Category: Receivers

By Alan Grant and Dana Goward 

In my “First Fix” editorial in the January 2022 issue of this magazine, I listed 10 questions about eLoran I had received from a PNT expert in response to an article about eLoran I wrote for the November 2022 issue. I encouraged eLoran proponents to address these questions. Two well-known authorities, neither of whom have a financial interest in the technology, stepped forward to help. Below, again, are my 10 questions about eLoran and their answers.

Alan Grant is head of Research and Development for the General Lighthouse Authorities of the United Kingdom and Ireland (GLA). He is an expert in radionavigation systems and leads the team that established the U.K.’s eLoran system, which operated from 2007 to December 31, 2015 in support of maritime users. 

Dana A. Goward is president of the Resilient Navigation and Timing Foundation and a retired U.S. Coast Guard Captain. He also served in the federal Senior Executive Service as the maritime navigation authority for the United States. He has decades of experience with navigation policy and leading government policy and programs. 

— Matteo Luccio, Editor-in-Chief

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Accuracy specifics. While my November article stated that eLoran would have a two-dimensional accuracy of “better than 20 meters, and in many cases, better than 10 meters,” is that RMS, 95%, or some other statistic?

AG: Like any radionavigation system, the achievable accuracy will depend on several aspects, including the user’s location with respect to the broadcast stations and how error sources are modelled. The GLA eLoran service, when in operation in 2015, provided positional accuracy in the order of 8-10m (95%) to seven ports on the east coast of the UK. These ports had local reference stations to help manage temporal errors and the ports had been mapped to correct for additional secondary factors (ASF).i 

DG: Others have reported greater accuracies using differential corrections.

 

2. Performance standard. GPS provides a commitment to users in a published performance standard. What specific measures of positioning accuracy, integrity and continuity would you recommend the proposed eLoran system be committed to provide (using the architecture described in the answer to Question 6)?

AG: The target performance would need to be tied to the target use cases to ensure the appropriate requirements are met. IALA provides guidance in this area for maritime services with general maritime requirements provided by the IMO within resolutions A.1046 and A.915. 

 

3. Coverage. Would you recommend this eLoran positioning performance hold for the entire United States (including Alaska, Hawaii, Puerto Rico and other territories), only for the “lower 48” states, or only parts of these 48 states?

DG: The primary goal of any effort to complement and back up GPS/GNSS would be to make the nation and its citizens safer in at least two ways. First, to provide an alternative PNT source or sources in the event that signals from space were not available for any reason. Second to make GPS satellites and signals (and therefore the nation) safer by “taking the bullseye off GPS.” Having one or more alternatives will greatly reduce incentives for malicious disruption. To achieve these two goals the alternatives must be widely available and easily accessed. How widely available and easily accessed the United States or any other country wants to make such systems is a policy decision. 

 

4. Current users. By number of users, the predominant common current civil uses of GNSS for positioning are consumer devices (mostly cellphones). By contribution to the U.S. economy, the predominant uses are high-precision applications. For what fraction of these uses would eLoran positioning be adequate? Could an eLoran receiver and antenna fit in today’s consumer devices?

DG: Lots of presumptions and assumptions in this question. Several overall thoughts, though. First, determining users’ real requirements can sometimes be difficult. I have a nice new full-size sedan. So, I think that is my requirement even though I could get to work almost as quickly and much less expensively if I owned a used compact car or caught the bus at the corner.  

Second, GPS/GNSS will, hopefully, always be the primary source. The questions then are 1) how accurate can eLoran positioning become with additional work, and 2) how accurate does a fallback system need to be? 

Finally, as to equipment size, I recall seeing a photo of the first GPS receiver sitting on a pallet with two chairs for operators. Today, receivers are made at chip scale. Huge reductions in C-SWAP have been the growth arc for all kinds of technologies as they are implemented more and more widely. 

In 2017 the Dutch company Reelektronika showcased a combination eLoran, Chayka, GNSS receiver that was only 6 cm long. This was achieved without a whole lot of investment in research and development. Who knows how low C-SWAP for eLoran receivers will go? 

 

 

5. Future uses. Emerging civil uses of GPS for positioning include autonomous ground and air vehicles, navigation to space and in space, and lane-accurate car navigation. Which of these could be served by eLoran?

