Landing Guidance

By William Reynish | June 1, 2008
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Of all the world’s major industries, there must be very few that have the same level of self-imposed standardization of equipment, operating practices and standards as that of aviation. Furthermore, under the guidance of the International Civil Aviation Organization (ICAO), which has no legal power to enforce its standards on nations or individual aircraft operators, the worldwide aviation industry has achieved a level of performance and safety that is probably unmatched in any other comparable endeavor.

Aviation’s constant drive for worldwide standardization has been the key here. And in this area, navigation and air-traffic control have been among the main beneficiaries, allowing aircraft to fly seamlessly from country to country, and continent to continent. Yet in landing guidance, probably the most demanding of all flight operations, standardization is now being reassessed, with the introduction of new technologies and with them, new capabilities.

Today, the international precision landing guidance aid is the Instrument Landing System (ILS). Developed in the 1940s, ILS has matured over the past 60 years to become the world standard, installed at thousands of airports large and small. As well, increasing numbers of ILS installations now support fully automatic "hands-off" autoland approaches to below 50 feet above the runway threshold in visibility, or Runway Visual Ranges (RVR) as low as 600 feet or less.

The differences in the guidance quality of landing systems are expressed as categories, or "CATs." Broadly, the CAT designation indicates to the pilot the combination of ceiling and RVR — commonly called "limits" — that a given system has been certified to support. On an instrument approach, pilots may not descend below the specified decision altitude/height (DA/H) or the reported RVR declared for the specific landing guidance system, unless the runway is clearly in sight. If the runway is not clearly visible at that point, they must execute a missed approach.

Operator-specific waivers, such as for a Head Up Display (HUD), or other considerations, can modify CAT values. As well, operations to CAT IIIa and IIIb limits may only be carried out by certified pilots flying aircraft with appropriately certified onboard systems.

There is not yet a CAT IIIc designation, denoting a true zero ceiling and visibility combination. While today’s autoland systems could perform CAT IIIc operations, there are currently no certified surface guidance systems that would then take the aircraft safely and accurately off the runway and guide it to the terminal.

Replace ILS?

With such remarkable performance, why should we need to replace ILS? The prime reason is that the ILS localizer beam, transmitted in the 112-118 MHz band, is vulnerable to reflections off terrain and manmade structures and, particularly, other aircraft.

The classic example of the latter problem occurred at London’s Gatwick airport many years ago — fortunately under clear skies — when the pilot of a DC-9 decided to practice an autoland approach as a preceding 737 was completing its landing roll. All went well until the 737 turned off the far end of the runway and placed its fin squarely in front of the ILS localizer antenna. This caused a major sideways deflection in the localizer beam, which the DC-9’s autoland system interpreted as the aircraft drifting off course, and responded with a hard bank to regain what appeared to be the ILS centerline. Only quick action by the crew at 600 feet above the ground prevented a tragedy.

Since that time, practice autolands require specific ATC permission, and significantly increased spacing is mandated between aircraft on ILS approaches in low-visibility conditions. As well, aircraft awaiting takeoff clearances are held well back from the ILS reflective areas around the active runways, by the use of "Hold CAT II" or "Hold CAT III" signs on the taxiways.

But these restrictions raise the second reason to eventually replace ILS. The increased bad-weather approach spacing, and the taxiing time spent by a departing aircraft to reach the runway from the "Hold" sign, reduce airport throughput and create delay, which is becoming less and less acceptable as traffic volume increases.

Currently, there are three alternatives to ILS. These are, first, variants of GPS; second, the Microwave Landing System (MLS) and third, the Transponder Landing System (TLS). Note that other than the military system mentioned, the GPS applications are equally achievable by Europe’s Galileo system, Russia’s GLONASS and other regional satellite navigation systems now being developed by China, India and Japan.

Landing Alternatives

  • LAAS. Called the Local Area Augmentation System (LAAS) by FAA, the Ground Based Augmentation (GBAS) by ICAO and GPS Landing System (GLS) by some operators, the concept uses four or five high-accuracy GPS receiver stations spaced around the airport to continuously and very precisely monitor the incoming GPS satellite signals to derive their individual GPS positions. These positions are then compared to the exactly surveyed locations of the receiver stations, to determine the error in the "raw" GPS positions. Any differences between the raw GPS and the surveyed locations is then processed and transmitted via a VHF data link to the aircraft as an accuracy correction. This is applied to the aircraft’s GPS, which then sends the corrected position to the aircraft’s flight management computer, and then to the pilot’s displays.

    Unlike ILS, LAAS/GBAS is immune to reflections and is therefore potentially capable of supporting closer approach spacing and the elimination of taxiway "Hold" signs. However, many years of extensive testing did not completely eliminate issues of signal reliability and overall integrity, causing FAA to withdraw its earlier intention to adopt the system as its future ILS replacement across the National Airspace System (NAS). LAAS manufacturers Honeywell and GM Merc A/S, of Denmark, are now understood to have overcome the earlier issues, and both companies are finalizing the development of FAR 171-compliant "non-federal" variants of FAA’s full LAAS specification, which they expect in 2009 to certify to CAT I standards.

    Honeywell prototypes are under evaluation in Germany, Spain, the United States and Australia, where Qantas 737s have been equipped, while a GM Merc system is installed at Norman, Okla., and used for FAA and United States Air Force tests.

