Orolia developed the Skydel GSG-8, a PNT test solution in its GSG family of simulators, to deliver GNSS signal testing and sensor simulation performance in an easy to use, upgradable and scalable platform. (Photo: Orolia)
We discussed complementary PNT with Erik Oehler, marketing director at Orolia.
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.
How does Orolia fit in that mix?
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 hardware-in-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.
Jackson Labs Technologies PNT-6200 Series, an STL-based time and frequency reference system installed in a 5G application. Photo: Satelles
We discussed Satellite Time and Location (STL) services and complementary PNT with Michael O’Connor, CEO at Satelles.
What is the problem with GPS/GNSS that Satelles aims to solve?
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 unique aspects in the areas where GPS could use complementary service. First, we provide a fully independent backup. We all know that things can happen, so we aim to provide an independent source of position navigation, and timing (PNT). Second, we focused the high-power aspect of STL to enable us to reach indoors and other places where GPS does not reach. Because STL comes from low Earth orbit (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. If you can ensure that the signal is coming from the satellite and not from a third party you can have a more secure and resilient solution.
To what extent can you replace GPS during an extended outage?
We have never considered LEO 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. That’s one of the services we can offer. It is probably not the most important thing to our customers, honestly. The service we offer is similar to GPS and GNSS in that we have a space segment (the satellites), a ground segment, and a user segment. We have space vehicles, user equipment, and ground infrastructure that supports the space infrastructure.
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 a time throughout the network. That’s one unique aspect we can take advantage of with this particular network, Iridium, which is pretty amazing.
Additionally, we have multiple ground infrastructure and monitoring sites and multiple sources of time at those ground monitoring and control stations. For example, 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.
How do you define “complementary PNT” and how does Satelles fit in that mix?
Several applications have additional needs beyond what GNSS offer. There are many technologies that can come to bear on that. There’s the LEO satellite base, which is where Satelles fits in, but there are also local and wide-area terrestrial radio navigation sources, network-based time transfer, signals of opportunity, and so on. They all have something important to offer, depending on the application. 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. The industry is working together to make sure that an awareness of these capabilities is propagated throughout the industries that we serve.
Besides the orbit height, which requires many more satellites, how does your system differ from GNSS?
We do not consider LEO PNT as something that might replace MEO PNT. The fundamental difference is being in lower Earth orbit, which results in a higher received power. That is what allows us to penetrate, just based on the 1/r2 losses. 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.
If I explained how GPS works to, say, a high school science class, how much of that basic explanation—about trilateration, spread spectrum, etc.—would also apply to your system?
It’s fundamentally the same. It relies on a lot of the same physics and geometry. 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. There are subtle differences—for example, a lower Earth orbit is faster—but it is very similar.
How would GPS user equipment have to be modified to make use of your service?
We don’t think of STL as something where we are modifying GPS user equipment. Rather, we think about what must be done in an end-user application to meet their needs. For example, one of our partners, Orolia, has a GNSS + STL secure synchronization product that we have delivered to customers in data centers and major stock exchanges around the world. Those are operational and in service. They integrate through standard interfaces, such as PPS or PTP, depending on the type of equipment to which they are connecting.
Ultimately, we don’t think of it is as replacing GPS user equipment. Rather, where a user has a need for PNT, they’re opting for this GNSS + STL solution because they have an indoor need, such as a data center, or they have a need for resilience in the case of a stock exchange.
Another example is Jackson Labs. The Jackson Labs 2600 is also a GNSS + STL solution that generally is integrating with existing 5g. It has a specialized transcoder interface that can work with any existing GNSS-type equipment. In some cases, we’ve taken a chip that was originally designed for GPS and modified its firmware.
Who are the earliest adopters?
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.
If your solution can survive on its own, why does it need GNSS at all?
In some cases, the user is not using GNSS at all. The product itself has a GNSS capability. User equipment is very affordable and the service is taxpayer-funded. In many cases, especially for indoor installations, the equipment that is installed is capable of tracking GNSS and STL signals, but often it relies on the STL signal itself for timing.
How do you predict STL spreading through various applications and industries?
