Hope that a global navigation satellite system (GNSS) could constitute a single navigational source so accurate and reliable that no other source would be needed, continues to fade. The Volpe National Transportation Systems Center provided a wakeup call in 2001, pointing out that the faintness of the GPS signal left the system vulnerable to disruption by anything from a solar storm to a disaffected individual armed with a cheap, commercially available jammer. The next-generation GPS Block III satellites and Europe's Galileo, though somewhat more robust, will not be immune, either. Following the Volpe report, the view has strengthened that "sole source" is no longer tenable and that an independent navaid backup will be necessary for high-integrity applications.
This begs the question of what system should be adopted as the backup. Other satellite-based systems, such as Russia's GLONASS and Europe's future Galileo constellations, might be insufficiently complementary in their vulnerabilities and failure modes. Ground-based augmentation systems like the U.S. wide area augmentation system (WAAS) and the European geostationary navigation overlay system (EGNOS) can be considered equally vulnerable because a GNSS signal is still required.
Experts therefore have suggested Loran, a terrestrial hyperbolic radionavigation aid first developed during World War II for LOng RAnge Navigation, as a complementary partner to GNSS. A Federal Aviation Administration (FAA) technical evaluation of Loran, though not yet public as of press time in early May, has been presented to the U.S. Department of Transportation (DoT). It is likely to support a favorable view of the system, if one can judge from the tone of a preliminary report and ongoing presentations from the FAA radionavigation task force that is looking into the matter. The task force finds that—provided Loran is enhanced to deliver levels of performance and reliability appropriate to critical aviation and maritime navigation, and provided it can be cost-effective—it should be considered for adoption as part of the future U.S. radionavigation mix. The Volpe Center also has turned over a parallel Loran cost/benefit study to DoT, which plans to make a public announcement regarding Loran policy in June.
The choice of Loran as a GPS backup might surprise older navigators who recall using unwieldy analog equipment with drawbacks like manual tuning and lane ambiguities. Loran is inherently subject to significant positional inaccuracies, which can be up to several hundred meters at times. But new receivers greatly reduce this problem. A contemporary, digital Loran-C receiver is to older systems what a modern digital radio is to a 1950s valve set. Navigators today require no awareness of the chains, pulse groups and master/secondary station relationships that the original Loran-A and successor Loran-C users had to contend with. Today's all-in-view receivers can monitor dozens of stations simultaneously from several Loran chains. Powerful correlation techniques enable receivers to pick out signals as low as 25 dB below what was previously possible, providing ranges up to approximately 190 miles (305 km) greater than those of previous-generation systems. Bringing more stations into the navigational net enhances accuracy, as the receivers automatically select an optimal combination of stations to obtain the best positioning solution.
Today's receivers mitigate much of the inaccuracy caused by the effects of terrain on the Loran signals. Modern digital receivers also deliver advanced signal processing, adaptive digital filtering, and high processing speeds. High system redundancy allows for autonomous integrity checking. Signals can now be accurately resolved to 1 nanosecond in time, 100 times better than older-generation receivers could achieve. This is crucial both because positions are derived from the signals' times of arrival (ToA) and because Loran not only backs up the positioning functions of GNSS, but also offers an alternative timing source.
Technology has equally transformed Loran's fixed infrastructure, with major improvements to transmitters, antennas, timers, networks and control functionality. The new solid state transmitters replacing tube-based systems are stable, compact and reliable, and can operate unattended. Because they deliver high power—from 250 kilowatts to over 1 megawatt in some cases—only a handful of stations are needed to provide regional coverage.
Loran is inherently complementary to GNSS/GPS. It is terrestrial rather than space-based. It operates in a very different frequency band and has dissimilar failure modes.
