GPS World • July 2022 • COMPLEMENTING GNSS

As GNSS have grown in importance and number — see our October 2021 cover story on how four complete GNSS enhance surveying accuracy and speed — so have concerns about their vulnerabilities and limitations. In parallel, the development and deployment of complementary sources of PNT continues apace, with increased encouragement by the U.S. federal government.

Complementary PNT sources are particularly capable as to the T in the abbreviation, and likely will never replace GNSS as to those P and N uses that require high accuracy. They include space-based and land-based systems, both existing and under development, some new and dedicated and some piggybacking on existing systems, such as Iridium satellites or such “beacons of opportunity” as television broadcast towers. They also include network-based time transfer.

I asked four companies — Spirent Federal, Satelles, Orolia, and OxTS—for their perspectives on this subject. Much longer versions of my interviews with the first two companies appear on our website.

New Approaches Improve PNT Resilience

What are some of the most promising approaches to complementary PNT sources and how does simulation technology help?

The vulnerabilities of GNSS have been recognized. Legacy GNSS are all operating on pretty much the same frequencies and power levels, so they have some significant common vulnerabilities. There is great interest in finding ways to complement or even replace those capabilities.

Dead reckoning, magnetic and inertial systems have been around for a long time. There are emerging markets to make use of alternative radio frequencies for navigation. In some cases, we are piggybacking on communications signals and deriving PNT from them. In other cases, we are using new PNT signals. They may be using different orbits, different frequencies, different encoding schemes that set them apart from the legacy GNSS systems, so that, used together, they provide greater resiliency and even stand alone when one or the other system may be affected by interference.

Not to be forgotten is inertial navigation. It’s been around for a long time and is still a standard of navigation. Together with GNSS, it makes a terrific navigation system. It almost defines complementarity because where GPS is vulnerable inertial can fill in the gaps and where inertial drifts GPS does not. So, paired, they make a very strong system.

What are some of the remaining challenges of integrating GNSS receivers with inertial sensors and, again, how does simulation technology help with that?

Inertial works by integrating sensor measurements that come in. Therefore, any errors that are present just accumulate over time and can corrupt your navigation solution. So, there’s a strong focus on updating error models and on translating them so that everyday users can use them and get real-life-type performance out of them.

There’s a tendency to think of integrating GPS-INS as putting everything together in one box. There are packages that do that. However, the push now is to go to more distributed systems that are integrated but not packaged in the same box.

Much work has gone into addressing the enduring challenge of urban canyons. How does simulation technology help?

Urban canyons are the worst nightmare for GNSS signals. If you’re surrounded by tall buildings, signals are blocked. You may have few or even no satellites in a direct line of sight and many multipath reflections. Of course, the more GNSS satellites you have, the better chance you have of getting good signals. But complementing that are radar and vision systems. Those are the ones that will stand out, particularly the vision systems that can read the street signs, see where the curb is, look for parked cars.

You can observe what’s going on in the environment and simulate it. You can also use our forecasting tool to look ahead.

You can’t know how your system will work by individually testing each piece. System is the key word here. Simulation enables you to generate the signals and bring them together into a sensor fusion engine. You can test different algorithms. It’s certainly much cheaper and quicker than trying to build this into a product and then test it.

High-Power Satellites Go Beyond

GPS and GNSS are amazing. We designed Satellite Time and Location (STL), the service that we offer, to complement those capabilities. We have focused on three areas where GPS could use complementary service. First, we provide a fully independent source of PNT. Second, we focused on the high-power aspect of STL to enable us to reach indoors and other places where GPS does not reach. Because STL comes from LEO satellites, the signals are naturally at a higher power. We also focused on improving the indoor penetration capability by enhancing the signal design and doing some other things. Third, we use modern cryptographic techniques to ensure the security and resilience of the system, specifically to intentional misdirection attacks.

We have never considered LEO (low Earth orbit) PNT as a replacement for MEO (medium Earth orbit) GNSS. GNSS are the primary domain of PNT but there are applications that have additional needs. The more independence you can get, the fewer the common modes of failure, if you can at least have some survivability in the absence of GNSS.

What’s interesting about the way we work with the Iridium satellite constellation is that the satellites themselves include inter-satellite links. That provides a lot of resilience to ground-based events. The satellites themselves have a time transfer capability between them. So, we don’t require a direct connection to every satellite to propagate time throughout the network.

Additionally, we have multiple ground infrastructure and monitoring sites and multiple sources of time at those ground monitoring and control stations. Some of them rely on GNSS combined with atomic clocks as their master timing source but we also have one installed at the National Institute of Standards and Technology facility in Boulder, Colorado. So, we have multiple primary time sources that we can integrate into our filtering across the network. That, combined with satellite links, allows us to maintain time for substantial periods independent of GNSS.

Besides the orbit height, which requires many more satellites, how does STL differ from GNSS?

The fundamental difference is being in lower Earth orbit, which results in a higher received power. The measurable Doppler signatures give additional observables for PNT calculations, and higher satellite dynamics that can help with multipath. This service relies on many of the same physics and geometry as GPS. We measure the time of arrival of a very similar signal. The signals from the Iridium satellites are even in the L band. Very often we’re using a GPS chip that’s been reprogrammed to track and utilize our service as well as GPS or instead of GPS.