    France’s Thales is planning to upgrade its system at Toulouse, where it is used in the Airbus test program. However, no plans had been announced to further develop the LAAS ground stations to meet CAT II or III standards, reportedly requiring significant investment, although Boeing has investigated hybrid LAAS/IRS/FMS combinations to achieve the same result.

  • SCAT-I. An early development in FAA’s original LAAS project was the basic, low cost, Special CAT I (SCAT-I) system. Although subsequently overtaken by a later, more sophisticated FAA specification, SCAT-I used a similar configuration and provided similar guidance. The equipment has since been reintroduced by Norway’s Normarc, a component of Northrop Grumman subsidiary Park Air Systems, and has been approved by the European Aviation Safety Agency as meeting ICAO CAT I performance requirements. However, SCAT-I is not signal-compatible with LAAS, and requires unique airborne receivers built by Universal Avionics.

    Naviair, the privatized Norwegian Air Navigation Service Provider (ANSP), plans to install 25 Normarc systems at "difficult" airports used by commuter airlines. Typically, these will serve remote coastal and inland settlements, such as the first installation at Bronnoysund, where surrounding high ground would make ILS impractical due to multipath effects.

  • JPALS. At the other end of the spectrum is the Joint Precision Approach and Landing System (JPALS), an advanced LAAS variant to meet the requirements of the U.S. military, and particularly the U.S. Navy. To be built to much more demanding specifications than civil systems, the Navy’s JPALS is intended to provide all weather, fully automatic landing guidance on aircraft carriers to manned and unmanned aircraft, with less sophisticated systems being produced for land installations, compatible with both civil and military GPS signals. Portable tactical variants are also included in the Pentagon’s projected $1 billion program.

    Under separate contracts, more than 13,000 manned and unmanned aircraft are expected to be installed with JPALS-compatible avionics. The JPALS ground station contract award was expected to be announced in May, following competitive bids from Honeywell and Raytheon.

  • WAAS. The fourth GPS system that is starting to attract attention in the landing guidance community is FAA’s Wide Area Augmentation System (WAAS). Extensive testing and analysis of a massive collection of WAAS data by FAA’s Atlantic City, N.J., Technical Center showed that, as a result of progressive signal processing updates and the introduction of two new high-altitude geostationary satellites optimized for NAS coverage, WAAS guidance accuracy had become comparable to CAT I ILS.

    Recent flight tests by Eurocontrol, using Europe’s equivalent EGNOS GPS augmentation system, have produced similar results. And while FAA officials are cautious about describing WAAS as a CAT I precision approach aid, the agency has nevertheless published WAAS approach procedures with 200-foot decision heights for several major airports, with more to follow.

    This raises some intriguing possibilities. Since WAAS requires only an airborne receiver and no ground facilities, it offers the potential of a 200-foot decision height approach aid to literally thousands of airports that could not afford ILS, and have only been able to offer an NDB or VOR non-precision approach to much higher limits. As well, WAAS could avoid the ongoing maintenance and other costs of a currently installed ILS, or avoid the investment in a new or replacement system.

  • MLS. Some may be surprised to see MLS in this overview, since it is often thought to have been superseded by GPS. In fact, MLS is the only current ILS alternative to be certified to provide CAT II and III guidance, and Thales CAT III systems are installed at each end of London Heathrow’s two main runways. In turn, British Airways A320s and A321s are approved for CAT IIIb landings at Heathrow.

    Like GPS, MLS is immune to signal distortion, which allows British Airways to gain closer spaced, and therefore additional, landing and departure "slots" in Heathrow’s high-density traffic stream, a major operational advantage. It is expected other airlines, including U.S. carriers, may install MLS for the benefits it provides today at Heathrow, and eventually at major European airports.

    Since the availability of CAT III LAAS is unknown, other than possibly between 2015 and 2018, Eurocontrol is planning to replace CAT III ILS with MLS as traffic demands increase in the future.

  • TLS. A TLS ground station near the runway interrogates an aircraft’s transponder to derive its vertical and horizontal bearing, plus its range. It then calculates and transmits ILS-like localizer and glidepath signals to the aircraft’s ILS receiver, appropriate to the aircraft’s position relative to the desired runway approach path. Pilots then fly their ILS displays in the normal way.

    TLS has FAR 171 approval for CAT I approaches, but its critical operational drawback is that only one aircraft can use the system at a time. It is reportedly successful in Alaska and other areas where landing traffic is light, but clearly has limitations for busier locations.

    ILS is clearly going to be with us for many years to come — one FAA official predicted in 1990 that "ILS will be around for at least another 50 years." While that may be an exaggeration, the system’s worldwide proliferation, and its reliability and performance, coupled with the carriage of ILS receivers in virtually every civil aircraft, mean that the transition to a successor can only occur over a very extended period.

    On the other hand, ILS signal reflection limitations would likely inhibit the expectation that in FAA’s NextGen environment, for example, landing traffic will flow as smoothly in low visibility conditions as it will in clear weather. At major high-density airports, therefore, it seems likely that ILS could be under pressure from both MLS and, when finally certified, CAT III LAAS. Or from some new technology yet to be revealed.

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