EAB Q&A: Diverse PNT sources essential for uninterrupted service

Image: GarryKillian/iStock/Getty Images Plus/Getty Images

Mitch Narins’ answer to the question, “If not GNSS, then what?” in the August 2024 issue’s EAB Q&A column, conveys an important message. As a result of their quality and availability, services offered by GNSS create dependencies and subsequently the expectation that of course they must always be there. However, recent experiences have shown we cannot rely on that expectation because either natural or hostile occurrences can disrupt GNSS services, no matter what measures are taken to protect them. That is why the U.S. Department of Defense (DOD), in its “Strategy for the DOD PNT Enterprise,” assessed that, “To combat man-made and natural threats to GPS, other sources of PNT information will be necessary to assure continuous PNT service …”

The strategy describes a layered PNT architecture using global (GPS), regional (eLoran), and local (or self-contained) sources of PNT information. It states, “The global PNT layer is space-based and available worldwide. The regional PNT layer may be implemented in areas … where PNT resiliency must be assured with backup capability. The local layer provides PNT information using man-made and natural information sources available for a limited time or over a limited area.” To achieve resilience from this layered architecture, the strategy provides an integration concept in which GPS and other GNSS are individually integrated with PNT information from the other layers into resilient applications to operate through the hostile environments they will encounter.

Unfortunately, the U.S. government is ignoring a major piece of this layered strategy in favor of space-based and local or self-contained solutions, as it has dismantled virtually the entire legacy Loran infrastructure in the United States and completely in Alaska, although GPS backups are lacking in the Arctic and northern Pacific regions. The loss of the Alaska sites is particularly concerning as the Arctic and northern Pacific Ocean coverage they would provide is a valuable backup to vulnerable GPS signals at a time when other nations are eying the sea lanes north of Alaska/Canada and conducting excursions around the Aleutian Island chain and the Alaskan coast. Coincidentally, the DOD has just published a new “Arctic Strategy,” which requires availability of PNT from GPS (at least) for its success, though “PNT” is not mentioned. However, without effective PNT, whether from GPS or other sources, the systems on which the strategy depends will fail.

As Mitch notes, there are those GNSS advocates who see strengthened GPS/GNSS as the best answer – but real-world events highlight the need for diversification, and now other nations are expanding their Loran-based regional systems. For its Arctic interests and domestic critical infrastructure as well, the U.S. must wake up to reality and do the same.