ATM Modernization, Business & GA, Commercial, Military

Safety in Avionics: Not Your Father’s Loran

By David Evans | December 1, 1999

Just as the value of real estate has been based on the word "location" repeated three times, the commendable safety record of the airline industry has been built on redundancy, redundancy, redundancy.

Regarding future navigation systems, therefore, any plan that does not mandate a backup would be tantamount to providing the airlines with a built-in incentive to decrease their safety. Yet, essentially, that has been the general thrust of the effort to utilize the global positioning system (GPS) as a "sole means" of navigation in the next century. Until recently, the plan was to decommission all ground-based navigation aids, leaving the entire aviation industry dangling, as it were, by a "single thread."

The plan to unplug everything on the ground horrifies former FAA Administrator Langhorne Bond, who has been most vocal about the danger of hanging the future of air navigation on the single thread of GPS. Recently, in his speech accepting the Glen Gilbert award at the Air Traffic Control Association’s annual convention, Bond went one step further, publicly calling upon the U.S. Federal Aviation Administration to outlaw GPS as a sole means of navigation as "a safety regulatory matter."

In his remarks, Bond applauded the FAA’s recent decision to continue all ground-based navigation aids until 2008, a move clearly generated by over-promising the benefits of GPS while recognizing, belatedly, the safety limitations.

"We need to put this issue to bed once and for all," Bond said, "by formally declaring that GPS as a sole means is not acceptable for most IFR (instrument flight rules) flight—forever. This will end the safety debate and will permit operators and manufacturers to come to agreement on a mixed redundant system. This won’t take long, once the safety baseline is set," he argued. "We can then decommission most of the old line-of-sight navaids on the ground and we can save a lot of money."

Note the significance of his reference to line-of-sight systems. That leaves the window open for Loran. The term is an acronym for long range navigation, the latest version of which is Loran-C. Just to make sure all cards are face-up on the table, Bond happens to be on the board of directors of the International Loran Association.

Be that as it may, the association’s president, Linn Roth, recently published an excellent paper, titled The Case for Loran, that lays out a concept for a truly redundant navigation system. Here are the highlights of his argument that today’s Loran is not some antique Oldsmobile:

  • Combining GPS with Loran would provide a hybrid national system with capabilities better than either system alone.

  • To date, no major carrier has equipped even half of its fleet with GPS receivers; the airlines continue to carry redundant, dissimilar systems for needed backup.

  • Recent improvements to range and reliability suggest that Loran can provide coverage of Northern Atlantic and Northern Pacific air routes.

  • New digital technology eliminates burst noise from lightning, and new magnetic antennas are not susceptible to static buildup on the airframe.

  • For carriers reluctant to install another antenna, a new antenna with a diameter of just 4 inches (10.1 cm) combines GPS and Loran antennas into a single compact unit. Separate antennas would, of course, assure absolute physical redundancy.

  • Loran can enhance GPS accuracy. For example, the Eurofix system developed in The Netherlands uses Loran to distribute GPS corrections about 540 nautical miles from a transmitter. Let’s take the matter further. Loran can do the same thing as the FAA’s troubled Wide Area Augmentation System (WAAS), without having to send correction signals to a master station, then out to satellites in geostationary orbit and Back to December 1999 GPS receivers on Earth. The U.S. Coast Guard has already tested Loran for this purpose, achieving 3-meter (10-foot) accuracy, which is about the same achieved by WAAS—when it’s working.

  • Loran can enhance GPS availability. As Roth explained, "If the planned improvement in the Loran infrastructure simply synchronizes Loran transmitters to universal time coordinated (UTC) as is done with GPS, then an integrated GPS/Loran system could treat Loran transmitters as if they were additional GPS satellites, or pseudolites. There are now 29 Loran transmitters in North America. So, in effect, a modern tightly-synchronized Loran infrastructure could supply 29 GPS pseudolites...such a system would mean a single, combined GPS/Loran receiver would track an additional seven to 10 ‘satellites’ at any one time.

    A GPS-only receiver would see perhaps six satellites, but a GPS/Loran receiver would see 13 to 16 satellites in the same position. With such a combined system, GPS availability in the National Airspace (NAS) would increase dramatically, even if no additional satellites, frequencies or signal strength were added to the current GPS infrastructure."

  • Finally, Loran’s signal, which follows the earth’s curvature, provides penetration into areas where GPS has difficulty because of line-of-sight blockage, e.g., urban and forested areas. Loran can even penetrate some buildings. These and other features enable Loran to serve as a backup timing signal. Presently, the nation’s telecommunications and electric power distribution systems are highly dependent on the timing signal in GPS—which also makes the national infrastructure vulnerable to GPS failure.

  • Above all, Roth maintains, Loran provides true GPS redundancy.

As Bond says, the loss of the GPS signal in an airplane cruising along with Loran would be almost undetectable to the crew.

That is, if the airplane were so equipped. Presently, while tens of thousands of general aviation aircraft carry Loran, the nation’s scheduled carriers do not. They employ inertial systems and also navigate based on line-of-sight signals from VOR/DME (VHF omnidirectional range/distance measuring equipment) stations.

Why should air carriers add Loran? Three reasons come to mind.

  • First, the retrofit to the open channel on most modern flight management systems (FMS) would facilitate installation at a miniscule cost of some $1,500, enabling a simple lateral switch from VOR/DME to Loran.

  • Second, the quarter-mile accuracy from multiple DMEs is bettered by Loran and, unlike line-of-sight DME, Loran is like GPS, providing a navigation solution all the way to the ground.

  • Third, 29 Loran stations can do the "work" of the present network of 1,200 VOR/DME stations, at considerable savings in ground support costs. This is the source of Bond’s "a lot of money" savings.

To be sure, Loran does not provide precision approach. While advanced inertial navigation systems hold the promise to fill this need, providing a backup to GPS-based landing systems, the technology is not yet ready for deployment.

The near-term course seems obvious: Some 640 airports in the United States are equipped with one or more instrument landing systems (ILS); of these airports, about 380 are served by America’s scheduled carriers. Those ILSs should be maintained against the day when inertial systems are capable of providing a precision approach backup to GPS.

The pieces all fit. And one thing is for sure—hanging the future of air navigation on the tenuous single thread of GPS not only violates the proven theology of redundancy, it’s fundamentally unsafe.

(Note: An unabridged, original version of The Case for Loran can be viewed on the following two websites: www.loran.org and www.locusinc.com )

David Evans is managing editor of Air Safety Week newsletter. For information or to subscribe to Air Safety Week, call 888-707-5812.

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