BEYOND GPS: A MULTILAYERED APPROACH TO ADDRESSING PNT VULNERABILITIES

The Global Positioning System is fundamental to the strength of American fighting forces. Nearly every asset the armed services deploy — from tanks to planes, from weapons systems to soldiers — may rely at some point on satellite-supported Position, Navigation and Timing (PNT) information.

But GPS is vulnerable. Iraqi forces used a jamming system to hamper allied PNT capabilities during the U.S. invasion of Iraq in 2003. North Korea reportedly used GPS jamming to disrupt U.S.-South Korean joint exercises in 2011. One European Union study detected nearly 500,000 interference events in 23 countries from 2016 to 2019.

“NATO’s adversaries have the ability to degrade or deny GPS-enabled capabilities,” NATO’s Jean-Philippe Saulay said in a recent statement. Western nations “must take appropriate measures to ensure Allied forces can operate in a degraded or denied environment.”

American political leaders are heeding that call. In the face of imminent and future threats, they are pushing the military to explore its options beyond existing GPS capabilities. Earlier this year the White House issued an executive order to that effect. Industry and government leaders have said the order signaled the United States is taking steps to mitigate the threat of disruption.

The military also recognizes the need. The Air Force for example lists among its technology areas of interest the ability “to enable resilient Positioning, Navigation and Timekeeping (PNT) in electronically contested, Navigation Warfare (NAVWAR) mission scenarios or where signals are otherwise unavailable, blocked or degraded.”

The Army Futures Command’s Assured PNT Cross-Functional Team wants to provide “enhanced and alternative GPS capabilities to military systems and platforms” across multiple armed services.

Another arm of Futures Command, the Army’s Command, Control, Communications, Computers, Cyber, Intelligence, Surveillance and Reconnaissance Center describes the imperative this way: “Soldiers conduct missions in austere and complex environments where GPS is not always available or accurate,” the C5ISR Center reported. “To accurately execute unified action, Soldiers need to know where they and their comrades are located, where they are going, and when to strike.”

Safeguarding GPS is only a first step. To ensure effectiveness of the battlefield, military leaders believe they need to think in more comprehensive terms. American troops should be supported by a multilayered PNT strategy, with a range of interwoven technologies and capabilities that work in tandem with existing GPS.

MULTILAYERED VISION

In modern warfare, it’s impractical to rely solely on GPS.

“The navigational satellites are up at the mid-Earth orbit, MEO, some 12,000 miles above the Earth. Because of that, and because it’s difficult to get much more than 100 watts of radio frequency-radiated power out of those satellites, it’s going to be a weak signal when it gets to the Earth,” said John Fischer, vice president of advanced research and development at Orolia, a company that focuses on PNT solutions. That weak signal is intrinsically susceptible to enemy interference.

Rather than rely on a single source of PNT information, one option may be to deploy a range of interlocking and overlapping solutions. In this multilayered vision, existing and emerging technologies in some cases will work in tandem with GPS, and in other cases can step in to fill a gap when GPS is unavailable or potentially compromised.

“With diversity, you always have your best solutions available, ones that give you the best reliability: You get multiplicity and you get redundancy,” Fischer said.

As a rudimentary example, consider that GPS is extremely accurate in the X and Y planes — forward and backward side to side — but less so in the Z direction — vertically. In this scenario, augmentation or a multilayered approach might call for the addition of a radar altimeter as a way to add altitude to measures of latitude and longitude. That’s just an example that a combination of solutions can bring a range of capabilities that a single-solution may overlook.

With GPS vulnerability in mind, Army Futures Command began evaluating next-generation PNT devices in mid-2020 to ensure they are effective before production and fielding. “We need to verify military devices don’t fail because of environmental stresses when they are needed most,” said Michael Panko, a C5ISR center mechanical engineer. “For service members in the field, if equipment in a military vehicle doesn’t turn on, or stops working part way through a mission, it could cost lives.”

But better devices are one part of the equation. To fully understand the emerging, multilayered approach to PNT, it’s helpful to approach the landscape from two different directions. Some are high-level approaches – large, systemic strategies to supplement existing PNT capabilities. Others have a narrower focus. These are the supportive technologies that ensure redundancy, reliability and accuracy in today’s contested environment.

SYSTEMIC ADAPTATIONS

The Army is looking to industry for a range of adaptations that will enhance military navigational capabilities. “A lot is happening here, a lot of good success,” said Willie Nelson, the director of the Assured PNT Cross-Functional Team (CFT) at Army Futures Command. He said the top priorities include getting the best equipment to “war fighters on the front lines and getting their feedback rolled back into the next generation.”

While that feedback today is focused largely on the performance of GPS devices, emerging solutions will deliver broader, more systemic solutions. These are the higher-level approaches to a reimagined, multilayered PNT capability and offer military leaders new ways to address the PNT challenge.

