eLoran and GPS

By Gene H. McCall, Ph.D.*
Thursday, May 28th, 2015 @ 9:13PM

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Foreward: “On May 22, 2015, the US Department of Homeland Security agreed with the Exelis and UrsaNav Corporations to put an eLoran signal (a terrestrial system) on the air for testing and demonstration.

The vigorous advocacy of the Washington-based Resilient Navigation and Timing Foundation, helped this along. The RNTP should also be credited with the introduction of the bipartisan National Positioning, Navigation, and Timing (PNT) Resilience and Security Act of 2015, H.R. 1678. Accordingly, the Secretary of Defense would be required “to work with the Secretary of Transportation to build a terrestrial complement and backup for GPS.”

Earlier this year, the DOT asked for public comments on Complementary Positioning, Navigation, and Timing (CPNT), highlighting eLoran. Several experts objected to eLoran. Still, according to Exelis press-release, the “Department of Homeland Security’s Science and Technology Directorate (DHS S&T), and the U.S. Coast Guard have entered into a cooperative research and development agreement (CRADA) for testing and demonstration at former LORAN-C sites.” 

Considering how much time it takes to get anything done in Washington, this is a considerable achievement for the eLoran supporters, who also call on the National PNT Executive Committee, which will meet in June, “to discuss how funding for a GPS complementary system should be reflected in the President’s 2017 budget.

Not so fast, says Dr. Gene McCall, former Chairman of the United States Air Force Scientific Advisory Board and Chairman of the Global Positioning System (GPS) Independent Review Team (IRT).

We share his concerned that funds that could be used to support R&D on new Positioning, Navigation and Timing (PNT) technology will be siphoned off to pay for the eLoran system.– R.E. 

Dr. Gene McCall’s paper on eLoran and GPS explains:

Abstract

The possibilities and implications for using eLoran as a backup positioning navigation, and timing (PNT) system to replace the Global Positioning System (GPS) if the GPS is not available are discussed, and some ways for enabling preparations for the use of eLoran are discussed.

 I. Introduction

This paper will discuss the relation of eLoran to the Global Positioning System (GPS).

Author’s caveat: Nothing in this paper should be construed to indicate that the author believes eLoran to be an important, or useful, technology. It is not. The technology known as eLoran is the result of resurrecting the loran technology of World War II, adding minor capabilities, and stretching it to its limits. The result is a positioning, navigation, and timing (PNT) system that is obsolete, inadequate for use in the 21st century, and has no upgrade possibilities. It is not completely clear why advocates chose eLoran instead one of the more accurate systems from the same, or slightly later, era, such as Hi-fix or Hyperfix. Although hyperfix would require more transmitters because its range is shorter than that of eLoran, it would provide sub-meter errors at distances of 700 km with a transmitter power of only 40 watts. Timing modifications would, no doubt, provide better accuracy than that of eLloran. One assumes that the GPS backup would only be supplied for users within the United States, however, so shorter range should not be an issue. It may be that the proponents of eLoran felt that the government funding possibilities were more favorable for eLoran and there would be no proprietary issues for the companies involved. Performance may be a secondary issue.

There is, however, an effort being mounted to identify eLoran as a government certified, and funded, alternative to the GPS, in spite of its severe limitations. Some considerations related to this proposal will be discussed below.

II. The Issues

The primary issue given as the reason for establishing eLoran a complementary system to the GPS is that the signal received from GPS satellites is very weak, and it is, therefore, easy to deny by intentional jamming, unintended interference in the L-band, or by natural processes.

eLoran, on the other hand operates at a frequency of 100 kHz and at much higher received power because of its high power land based transmitters. No one denies this assertion, but it should be noted that any radiofrequency device can be jammed. The primary interference effect appears to be the result of eLoran signals themselves [1]. Thus, nearby transmitters, which transmit, simulated signals can be dangerous. eLoran receivers should consider the possibility of such signals and employ methods to reject them. The assertion of unjammability may, itself, generate vulnerabilities. The possibility of cyber attack against control and monitoring stations must also be considered.

The GPS has, certainly, matured into part of the critical infrastructure of American life and economy, and, even, that of much of the civilized world. However, the approach taken as the result of PNT vulnerabilities appears to differ greatly from that assumed as the result of the vulnerabilities of other important systems. The Director of the National security Agency and U.S. Cyber Command has stated publicly that the U. S. power grid is vulnerable to hacking, and others, such as former CIA director James Woolsey, have identified the sensitivity of the grid to electromagnetic pulses (EMP). There appears to be no one who is advocating that the U. S. Government fund a program to supply electric backup generators to every house in America, perhaps along with a substantial supply of candles and kerosene. Rather, all emphasize the importance of protecting the power grid against attack. Why then are so many adopting a sky is falling, and must have an alternative attitude in the case of PNT? Is it, simply, because no one has presented a better idea?

