GPS signals are by far the single most widely used and most accurate source of navigation, positioning and timing (PNT), and this capability is deeply integrated into every aspect of our society. In particular, the timing service provided by GPS, while virtually unknown to the general public, is essential for a variety of digital operations — from performing financial transactions to operating cell phone networks to running the internet.
Of course, GPS — originally developed to guide nuclear submarines — is now vital to most military missions, and the system’s vulnerabilities are a source of great concern.
GPS has been remarkably reliable over the past quarter century. Solar flares are rare, multipath can be largely mitigated, and obstructed line-of-sight to the satellites is an acute problem only in certain environments, such as urban canyons.
The most serious intentional threats to GPS are spoofing and jamming. Jamming is more widespread — it is more easily accomplished intentionally and it also occurs unintentionally. In the defense sphere, intentional jamming is a regular occurrence. It is expected as a routine aspect of electronic warfare operations to disrupt and deceive, typically just before the shooting begins. Unintentional jamming includes recently re-emerging concern about potential interference by ultra-wideband devices.
Experts at NovAtel, Collins Aerospace, L3Harris Technologies and Honeywell address the challenges posed by jamming and the relative effectiveness of various anti-jamming approaches.
NovAtel
Tackling Jamming on Multiple Levels
Disruption by jamming of GPS’s PNT data “is occurring with a growing regularity,” said Dean Kemp, Defense Segment manager at NovAtel, part of Hexagon’s Positioning Intelligence division. The problem will only increase, given our reliance on GNSS and increasing demand for precision. In the military sphere, electronic warfare in Syria, as well as jamming in Ukraine, Korea, and Finland, “have shown that modern, high-power equipment is routinely being used to disrupt the military.”
In the civilian sphere, interference is a growing issue because of cheap and effective jammers available via the internet. People use these so-called personal privacy devices to defeat vehicle tracking devices for purposes ranging from avoiding supervision all the way to hijacking vehicles.
GNSS signals are vulnerable because the received power is so small that receivers can be disabled with an incident power in the picowatt (10-12 W) range. “Jammers come in many different forms,” Kemp said, “from low-power civil devices to complex and powerful military-grade electronic warfare systems that can disable civilian receivers from a few hundred meters to hundreds of kilometers.”
Situational Awareness. Users can fail to recognize that their GPS is being jammed, Kemp said. Beyond defending against possible jamming scenarios, it is also necessary to “identify, find, and characterize the source of interference and to provide this information to the user so that it can be used appropriately.” In the defense field, this is known as situational awareness.
Emerging jamming threats, Kemp explained, can be understood within the context of cyber and information warfare using the Cyber Electromagnetic Activities (CEMA) layered approach. It recognizes a cognitive layer — a human decision based on PNT data; a virtual layer, in which PNT data are used to inform or support networked systems; and a physical layer, the hardware used to provide and protect PNT data.
Therefore, effective anti-jamming requires that:
- users understand the system’s vulnerabilities and identify when they are being jammed, so that they can resort to traditional means for positioning and navigation (but not timing)
- PNT data be protected and verified before being trusted
- on the physical level, there be a multi-layered and heterogeneous approach that provides assured PNT information in the presence of jamming and spoofing without quantifiable loss of accuracy.
By combining these considerations at each layer, “they form a unified view on capability,” Kemp said.
Spoofing with Pokémon. Jamming threats are evolving, employed by both civilian and state actors. Worse, these threats are augmented by spoofing. While spoofing is harder to achieve than jamming, it is potentially more concerning. “Spoofing the receiver by rebroadcasting the GNSS signals or by generating them from a simulator has become a regular occurrence,” Kemp said.
Spoofing came to public attention in 2016 when enterprising programmers designed location-deception apps to hack the Pokémon Go mobile game. Instances have since been reported worldwide. Because early spoofing demonstrations were conducted against simple GPS L1 C/A-code receivers, it was initially hoped that spoofing could be defeated by using dual- or multi-frequency receivers.
However, it has been demonstrated that multi-frequency receivers using commercially available components can also be spoofed, “at least when the receiver is using multiple frequencies of GPS,” Kemp noted. “Adding further GNSS signals will help, but the best defensive measure is to employ, if authorized, an encrypted military signal.”
Coverage Improvement Factor. Typically, the effectiveness of an anti-jam system is assessed on the basis of the jamming to signal ratio (J/S) figure in decibels, which depends on variables such as the receiver’s front-end RF bandwidth, the signal type being tracked (C/A versus P(Y) code), the signal tracking threshold of the receiver, the receiver platform dynamics, the choice of receiver oscillator, the interference type and antenna characteristics.
Difference in how manufacturers calculate J/S led to the invention of the coverage improvement factor (CIF), adopted by the GPS Joint Project Office. “CIF gives a single number that describes the effectiveness of an anti-jam system for a particular jammer scenario, given that space vehicle positions vary by elevation and azimuth,” Kemp said.
However, the use of CIF to assess the anti-jam performance is a highly technical process and the results are usually classified. He discussed current approaches to anti-jamming.
- Multi-element, controlled reception pattern antennas (CRPA), which pass the good signal to the receiver while nulling out the interference, are the first line of defense. “The system can dynamically change the gain pattern of the antenna so that as the platform and jammers move, the gain pattern adapts so that nulling continues effectively.”
- The use of multiple constellations and frequencies can be an effective tactic to mitigate interference, “but relies on the jammer not covering the bands of interest.”
- “Obtaining actionable data on interference is almost as important as mitigation,” because it enables users to modify plans. However, “interference effects can be difficult to diagnose and complicated to track down.”