AG: The overall concept of having a mix of dissimilar position sources remains sensible for all modes. GNSS is expected to remain the primary means of position determination, with different use cases selecting different complementary systems based on their needs. eLoran may support some use cases but may not be the answer for all.   

DG: Many believe GPS alone is not sufficient to serve some of the applications cited. This is the basis for language in both the European Radionavigation Plan and a U.S. Presidential Executive Order cautioning against over-reliance on GNSS. Perhaps GPS and eLoran together might be deemed sufficient. Or, perhaps a more diverse and resilient PNT architecture will give rise to additional applications such as precise positioning from 5G that will be sufficient.  

 

6. Architecture. To maintain accuracy during a prolonged GPS outage, eLoran would require reference stations to calibrate time-varying propagation errors, as well as a certain number of transmitters for good nationwide geometry and for redundancy, ensuring service even if a transmitter is attacked or is taken off-line for maintenance. What architecture would you recommend to achieve this?

AG: The MarRINav project considered a similar question for the UK and the project’s approach could be employed to consider this question for the United States.iv 

DG: A good starting point for the United States might be the sites used by the shuttered Loran-C system. The federal government still retains custody of most of them. Also, considerable thought has been given to the questions of eLoran reference stations and integrity in the United States. PNT expert Mitch Narins, formerly of the FAA and now Strategic Synergies, advises that much of this work has been done. The FAA and Coast Guard conducted a study to deploy eLoran in the United States to support aviation non-precision approach, maritime harbor entrance and approach, and precise time and frequency users. The proposed architecture supported aviation’s demanding integrity requirement (1×10-7), maritime’s demanding accuracy requirement (8-20m), and time and frequency users’ precision requirements (100 ns/Stratum 1). 

 

7.  Infrastructure cost. What would be the cost of installing the required transmitters, power supplies, reference stations, communication links and control system for the architecture described in the answer to Question 6? Can you reference a recent and independent estimate? To a ballpark figure, what cost fixed-price contract would you accept to implement it? Similarly, what would be the annual costs for operating and maintaining this infrastructure? 

AG: The MarRINav project produced a cost-benefit analysis report that addresses some of these questions, albeit aligned to the approach proposed for the UK. The documents are open source and available on the MarRINav website.  

DG: To quote President Kennedy, “There are costs and risks to a program of action, but they are far less than the long-range risks and costs of comfortable inaction.” I agree with Dr. Grant that the capital costs in MarRINav are roughly transferrable to the United States. As another data point, the 2010 operating cost for Loran-C in the United States was about $36M/year. That number included several hundred employees, though. Plans to automate the system projected reducing annual costs to $15M/year in 2010 dollars. 

 

8. Impact.  eLoran transmitters are large and high-power. Providing positioning across the United States could require building some of them from scratch or significantly reconstructing old Loran sites. What issues — such as environmental, aviation safety and security — would this raise, and how would you recommend they be addressed?

DG: These issues would be dealt with the same way they are for any construction project. eLoran transmission sites are essentially the same as commercial AM radio stations. Reusing sites still owned by the government could make the process even easier. Compared to the cost and difficulty of putting PNT assets in orbit, these challenges should be relatively easy to overcome. 

 

9. Receivers. Assuming all the above were achieved, it would accomplish nothing unless eLoran receivers were widely purchased, installed and used. How much would that cost? Who would pay? Should we assume that “if we build it, they will come”?

AG: This is a valid concern and has different answers depending on the planned use case and the level of national/international standardization required. Within the maritime sector, the IMO has approved a multi-system receiver performance standard that supports the use of all GNSS and terrestrial systems within one device, rather than having a separate eLoran receiver.   

DG: I completely agree — adoption and use are absolutely key. Fortunately, government leaders have a wide variety of levers to influence adoption and use. These range from education and encouragement to regulation, legislation, and subsidies.  

 

10. Alternatives. Given the widespread development of other positioning technologies over the past decade, much has changed since the earlier recommendations for eLoran. How do we know that eLoran is the right investment — or even a needed part of the solution or needed system in a system of systems — for the future of U.S. PNT?