III. Is there a solution?

It has been more than21 years since the GPS was declared operational, and, as yet, the incidences of intentional jamming have been very localized and, essentially, fall in the vandalism category. It is likely that the most effective way to address GPS jamming issues is to, as the NSA director suggests for cyber attack, fight back. Most agree that our important infrastructure systems must be protected, and attackers must be repelled. It is of interest to explore that philosophy as it relates to PNT.

Some argue that the GPS signal is easy to jam because it is weak. It is important to explore the implications of this statement.

A GPS receiver processes the received signal to achieve a gain in signal to noise ration of, approximately, 40 dB, or a factor of 10000. The processed signal exceeds the thermal noise background. Thus, a jammer signal must generate a received power that is substantially above thermal noise if it is to overcome the processed GPS signal. The result of this requirement is that a simple receiver easily detects the jamming signal. Following detection, it is rather easy to locate the source of the jamming signal. A number of investigations have shown this to be true. (See, for example ref.[2]).

Jammer detection and location receivers are inexpensive, and a network could be established throughout the United States and coupled to a first responder alert system, which would show the jammer location. First responders could be trained local police or fire department employees who would find the source of the jamming and eliminate it. It is possible that they would be equipped with handheld locators to make the final identification of the jammer. Under these conditions, jamming would be short lived, and jammer equipment and its users would be quickly seized. It is certain that the equipment, networking, and training of first responders would be substantially cheaper and more effective than the creation of a nationwide loran system.

The simultaneous response to the threat of jamming should be the development of a nearly invulnerable GPS follow-on system. It should be recognized that the GPS technology is more than 50 years old. It is time to develop the system for the next century. It is very likely that the next technology will consist of the merging of networking technology with satellite PNT technology. Rather than mandating a solution, however, the nation’s industry and universities should be enlisted to propose and to develop the protected PNT technology for the 21st century. The situation is somewhat like that described by Edward Teller when discussing the atom spies of

World War II. He said that the spies were effective in delivering nuclear weapon design ideas to the Soviets. We developed new security measures, and we caught many of the spies, but the truly effective response was to work harder and to use our superior abilities in science and technology to stay ahead of, and to outdistance, our adversaries, and through this method to, ultimately, win the cold war[3]. The message is appropriate for the current discussion:

Now is the time to protect, defend, and develop. It is not the time to surrender to hypothetical adversaries and to retreat.

Continuing the discussion of eLoran, however, it is appropriate to observe that the position accuracy of eLoran is adequate for marine navigation where channels tend to be much wider than the beam of a ship. Aviation, however, requires a separate treatment.

IV. The Aviation Situation

First, it should be noted that eLoran is severely handicapped when considered for aviation applications. It is, fundamentally, a two-dimensional system, which produces no vertical component of position. Even in two dimensions, however, the performance is not impressive. Figure 1 shows a graphical representation of the two dimensional errors measured for eLoran, GPS, and the FAA Wide Area Augmentation System (WAAS), which enables precision-quality approaches at many airports that never before had such an approach.

Currently there are 3547 WAAS approaches at 1730 airports in the United States. If the GPS signal is denied nationwide, or, even, replaced with eLoran, all of these approaches go away immediately. Only 600 airports have the old Instrument Landing System (ILS) equipment, and, therefore, 1130 airports will no longer have a precision approach capability at all. Regional airline service will be severely impacted.

It is said that eLoran provides navigation accuracy adequate for non-precision approach, or as pilots say, dive and drive. While not a very impressive statement, it is, also, not the end of the discussion. The existence of navigation instrumentation is only the first requirement for enabling an instrument approach. The area around the airport must be examined for obstacles. A safe path, which results in reasonable alignment with a runway, must be determined. The path must be test flown by FAA pilots to assure its utility and safety, and an approach plate showing altitude and direction changes must be prepared by a group at the FAA Oklahoma City facility. The procedure is then verified by altitude flown by experienced FAA test pilots and FAA aircraft. Even this fairly complicated description is a rather abbreviated version of the actual process. The author followed the process rather closely in the early days of the generation of GPS non-precision approaches. The cost per approach is, approximately, $50,000. If, at a minimum, 1000 eLoran approaches must be generated, the cost of preparing for the use of eLoran at, even, a minimum number of instrument approaches is 50 million dollars. The cost of instrumenting aircraft with the necessary eLoran equipment is also problematic in this era when airline companies concentrate so intently on the return on investment (ROI) of any new equipment installed in an airplane. It must be decided whether the government will undertake the financial risk of providing equipment to the airlines, or, alternatively, undertake the political risk of mandating eLoran approach equipment on airliners. Equipment that we all hope will never be used.