- Monitoring automatic gain control can indicate jamming.
- “Coupling a GNSS receiver with a robust inertial measurement unit (IMU) will provide a higher level of protection for GNSS signals due to the IMU providing reliable position, velocity and attitude even through short periods when satellite signals are blocked or unavailable.” However, IMUs are liable to drift, resulting in degraded performance.
There are many approaches to designing anti-jam systems. They must be balanced against user requirements, which vary significantly. “A layered approach is the best form of defense against jamming and spoofing,” Kemp said, starting with protecting the incoming GPS signal. “One of the highest levels of protection is from an anti-jam antenna system paired with a GNSS receiver that is tightly coupled with an IMU.”
Finally, given that jamming attacks are now to be expected on the battlefield, it is critical to train users on the best response.
Collins Aerospace
Artist’s concept: Collins Aerospace
While sources of deliberate jamming are on the rise, the vast adoption of GPS means that “even the non-deliberate sources of jamming will have an asymmetric impact on end users,” said Sai Kalyanaraman, Ph.D. and Technical Fellow at Collins Aerospace. Challenges posed by jamming depend on the receiver, mission and performance needs, while the source of unintentional jamming could be “something as simple as a TV antenna that is transmitting harmonics into the GNSS band.”
Kalyanaraman outlined viable approaches to interference mitigation and anti-jamming:
- Integration with inertial navigation systems (INS) can provide the platform’s attitude, which is required for beam forming. This, in turn, is required for some of the CRPA GNSS Anti-jam signal processing modes. It can also alert the user of jamming when the INS position diverges dramatically from that provided by the GPS receiver.
- Use of multiple frequencies is a form of robust design against interference.
- For authorized users, M-code will provide additional limited capabilities against jammers.
- Integration of GNSS with other PNT sensors to help address GNSS-denied environments.
GNSS signals have the advantage that the true signal is well under the noise floor; therefore, “as long as you can characterize the noise floor adequately from the receiver design/installation perspective, anything that shows up above the noise floor typically does not belong in that slice of the spectrum,” Kalyanaraman said. Combining a CRPA, a platform orientation sensor (like an INS), and a GPS/GNSS receiver, “you have a fairly potent triumvirate of tools that you can use to help mitigate the impacts of jamming and potentially spoofing.”
Collins produces multiple variants of its digital integrated GPS anti-jam receivers (DIGAR). “Depending on which variety you choose, you can essentially have a receive apparatus that can perform basic nulling all the way up to beam-forming and direction finding and help provide resiliency against high jamming signal levels and other threats that emulate a GNSS-like signal in space,” Kalyanaraman said.
L3Harris
L3Harris develops gun-hardened anti-jam solutions for the M1156 Precision Guidance Kit Modernization program. The kit turns 155-mm artillery shells into smart weapons. Here, soldiers test the kit for accuracy. (Credit: U.S. Army/Spc. Robert Porter)
Field Tests Verify PNT Reliability
Dealing with deliberate and unintentional interference with GPS requires agreeing on the level of enhancements required, reducing the time and cost needed to integrate them into systems of systems, and “centralizing PNT generation and distribution functions on a platform to reduce user equipment redundancies and increase the leverage of future PNT enhancements,” said Dave Duggan, president of the Precision Engagement Sector at L3Harris Technologies.
The increase in interference “creates a cascading negative effect to PNT client mission systems,” Duggan said, including the systems of systems for sensing, maneuver and fires [military-speak for the use of weapon systems].” The capability of anti-jam countermeasures “scales across a range of performance, size, weight, power and cost points and can be tailored to a given threat space, improving the performance of even legacy user equipment.”
Spoofing, which inhibits receivers from forming a solution or, worse, tricks them into passing misleading PNT solutions to other systems, is a bigger challenge than jamming because it can result in aborted missions and loss of life and usually requires new receivers, Duggan said.
Duggan defines a reliable anti-jam/anti-spoof capability as one that “provides a PNT solution with a high level of confidence in its accuracy, authenticity and integrity for their applications and anticipated threat environments — all at a reasonable cost/performance point.” Confidence in the solution requires “extensive analysis, threat modeling, simulation and testing of the anti-jam/anti-spoof capability.” For this reason, “L3Harris has worked extensively in developing simulation and testing environments of the highest fidelity and continues to participate in numerous live field test events to establish that foundation.”
L3Harris develops and produces digital anti-jam antenna electronics for U.S. and allied end use.
Honeywell
Honewell’s HGuide micro-electro-mechanical system (MEMS) inertial measurement units (IMUs) and INS are designed to be integrated with GNSS receivers. (Photo: Honeywell)
Integrating GNSS with Inertial
Heightened awareness of intentional and inadvertent jamming threats has less to do with new types of threats and more to do with the increased importance of precise PNT coupled with more frequent instances of jamming, according to Chris Lund, senior director, HGuide Navigation and Sensors at Honeywell Aerospace.
“As applications become more reliant on highly accurate and reliable position and timing information provided by navigation systems, the consequences associated with the data not being available or not being correct quickly escalate,” Lund said.
The best way to measure the impact of a jamming threat and the capabilities of countermeasures is “to determine in actual real-world use cases whether the desired application outcome can still successfully be achieved,” Lund said.
The most promising approach to anti-jamming is integration of GNSS receivers with inertial navigation systems (INS) and other PNT systems. “Given the complementary aspects of many of the available approaches in the anti-jamming toolkit, it’s often best to leverage however many tools are available and needed to allow the application to achieve its desired outcome,” Lund said.
See also:
New CRPA concept antenna designed, By Tony Murfin
J-Shield filters out interference, By Tracy Cozzens
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