Business & GA, Commercial, Military

Loran-C: Will I Be the Savior of GPS?

By Adrian Gerold | July 1, 2002
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To many, the notion that Loran-C could be the savior of the satellite-based GPS system would seem a little ridiculous. After all, didn’t GPS replace Loran-C in most airplane cockpits several years ago?

So why are some serious people in the U.S. Federal Aviation Administration (FAA); the U.S. Department of Transportation’s (DoT’s) Volpe National Transportation Systems Center; the civil telecom and financial sectors; Ohio, Stanford and Rhode Island universities; Draper Laboratories; avionics firms like Rockwell Collins; and even airframe giants like Boeing talking about Loran-C as a potential backup to GPS?

Basically, it is because all these organizations and many others have recognized one important fact: while we are becoming more and more dependent on GPS in all aspects of our lives, the system itself is not perfect, and we can not rely on it totally in the future.

Emphasis on Backup

True, GPS has proven itself to be more reliable than most other navigational systems. But after the Sept. 10, 2001, Volpe Center report on the system’s vulnerability to accidental or deliberate interference, followed by the terrorist attack on the next day, an old watchword–"backup"–has attained new prominence. "Sole means GPS," said a senior FAA official earlier this year, "is no longer a part of our vocabulary."

In aviation, of course, backup or redundant systems have long been a way of life, as today’s levels of safety attest. Dual VHF, VOR, DME, ILS and other avionics systems have been standard for years in virtually all airplanes. But the main reason for their duplication has been to have a backup should their onboard companions fail. Much less emphasis was given in the past to guarding against the loss of external signals arriving from the ground facilities.

This is where the big difference lies when the subject of a GPS backup is discussed. Certainly, dual GPS avionics will be required in future installations, even though it seems likely that new technology will make them extremely reliable. However, should the incoming satellite signals be jammed or suddenly disappear, even having 10 GPS units on board would not help. If such a situation occurred today, of course, pilots still could navigate with VOR, DME, ADF and, in larger aircraft, inertial systems.

But in future busy airspace, where terrestrial navaids have been reduced and the majority of aircraft are GPS-dependent, loss of GPS would preclude both accurate navigation and safe air traffic separation. Even inertial-equipped aircraft, when forced into extended holding patterns, slowly lose positional accuracy after several orbits. For this reason, FAA is planning a series of GPS outage studies to assess the impact on air traffic control in high-density airspace (December 2001, page 31).

What Loran Can Do

So how does Loran fit into this scenario? Basically, Loran can provide stable, high-powered and essentially unjammable signals throughout the continental United States (CONUS), from ground level to above jet altitudes. It does not suffer from the line-of-sight limitations of VOR, DME or GPS.

Quite the reverse, Loran’s 100-KHz very low frequency signals do not require line of sight and can follow terrain contours and circumvent high-rise buildings. Loran provides signals in deep valleys and urban canyons alike.

Loran is not so precise as GPS, but its one-third mile accuracy is better over a much larger area than any of the ground-based alternatives. And Loran’s accuracy is improving; recent flight tests conducted by the University of Ohio have demonstrated accuracy down to 10 to 15 meters (33 to 49 feet), which is about as good as GPS.

Loran positioning is in the horizontal plane, of course. Like other terrestrial navaids, Loran doesn’t have a vertical component equivalent to GPS, and so it cannot provide precision approach guidance. However, it could handle non-precision approaches, missed approach procedures and all other en-route demands.

Always a Backup

But the key point here is talk of Loran as a backup system–one that, if the GPS signals should go out of tolerance or be lost, would automatically pick up the navigational reins until normal GPS service returns. Loran is not regarded as an alternative to GPS, and we probably will not see "stand alone" Loran-only receivers again–except perhaps in China, which has made massive investments in Loran and is politically disinclined to rely on GPS.

The current design philosophy points towards having a Loran circuit board embedded in the GPS receiver. There it would operate automatically and continuously, from start up to shut down, as a "shadow" system, running in the background. No crew action would be required to transition from GPS to Loran. Should the GPS signal be lost or interfered with, the transition would appear seamless to pilots except for a "GPS Fail" annunciator to advise the crew of the situation. In April, FAA signed a contract with Rockwell Collins to develop the first GPS/Loran multimode receiver, to evaluate these concepts.

All New Loran

Many may view Loran’s advancements as sheer science fiction, since their exposure to Loran has been limited perhaps to the rather clunky–and occasionally quirky–receivers designed 10, 20 or more years ago. Then Loran depended on tracking signals transmitted from a regional network, or "chain," of three or four transmitters. Moving from one Loran chain to the next required pilots to manually rechannel their receivers–usually by instinct or experience. The receivers didn’t perform transfer chains automatically, or tell when a transfer should be made.

