Matteo Luccio
Alternative. Complementary. Backup. Co-primary. These are some of the terms used to refer to sources of positioning, navigation and timing (PNT) data other than GNSS satellites.
The four current GNSS constellations — supplemented by two regional ones and by public and private augmentation systems — have firmly established themselves as the primary source of PNT data by virtue of their accuracy, reliability, global coverage and ubiquitous use. Yet, this widespread dependency on them — especially on GPS — coupled with their well-known vulnerabilities to jamming, spoofing, other RF interference, multipath, solar flares and space debris (see page 10) — make the development of alternative sources of PNT data imperative. In fact, the U.S. Congress has repeatedly mandated it.
Typically, when talking about alternative PNT, we are referring to sources of PNT data that either were not originally developed for navigation purposes — such as television broadcast towers used as “beacons of opportunity” — or that use a higher broadcast power or a different frequency band than GNSS. They include legacy systems and new versions of legacy systems, such as eLoran.
“The only replacement for a GNSS is another GNSS.”
Other non-GNSS sources of PNT data have a wide range of benefits, limitations and costs, including infrastructure requirements. Most provide only the P and the N, or only the T, in PNT. Inertial systems, for example, once initialized can provide positioning and navigation, but need to be periodically re-initialized to compensate for their drift. Therefore, while excellent for maintaining the navigation solution during short GNSS outages and very helpful in identifying false GNSS measurements due to multipath, they are no replacement for GNSS. Cameras, radar and lidar, while often excellent sources of relative positioning, cannot provide absolute positioning.
It is even harder to replace GNSS when it comes to timing. Already enormously important in synchronizing the Internet, financial transactions and broadcasting, this service is essential to the development of complex new systems, such as integrating autonomous and legacy vehicles into digital traffic networks.
As in other human enterprises, the key to resiliency in PNT is diversity: a mix of systems based on sufficiently distinct technological foundations so that a threat to one does not imperil the other ones. Additionally, having a variety of available sources of PNT data will enable users to choose the ones most suited to their platforms.
However, we need to distinguish between technologies that can assist GNSS, such as inertial, and those that could substitute GNSS. I agree with Chuck Schue’s definition of the latter (see cover story, page 28): “an alternative PNT solution is one that is readily available; provides an easy and seamless transition to/from the primary or other alternatives; allows continuity of operation at a possibly degraded, yet usable, level of accuracy, availability, integrity or continuity; and is dissimilar enough from the primary solution to withstand the effects that might be affecting it.”
Ultimately, Schue pointed out to me, “the only replacement for a GNSS is another GNSS.” So, let us stop referring to systems that are not true substitutes for GNSS as “alternative PNT.” Complementary is a more appropriate adjective.