Building the future of resilient PNT

When the US Navy launched the first navigation satellite in 1960, it was focused on a relatively narrow purpose – providing a new navigation option for warships, missiles, and other military assets.  Sixty years after that first US satellite went into orbit, navigation satellites from Russia, Japan, European Union, China and India together comprise what is referred to as the Global Navigation Satellite System (GNSS) –a key cornerstone of the world economy.

Most people outside of the geospatial community know the acronym GPS and have a basic appreciation for what it does.  Yet few appreciate just how critical a role it plays in our everyday lives.  The position, navigation, and timing (PNT) services provided by GNSS infrastructure go well beyond the maps on our phones.  They are integrated into the very fabric of our digital world.  Soon, the “digital twins” of buildings will form the foundations of tomorrow’s smart cities.

Take timing for example.  GNSS is the source of accurate, synchronized time for everything from cellular networks to power grids to financial transactions.  As these systems start to rely on automation for day-to-day operations, a common timing source is needed to orchestrate complex processes and ensure interoperability.  The impact of accurate timing in all of these systems extends to everything from the clock on your phone to autonomous vehicles.

The economic benefits of GNSS are staggering, and will only continue to grow as new technologies and use cases are developed.  A recent NIST/RTI study estimated the value of GPS-enabled technologies across the entire US economy at $218 billion in private and public benefits between 2007 (the first year for which benefits could be reliably calculated) and 2017.  The estimated the overall economic impact of GPS on nine critical US industries was $1.4 trillion dollars.  A similar study found that economic contributions of GNSS to the UK economy were £6.7 billion every year.

The need for resilient PNT solutions

While the overwhelming value of GNSS technology and its importance to the world economy is certainly cause for celebration, our increasing reliance on GNSS services should also give us pause.  Any disruption in GNSS availability, reliability, resilience, and integrity would weaken the critical infrastructure that sustains our national security, business operations, and public safety.

In January 2015, a blip of just thirteen millionths of a second resulted in a cascading outage of GPS services.  Police, fire, and EMS services were disrupted in parts of the US.  The BBC’s digital radio service was out for two days.  Anomalies in the power grid were minimized only because short term backup timing systems were available.

Civil aviation relies heavily on GNSS as well.  In this case, the vulnerability of unencrypted signals to GPS spoofing and jamming are a significant challenge.  A steady parade of minor incidents and near-misses over the last decade indicate that a major accident is probably just a matter of time.  The impact of GNSS outages on drone aviation are also cause for concern.

A study by RTI International on the economic impact of GPS estimated that a 30-day outage of GPS could cost about a $1 billion per day in economic activity.  Beyond the impact on critical infrastructure like power, telecommunications, and financial systems, the follow-on effects on mobile devices, automobiles, and autonomous systems would also be significant. A disruption to GPS service would have an immediate impact on anyone that depends on these services for their employment, their ability to participate in commerce, and the lifestyle improvements associated with mass-market availability of GPS services.

The US government is starting to take notice of this potential threat to national security and the world economy.  Over the last five years, several government agencies have reported on the nation’s increasing reliance of Position, Navigation and Timing (PNT) capabilities to support the services nearly every American now takes for granted.

In February 2020, Executive Order 13095 laid the first policy marker for resilient PNT services as a national security concern.  The order tasks Federal agencies with creation of strategies to “ensure critical infrastructure can withstand disruption or manipulation of PNT services” and encourages the use of additional PNT layers to supplement GNSS.

There are many different pathways that alternative PNT systems can take.  At a basic level, the satellite positioning systems used by other countries (Galileo, GLONASS, Beidou, and others) provide redundant services which operate in the same manner as GPS.  Smaller, custom-built contingency solutions are often used in specific industries as well.

Terrestrial PNT systems also provide distinct advantages.  Since they aren’t in outer space, terrestrial systems provide stronger signals – an important factor for positioning in urban and indoor locations.  Terrestrial PNT also brings a higher level of resilience at a lower cost, allowing for more customized deployments which meet the needs of specific areas.

The framework for resilient PNT

What would a resilient PNT system look like, who would build it, and (perhaps most importantly) who would pay for it?

In April 2020, the US Department of Homeland Security (DHS) issued a report to Congress which lays out potential roles for the Federal government and private industry in creating and operating contingency layers for PNT. The position and navigation functions in critical infrastructure are so diverse that no single PNT system, including GPS, can fulfill all user requirements and applications.

The report shows that the uses of position and navigation data in critical infrastructure are so diverse that no single PNT system (including GPS) can fulfill all user requirements and applications.  In this light, the report recommends a series of application-specific PNT systems rather than a single alternative.  Given the diversity of operational needs, DHS recommends that requirements be developed in coordination with industry owners and operators, with regulatory and financial incentives to encourage adoption.

Building the future of PNT

As noted in the DHS report and elsewhere, no single technology can account for the wide variety of use cases that PNT services address.  The ultimate solution will require multiple technologies working in concert to serve as both complements and supplements to GPS.

With this in mind, NextNav is deploying a terrestrial system which both complements existing GNSS technologies and offers new functionality for a broad range of industry use cases.

Our TerraPoiNT solution consists of a network of long-range, low-cost broadcast beacons that transmit a GPS compatible signal on NextNav’s 920-928 MHz near-nationwide licensed spectrum – the equivalent of “terrestrial GPS”.   TerraPoiNT is a standards-based technology, supported in specifications generated and maintained by the 3rd Generation Partnership Projection organization (3GPP) starting with Release 13.

NextNav’s beacons are typically placed on cell towers and rooftops and are deployed and managed to deliver PNT services with multi-layer reliability. The TerraPoiNT system uses multiple transmitters in a given geographical area for system level redundancy, an onboard atomic clock that allows it to operate independent of GPS, and power supply backups to ensure continuity during power outages. The MBS system also provides multiple levels of cybersecurity protection.

The data broadcast by TerraPoiNT transmitters includes all of the necessary information for standalone timing, ranging and positioning of the receiver, including time, precise latitude, longitude and altitude of the transmitter.  A fusion architecture at beacons uses diverse time and frequency sources to enable a seamless transition between clock sources.

Unlike GPS, which relies exclusively on range measurements to estimate position, NextNav also broadcasts environmental data collected at each transmitter site. This allows devices such as phones and tablets equipped with low-cost micro-electromechanical system (“MEMS”) sensors to accurately compute their altitude with “floor-level” (1~3m) accuracy.

Mapping the journey

Building resilient PNT services will require time, coordinated effort, and adequate funding.  It will also require long-term commitments by both the Federal government and private industry to maintain the infrastructure and improve its capabilities.

Recognizing the scope of this challenge is only the beginning of a long journey.  Yet as PNT becomes even more important to the economy, the effort to build resilience into these systems will only become more crucial.  That’s why NextNav is building its vision for the future for PNT.