We have our hands full with the markets we’re going after now, but there are certainly going to be other markets in which the customers will recognize that they have a critical need to implement a backup solution.
In the long run, could LEO satellites replace MEO ones for GNSS?
Sometimes there have been misperceptions in the industry. I’ve never considered that LEO PNT satellites might replace MEO ones. There are excellent reasons why Brad Parkinson, Jim Spilker, Gaylord Green and others decided almost 50 years ago to put GPS in MEO. Those physics haven’t changed. You can cover a large portion of Earth with each satellite. LEO will not replace MEO, but it has unique characteristics that make it a great complement to the GNSS MEO solutions.
Do you have any additional comments about complementary PNT?
It’s good to see that the federal government is encouraging the adoption of complementary PNT, which they often call “GPS backup.” It is encouraging to see the amount of activity on this issue that’s been going in Washington over the last couple of years. Although our company is very focused on delivering a LEO-based PNT service, which has several advantages for customers that need a global capability, many technologies can play an important role in those solutions.
The U.S. Department of Transportation did a fantastic job of looking at several of those technologies across those different categories. The European Union has also had a similar activity recently. Some reports will be coming out soon about that. It is very important that the government understands that this is an important issue for our society and encourages industry to adopt these solutions and is even starting to make some investments toward that. That includes executive order 13905 and some recent funding increases by Congress.
All of that has been very important and positive, as has modifying some of the legislation to be more inclusive of multiple technologies, such as removing the words “land-based” from the National Timing, Resilience, and Security Act this year.
I am involved in an industry consortium, the Open PNT Industry Alliance, with several other companies whose CEOs are in alignment that there is no single answer. Having a thriving ecosystem of technologies and companies trying to solve this important problem is incredibly important and it’s exciting to see.
Data shows how successful baseline validation testing of Spirent’s inertial simulation model as compared to real world inertial system performance. Photo: Spirent Federal Systems
We discussed complementary PNT with Roger Hart, head of engineering and Jeff Martin, head of sales at Spirent Federal.
What are some of the most promising approaches to complementary PNT sources and how does simulation technology help?
Roger Hart: 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. A couple that we’ve been focusing on are the alternative navigation systems.
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 it 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.
At Spirent, we’ve been working with customers to provide a variety of options for both those alternative navigation systems and inertial. Both are a very active field of development and we’re keeping abreast of that.
Jeff Martin: Some good points, Roger. This is something we’ve been engaged in for quite a long time. Since we provide test equipment to the community, it’s critical that we understand what they’re worried about, what the vulnerabilities are. It keeps things exciting, it keeps us on our toes and looking ahead to what’s coming.
What are some of the remaining challenges of integrating GNSS receivers with inertial sensors and, again, how does simulation technology help with that?
Hart: 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. One example is the all-source positioning and navigation standard that is being developed by the Department of Defense. It will allow you to swap one sensor for another as long as they adhere to the standard. That information all goes back to a sensor fusion engine.
Martin: We have known GNSS simulators well for about four decades. We have been playing in the inertial sandbox for at least a couple of decades as well. This has given us the opportunity to build relationships with the with the key manufacturers and designers of inertial systems. Those relationships have been expanding well beyond inertial to many other sensors and systems that are now coming online. It’s been exciting.
Much work is going into using low Earth orbit satellites for PNT—whether piggybacking on the Iridium satellites or launching new ones. How does simulation help with that?
Hart: It certainly helps with the development of the receivers. The groups that are using these alternative RF and LEO or MEO systems need simulation as they develop the receivers. It gives you the ability to try things certainly before you launch them. At this conference there is considerable interest in making things reprogrammable. We have the NTS-3 satellite, which will be running experiments for different waveforms that can be generated. Even M-code is a step in the direction of giving more flexibility to the signal. It has a lot more flexible cryptography and signal generation than the legacy system with the C/A and P/Y codes.
Our simulation platforms are software based, so we can generate and receive data that can be useful for developing software-defined receivers. It gives you the opportunity to try different waveforms. We have already delivered a satellite-based alternative navigation system simulator. Now, we can build on that one to help the other Leo constellations as they come forward.
