At one time, GPS was expected to supplant a wide range of navigation technologies in the world’s positioning, navigation, and timing (PNT) portfolio. But an unexpected thing happened along the way.

As GPS — and more recently, GNSS — moved from concept, to development, to reality, its vulnerabilities became more apparent, along with its remarkable qualities of accessibility, accuracy, and affordability. Interference to low-power GNSS signals, fear of spoofing attacks and intentional jamming, diminished performance in some operational environments — these and other factors have led policy makers and some user communities to reconsider their expectation of GPS universality and, instead, to seek alternative PNT (APNT) resources. In the United States, a 2004 presidential directive mandated creation of a backup system for GPS to ensure the uninterrupted provision of PNT services.

We called on Sherman Lo help us understand what is at stake in the search for APNT. Lo is a senior research engineer in the GPS Laboratory at Stanford University, where he earned his Ph.D. in aeronautics and astronautics. For the last several years, he has served as an investigator for the Federal Aviation Administration’s evaluation of APNT alternatives.

Alternative PNT