The Case for Loran
 Dr. G. Linn Roth, President, LOCUS, Inc.; October, 1998
Since its inception, and as its applications and benefits continued to grow, GPS has created a growing fervor of excitement and promise. It seemed as if GPS alone Ė albeit with some specific augmentations - could meet virtually all our national aviation, marine, and terrestrial radionavigation needs.
However, over the last few years there has been much debate about GPS potential use as the nationís sole-means radionavigation system, and even the term "sole-means" has undergone a redefinition. During the same period, international GPS support has waned, and world-wide acceptance of GPS as a sole-means system has not taken place for numerous performance, liability, and control reasons. The recent Johns Hopkinsí GPS Risk Assessment Study (GRAS) generated as many questions as answers, and there is growing realization that GPSí evolution to a sole means system Ė regardless of future augmentations and how sole-means is redefined Ė will not occur for well over a decade, if ever.
While such GPS issues can be reviewed within a strictly aviation framework, it is necessary to take a broader view, because GPS policy has major implications in marine and terrestrial applications too. A less well known but extremely important application is in telecommunication and power distribution systems, where GPS receivers generate timing signals used to synchronize massive networks affecting the daily lives and businesses of tens of millions of Americans. Without question, GPS pervades the deepest parts of our national infrastructure, and important risks associated with GPS dependency were identified in a report by the Presidential Commission on Critical Infrastructure Protection (PCCIP) in October 1997. Today those risks have grown, not abated, and critics of the GRAS study such as the International Federation of Air Line Pilots Associations (IFALPA) and Litton Aero Products have uniformly condemned reliance on a single technology, with one author calling the concept "fundamentally unsafe".
In this brief article, I will try to convince you that Loran should be viewed as an excellent complement to GPS, and that combining GPS with Loran would provide a hybrid national system with capabilities much better than either alone, regardless of how many GPS augmentations can reasonably be implemented. Although I will emphasize aviation applications, I will also try to adopt a broader perspective, as both GPS and Loran are critical national assets. Here is my reasoning:
GPS Requires a Backup
No matter how the definitions of sole-means, sole-service, and related terms evolve over the next decade, one thing seems clear: GPS requires a backup now and for at least another 15 years. While public debate over national GPS policy appears to be moving away from GPS as a sole-means system, it is interesting to note that industry has recognized the realities of the situation and provided backups all along. For example, commercial airlines carry a number of redundant, dissimilar systems, and no major carrier has equipped even half a fleet with GPS receivers, choosing reliance on these other systems until GPS augmentation programs are clarified, implemented and proven. Telecommunication and power grid systems rely on GPS as their primary timing reference for network synchronization, but incorporate Loran receivers or Rubidium oscillators to carry over in case of GPS interruption. Although not generally understood, GPS-based car navigation systems typically integrate differential wheel counters and digital map matching technologies in order to compensate for GPSí poor penetration into urban environments. 
Recognizing these realities is certainly not an indictment of GPS, but a necessity in todayís world. No technology can be made to perform flawlessly. In this context, I believe the US has two fundamental issues to address regarding GPS: 1) how much time and money are we willing to expend to make the system as close to perfect as possible? and 2) is it good public policy to base a complete national infrastructure on a "single-thread" technology?
With regard to the first issue, our history demonstrates we cannot accurately predict how much time or money GPS enhancements will take, or the real life performance they will ultimately offer. Engineering problems, component failures, schedule delays, etc. are facts of life in the world of technology. We also cannot predict societyís future commitment to such a program in the face of increasing budget pressures for social and other needs, and unexpected events such as Kosovo that could engulf tremendous national resources.
With regard to the second question, I offer a basic analogy. Hospitals provide backup power generation equipment because they must ensure safety of life and continuity of service for their patient and employee populations. Total reliance on the local utility is not an option. Today, there seems to be growing recognition that the United States should not make such a choice for a national radionavigation system. Instead, as suggested by some of the GRAS critics, we should determine what combination of systems makes the most sense for the nation, and here is where Loran fits so well. 