AG: The MarRINav project researched and compiled details of different positioning, navigation and timing technologies supporting maritime navigation, within Deliverable D4. The recommended system-of-systems approach recognized that there was no one-fit-all solution, rather it sought to allow for a scalable solution that reflects users moving from location to location and between systems. It considered global, regional and local solutions, recognizing the cost vs. usable coverage tradeoff for each. The proposed solution of GNSS, supported by eLoran in combination with VDES R-Mode and radar absolute positioning, was deemed as the most appropriate mix for the UK, given geographical and political constraints. The approach can be ported to investigate the appropriate options for the United States. 

DG: The U.S. Department of Transportation’s January 2021 report to Congress has findings similar to those in MarRINav. It described a system of systems that included fiber, satellites and terrestrial broadcast. The department subsequently said that a critical factor for a terrestrial broadcast system would be the coverage area per unit of required infrastructure. Of the systems discussed, eLoran met this criterion best. This recent finding is consistent with numerous other government reports, two previous government announcements that it would build eLoran, two recommendations from the President’s National Space-based Positioning, Navigation, and Timing Advisory Board and the technology’s on-going use around the world. Likely someday there will be something to replace GPS and other legacy technologies. We must work with the combination of technologies we have now until that day arrives.  

 

Common Threats

Common threats to GNSS and eLoran could include the following:

1. Cyber attacks. Given that GPS’s OCX is said to be the most cybersecure system built by the U.S. Department of Defense, how would eLoran’s control system be even more cybersecure than OCX, to avoid a common cyber-vulnerability? 

AG: Cybersecurity is a key concern and one that any navigation and safety of life system must consider. I will leave manufacturers of each system to comment on how secure they are. However, if we consider signal interference and data manipulation within this category, then using a stronger signal at a different frequency to GNSS provides some protection against jamming. While any radio signal can be jammed, the perpetrator would need more power and physically larger equipment to jam at lower frequencies.   

DG: Yes, the security of control systems is very important and must be included in the design up front. Authentication and security of signals, and the cybersecurity of receivers must be as well. This is especially true for complementary systems for GPS since GPS signals are so open and vulnerable, and so many receivers are largely unprotected. We will have the opportunity to do better with a new system and avoid the huge expenses of OCX, the new GPS control system. 

Additionally, let us not forget that cybersecurity is needed for much more than control systems. Signals and receivers need to be much more secure than civil GPS is right now. A new system, be it eLoran or another technology, will be able to build cybersecurity in from the beginning. 

 

2. Physical attacks. Given concerns about possible physical attacks on GPS satellites, which move at multiple km/sec 20,000 km from Earth, would it not be easier to physically attack eLoran transmitters, which are stationary, terrestrial, in remote locations, and hundreds of feet tall and require massive power sources?

AG: We should not lose sight that any ground infrastructure can be attacked, regardless of whether it is a satellite uplink station or part of a terrestrial communications or positioning system. Careful selection of the transmitter location, along with suitable site security options should help deter the attack and mitigate the impact where possible.  

DG: Every physical asset and every signal is vulnerable to some degree to attack by a host of malicious actors, and damage by a variety of natural occurrences. The key to resilience and making PNT sources less attractive targets is to have diverse sources with the smallest number of common failure modes. 

  

3. Space weather. GPS is potentially vulnerable to severe space weather that could damage satellites or temporarily hinder signal propagation from space to Earth. However, severe space weather could also damage the power grid upon which megawatt eLoran transmitters rely. How would eLoran service be protected from the effects of severe space weather, such as a Carrington Event?

AG: Space weather has the potential to affect all radio broadcasts. Depending on the type of event it can affect performance several different ways, including ionospheric scintillation, applying forces to satellites or disrupting power networks. The aim is to use systems where the underlying failure modes are as different as possible. Using a combination of satellite and terrestrial signals, at different frequencies, with local power generation where possible can help mitigate the impact.  Whether it’s possible to mitigate all the implications of a Carrington type event is not clear and perhaps one for the experts.  

DG: With the available warnings about solar events, it is conceivable that both GNSS and terrestrial systems could be powered off or otherwise secured for such an event to minimize damage. A new-build terrestrial system could also be constructed with surviving a Carrington Event in mind. And, of course, terrestrial systems will be easier to access, repair, and replace than those in space. As for other possible issues with the power grid, generators, uninterruptable power supplies, and other backup methods can easily be installed. Before 2010, several U.S. Loran-C transmitters were in such remote locations, they never had grid power and were always powered by generators.