Building the Future of Geolocation: The Technical Journey to 5G-Powered 3D PNT

Highlights

• NextNav provides a unique terrestrial 3D PNT solution based on standards-based 5G that delivers timing and positioning capabilities independent of GPS.

• The company's solution rests on more than 150 patents that enable 3D PNT capabilities, offering a unique blend of attributes that could support commercial widescale deployment in the near term.

• NextNav has demonstrated GPS-free 3D PNT in multiple field tests, including those conducted in partnership with government agencies, and expects the solution's performance to improve further as the company transitions to its 5G-powered 3D PNT solution.

NextNav was founded in 2007 with an ambitious goal: to create the future of geolocation. America's principal positioning, navigation, & timing (PNT) system, GPS, is foundational to national security, public safety, and the US economy. But GPS signals are vulnerable to spoofing and jamming and have limited coverage indoors and in dense urban environments. The company provides a unique ground-based technology solution based on standards-based 5G that delivers timing and position solutions independent of GPS.

Our journey began to solve that problem with a ground-based complement and backup to GPS that could work where GPS couldn't. That's led us to develop our Pinnacle commercial z-axis solution, which is today used by FirstNet, among others, to provide accurate vertical location information to support public safety, as well as our 3D TerraPoiNT beacon technology. We have evolved those technologies into a 5G-powered 3D PNT solution ready to deliver a widescale terrestrial solution offering accurate positioning, timing, and 3D geolocation capabilities over 5G networks that currently cover the United States.

NextNav's PNT solution is both standards-based and highly differentiated. Our solution rests on more than 150 patents that enable 3D PNT capabilities, offering a unique blend of attributes that could provide commercial widescale deployment in the near term. NextNav's PNT system is based on downlink signals that use positioning reference signals (PRS) and leverages extensive licensed spectrum holdings in the Lower 900 MHz band and the global 5G ecosystem to enable a commercially viable solution at no cost to taxpayers.

The Timing Challenge
Any PNT system - whether satellite or terrestrial - depends on precise time synchronization and traceability to Coordinated Universal Time (UTC). You need to know exactlywhen a signal was transmitted to calculate accurate positioning information. In addition, traceability to UTC is required for delivery of accurate timing information. All terrestrial and satellite systems that offer a backup to GPS timing need a GPS-independent source of timing.

NextNav's core technical breakthrough was the development of a leader-follower network architecture that maintains time synchronization wirelessly, independent of GPS. The solution can provide GPS-independent timing accuracy and a holdover capability of 30 days when using high-quality atomic clocks at the leaders.

Here's how it works: Leader base stations anchor the network with UTC-traceable timing sources. UTC-traceable sources could include a National Institute of Standards and Technology (NIST) disciplined cesium atomic clock or fiber connections to the U.S. Naval Observatory (USNO) or NIST. Follower base stations then maintain relative synchronization by listening to neighboring stations' positioning reference signals in a wireless mesh network.

This architecture delivers three critical capabilities. First, it provides precise relative timing synchronization in the network needed for accurate 3D positioning. Second, it provides accurate wireless time distribution with UTC traceability with bounded time accuracy. Third, and perhaps most importantly, it continues to provide positioning and time distribution during GPS outages.

NextNav developed these time synchronization concepts as part of our previous-generation TerraPoiNT beacon technology which we have demonstrated in multiple tests, including those conducted in partnership with government agencies.

For instance, NextNav demonstrated GPS-independent timing and positioning through rigorous government testing over the past five years. The Department of Transportation's (DOT) Complementary PNT testing in 2020 demonstrated our 3D PNT technology, including the company's wireless time transfer techniques that enable GPS-independent operation. The DOT's independent report consistently rated NextNav as the top-performing complementary PNT technology across all timing and positioning categories the agency tested.

The DOT continues testing complementary PNT technologies, and NextNav was one of the select few that participated in its Rapid Phase I testing. That test examined the performance of the company's existing TerraPoiNT network operating in the San Francisco Bay Area. This environment offered a testbed outside of a controlled testbed in the DOT Volpe Center's PNT and Spectrum Lab in Cape Cod used by the majority of other participants. We look forward to seeing the results from this testing and are confident that the company's next generation solution will further improve on the previous TerraPoiNT network performance by offering a widescale solution delivered over existing 5G network infrastructure.

Timing Synchronization and 5G PNT
As discussed, without an independent timing anchor, you're still vulnerable when GPS goes down. Here's how NextNav will tackle that.

Our 5G-based 3D solution builds on the leader-follower architecture we proved with TerraPoiNT, and evolves it to scale across commercial 5G infrastructure. Under our solution, leader base stations would be equipped with atomic clocks through fiber connections and would support one or more UTC sources in addition to GPS. Leaders and Followers would maintain relative synchronization through our wireless mesh, a capability that operates completely independent of GPS.

5G networks already have fiber backhaul connecting base stations-the perfect infrastructure for distributing backup UTC traceable timing. These same fiber networks could be linked directly to NIST or USNO for UTC traceability, providing the accuracy level most applications require (within a few microseconds). For even greater resilience, leader base stations can incorporate diverse timing sources with different failure modes-including timing over LEO satellites, and if available in the future, other terrestrial systems. The number of Leaders in each geographical area could also be increased if there is a need for increased resiliency.

By adding precision time protocol connectivity and our wireless synchronization approach, we can turn existing 5G infrastructure into a resilient PNT network covering the vast majority of the U.S. population.

Like any terrestrial system, coverage is optimized where population and critical infrastructure are concentrated, but our solution can scale to rural areas as commercial 5G networks expand. Plus, it complements space-based solutions by providing the indoor and urban canyon coverage GPS cannot deliver in the places where most Americans live and work. Amongst terrestrial systems, a cellular based 5G based system provides the best available coverage driven by communication needs.

Availability of Signals for Positioning and Trilateration
While it may appear that a positioning system based on downlink cellular signals might require significant densification for positioning signal availability overlap for trilateration, that thinking is flawed since it assumes that the dedicated positioning signal has the same coverage as downlink cellular signals (such as Synchronization Signal Block). The downlink positioning reference signal (PRS) being a reference signal with known transmitted bits that stretches across the full bandwidth, provides the ability to detect low level signals using the high processing gain available during PRS range processing. The orthogonality introduced through comb and muting pattern PRS configurations, when chosen optimally, provides significantly better interference rejection which enables larger positioning signal coverage.

Additionally, a downlink cellular signal will have significantly better coverage than uplink cellular signals due to the significantly higher powers available on the downlink from the base transceiver station when compared to the low power available on user equipment.

Localized solutions such as Wi-Fi routers are available in some situations that can provide indoor positioning, but these are limited because they are not deployed everywhere. Even where deployed, their reliability can be limited due to issues like network reorganizations and power outages to name a few examples. A wide area solution like NextNav's will continue to provide positioning when localized solutions are not available and augments them when they are.

Another point to note is that while 5G PRS can technically operate across multiple spectrum bands, the Lower 900 MHz band offers superior propagation characteristics for indoor penetration and wide-area coverage, thus making it suited for a wide area positioning solution.