Contemporary Loran Performance 
Although GPS is unquestionably the best radionavigation system and should function as the foundation of our national infrastructure, Loran is considerably better than generally appreciated, and it has not yet reached its technological limit. Contemporary Loran receiver performance is vastly improved beyond the 10-20 year old technology commonly used in general aviation aircraft. For example, new receivers are all-in-view and track 30-35 Loran transmitters simultaneously in North America.
From an aviation perspective, range and availability have been increased significantly, and it is now likely Loran can provide coverage of Northern Atlantic and Northern Pacific air routes. In addition, modern digital technology eliminates burst noise from lightning, and new magnetic (H-field) antennas are not susceptible to precipitation static build-up on an airframe. In fact GPS antennas and magnetic Loran antennas can be combined into a single small device, thereby minimizing aircraft installation costs and mounting holes. Figure 1 shows a small Loran H-field antenna to illustrate how far the technology has progressed.
H-field Antenna
Figure 1. Loran H-field antenna with outer diameter of approximately 4 inches. Photograph is courtesy of Megapulse, Inc.
Another area of performance advancement is repeatable accuracy, which has been one of the reasons for Loranís widespread popularity. However it is not well known that in a good coverage area, Loranís repeatable accuracy can be better than GPS (see Figure 4), even without selective availability (SA). Indeed, contemporary Loranís repeatable accuracy can be improved further, but that requires modernization of the Loran transmitter control system. In others words, the current Loran infrastructure now limits the overall system performance, not receiver technology, and we still do not know how good Loran can be in the US.
Loran Complements GPS 
From a physical perspective, Loran and GPS have very different characteristics: ground vs. satellite based, low vs. high frequency, high signal level vs. low signal level. Consequently they do not suffer from the same modes of failure, and Loran or GPS will be available or provide better performance under conditions where one system might be compromised. 
Such dissimilarities mean Loran can enhance GPS performance in a variety of ways, if we take advantage of each systemís properties. For example, the Eurofix system developed in The Netherlands uses Loran to distribute DGPS corrections about 1000 km from a transmitter; comparatively, this coverage area is very much larger than what can be provided by the United States Coast Guard (USCG) marine radiobeacon DPGS system. Four European Loran transmitters are expected to be outfitted for Eurofix operation in the near future, and Figure 2 shows the projected North American Eurofix coverage.
North America Integrity Coverage
Figure 2. Projected North American DGPS and/or external GPS integrity coverage provided by Eurofix system using existing 29 North American Loran transmitters and 1000 km range. Note that Loran navigation coverage (i.e. disregarding the Eurofix correction range) extends over a much larger area than shown, typically about 1800-2000 km from the transmitters. Illustration courtesy of Megapulse, Inc.
In addition, the Eurofix system enables Loran to communicate external GPS integrity messages and meet those performance specifications required by the Wide Area Augmentation System (WAAS). Importantly, Loran can simultaneously function as an independent radionavigation system while providing DGPS and/or GPS integrity messaging to enhance GPS performance, a capability not inherent in these other GPS augmentation systems. 
Although Loranís physical properties increase overall availability in a combined GPS/Loran system, it can significantly increase GPSís availability in another way. 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 within a combined system, and the locations of those pseudolites are shown in Figure 3. Because of coverage and geometry limitations, such a system would likely mean a single, combined GPS/Loran receiver could track an additional 7-10 "satellites" at any one time. That is, a GPS-only receiver would see perhaps 6 satellites, but a GPS/Loran receiver would see 13-16 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. 
Loran Transmitter Locations
Figure 3. The location of 29 potential GPS pseudolites comprised of the existing North American Loran transmitters, assuming they were simply synchronized to UTC. A combined GPS/Loran receiver would likely track 7-10 of these individual pseudolites within the Loran coverage area, in addition to whatever number of GPS satellites were currently in view.
Another practical advantage of combined GPS/Loran receivers would be the sharing of common components such as the oscillator and user interface, which reduces overall system costs and simplifies operation. Integrating these technologies also enables the user to purchase a single device, and minimizes panel space and installation costs in an aircraft.
