Between the escalating conflicts in Ukraine and the Middle East and the growing geopolitical competition between the United States and China, there has been a collective realization about the fragility of GPS and other Global Navigation Satellite Systems (GNSS).
Spoofing and jamming have not only bedeviled positioning on the battlefield, but they have also spilled over into civilian life by wreaking havoc on commercial aviation. The challenges of operating GPS in adverse environments have led to a call for AltPNT — Alternative Positioning, Navigation and Timing — that doesn’t rely on the GPS constellation.
We are now beginning to see a proliferation of technologies that focus on non-GPS navigation. This is a significant departure from the recent past, where the focus was more about “augmenting” PNT instead of creating an alternative that is totally disconnected from GNSS.
But how feasible is this notion of entirely replacing GNSS? Realistically, there is no singular solution to replace GPS, but we can make it more resilient with complementary technologies.
Essential requirements of AltPNT
It is important that we be clear about what really constitutes AltPNT, so as not to confuse it with augmentative “AugPNT.”
To begin with, the technology must be truly independent of the GPS constellation and must not require periodic GPS updates in order to avoid drift. It must also be comparable to GPS in accuracy, speed and global coverage. And it should not be susceptible to the same problems that could degrade or deny GPS signals. Otherwise, what’s the point?
Few current technologies meet this definition. Most AltPNT solutions are merely supplements to GPS, not replacements for it. As Nikki Markiel, senior GEOINT authority for geomatics at the National Geospatial-Intelligence Agency recently told Breaking Defense, “There is no silver bullet” when it comes to replacing GPS.
Satellite alternatives
Private companies like Xona and TrustPoint are building new commercial PNT services using low Earth orbit (LEO) satellite constellations. Researchers have also discovered that SpaceX’s Starlink satellites could be used to deliver PNT services from LEO.
As part of a multilayer satellite navigation (satnav) system, these constellations offer a lot of promising potential. Due to their lower position in orbit, they can deliver faster signal transmission, reduced latency, higher signal strength and better positioning accuracy over medium-Earth orbit (MEO) GNSS satellites. This combination will improve the experience for users. They could also spur development in advanced technology fields like autonomous vehicles and augmented reality which require more precise PNT measurements than GPS/GNSS can offer.
But LEO PNT satellite constellations are not without drawbacks — and risks.
The most important issue is cost. Because of their LEO orbits, these services will require larger constellations to achieve global coverage. They also have a lower lifespan than medium Earth orbit (MEO) satellites and therefore require more frequent replacements and repairs. The need for frequent handoffs between satellites also creates higher operational costs and complexities.
A large constellation (with supporting ground stations) also presents a larger attack surface for cyber warfare. And while LEO satellites produce stronger signals than MEO satellites, they are still vulnerable to malicious interference, such as jamming and spoofing.
LEO PNT satellites also face greater problems with atmospheric interference, specifically ionospheric and tropospheric delay. Because LEO is a more crowded operating environment, the risks of accidental collisions with other satellites and space debris are at least three orders of magnitude greater than in MEO.
Aviation alternatives
In the civilian arena, the most significant impact from GPS denial has certainly been in the aviation sector.
So what are the options? Most commercial airlines have supplemental systems that enhance and improve GPS/GNSS-supplied PNT data, such as the Instrument Landing System, Ground Based Augmentation System), Wide Area Augmentation System, Distance Measuring Equipment, Non-Directional Beacons and VHF Omnidirectional Range.
One of the most significant non-GPS capabilities comes from Inertial Navigation Systems (INS), which most airlines rely on as their primary backup to GPS. This technology utilizes accelerators, gyroscopes, barometric altimeters and magnetometers to calculate accurate and reliable PNT data.
However, there are two major limitations with INS that undermine its ability to replace GPS: the need for initial PNT data from an external source like GPS, and the tendency for INS to drift if it does not receive periodic GPS updates. The latter could be significantly reduced by new quantum-enabled INS technologies. However, it’s still too early to tell how effective this will be.
