx.

Although FAA’s TSO for GLS airborne systems has not been finalized, Kendrick says Collins "will need to update some items, but certification should not be an issue." Most of the remaining changes expected in the LAAS system will concern the ground stations, he says.

Since last January, the FedEx aircraft has flown many precision approaches, Kendrick says. The 727-200 has flown against a Honeywell LAAS ground station and Raytheon-built, military Joint Precision Approach and Landing System (JPALS) ground stations. Boeing also has been performing GLS Cat I landings with its 737 NG testbed aircraft against a Honeywell ground station, using a pre-production Collins MMR at Moses Lake, Wash. Boeing also has tested GLS at Salt Lake City against a Raytheon LAAS ground station to demonstrate interoperability.

Thales has participated in a French development program to validate ICAO’s ground-based augmentation system standards and recommended practices, using a Thales GLS ground station in Toulouse. The ground station is expected to receive a GLS Cat I type certificate in early 2004. The company is working with Airbus on the Airbus GLS/LAAS technical specification.


Although Collins’ MLS module can be installed in any aircraft type, the driver has been military. "The air transport industry has yet to establish clear benefits from equipping," Kendrick says. And the expected ILS degradation at Schiphol Airport in Amsterdam "is materializing slower than projected." Collins expects to begin delivering "black label" (production) MLS units to military customers in the next few months.

Boeing has no MLS program, Thales says, but Thales has an MLS program for civil air transport with Airbus. Thales expected a TSO by March 2002 and plans to certify an MLS MMR on a British Airways’ A320 aircraft by April 2004. This certification then will be extended to the A318, A319 and A321, Perret says. "MLS is the only existing Cat IIIB alternative to ILS."

Schiphol has installed two MLS stations and Heathrow has ordered four, Perrine says. MLS acquisitions also are expected from airports in Toulouse, Munich and Paris (Charles De Gaulle and Orly), as well as from other facilities in the UK, Switzerland, Spain and northern European countries.

Advanced Approaches: Peeling the Onion

Aircraft operators want to get the most from their expected investment in GPS landing system (GLS) avionics making use of the Local Area Augmentation System (LAAS). They want to reduce flight leg lengths and thereby fuel costs. Airport operators want advanced approaches even more for noise abatement. Pilots also want these curved, angled/offset and segmented approaches and the ability to land on closely spaced, parallel runways, as well as the originally envisioned straight-in, "ILS-look-alike" approaches.

LAAS benefits will be substantial. If the technology can provide the accuracy, integrity and continuity required for autoland, it certainly can provide the navigational information for advanced approaches. But the devil is in the details –how the system will be operated and what the division of labor will be between the air and the ground. "We’re just now peeling the onion to figure out the most efficient way to do this," says Ted Urda, LAAS Cat II/III R&D lead with the Federal Aviation Administration (FAA). "What needs to be made clear is that we are breaking new ground with this. Nobody has certified a curved or segmented approach. There are an infinite number of ways to implement this."

One question is, how do you want to use LAAS messages and what is the most efficient way to achieve the desired performance? "Do we want the ground station to transmit other path points for curved or segmented approaches or have the aircraft construct the approaches?" Urda asks. "Standards need to be established and agreed upon between the ground station and avionics folks as to what the operational concept will be and how to implement it."

Now that the basic performance requirements and design characteristics are fairly mature, these implementation issues are being addressed. FAA has started an initiative to determine the viability and schedule for advanced approaches. One of the first items of business will be to meet with groups such as the Air Transport Association, airlines and the air cargo companies "to better define specifically what they are looking for in advanced procedures," says Steve Hodges, FAA’s LAAS product team lead. Then the agency will "determine if LAAS can technologically provide the approaches, how much money they will cost and how long it will take to develop them."

LAAS can support complex approaches in basically two ways, explains Tim Katanik, manager of business development-navigation and landing systems for LAAS ground system developer, Raytheon. Under method No. 1, standardized for straight-in Cat I precision approaches, a ground station broadcasts differential corrections, integrity data and precision path points. The aircraft uses this information to correct its position measurement, verify LAAS data integrity and follow the uplinked flight path down to a landing. The uplinked path points come from a high-integrity database that is verified and locked into the ground station during commissioning flight inspection. Although the flight path message formats are only standardized for straight-in approaches, "the ground station could uplink more complex paths," Katanik says. LAAS message types have been reserved for such purposes. Nonetheless, "if users want 10,000 waypoints, there aren’t enough words," Urda says.

A second method, known as the differentially corrected position service (DCPS), uplinks the same information as the first method, but requires an onboard pathpoint database to navigate, rather than using the uplinked path points for advanced approaches. Aircraft would use uplinked differential corrections and integrity parameters, in combination with airborne GPS position measurements, to determine where they are, but then use an airborne database to determine the desired flight path and navigate until intercepting the final, straight-in segment needed for aircraft stabilization, Katanik explains. At that point the aircraft probably would transition to the uplinked path points to achieve the database integrity levels required for precision approach.

Method 2 requires GPS/RNAV-capable avionics. This method could be used to fly complex initial approach procedures "and then transition to a short precision final approach segment," using the straight-in, standardized method, if necessary to achieve full Cat I (or better) minimums, Katanik says. Methods 1 and 2 also can be viewed as two ends of the spectrum, Urda adds. "The ground station could broadcast a ‘word’ in the message that authorizes a left or right curved approach. The avionics could then construct the curved approach based on the straight-in path points and an agreed-upon standard for calculating the curved position of the approach."

Dwarfing these issues, however, are the operational concerns. How do you integrate the information and display it in the cockpit? How would controllers deal with different advanced approaches coming in rapid succession?

"Right now, aircraft come in lined up on the same path through space," Urda explains. "If odd-numbered aircraft want a left hook approach and even-numbered aircraft, a right-hook approach, how do controllers deal with that, with the spacing?"

However, once LAAS and Wide Area Augmentation System (WAAS) ground stations have been certified and are operational, and corresponding avionics have been installed in a large number of airplanes, "you will see a rapid increase in the number of complex procedures approved," Katanik predicts. "The difficulty lies in the transition period, when controllers have to deal with mixed aircraft equipage and must support the needs of both advanced and conventional users."

Maintaining Integrity

Rockwell Collins’ multimode receivers (MMRs) are "designed to meet 10-8 and 10-9 integrity requirements for flight-critical operations regardless of the mode selected–ILS, MLS, GLS or [en-route] GPS," says James Kendrick, Collins’ marketing manager for precision landing programs. The design also provides GNSS monitoring, which "independently monitors and compares each satellite’s determined range and time against each other," enabling the calculated position, velocity and time to meet these integrity limits. High integrity assures that an undetected error is not present.

Collins’ MMR design also is the first that is fully integrated into the navigation and landing systems, Kendrick maintains. The company has 34 GLS-capable MMRs in the field and has demonstrated deep integration in a FedEx Express installation. On the FedEx 727-200, the MMR "is installed in place of the previous landing system," Kendrick says. That Collins MMR–installed with ILS, VOR, GPS en-route and GLS–was "fully integrated… into the [aircraft] by replacing the ILS/VOR approach and landing system with the [MMR]," Kendrick says. The aircraft was then recertified to VOR and ILS Cat IIIA." Then GLS software was incorporated and the MMR’s GLS precision landing function was "operationally certified."

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