Satelles’ LEO satellite solution is available today, has global coverage, and is relatively affordable. It leverages the capital investments that have been made to launch the satellites to provide this service globally. Data centers, stock exchanges and cell phone providers are implementing these capabilities today. The major wireless operators are seeing that more and more of the 5G infrastructure they roll out is going indoors, where GPS doesn’t reach. We provide a solution that integrates with their existing solutions and can provide reliable timing capabilities.

5G Promises Deeper Connections

What are some of the most promising approaches to complementary PNT and how does simulation technology help?

5G is the most promising for the future. I believe the benefits in infrastructure, speed, precision, reliability, and the industry incentives 5G offer are superior to GNSS. Alternative signals of opportunity and new commercial satellite-based providers are always valuable as extra layers of resilience. However, PNT from 5G is not quite ready yet. There will be a transition period during which systems use GNSS and these signals of opportunity simultaneously, so simulation enables receivers of any complementary signal to be tested in the same environments and with the same potential threats faced by primary constellation signals.

Orolia has the most atomic clocks in orbit, including those aboard the Galileo constellation. We integrate anti-jam antennas and build Interference Detection and Mitigation (IDM) into our products. We partner with companies that offer alternative signals, such as STL from Satelles. Our SecureSync NTP and PTP time servers live in the world’s biggest data centers and support encrypted signals, such as M and Y code for our militaries. We innovate with industry leaders such as Meta on building a better PCIe Time Card. We offer edge time servers with the ability to automatically add Hoptroff’s Traceable Time as a Service. If 5G PNT becomes a standard, we are already providing industry leaders such as Anritsu with solutions for acceptance testing on a major carrier’s backbone. With our pending acquisition by Safran and access to a world-leading portfolio of INS components, we are one of the most qualified companies in the world to solve nearly any PNT challenge.

What kinds of complementary PNT are most useful in addressing specifically the challenges posed by jamming and spoofing, and how does simulation help?

In two technical notes published by NIST, they recognize STL as one of four recommended solutions for PNT resilience and the only one being both independent of GNSS and capable of sub-microsecond accuracy. Being closer to Earth, it is a stronger signal, making it 1,000 times less susceptible to jamming. Additionally, because it is encrypted it is inherently immune to spoofing. The aforementioned Hoptroff TTaS is time delivered over VPN, removing the outside environment component completely. For positioning and navigation, the integration of an IMU provides a contiguous PNT solution even during periods of GNSS denial, analogous to how an atomic clock provides precise time holdover during these denial periods. Combined with anti-jam antenna technology and IDM software, a robust PNT solution is always available.

Simulation helps by (1) identifying the vulnerabilities your PNT system might have (or could have in the future to evolving threats) and (2) verifying the total integrated resilient system. Our GSG-8 Advanced GNSS Simulator supports hundreds of GNSS full spectrum signals, custom signals, and hardwarein- the-loop testing of integrated IMUs at up to 1000 Hz iteration rate. Our Skydel Wavefront and Anechoic simulators can verify the most complex GNSS anti-jam antenna systems.

Ultra-Wideband Brings Signals Indoors

What are some of the most promising approaches to complementary PNT and how does simulation technology help?

There are two approaches of particular interest. The first is looking at LEO satellite systems that can provide supplementary and potentially more secure methods of navigation, with global coverage from a single system. But these will still suffer from some of the issues GNSS systems experience, namely, what happens when you can’t obtain a signal?

The second is the use of visual aiding through sensor fusion, such as lidar and cameras, that can provide relative positioning (or absolute positioning once you have a space mapped) using SLAM algorithms. While this may increase onboard hardware dependencies, it creates a localized navigation system that can be better protected from malicious actors.

In contrast, closed-loop systems can look to an infrastructure-based system, allowing free movement within the specific area in which the infrastructure is located and a potentially more reliable source of PNT, especially indoors, where GNSS is not available. Ultra-wideband is definitely the up-and-coming technology here, but systems using Wi-Fi, cameras, Bluetooth and others also are being used.

Simulation, as within many domains, allows users to test on a large scale with fewer barriers to entry than real-world testing and an ease in making iterative changes to find an optimal solution. Whether that is to benchmark performance in locations of interest or to change configuration settings to improve visibility or positioning, simulation allows you to do this without the expense of going straight into the environment itself or configuring the actual vehicle under test.

OxTS provides customers with the ability to navigate anywhere; whether for reference data in R&D, georeferencing for survey and mapping, or active navigation of autonomous solutions. To do this we provide an IMU-first offering that we then complement with other technologies. Traditionally, this is with GNSS, to form an INS that can provide centimeter-level accuracy. However, we are also aware of the vulnerabilities of GNSS. For us, this is when it becomes an unreliable source of PNT in denied areas, such as indoors, in urban canyons or under tree canopies.

Because of this, we are also investigating and developing complementary solutions that can enhance our offering for users who need confidence in their position even when GNSS is not available. Whether that is through sensor fusion, our Pozyx UWB solution for indoor navigation or other proprietary software and firmware capabilities.

What kinds of complementary PNT are most useful in addressing specifically the challenges posed by jamming and spoofing and how does simulation help?

We need to look at systems that cannot be impacted by, or have mitigations from, the impact of jamming and spoofing. Solutions that are independent of radio communications or satellite use are then valuable in providing this layer of protection. This is where we could look toward OxTS’s use of IMU technology and visual aiding systems. Simulation technologies would then allow you to run hardware-in-the-loop testing, where the primary GNSS solution can have simulated jamming and spoofing to understand the performance of your complementary and protected systems when GNSS cannot be trusted.