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.

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.