Finally, Loranís wavelength and signal strength enable it to penetrate into areas where GPS has difficulty because of line-of-sight blockage, e.g. urban and forested situations, and Loran can even penetrate some buildings. In fact, the Defense Advanced Research Project Agency (DARPA) has explored combined receivers that could be used to locate troops in urban environments. Loran penetration into cities and its ability to provide an indefinite backup to GPS in timing applications are two additional advantages Loran provides in telecommunication applications.
GPS Complements Loran
Now let us turn to how GPS complements Loran. Among other advantages, GPS has better absolute accuracy than Loran, and obviously provides much more global coverage. Currently Loran is limited to the Northern Hemisphere, where it covers substantial areas of the Pacific Rim, China, Russia, Northern Europe, North America, and there are smaller systems such as those in India and Saudi Arabia. Within those substantial global areas where the two systems overlap, and I will specifically consider the NAS for this paper, GPS can be used to improve Loranís absolute accuracy so it is comparable to GPSí. How is this possible?
First we need to summarize how the physical nature of Loran limits the absolute accuracy of the system. As a terrestrial system, Loranís ground waves are affected by the earthís conductivity over the ground wave propagation path, resulting in an inherent bias that compromises Loranís absolute accuracy. So called additional secondary factor (ASF) tables are often used to correct for this accuracy bias.
Since GPS is not subject to these same influences and is such an accurate system, it can be used to "calibrate" Loranís absolute accuracy in a number of ways. In an airplane for example, a combined GPS/Loran receiver could integrate range measurements or positions from each system. Another method would be to use DGPS for calibration and to generate a national table of Loran ASF corrections, which could be stored in a receiverís memory and accessed periodically. In fact a program to generate ASF correction tables is currently underway in Europe. But regardless of the method implemented, GPSí capabilities can be used to remove Loranís positional bias due to variations in the earthís conductivity. GPS calibrated Loran would therefore enable Loran to operate with an absolute accuracy comparable to GPS, and most importantly, to function as a highly accurate, independent radionavigation system in situations where GPS was unavailable.
Figure 4 shows GPS and Loran data taken over the same 24 hour period, and graphically suggests what a combination of the two systems might achieve. The GPS data were generated by a modern 12 channel receiver, and the Loran data were generated by an all-in-view, but technologically outdated receiver. Three aspects of the data are immediately apparent: 1) the effects of Selective Availability (SA) on the GPS scatter plot; 2) the tight repeatable accuracy of Loran; and 3) the Loran position offset of approximately 135 m to the East and South. That offset is due to earth conductivity differences as described earlier.
Loran and GPS Scatterplots
Figure 4. One day GPS and Loran scatterplots generated at 1 minute samples on Dec. 29, 1998. GPS data show occasional outlier points due to effects of SA, and the Loran data are all contained within the single cluster as shown. Data courtesy of SatNavLab, Lincoln Laboratory, Massachusetts Institute of Technology.
As mentioned above, GPS could be used to calibrate Loran, essentially eliminating the inherent Loran bias and moving its positions over the center of the GPS scatterplot shown. Moreover, three related aspects of the Loran data strongly suggest that Loranís repeatable and absolute accuracy are better than illustrated, i.e. the potential for combining these technologies is likely better than these data indicate. First, the Loran receiver technology and its incorporated single chain navigation fix are outdated. A contemporary receiver would use a multichain or chain independent calculation, and either of these calculations would tighten up the Loran scatter plot. Second, no ASF correction was used in the navigation calculation, so the absolute positional bias would have been substantially reduced if such a factor were applied Ė even before GPS calibration. Finally, control limitations at the current Loran transmitters make the repeatable accuracy poorer than it really can be, as documented by numerous recent tests. In other words, the uncorrected Loran data would be better than illustrated if a better receiver and upgraded infrastructure were in place. Obviously, removal of SA would significantly contract the GPS scatterplot too. But regardless of the absence or presence of SA, the power of a GPS/Loran combination is apparent.
Of course the same physical differences in the two systems ensure that GPS will certainly be available at times Loran is not, or is somehow compromised. Again, a combined GPS/Loran system would greatly increase the availability and continuity either system can offer alone, regardless of the application.