  • Ground based signal sources: This idea considers what would happen if PNT data were not wholly the product of a satellite broadcast, but rather were tied to terrestrial sources. A ground-based network connection, ideally a fiber-optic line, could deliver extremely precise time measures via PTP, or Precision Time Protocol, a software-based methodology for measuring timing. With the timing down to a billionth of a second – a nanosecond – location measures become significantly more precise. A ground-based approach could augment existing GPS with a stronger signal and could potentially drive greater accuracy in GPS-enabled systems.
  • Network connectivity and crowdsourcing: War fighters already require connectivity to complete a mission. If network users share their positions with others, those that don’t know their own positions can infer it based on proximity, and then the military could begin to crowdsource positioning. At the systemic level, improved network connectivity could support new sources of PNT data.

In this scenario, war fighters can gauge their position against a known, fixed object and against one another, layering in precision timing tools to get those proximity distances exact. If there are enough known positions, people or other assets, it’s possible to generate PNT from the network. Then if GPS is compromised entirely, it doesn’t matter: With one known, fixed site, all users can triangulate from each other.

  • Inertial navigation: An inertial navigation device is a self-contained collection of accelerometers and gyroscopes. Such a device makes it possible to navigate by measuring the forces on the vehicle, the aircraft or the soldier. For example, they would know if they were being pushed ahead, pushed left or pushed right.

The Defense Advanced Research Projects Agency is investigating this through its Micro-PNT project. That effort “is developing high-performance miniature inertial sensors to enable self-contained inertial navigation for precise guidance in the absence of GPS,” DARPA reports. “The program focuses on creating low cost, size, weight and power solutions for precision navigation in harsh environments relevant to Defense Department needs.”

Because there are no outside influences, such devices are virtually impossible to defeat – but their long-term accuracy is poor. By combining inertial navigation with GPS, however, it’s possible to do sporadic course corrections – leveraging the power of the self-contained unit, while periodically tapping GPS to ensure accuracy over time. This is the essence of the layered approach to PNT: Taking the strengths of one system, in this case the resilience of inertial navigation, and combining it with the ubiquity and accuracy of GPS.

  • Low-Earth orbit satellites: Also known as LEO PNT, low-Earth orbit satellites operate a few hundred miles above the Earth, versus the 12,000 miles where GPS satellites traditionally reside. “Now you have a super strong signal 100 watts, and it’s a lot harder to jam,” Fischer said. An adversary could still try to jam a signal from an LEO satellite, but that jammer would have to send a much stronger signal, effectively betraying the adversary’s position.

The Army already is aware of the potential here.

It seems likely that the military would want to deploy LEO PNT in tandem with GPS. Geometry dictates that GPS satellites must operate from high in the sky to cover as much ground as possible. LEO PNT would have a narrower view of the landscape, and such satellites could be deployed fairly inexpensively.

KEY TECHNOLOGIES

In addition to these systemic adaptations, the military is also looking at specific technology upgrades to improve PNT capabilities.

The Army is casting a wide net in search of such solutions, for example through its xTechSearch competition. This year’s winner, TRX Solutions, announced in September, offered a solution for situations in which GPS is not reliable or available.

“We are looking to take this technology and show how it can really change the impact of operations in GPS-denied environments,” said senior technologist- senior scientist for the Army Research Office, and one of the xTechSearch finalist judges.

Along these same lines, the U.S. Space Force’s Space and Missile Systems Center made multiple contract awards in mid-2020 for an advanced hand-held GPS device. The goal was to produce a Joint Modernized Handheld smaller in size with low power consumption, increased military code capability, and improved anti-jamming and anti-spoofing capabilities compared to the current equipment.

“The advanced capabilities of this device will allow our airmen, soldiers, sailors and marines to conduct operations in GPS-challenged environments,” said Col. Clifford Sulham, the user products division chief.

While such end-user solutions are a piece of the puzzle, several other enabling technologies also will play a role in supporting PNT across the armed services. Each of these components adds a layer of resiliency, redundancy or precision to existing GPS solutions.

  • Advanced antennas: With enhancements, antenna deployments can be made more resilient to GPS jamming efforts. Most military and civilian GPS units have an “omni antenna,” intended to seek out signals across the entire sky. However, such a setup will, also be equally vulnerable in all directions. Emerging advanced antennas take a different approach.

An advanced antenna creates focused beams, then points those beams only where the satellites are, and away from any potential interference. This approach could work well when used in tandem with existing GPS, creating multiple layers of visibility while reducing the potential for interference.

  • Encryption: Military-grade GPS devices generally are encrypted, shielding them from potential adversarial action. But encrypted devices are expensive, and the management of encryption keys can be labor- and time-intensive. As a result, many military end users depend on commercial products to deliver GPS information – products that lack intrinsic encryption.

By layering consistent encryption methodologies across the military user base, it would be possible to harden GPS against attack. The military already recognizes the value of this approach: Navy PNT policy for example states that a GPS system must always be used “in an operating mode that makes maximum use of the encrypted GPS signals”.

