New Backward-Compatible Technique to Develop GPS+eLoran User Base

November 30, 2009

GPS researchers have developed a technique that could expedite adoption of Loran as a backup to GPS, demonstrating a cost-effective backward-compatible way to exploit eLoran to robustify GPS. Discarding the Loran-C infrastructure — as may be done soon by Department of Homeland Security Secretary Janet Napolitano — would be a regrettable waste of a valuable insurance plan against GPS outages and interference. The new method offers a way out of the current catch-22: There are presently not enough eLoran users to justify devoting more federal funds to developing and maintaining the eLoran system, but without a federal commitment to eLoran, no receiver manufacturer will risk the investment required to develop cheap eLoran-GPS receivers. The new technique paves a low-cost, backward-compatible way for the average GPS user to become a GPS+eLoran user. This blunts the criticism that Loran has a scant user base by showing that a user base could materialize quickly.

Todd Humphreys of the University of Texas and Brent Ledvina of Coherent Navigation have developed a technique that would expedite adoption of Loran as a backup to GPS. They have repurposed their civil GPS spoofer (presented in Profile of a Spoofer: Assessing the Spoofing Threat cover story, GPS World, January 2009) to generate “friendly” spoofing signals whose implied navigation solution is derived from a fusion of GPS and eLORAN observables. One can route the output from this so-called GPS Assimilator into the RF input of existing GPS equipment to immediately robustify the equipment against GPS outages and interference. The beauty of this approach, the researchers say, is that no hardware or software changes of the existing equipment are required: one simply replaces the GPS antenna of existing equipment with the assimilative device, which includes GPS and eLoran antennas and a digital signal processor.

"My colleagues and I continue to investigate the likelihood and effects of civil GPS spoofing and jamming," says Humphreys. "We recognize that eLoran, with its megawatt transmitters, low frequency, and signal authentication overlay, outstrips other current backup options in robustness against jamming and spoofing. Of course, others have also drawn this conclusion — notably the Independent Assessment Team whose report was released earlier this year."

“We've built a prototype assimilator that receives L1 C/A and L2C signals and outputs simulated L1 C/A signals. Here at the University of Texas Radionavigation Lab, we're working to extend this software-defined prototype to capture and process eLORAN signals whose observables will be assimilated into the navigation solution that drives the embedded L1 C/A signal simulator (the erstwhile spoofer).”

The attached PDF of their paper describes the method in more detail. Development of the assimilator technology is a joint effort between the Radionavigation Lab at the University of Texas at Austin and Coherent Navigation, a global navigation and communication solutions firm in San Mateo, California.

“What this method offers,” Humphreys continues, “is a way out of the current catch-22: There are presently not enough eLoran users to justify devoting more federal funds to developing and maintaining the eLoran system, but without a federal commitment to eLoran, no receiver manufacturer will risk the investment required to develop cheap eLoran-GPS receivers. The assimilator technique breaks this logjam by demonstrating a cost-effective backward-compatible way to exploit eLoran to robustify GPS.”

Humphreys plans to write to Secretary Napolitano to alert her about this research, in an effort to persuade her and others that an eLoran user base is just around the corner. One objection raised to continued Loran funding has been that the technology has few current users. The latter argument ignores, however, that wireless carriers use Loran as a timing backup, critical to all cell-phone use, and thus Loran could be counted to have hundreds of million of users — in the event of any GPS disruption.

“It will take enormous effort to come up with a backup system as good as eLoran.”