Martin: Roger put it well. This is where things get fun. People are concerned with PNT vulnerabilities, so we’re seeing these alternative navigation solutions coming forward. Spirent has done a good job over its nearly 40 years of existence of manufacturing and designing its own hardware and software. It has given us the opportunity to respond quickly. These things are coming fast. People need solutions quickly. We have some solutions already and the platform that we have created gives us the flexibility to develop more. We’re seeing more and more ideas come to fruition and people need to test them. So, this is where it gets fun. We’re excited.
Much work has gone into addressing the enduring challenge of urban canyons. How does simulation technology help?
Hart: 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. So, diminished and corrupted signals are available to you. 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. All those kinds of things will help fill in when you have poor GNSS coverage.
You can observe what’s going on in the environment and simulate it. You can also use our forecasting tool to look ahead.
Martin: This is where things get exciting, isn’t it? In these terrible environments where GNSS is contested—whether it’s an urban environment or one with intentional jamming—there is a lot we can do to help our industry. When this happens in real life, it’s bad news. But when you create that scary situation in the controlled environment of a laboratory, it is great. You can pick things apart and see where you need to improve. I get excited about it. It’s probably the geek in me. It gives us and our partners a lot to look forward to.
How does simulation technology help with sensor fusion?
Hart: It definitely helps you put all the pieces together. 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. Over the decades, simulation has proved itself as a very valuable way in both basic development and integrating the final product.
Martin: That system-wide fusion is where the magic happens.
It sounds like simulation technology—and Spirent Federal in particular—are very much at the center of a lot of the current developments and discussions about complementary PNT. Do you have any final comments?
Hart: As Jeff said, it’s an exciting time. There are many things going on—new technologies, new ways of communicating. It’s a busy time and a bit of a scramble sometimes to keep up with all the new things that are coming.
Martin: People look to Spirent to be their testing resource and it puts us right in the middle of it.
Reading her biography, you might not think that Jennifer Wexton, U.S. Representative for Virginia’s Tenth District, would be very connected to technology issues. As an attorney and government leader, she has spent a lot of time advocating for families, veterans and federal workers.
Yet she is also co-founder of the Congressional Task Force on Digital Citizenship and has become an advocate for protecting the nation’s vital positioning, navigation and timing (PNT) services.
“We all use GPS every day whether we know it or not, and we need to ensure it stays the gold standard for satellite navigation,” she said. “But as the National Security Council said last year, it has become a single point of failure for America. So much of our critical infrastructure depends on GPS, from the navigation apps on our phones to the military to our financial and energy sectors, and it is past time that we address its critical vulnerabilities and develop and implement reliable backup technologies.”
Wexton says that she has been aware of and concerned about the issue almost since the day she took her seat in Congress.
“I am a member of the House Appropriations subcommittee that oversees funding for the Department of Transportation, and civil GPS and PNT issues fall squarely in DOT’s jurisdiction,” she said. “In 2018, Congress passed a law requiring DOT to ensure we have a backup for GPS, and we updated that law earlier this year to ensure that all appropriate types of technologies are included in this effort. I don’t know how we can be comfortable with autonomous drones and self-driving cars if we don’t have technologies in place to support and back up GPS. They are certainly available.”
The fact that her district includes a huge swath of Virginia’s high-tech corridor has added to her knowledge of the problem and range of possible solutions. In addition to many big names in technology and government contracting, several smaller companies focusing on resilient PNT have offices in her district. These include Echo Ridge, Hellen Systems, Satelles and UrsaNav. All four participated in the Department of Transportation’s Complementary PNT and GPS Backup Technologies Demonstration.
Adding to her concern are increasing threats to GPS satellites and signals. At a recent hearing with DOT Secretary Pete Buttigieg, she mentioned Russia’s anti-satellite tests and ongoing GPS jamming in Ukraine as examples.
“And that might not even be the worst of it,” she later said. “We need to be cognizant of Chinese capabilities in cyber and in space. They have shown how they can physically grab one satellite with another and toss it out of orbit.”
At the hearing, she gently prodded Secretary Buttigieg about DOT’s progress toward protecting the nation.
