The defunct, obsolete LORAN navigation system is being revived with enhancements to overcome GNSS’s potential performance and availability risks.
Congress debated whether to retain and upgrade the LORAN-C infrastructure to become enhanced LORAN (eLORAN), a national backup to GPS. In October 2009, Congress enacted a Department of Homeland Security appropriations measure allowing LORAN-C termination. The Coast Guard began shutting it down in February 2010.
Q: Is that the end of the LORAN story?
A: Not at all. Just five years later, the U.S. House of Representatives reopened discussions of an enhanced version of LORAN called eLORAN (Figure 1).
In 2018, President Donald Trump signed into law the National Timing Resilience and Security Act, as part of the Frank LoBiondo Coast Guard Authorization Act, which mandates the Secretary of Transportation to establish and sustain a land-based timing system to provide a backup to GPS, namely “to ensure the availability of uncorrupted and non-degraded timing signals for military and civilian users in the event that GPS timing signals are corrupted, degraded, unreliable, or otherwise unavailable.”
Q: Is eLORAN radically different than LORAN-C?
A: Yes and no. It uses many of the same principles of intersecting hyperbolas but with more sophisticated coding, timing, and formats, all made possible by modern technologies.
Q: What does eLORAN offer?
A: The core eLoran system comprises modernized control centers, transmitting stations, and monitoring sites. The transmissions are synchronized to an identifiable, publicly-certified source of Coordinated Universal Time (UTC) by a method wholly independent of GNSS, so the system operates on a time
scale synchronized with—but operating independently—of GNSS time scales.
Q: What is the primary difference between eLORAN and LORAN-C?
A: The principal difference between eLoran and traditional Loran-C is the addition of a data channel on the transmitted signal. This provides application-specific corrections, warnings, and signal integrity information to the user’s receiver. In addition to positioning, navigation, and timing, the LORAN Data Channel (LDC) acts as a channel within eLORAN. It enables one-way data communication over 1,200 miles, with nearly 100% of its strength at that distance. The LDC includes time of day and clock corrections; encrypted, mission-specific data messaging and updates; authentication of eLORAN broadcasts (anti-spoofing); secure, un-jammable command and control; and can penetrate nearly all structures across all weather conditions.
Due to this data channel, eLoran can meet the very demanding requirements of landing aircraft guiding ships. eLORAN can also provide the exceedingly precise time and frequency references needed by the telecommunications systems that carry voice and Internet communications. eLORAN uses efficient, solid-state transmitters and three cesium-based atomic clocks per transmitting station.
Q: Isn’t eLORAN really just a modest upgrade to LORAN-C?
A: Not really; several points show that eLoran is not just a somewhat “modernized” Loran-C:
•It requires a different timing strategy, control strategy, and new equipment to meet more stringent
•It specifies tighter timing tolerances
•Transmissions are synchronized with respect to UTC (Coordinated Universal Time)
•It has a data channel for the broadcast of application-specific data
•It includes differential eLoran monitor stations and area-specific maps to provide optimum accuracy in key areas such as marine ports or airports)
Q: Why is eLORAN an attractive backup alternative to GNSS designs?
A: Right to the point: they have absolutely nothing in common. eLORAN is literally at the other end of the spectrum from GNSS and has completely dissimilar failure modes. It is a very high-power, low-frequency pulsed transmission, whereas GNSS transmissions are low-power, in the UHF band, and with multiple modulation schemes:
- GNSS is space-based, and eLORAN is terrestrial-based.
- GNSS is high frequency, eLORAN is low frequency—it’s 90 to 110 kilohertz.
- GNSS is low power; eLORAN is high power; at a typical receiver, eLORAN signals are three to five million times stronger than GPS/GNSS and have 99.999% availability and reliability
- GNSS can’t penetrate buildings and go indoors, while eLORAN signals can because they use a low-frequency wave and high power. It can penetrate through buildings, go underground, go through tunnels, and go underwater, and is unaffected by space phenomena.
Q: What about jamming or spoofing or eLORAN versus GNSS?
A: eLoran is exceptionally difficult to spoof or jam, and it is nearly impossible to do so at a distance. In contrast, the equipment required to spoof and jam GNSS only needs to mimic the relatively low-powered GNSS transmissions. Spoofing and jamming eLORAN requires very high-powered transmissions over a large area.
