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
|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"
|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.
|Figure 1. Loran H-field antenna with
outer diameter of approximately 4 inches. Photograph is courtesy of Megapulse,
|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
|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.
|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
|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.
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
|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.
|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.