Many PNT sources from multiple providers needed

“We cannot have GPS signals be a single point of failure for transportation and other critical infrastructure sectors. More safety applications will depend on PNT in the future. Public confidence in these will be critical.

“People will not be comfortable getting into an automated vehicle or with platooning driverless trucks heading down the highway if they think that their invisible hand is not reliable and that their GPS might be spoofed.

“Getting public adoption of other PNT capabilities — space-based, terrestrial, and self-contained — integrated with GPS technology will be critical to the success of any such system.”

— Diana Furchtgott-Roth, Deputy Assistant Secretary for Research and Technology, U.S. Department of Transportation, Nov. 20, 2019, Edinburgh, U.K.

A Single Point of Failure

The Department of Transportation (DOT) is responsible for leading civil positioning, navigation, and timing (PNT) issues for the United States. At the moment, the U.S. GPS provides the vast majority of PNT services in the U.S. and around the world. Yet, like all space-based systems, its signals are weak and very vulnerable to interference.

A recent example of how dangerous that can be in automated transportation systems was revealed recently in an accident report released by the British government. Interference from an unknown source caused a 15.5 kg drone to get away from its operator and crash. Fortunately, no one was hurt. The report cited analysis showing that such a weight could have easily killed someone on the ground.

Even more concerning, GPS signal characteristics are well known and therefore easy to imitate. Thousands of cases of “spoofing” have been documented with government and malicious actors causing receivers to report they are far from their actual location. In the worst cases, this can cause accidents or enable criminal acts.

One result of all of this is the President of the United States issuing an Executive Order encouraging “responsible use” of PNT systems. It also directs steps to encourage development and adoption of alternative systems. This includes a White House-level plan for research and development of non-Global Navigation Satellite System (GNSS) PNT.

In Europe the European Union (EU) has warned that space based PNT alone is insufficient for “…critical applications requiring continuous availability and fail-safe operations.” The EU has also established a monitoring system to detect sources of GNSS interference, and the European Space Agency (ESA) has established an on-going program funding study of both space and terrestrial alternate PNT systems.

Multiple Cooperating Systems

The ultimate solution, though, according to senior government officials, will be development and use of many diverse PNT systems working together to ensure users have what they need when and where they need it.

Image: DOT

The National PNT Architecture, jointly developed by the US departments of Defense and Transportation, envisions a multitude of PNT sources ranging from GNSS provided by national governments, to inertial and clock suites acquired by users as needed.

“Many people are fond of talking about a GPS backup,” said one administration official.

“It’s more appropriate to use the plural ‘backups’ since one system isn’t going to meet everyone’s needs. Even GPS doesn’t meet everyone’s needs which is why we require complementary PNT capabilities.”

The idea that multiple redundancies are required for an essential function as long been a core principle of systems engineering. This is clearly foundational in the National PNT Architecture.

It is also a feature in more recent documents.

One example is the U.S. Department of Defense’s (DoD) PNT strategy publicly released in August of last year. It envisions use of a multitude of systems as a way of “Ensuring a U.S. Military PNT Advantage.”

Image: DOD

It categorizes these in three layers. A global layer of GNSS and other satellites, a regional layer that includes STOIC and eLoran, and a local/autonomous layer populated by inertial, clock, lidar, radar, scene matching and beacon-based systems.

Another project taking the architecture approach is described in detail by the recently completed MarRINav report. Sponsored by the European Space Agency, it analyzed the PNT needs of maritime commerce in the United Kingdom.

The project concluded that a “hybrid approach” using GNSS, eLoran, and the short-range R-mode VDES would be the best and least expensive combination for maritime. It also recommended a local navigation system such as Locata for port cargo operations. The study found that such a combination of systems would also benefit other transportation and infrastructure sectors.

Implementation

Yet identifying solutions is often much easier than making them happen. Especially for national projects with dozens of stakeholders. Stakeholders who may often have…