Terrestrial alternatives
Another alternative capability is eLORAN: a modern implementation of the hyperbolic radio navigation system originally developed in the 1940s. It uses a network of fixed terrestrial beacons, and its signals are stronger and more resilient to jamming and spoofing than GPS.
However, eLORAN has several drawbacks. It is largely limited to land and littoral regions, it requires a heavy infrastructure investment and it only provides lateral positioning, not vertical navigation capability. However, it’s worth noting that the U.S. military is currently experimenting with new deployments of eLORAN that could provide complete coverage across the continental U.S. with only three terminal stations.
Terrain-based navigation (TBN) is another promising technology that uses sensor input matched against pre-mapped terrain to guide navigational systems. TBN has a fixed reference based on the altitude of the terrain that has already been mapped in order to reduce the potential for drift. However, one important limitation with TBN is that it is less effective in flat terrain, where there is less altitudinal variation, as is the case in Ukraine. Additionally, heavy bombing, especially in cities, can change terrain maps and complicate navigation. It is possible to use real-time mapping with SLAM (simultaneous localization and mapping) — but that only provides relative positioning, so the user will still need an anchor with known positions.
Other alternatives
Other emerging technologies are also entering the picture, and may someday serve as viable GPS alternatives.
One of the more exciting areas of development is in computer vision technology. While this is a broad field, it has immediate applications in PNT with Visual Positioning Systems (VPS) — already in use by such companies as Google, Meta, Apple, Microsoft, Snap and Niantic.
VPS works by matching pixels seen through the camera with pixels in a feature database to calculate position and navigation. Since the position is tied to a real world coordinate system, it can provide persistent tracking wherever a user goes. However, in order for this to work, all the pixels generally have to be mapped out ahead of time with highly accurate geospatial coordinates. When operating at a large geographic scale like a city or region, this presents a significant logistical and cost challenge. It also poses unique challenges in a wartime environment — after all, if an area’s key landmarks are destroyed by bombs, missiles or fire, the pre-mapped pixels will no longer be accurate.
These factors make a fully independent system quite difficult, which is why the most advanced tech companies in the world take a layered approach that integrates VPS with other systems.
LiDAR, or light detection and ranging, is another promising technology. When fused with INS, this technology could temporarily support navigation in a GPS denied environment.
LiDAR emits pulsed light waves from a laser to create a real-time 3D map of the environment. This allows it to accurately detect obstacles and determine depth. By incorporating INS, it can also calculate position , orientation and velocity. This navigation system could also be further enhanced by integrating VPS and other technologies like radar.
But in order to operate over large geographies, LiDAR typically needs to have the region already mapped so that the device can localize. This can be expensive and also causes limitations since it requires a clear line of sight to function properly (which can be impeded by obstacles and weather conditions) and its effectiveness diminishes over longer ranges.
Ultimately, none of these technologies on their own offer a complete replacement for GPS or other existing GNSS. It’s important that we not confuse the marketing to ditch satellites with the reality of how hard and complex it really is to establish a true AltPNT system that can maintain high accuracy and reliability in the absence of GPS or other GNSS. The reality is that we need a layered approach to PNT that interconnects multiple technologies to enable graceful degradation in adverse conditions.
There are a lot of talented people developing new AltPNT technologies, and a replacement for GPS/GNSS is probably in our future. But in the meantime, we must continue to invest in a more resilient and robust GPS system, especially by developing a diversity of AugPNT technologies that will be the key to secure navigation years down the line.
Sean Gorman is the CEO and cofounder of Zephr, a developer of next-gen location-based solutions. Gorman has a more than 20-year background as a researcher, entrepreneur, academic and subject matter expert in the field of geospatial data science and its national security implications. He is the former engineering manager for Snap’s Map team, former Chief Strategist for ESRI’s DC Development Center, founder of Pixel8earth, GeoIQ and Timbr.io, and a Director at Maxar. Gorman served as a subject matter expert for the DHS Critical Infrastructure Task Force and Homeland Security Advisory Council. He is also a former research professor at George Mason University.