From a security standpoint, hostile forces would find it hard to disrupt land-based and space-based infrastructures simultaneously. Loran installations can, in most cases, be repaired or replaced repeatedly, whereas the consequences of any successful assault on a satellite infrastructure are likely to be prolonged. GPS signals remain, despite improvements in countermeasures, vulnerable to jamming, whereas Loran, with its strong, dispersed signals, large ground antennas and 90- to 110-KHz medium-wave operation, is thousands of times harder to jam.
Because of their complementary nature, the two systems combined should improve availability. According to Linn Roth, president of Loran receiver manufacturer, Locus Inc., Madison, Wis., such an approach offers "significant opportunities for creating an integrated system with better availability, integrity and continuity than either system alone [could provide]." Locus collaborated with Rockwell Collins to produce a prototype GPS-Loran receiver as part of the U.S. government's Loran evaluation effort. Locus also is working with FreeFlight Systems, Waco, Texas, on an integrated GPS/WAAS-Loran receiver.
Meeting the Standards
The continuation of Loran service in the United States—in navigation, where the U.S. leads, other countries follow—is conditional on the system's ability to deliver certain standards of performance and at competitive cost. Mitch Narins, program manager for the U.S. radionavigation task force, tells Avionics Magazine that further Loran enhancements will be required to meet some of these standards. For example, the need to meet non-precision approach accuracy requirements will call for corrections to terrain-induced delays to the Loran signals. These are called additional secondary factors (ASFs). ASF values can be derived from physical measurements and computer models. In fact, the Collins system uses GPS to generate Loran ASF corrections in real time. Understandably, measurement and modeling of ASFs were substantial parts of the FAA study.
A recapitalization plan should secure an "Enhanced Loran" infrastructure that can meet the requirements. Upgrading remaining transmitters to new, solid state models and providing uninterruptible power supplies for transmitters and control networks will help meet the availability requirements of both FAA for airborne navigation and the U.S. Coast Guard for maritime purposes. New digital receivers will be able to recover from a station loss in seconds rather than minutes, benefiting continuity. Parallel redundancy will help secure the very high integrity needed for aviation. Under the plan, coverage of the enhanced system would be extended throughout the continental United States and Alaska.
According to Narins, the radionavigation task force has discovered no technical show stoppers for Loran. This means that an enhanced system could indeed meet future navigational and timing requirements and serve, in line with Volpe recommendations, as a cross-modal radionavigation complement to GNSS. The cost/benefit analysis will help establish the system's ability to compete for funding against other systems that are also being considered. These include a national differential GPS (DGPS) service for the United States—indeed NASA is even considering a global DGPS able to deliver a 33-foot (10-meter) accuracy—and WAAS, which can achieve some 25-foot (7.5-meter) accuracy. Inertial systems—while self-contained, independent of either terrestrial or space-based radio systems, and unjammable—are likely to be more expensive and will remain prone to long-term drift. Therefore they require periodic updating from other systems.
Combined Aviation Receivers
The prototype receiver developed by Locus-Rockwell Collins and evaluated in flight tests last year illustrates the advantages of the combined system approach. The receiver can produce GPS-only, Loran-only, and combined GPS-Loran solutions, including GPS-calibrated corrections for large ASF errors. The receiver's all-in-view Loran portion typically tracks 20 to 30 stations in North America using a subset of those stations to provide the navigation solution. Locus developed a GPS-Loran antenna that operated with the GPS sensor during the flight trials.
Initial flight tests using the receiver with an H-field (magnetic) antenna provided accuracies well within the non-precision approach standards desired by FAA of 0.3 nautical mile (RNP 0.3, according to required navigation performance guidelines). If the U.S. government gives its advancement the green light, Loran could appeal to aviation safety regulators and operators alike as a viable navigation second source alongside GNSS. In air transport-category aircraft it could become an alternative to costly inertial reference systems (IRS) in future RNP 0.3 environments, as further regulations come into play.
Given the right conditions, a niche for Loran receivers could exist in lower-end business and regional aircraft whose operators are unable or unwilling to afford expensive inertial navigation equipment, according to Jim Doty, a principal systems engineer with Rockwell Collins' Advanced Technology Center.