As military leaders begin to envision a more robust PNT ecosystem, they could be working in tandem with industry to find affordable solutions with an eye toward making encryption standard across the entire spectrum of military GPS use cases.

  • Threat detection and mitigation measures: Just as in the military world, GPS in the commercial sphere is threatened. Take for instance the long-haul truck driver who installs a jammer to obscure his or her location. Commercial vendors are developing threat detection and mitigation countermeasures that also could be leveraged in the military space to mitigate potential threats.

Various algorithms can drive improved filtering and help operators detect potentially malicious interference in navigational systems. These methodologies are constantly evolving as new attack vectors emerge. Because many war fighters are using commercial receivers, these commercial-side improvements can apply to securing military GPS as well.

  • Improved signal processing: As consumer connectivity increases, the ability to process radio frequency signals has improved dramatically. Emerging tools on the consumer side will filter out interference more effectively than in the past, for example, and these same tools are beginning to show up on military receivers.

The NATO Research and Technology Organization points to improved signal processing as one of several key measures needed “to boost the resistance of GPS to [adversarial] jamming technologies.” High-end simulators can help the military to test such capabilities and get them into the field more quickly.

Improved signal processing could also support more robust uses of PNT data, an idea in which the Army has expressed interest. “Using analytics and modeling and simulation analyses, the CFT is incorporating new ways to challenge the [PNT] problem set and has been yielding excellent results in developing a requirements document that will provide the soldier with the right amount of PNT information at the right time under any condition,” Nelson said.

  • Chip-sized atomic clocks: While often overlooked in the “P-N-T” equation, the “T” for timing is an integral part of any navigational technology. More accurate timekeeping leads directly to more accurate measures of distance. Chip-sized atomic clocks, or CSACs, make it possible to embed such timepieces in GPS devices.

By keeping time via atomic clocks, users don’t need to rely on GPS for timing. Once a device is keeping accurate time internally, it can go for days or weeks without connecting to GPS, and still have near-perfect time.

The military recognizes the value of CSACs to the war fighter. “New applications and technologies like 5G networks and GPS alternatives will require precise timekeeping on portable platforms, driving a demand for miniaturized atomic clocks with a high degree of performance,” according to DARPA documents, which note that today’s commercial CSACs “offer unprecedented timing stability for their size, weight, and power.” Second generation CSACs are also now available, which are ~ 10 times more accurate for missions that require the most precise timing.

LOOKING AHEAD

The Army has created a sense of urgency around the efforts to enhance its navigational capabilities. “Today’s soldiers are already operating in GPS-challenged environments,” said Ben Pinx, the integration branch chief for the APNT Cross Functional Team. “Army operations rely on the ability to shoot, move and communicate, and when GPS is challenged, so are these core functions.”

The military wants the ability to develop and field emerging technology when it comes to PNT.

Army Futures Command for example announced earlier this year that it was developing an improved process for quickly delivering PNT capabilities to the field. C5ISR center has teamed with Project Manager PNT to build a prototype with enhanced PNT technologies for integration into a Stryker armored vehicle for evaluation in a matter of months.

Even as the various PNT solutions come to fruition, there are steps that the military can take to begin implementing a multilayered strategy in support of GPS-challenged war fighters.

“An important first step would be to move toward uniform circuit cards in all GPS-enabled devices, Fischer said. Standardizing cards now would offer advantages. The military could predefine navigation slots – for GPS, for LEO PNT, for other signal sources – and would free itself to mix and match PNT resources as they become available.”

That would accelerate the time to deployment for emerging solutions, while also simplifying the ability to manage these otherwise heterogenous devices.

The military can also keep up the pace of rigorous test and evaluation that it currently applies to solution sets related to GPS. “They are running field tests out in White Sands and Aberdeen, doing a great job of evaluating and analyzing the results. You need to know the vulnerabilities of your systems, know where the weak points are, before you can address them.” Fischer said.

Take for example the Army’s PNT assessment exercise, used to assess the performance of weapon systems in GPS-degraded and even fully denied environments. “It’s an opportunity for industry to come and join us …to evaluate the performance of their research projects,” Nelson said. “We continue to challenge ourselves.”

Industry partners see a place where they can help to support this rigorous analytic environment. “We have test equipment for all kinds of scenarios. In simulations we are able to say: ‘Here’s what jamming looks like, here’s a different spoofing attack,’” Fischer said.

A strong collaborative approach is especially important when developing a multilayered strategy, and it makes particular sense in the case of GPS, the history of which is rooted in military-civilian cooperation.

“GPS is a military system, but it was made available to the civilian world, and that was what made it really successful. The fact that the war fighter is able to get a $10 GPS receiver is because it was used in the civilian world, and the civilian companies worked alongside with the military to push forward with new developments,” Fischer said.

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Adam Stone
ABOUT THE AUTHOR
Adam Stone

Annapolis-based writer Adam Stone covers technology, government, defense, and education.

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