Flying from, say, Florida to Alaska required four or five selections of chains and individual ground transmitters. And woe betide pilots who flew into heavy rain or near thunderstorms en route, since either could create levels of precipitation static around their Loran antennas sufficient to cause signal dropouts. "Loran’s a great system," the saying went, "as long as you stay VFR." Obviously, such a system would be a totally inadequate backup for GPS.

For some time, Loran appeared to be in a rather paradoxical time warp, overtaken by new technologies that delivered cheap, pocket-sized GPS units, tiny cell phones and incredibly powerful home computers. Recently, however, Loran technology has changed. New digital signal processing (DSP) receivers, developed by industry pioneers such as Locus Inc., Madison, Wis., have introduced GPS-like "all in view" units, coupled with new "H-field" precipitation static-resistant antennas, some combined with GPS antennas.

These units no longer simply track signals from the three or four transmitters in a local Loran chain, but now automatically track every station that can be received, even thousands of miles away. In CONUS, this typically means tracking signals from 30 or more widely separated transmitters, all of which the receiver analyzes for a "best fit" position solution. And the system’s "sky waves"–waves that travel into space and may or may not return to earth by reflecting off the ionosphere–once the bane of longer-range Loran operations, are now accommodated and used in the solution. Remarked one Loran engineer, "This is redundancy, in spades."

The U.S. Coast Guard, moreover, under a congressional mandate, is upgrading the earlier tube-type transmitters to solid state units. This, combined with the installation of new cesium atomic timing standards, will assure greatly improved signal stability and accuracy. In fact, one rather subtle consequence of all this is the gradual dropping of the -C suffix from the system’s name. "This is no longer the Loran-C system of 20 years ago," said Locus President Linn Roth.

How Loran Plus GPS Adds up

The advances to Loran give several organizations reason to advocate the navigation system as the most promising backup to GPS. There is also a lesser-known benefit: Loran’s newly demonstrated ability to provide pilots with the GPS failure warnings and accuracy corrections planned to be provided by the FAA’s Wide Area Augmentation System (WAAS). Flight tests by FAA’s Technical Center in Atlantic City, N.J., over northern Alaska, where WAAS reception is hampered by the low elevation of the equatorial orbiting WAAS satellites, showed excellent reception of WAAS messages transmitted from a Loran station.

Yet the navigational benefit of Loran is just the tip of the iceberg. Today, we take for granted things such as instantaneous banking transactions, utilities, communications, TV, radio and other services. But these, along with the massive, nationwide, government and commercial infrastructures, are dependent for their continuous operation on high-accuracy timing, which must today meet the so-called "Stratum 1" standard.

Currently, GPS provides this capability. Only two other timing methods meet Stratum 1’s exacting criteria: cesium atomic clocks, which are both expensive and impractical for many applications, and Loran.

Over the past five or so years, several organizations, including a U.S. presidential commission, have warned against placing total reliance on a rather vulnerable military satellite system, since loss of its timing capability, even for brief periods, would have serious economic impacts. The Europeans have similar concerns, but these are based less on the loss of GPS signals than on their own infrastructure’s growing dependence on a satellite system that is controlled by a foreign military body, however friendly. Europe’s GPS-like Galileo system will, they feel, give them the independence they need. (In much the same way, non-U.S. attendees at last January’s Institute of Navigation [ION] meeting in San Diego probably didn’t share the enthusiasm of an Aerospace Corp. speaker, who proclaimed that GPS would be "a critical part of the worldwide infrastructure.")

In these infrastructure applications, Loran advocates see their system–which, prior to GPS, provided this terrestrial timing function and still does in many instances–as a key future backup component. Loran also could provide a critical backup to elements of FAA’s Operational Evolution Plan, which sees major roles for Controller/Pilot Data Link Communications (CPDLC) and the Next generation air/ground Communications (NEXCOM) system, both of which depend on very precise timing for their operation.

In 1996 the U.S. Department of Transportation (DoT) declared that Loran would be shut down on Jan. 1, 2000. It was a near-death experience for the Loran industry until DoT later relented, stating that the system would stay on the air in the short term, while its future was evaluated. A wide range of FAA investigations is under way, including the Alaska WAAS tests and the Rockwell Collins multimode receiver development, and the agency is to report its conclusions to DoT by the end of December 2002.

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