Q: Which companies are involved with eLORAN?
A: Among the participants are Hellen Systems and UrsaNav, Inc. Hellen has a contract to perform a GPS backup demonstration for the Department of Transportation. Their solution will include a solid-state eLoran transmitter from Continental Electronics Corp. integrated with advanced timing and frequency products from Microsemi Corporation, a wholly-owned subsidiary of Microchip Technology Inc. Hellen Systems and program integrator L3Harris will manage the demonstration, with Booz Allen Hamilton providing technical and engineering leadership.
UrsaNav, Inc. supplies eLoran, LFPhoenix, and low-frequency technology for very wide-area, GPS-independent, PNT data, and frequency services. The Department of Transportation Volpe Center selected UrsaNav plans to demonstrate wide-area UTC synchronization eLORAN signal distribution using a former Loran site in Wildwood, New Jersey, and show it can cover at least 700 miles or more.
Each participant works with other partners who provide additional specific expertise in components, system integration, installation, and evaluation.
Q: LORAN transmitters required significant on-site staffing (a challenge in the many remote locations). What’s the corresponding situation for eLORAN transmitters?
A: eLoran transmitters are autonomous, unstaffed, self-controlled, self-supporting, and remotely monitored.
Q: What’s the signal shape and format for the transmitted eLORAN signal?
A: It’s simultaneously simple, sophisticated, and complex. The basic signal uses a known Loran envelope shape to identify a reference zero-crossing, which is synchronized to UTC (Figure 2).
It is based on a pulsed signal with a 100-kHz nominal carrier frequency, with a group of eight pulses with one-millisecond spacing in a TDMA arrangement. The transmission of groups repeats every Group Repetition Interval (GRI), and up to 5 stations may share the same GRI to form a chain.
Q: What is done to support Master/Secondary signal identification and minimize the skywave problem?
A: The transmitted signals are phase coded (0° or 180°) for Master/Secondary identification and rejection of multiple hop skywaves (Figure 3).
Given the widespread use of GNSS and GPS, the low cost of implementation by the end user, and many other factors, eLORAN will not directly compete with it in adoption and popularity. However, for mission-critical situations, the availability of an orthogonal, totally independent alternative is an exceptionally attractive, desirable, and even necessary proposition.
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- S. Department of Transportation, “What is Positioning, Navigation and Timing (PNT)?”
LORAN and eLORAN
- Smithsonian, Time and Navigation, “Why Was LORAN Such a Milestone?”
- Smithsonian, Time and Navigation, “Hyperbolic Systems”
- Wikipedia, “LORAN”
- Britannica, “loran”
- Web Pages Of Jerry Proc, “LORAN-A”
- Military & Aerospace Electronics, “Public-private partnership to launch eLORAN technology to backup and accompany GPS.”
- L National Bureau of Standards, Monograph 129, “The Development of LORAN-C Navigation and Timing” (a 1972 source document, 166 pages)
- GPS World, “GPS backup demonstration projects explained.”
- UrsaNav, Inc., “eLoran System Definition and Signal Specification Tutorial” (47 pages PowerPoint, technically detailed)
- UrsaNav, Inc., “About Enhanced Loran” (brief overview)
- UrsaNav, Inc., “eLoran Points of Light” (16-page facts and myths – very useful)
- UrsaNav, Inc., “Resource Vault” (links to papers and tutorials)
- UrsaNav, Inc., “eLoran” (excellent two-page overview)
- S. Coast Guard, U.S. Department of Homeland Security, and Federal Aviation Administration, U.S. Department of Transportation, “Benefit-Cost Assessment Refresh The Use of eLORAN to Mitigate GPS Vulnerability for Positioning, Navigation, and Timing Services – Final Report (2009)” (37 pages with lots of bureaucratic content but also some nuggets)
- gov, “LORAN-C Infrastructure & E-LORAN”
- Spirent Communications, “Working With the Strengths and Weaknesses of Satellite Navigation Systems”
- Bliley, “What’s The Differences Between the 5 GNSS Constellations?
- GNSS spoofing Septentrio, “OSNMA anti-spoofing technology now on PolaRx5 GNSS reference receivers” [Open Service Navigation Message Authentication]
Hyperbolas and math