"In crowded airspace, with certain kinds of approaches and holding patterns, you're probably going to get preferred operational capability with an [inertial nav] backup," Doty predicts. But operators who reject inertial systems, for whatever reason, might consider using Loran. An airplane with a GPS-calibrated Loran receiver, but without other means of navigation, would hold its accuracy (within a few meters) longer, within a limited area, than it would simply coasting on inertials.
If there were a temporary or partial loss of the primary system, GPS, "Loran would be able to perform with adequate horizontal positioning capability within an area to maintain navigational continuity," says Patrick Hwang, principal systems engineer for navigation with Collins' Advanced Technology Center.
After the conclusion of the FAA flight test program, Collins also flight tested a receiver that combined integrated GPS-Loran elements with a low-cost inertial measurement unit (IMU). Although this experiment was limited to four flights over three days, it showed that IMU data can be used to reduce small positional Loran errors, or "noise," which remains even after large ASF errors have been removed. This further improves the performance of a GPS-calibrated Loran receiver in the absence of a GPS signal.
Present hopes of renewing Loran are fueled by the system's transformation from an analog relic to a highly capable digital system fit to be integrated with present and future GNSS. This "integratability" is crucial since Loran is no more suitable than GNSS to be a sole source for navigation. Combining the two together, however, would enable Loran to cover any GNSS outages, while GPS (or other GNSS) could be used to continually calibrate Loran and thereby enhance its accuracy.
There are remarkable synergies between the two systems. Europe has demonstrated one of these in a highly capable system known as Eurofix. First developed during the late 1990s by Delft University in the Netherlands, Eurofix distributes GPS differential corrections and integrity messages within the Loran-C signals, thereby extending DGPS coverage. Eurofix messages are currently broadcast on four transmitters of the Northwest European Loran System (NELS), providing DGPS coverage over the whole of northwest Europe. Eurofix also will be implemented on three Saudi Arabian transmitters later this year, so that a large section of the Middle East will be covered. The system also has been trialed successfully with Russian Chayka stations, which, with their high power, could extend augmented GNSS coverage towards the poles to serve the growing number of commercial transpolar flights.
At medium range from the transmitter and reference station, Eurofix has demonstrated accuracy of better than 10 feet (3 meters), with a 95 percent level of confidence. Should the GNSS signal be lost or obscured, a combined Loran-C, GPS and Eurofix receiver could still provide calibrated Loran positions accurate to approximately 33 feet (10 meters), with 95 percent level of confidence, for several hours.
Unlike GPS signals, Loran signals are not restricted to line of sight. Their ability to propagate into valleys will help compensate for GNSS deficiencies in mountainous areas where high ground alongside airports can interrupt line of sight to satellites that would otherwise be in view. Loran can therefore benefit operators in places like Alaska, where positioning difficulties have been experienced. Because Loran coverage extends right down to ground level, it can also serve aviators making non-precision approaches to destinations lacking VOR/DME coverage. This would have special merit for general aviation (GA) pilots, for whom the multi-DME alternative to GPS typically used by airliners at low levels is not readily available. Loran also provides area navigation (RNAV) capability, in line with airworthiness authorities' desire to move towards an RNP-based "free flight" environment.
It is also significant that Loran can penetrate into urban canyons, where GPS is often ineffective, and that it can be received inside vehicles and buildings without the use of external antennas. One of the drivers for a possible Loran revival is the potentially substantial growth market in land positioning applications. Moreover, Loran has long been a maritime navigation aid, and an improved system would increase its uptake within this sector. A diversity of applications will improve the system's economics for all users.