“…in the FY-22 Omnibus we created a program which is housed at DOT and provided $15M to help develop the needed requirements and standards and conduct additional testing. […] How’s it going?”
Buttigieg temporized saying the department was working on the Congressional tasking in the appropriation with the funding available. That tasking focused on testing and developing standards as opposed to contracting for signals to begin making one or more alternatives to GPS widely available for adoption.
Funding for such an effort has been problematic since a 2018 law required DOT to establish a timing backup for GPS. Administrations have justified inaction by saying that Congress had not funded the effort. Yet hill staff report that for several years administration representatives vigorously opposed any funding at all during annual budget development negotiations. The $15M to establish the program within DOT this fiscal year, which Wexton supported, was the first time any allocation had been made.
Rep. Wexton is also a big supporter of GPS modernization. “GPS is the gold standard for satellite navigation and America’s gift to the world,” she said. “We have to continually modernize it to support both military and civil uses. That’s a no-brainer.”
She also sees deploying alternative timing and navigation systems a step to making GPS more secure. “Unlike the Russians and Chinese, we don’t have one or more systems widely deployed domestically if GPS is unavailable for some reason. This makes GPS satellites and signals prime targets for our adversaries, terrorists, and criminals. Having one or more alternatives will create redundancy and make GPS a much less attractive target.”
Providing one or more alternatives that could be widely adopted could be a relatively inexpensive proposition for the government, certainly when compared to the cost for GPS. More than $2 billion was appropriated for GPS operations and modernization this year. Industry sources estimate that contracts for the suite of alternative PNT sources described in the 2021 DOT report would cost a small percentage of that.
Dana A. Goward is President of the Resilient Navigation and Timing Foundation.
The Air Force Research Laboratory’s complementary positioning, navigation and timing (PNT) AgilePod prototype achieved three important objectives in flight tests conducted at Edwards Air Force Base Nov. 1-10, 2021.
PNT AgilePod helps develop advanced navigation technology independent of GPS, according to Maj. Andrew Cottle, Air Force Strategic Development Planning and Experimentation (SDPE) office. This technology provides reliable, resilient PNT navigation signals through alternative means, increasing mission effectiveness in scenarios where access to GPS is not guaranteed.
The test team — representing a broad base of Air Force, Navy and vendor organizations — successfully executed eight sorties aboard a T-38C aircraft, which included:
the first test of the PNT AgilePod on a high-dynamic-range platform
the first test of fully remote interfacing and alt-PNT data transmission
the first demonstration of overland/overwater transition performance.
He said the tests demonstrated the operational utility of a fused alt-PNT system incorporating multiple technologies within a single government-owned open-architecture prototype.
A PNT AgilePod attached to a T-38C successfully demonstrated remote interfacing and alt-PNT data transmission, as well as performance over land and water. (Photo: USAF/2nd Lt. Bowen Lin, 586th Flight Test Squadron)
AgilePods Designed for Flexibility
AgilePods are comprised of a series of compartments and can be configured to meet a wide variety of mission requirements for many aircraft platforms. Experimenters can fill the spaces with plug-and-play sensors they need for a mission — high-definition video, electro-optical and infrared sensors, and devices with other capabilities — including PNT.
The AgilePod has an open hardware architecture. For the complementary PNT prototype, it was combined with an open software architecture that allows a wide variety of alternative PNT technology to integrate and pass information. These capabilities enable rapid integration of sensor technologies through standardized software and hardware interfaces, allowing the pod to seamlessly integrate on platforms that leverage the standard architectures.
In this way, one pod can perform hundreds of different mission sets with additional benefits of cost savings and increased sustainability, Cottle said.
The project directly supports the AFRL PNT Enterprise and the Air Force PNT Cross-Functional Team as they work to ensure reliable navigation within GPS-contested operational scenarios critical to the success of future Air and Space Force missions.
A PNT AgilePod attached to a T-38C successfully demonstrated remote interfacing and alt-PNT data transmission, as well as performance over land and water. (Photo: USAF/2nd Lt. Bowen Lin, 586th Flight Test Squadron)
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.