Loran Can Support the National Infrastructure
As indicated earlier, GPS is used throughout the national infrastructure, and our dependence is growing rapidly. As a result, any system failure might have profound ramifications well beyond those affecting a single user group. For example, commercial and general aviation air traffic, car and train navigation, small and large vessel marine operations, and millions of individuals using GPS-based telecommunication and power distribution systems could be simultaneously affected by a GPS failure. As reviewed in the Johns Hopkins study and by members of the Presidentís critical infrastructure committee, GPS is subject to natural, as well as intentional and unintentional man-made interference. Given our national infrastructure now substantially depends on GPS, and also given GPSí vulnerability to intentional jamming, is it good national policy to ratchet up our GPS dependence with no provision for backups? Many would argue that such a policy makes an attack on GPS all the more appealing, and therefore all the more likely.
Here again, Loran has some important capabilities. Although not widely recognized, Loran is the one existing radionavigation system that can be used to complement GPS in all applications of significance to the national infrastructure. Perhaps 100,000 GA aircraft are equipped with Loran receivers, about 10 times as many boats have Loran systems, and Loran receivers are used to support GPS receivers in cellular phone base stations carrying telecommunication traffic involving millions of Americans. But modal-specific systems such as VORs and marine radiobeacons cannot support other applications. In contrast, Loran is particularly well suited to aviation, marine, terrestrial, and time and frequency applications, and provides true GPS redundancy in all these roles. Moreover, Loranís infrastructure is established, operational, proven, and extremely cost-effective.
Can GPS be made to perform at increasingly higher standards and vulnerabilities eliminated? Certainly performance can be enhanced with progressively greater expenditures, but vulnerabilities cannot be eliminated. Technologies are simply not foolproof, and spending incredible amounts of money will not make them so. As suggested in the Litton review of the GRAS study, perhaps our national efforts should be spent determining the best combination of systems.
I have briefly reviewed some ways in which combining dissimilar technologies such as GPS and Loran can produce remarkably high performance, robust systems; creating such a system can also be done for a remarkably low cost. In a recent Booz-Allen & Hamilton study, it was estimated expenditures to decommission or upgrade Loran would be about the same (~$100M), and it is likely upgrade funds would be spent over about 5 years (i.e. ~$20M/year). For comparison, $20M represents about 0.04% of each annual DOT budget during that 5 year period ( FY2000 DOT budget request is around $50B). From an ongoing support perspective, Loranís operations and maintenance (O&M) would be about 3-4% (i.e. $15-20M) of the projected annual $500M O&M costs for the entire GPS system with augmentations, if you assume those projections will prove accurate. In the context of todayís multibillion dollar budgets, these numbers are extremely attractive for a proven, reliable system that could function not only as an insurance policy for the entire national infrastructure, but also as a GPS enhancement.
A Hybrid GPS/Loran System Is Good National Policy
In summary, GPS is a most remarkable system and should constitute the foundation of our national radionavigation infrastructure, but it should not be the only technology. For numerous technical, economic, and pragmatic reasons, Loran is uniquely capable of not only supporting GPS in the NAS, but also throughout the national infrastructure. Contemporary Loran performs much better than previously appreciated, and that level of performance can be improved. Most importantly, GPS and Loran can be integrated in a variety of ways, and a combined system outperforms what either can do alone, i.e. these are truly synergistic systems. We need to recognize this synergism and use it. US radionavigation policy should recognize and support Loran as a national asset Ė and as an asset to GPS.
 
Biography:
Linn Roth is Past President of the International Loran Association (ILA). He has been a member of the ILAís Board of Directors since 1995, was Vice-President in 1996, and is Chairman of the ILAís Committee for a Balanced Radionavigation Plan. He has received the ILAís Medal of Merit and Presidentís Award. Roth is also president of Locus, Inc., a Madison, WI company that develops and manufactures spread-spectrum radio modules for integration into industrial, utility, GPS and other OEM products and high performance digital Loran receivers for navigation and timing applications. He has a B.A. from the University of California - Berkeley and a Ph.D. in Neurophysiology from the University of California Ė San Francisco.