Screen Shot 2015-05-28 at 3.15.30 PM

Figure 1: Graphical representation of two-dimensional eLoran and gps errors as measured by the FAA. The red curve is eLoran, blue is GPS, and green is WAAS.

V. Positioning

Before discussing time transfer, it is instructive to address relevant accuracy issues.

GPS position measurements are related to positions on the surface of the earth through well-known models of the shape of the earth. For a discussion of this process, one is referred to ref. [4]. Errors in position result from ionospheric effects and from imprecise knowledge of the satellite ephemerides. The ionospheric error can be corrected by measuring phase differences of the arriving signals at two different frequencies. Thus, the ionospheric error is corrected within the system, itself. The primary source of the remaining error is the inaccuracy of the satellite ephemeris. This error can be reduced by decreasing the latency time of the ephemeris measurements. It has been demonstrated that use of the ephemerides determined by the Air Force every 15 minutes can reduce the position error to a reliable, and continuous value of 0.89 m., or less.

The situation with eLoran is quite different. Position measurements depend on measurement of the arrival times of surface wave signals from different transmitters. Thus, the velocity of the wave between the transmitter and the receiver is critical to the position measurement. The velocity over water is well known, and it can be calculated accurately. If, however the wave travels over the ground and structures, the accuracy becomes much poorer, and calculations must be calibrated by making many accurate measurements. It is clear that the error in the time required for the wave to travel from transmitter to receiver effects both the position and the time errors. A 20-meter position error. then, produces a time error of, approximately 70 ns. The time error is, thus, dependent on the distance of the receiver from the transmitter.

VI. Timing

An important feature of modern PNT systems is the transfer of accurate time from a reference station to a user. eLoran time error is usually quoted as being less than 100 ns, and it is said that this value can satisfy the requirements established by governments. While this statement is, undoubtedly, true, experience with the GPS performance has shown that the system has, historically, performed better than its government-specified requirements. Further, users have taken advantage of this additional performance and have used the improved performance immediately after it has been shown to be reliable and reproducible. Current time accuracy of the GPS is near one nanosecond [5], and errors greater than 10 ns are unacceptable.

If GPS protection measures as described above are taken nationwide, it is unlikely that GPS outages much greater than an hour will occur, at least over large areas of the nation. A timing backup consisting of an atomic clock on a chip calibrated regularly, at least once an hour, would be a more reliable time backup system than eLoran in the unlikely event of the absence of the GPS signal.

VII. A Way Ahead?

Given that there are those who believe fervently in the utility of eLoran as a GPS backup, the question arises as to how the service can be made available to those who need, and want, it in case of emergency.

It is recommended that the Decca navigation model be adopted.

eLoran is being proposed, established, and tested by a very small number of companies in the United States. Most of the work is being done by only two companies, Exelis and Ursanav, Inc., It is possible for the government to, say, lease the old loran-C sites, buildings, and hardware to them for a dollar a year and give them the sole right to produce and sell eloran receivers in the United States. The companies would receive all the profits from the sale, or lease, of equipment to those who feel the need to back up their GPS capabilities. If the urgent need that the proponents claim truly exists, the companies will recoup operating expenses of the eLoran system and make a healthy profit, as well. Whether the government should mandate backups in specific critical areas is another issue.

VIII.  Conclusion and Recommendation

Let the marketplace decide.

—-

References

[1] j. Safar, F. Vejrazka, and P. Williams, International Journal on Marine Navigation and Safety of Sea Transportation, 5,March 2011.

[2] A. Brown, D. Reynolds, D. Roberts, S. Serie, Proc. of the ION ‘99, September 1999, Nashville, TN.

[3] Edward Teller, private communication in conversation with the author

[4] E. Kaplan and C. Hegarty, editors, Understanding GPS: Principles and Applications, Second Edition, Artech House, London, 2005.

[5] Lewandowski, et al, Indian Journal of Radio and Space Physics, 36(4), 303, 2007.

—-

Gene H. McCall is Laboratory Fellow (ret.), Los Alamos National Laboratory. He can be reached at ghm7723@gmail.com

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