GPS Backup: Is eLoran the Answer?

Avionics Today, April 12, 2012, by Callan James

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Recent headlines about possible threats to the GPS network is bringing new life to the discussion about the vulnerability and potential backups to the ubiquitous system. Despite the defeat of the LightSquared broadband proposal, the threat of GPS interruption, intentional or otherwise, remains a reality for the aviation industry, experts say.

According to researchers in the U.K., using its Sentinel GPS monitor network, between 50 and 450 jamming incidents were detected every day over a six month period last year. The Department of Homeland Security (DHS) in the coming years plans a similar monitoring scheme across the United States, called Patriot Watch. Unquestionably, such continuing investigations can only be an endless cat-and-mouse game between adversaries, during which time experts anticipate civil GPS jamming will continue to escalate in intensity, coupled with decreasing availability and integrity for users, while underscoring the increasingly urgent need for an unjammable GPS backup for aviation and all other critical national needs for positioning and timing.

This has brought about the recent Federal Register request for public comment on the FAA plan to reduce the size of the national VOR network — while retaining DME — in readiness for NextGen’s Performance-Based Navigation (PBN) plan, and as a skeleton backup to GPS and Wide Area Augmentation System (WAAS), should jamming render them unusable. Under the plan, most ILS installations would be retained as backups for GPS Local Area Augmentation System (LAAS) for the same reason. But reducing VOR numbers down to FAA’s Minimum Operational Network (MON) has the unfortunate effect of raising the VOR’s nationwide reception “floor” to above 5,000 feet, due to line of sight limitations. Losing GPS at night while cruising at 3,000 feet sounds challenging

Experts say eLoran, an enhanced version of the Loran-C long-range, ground-based navigation system, could provide a backup to GPS from the ground up and the essential timing signals for the nation’s critical infrastructure. ELoran was not included in FAA’s plan. (The U.S. Coast Guard terminated the transmission of all U.S. Loran-C signals in 2010.)

“Today, eLoran is the only system that can fully back up GPS, and all other GNSS systems planned or in use in other countries,” said UrsaNav President and CEO Charles Schue. UrsaNav, a diversified technology company based in Chesapeake, Va., has assembled what could probably be described as one of the world’s leading centers of excellence in Loran-C and eLoran.

“What’s more, it’s becoming clearer and clearer that GPS interference, whether inadvertent or deliberate, will continue to grow, thereby causing GNSS to become less and less valuable. In that arena, eLoran provides two benefits. First, it can prevent the interruption or loss of vital satellite services. But second, and less widely appreciated, it can vastly reduce the incentive to jam GNSS systems when it becomes known that they have constant and essentially unjammable backups,” Schue said.

ELoran retains the powerful long range — and unjammable — 90-110 KHz low frequency signals of its predecessor, plus their solid coverage from the surface to well above 60,000 feet, and out to over 1,000 miles, it is otherwise totally different.

From a pilot’s point of view, Loran-C suffered from two serious drawbacks — precipitation static and the transmitter station’s inconvenient configurations in “chains” or groups of three or four stations to provide regional navigation coverage. P-static was eliminated by H-field antennas, such as semi-flush ADF loop antennas. ELoran dispenses with the old “chain” concept. Every eLoran transmitter would be totally independent of all others, with the system operating on an “all in view” basis, exactly like GPS, where incoming signals are selected for their best fix geometry.

But while the old Loran-C concept provided fix accuracies of less than a quarter mile, differential eLoran has met and exceeded International Maritime Organization standards of 8 to 10 meters for harbor entrance applications, with that performance attributed to its much higher signal stability of its predecessor. Similarly, its +/- 100 nanosecond accuracy at the receiver antenna underscores its unique timing capability, making it one of just three methods of achieving the US national Stratum 1 standard, with the others being GPS and certain laboratory atomic clocks. One consequence of this is that a U.S.-wide eLoran network could provide continuous RNP 0.3 performance throughout the CONUS.

“It is now generally recognized that advanced low frequency signals, of which eLoran is one example, can provide alternative timing — either as a stand alone service or as a component of an existing positioning, navigation and timing service,” said Don Jewell, retired U.S. Air Force and GPS subject matter expert. Jewell has been involved in GPS since its inception; he has served as a commander at Schriever Air Force Base, Colo. — the “home” of GPS — before retiring as deputy chief scientist at Air Force Space Command.

“High power, virtually jam and spoof proof low-frequency signals operate independently of GPS and provide a Stratum-1 Universal Coordinated Time (UTC) reference. The recognition of the criticality of time to many aspects of our overall critical infrastructure beyond simply GPS has hopefully finally led to an evaluation of the benefits of a nationwide low-frequency timing system,” Jewell added.

Airborne eLoran operating procedure concepts would also be very different. Many see it as a totally autonomous sensor on a chip, running in parallel and cross checking with the GPS — possibly even embedded in that unit — but with the FMS automatically switching to eLoran should tracking divergence due to GPS jamming be indicated. When the FMS determines that the GPS is no longer being jammed, it switches back to satellite navigation. And there would be no pilot eLoran controls or displays, other than perhaps a flight deck annunciator indicating a GPS sensor outage.

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