Matter of Timing
In addition to providing an invaluable positioning source, Loran can contribute in the equally crucial area of timing. With three cesium atomic clocks in every Loran station, and using the latest receiver technology, the system has the top telecommunications, "Stratum 1" timing capability, making it a suitable backup for the other major timing sources. Loran can provide Universal Coordinated Time (UTC), which the major GPS timing backup, rubidium, cannot. Some countries may adopt Loran as the primary source, thereby gaining an independent means for synchronization in telecommunications and other major systems. China, for example, has been developing Loran-C "time receivers." The present heavy reliance of the telecommunications industry on GPS for timing makes it vulnerable to any GPS failure, with consequences that could be globally severe. Loran's timing capability also could benefit the proposed next-generation, digital air/ground communication system, NEXCOM, and back up both the timing and navigation functions provided by GPS in the automatic dependent surveillance-broadcast (ADS-B) system.
Opponents of Loran point to the expense of operating a dispersed ground infrastructure. Historically this may have been substantial, but the ability of modern transmitters to operate unmanned should reduce this burden. Moreover, the cost of decommissioning the present system would be significant—some $100 million, according to informed estimates. The objection that terrain can influence signal propagation and degrade accuracy is being addressed by modeling and compensating for these influences in software. Notably, Prof. David Last at the University Of Bangor, UK, has improved computer models used to generate ASF corrections.
Because "sole means" GNSS isn't likely to be able to satisfy all the requirements for safe navigation, future nav receivers are likely to feature combined-system operation. As well as the receivers already mentioned, a number of integrated receivers are under development, with support from the U.S, European Union, Russian and other authorities. At the GNSS 2003 conference in Graz, Austria, the German company Detectis presented a new Loran-C receiver no larger than a handheld computer, saying that an integrated Loran-C/Eurofix/GNSS receiver would follow. Megapulse, the U.S. company responsible for much of the Loran infrastructure, likewise discussed the integrated receiver technology it is developing with subcontractor Reelektronika BV of the Netherlands.
Outside the United States the future of two of the world's four major Loran systems (five if Chayka is included) is unclear. Some of the worldwide infrastructure has been closed down and replacement has been patchy. The Southern European Loran System (SELS)—effectively the Mediterranean chain—has been degraded by the loss of its Spanish station and by two Italian stations' going off the air, while the Northwest European Loran System (NELS) is secure only until the end of 2005, when present contracts expire. One official describes the outlook for NELS as "not hopeful." The Netherlands, Norway and Ireland have said they will withdraw their support. A station planned for Ireland was never built, and the UK, though it has shown interest, has not yet committed to the system. Informed opinion suggests, however, that if the U.S. opts for the system, sentiment in Europe will become more positive. The long-term future of the North American system has not yet been decided, though it is certain to survive until at least 2008. This leaves only the Far East Radionavigation Service (FERNS) in a clear expansionary mode at present.
Proponents, Take Heart!
Loran proponents and all who have counseled against overreliance on satellite-based navigation can take heart. After all, the U.S. once was expected to phase out its Loran by 2000, following declaration of full operational capability for GPS five years earlier. But the satellite system's highlighted vulnerability prompted a stay of execution. Concerns were strengthened as a result of 9/11, and over the last seven years the U.S. spent about $120 million on Loran, mainly on infrastructural improvements, including new transmitters.
Elsewhere, Japan is modernizing its Loran system, China is considering doing so, as well, and Korea is testing Eurofix. Russia continues to support its Chayka system. In Europe, technical development continues, while France, Germany and Italy are discussing how some of the present NELS coverage could be maintained after 2005. France, a considerable Loran driver within Europe, wants operation to be maintained until at least 2015 and is planning to establish two new stations to help it address marine and land, as well as aviation navigation requirements. Germany is working for a solution that is both publicly and privately funded. The UK, still a watchful bystander, is taking careful note of U.S. and European developments. Navigational authorities are interested in Loran usage on land, sea and in the air, and policy makers have been urged to take into account this multimodal potential in finalizing the European Radio Navigation Plan, whose delayed publication is now eagerly awaited.