Assured and Resilient PNT Techniques for GNSS Attack Mitigation

With position, navigation, and timing technologies being placed at the heart of energy, finance, transport, and communication networks, satellites and associated systems should be seen as among the most important of critical infrastructure. 

In 2019, the cost of a knockout to the U.S. economy was estimated by the U.S. Department of Commerce to be over $1 billion per day. The technology’s importance has only increased since then, with a Brattle Group analysis from 2024 suggesting this figure had risen to $1.6 billion for a single day outage, with a seven-day outage costing over $12 billion, and a 30-day outage $58.2 billion. 

The Brattle Group figure may be a conservative estimate. The U.K. Government’s 2023 study, for example, calculated a seven-day outage would cost the U.K. economy, whose GDP is one-eighth that of the US, £7.6 billion ($10.2 billion). 

Position and Navigation
GPS is used for positioning by all branches of the military for land, sea and air operations. This includes for autonomous and manned vehicles as well as by infantry in the field. As such, GNSS jamming and spoofing, and navigation warfare (NAVWAR), is now a core element of electronic warfare and the US government, for example, has established the Joint Navigation Warfare Center as part of attempts to counter these.

In commercial applications, the technology is not only used for aircraft and maritime vessel navigation, but also in location-based services for logistics, private vehicles and phones. Beyond this, it is used in a raft of applications such as soil mapping in agriculture. According to the Brattle Group report, the two most affected industries in the event of disruption or loss of position and navigation information would be Agriculture ($19.5 billion lost from a 30-day outage) and maritime/shipping ($13.4 billion). 

Timing
The timing signals given by the GNSS network are also vital to a huge array of organizations. These notably include financial institutions, which use the timing signal for timestamping, sequencing, and regulatory compliance (e.g. MiFID II), and electricity grids, which use it to prevent blackouts through wide-area monitoring, fault detection and the balancing of electricity loads. 

Timing signals also play a crucial role in telecommunication networks and AI data centers, enabling the nanosecond-synchronization of data packets and coordination of complex distributed workloads.

The MEO satellites used for PNT orbit at a height of c.20,000 km. By the time they reach the earth’s surface, the RF power level is in the region of -130 dBm or less, which is below the thermal noise floor of many receivers. 

For them to be used at all requires large processing gain of spread-spectrum correlation. By introducing a more powerful interference signal, these genuine GNSS broadcasts are easily drowned out. 

PNT GEO Thermal Noise

Figure 1: Being below the receiver’s thermal noise floor level, and requiring correlation techniques to reconstruct, GPS signals are inherently prone to jamming and spoofing attacks

Adding to GNSS’s vulnerability is the open nature of its design, with civilian L1 (1575.42 MHz) signals lacking encryption. Even with encrypted P(Y)- and M-code used for military equipment the physics underpinning RF saturation techniques such as jamming remains a constant threat.

In 2024, there were as many as 700 GPS jamming and spoofing attacks taking place each day according to an analysis of ADS-B reports by the Zurich University of Applied Sciences. In 2025, this number is predicted to have risen to between 1,000 to 1,500 per day, affecting air, land and sea operations for both military and civilian use cases.

Figure 2: A military jamming and spoofing complex for low-flying military targets

Attack methods

Jamming
Jamming is a denial of service (DoS) attack, that takes brute force approach to overpower the weak GNSS signals. This is achieved by transmitting a signal on the same L1 (1575 MHz), L2 (1227 MHz) or other relevant band as the PNT satellites to drown them out. 

This prevents the calculation of a position and therefore makes GPS services unavailable.

Spoofing
Rather than trying to just block GPS services, spoofing mimics the structure of an authentic satellite signal, but uses false timing and positioning data. 

Like jamming, the spoofed signal still needs to be more powerful than the legitimate PNT transmission, with navigation systems repositioning themselves according to this false signal. This can be an instantaneous jump, but more sophisticated attacks will cause slow changes over time to prevent detection.

Other Methodologies
Meaconing is the rebroadcasting of an authentic signal with the delay and shift in position to affect navigation systems. The benefit of this approach is that, by using authentic digitally signed signals, it is possible to overcome some cryptographic authentication checks. 

Replay attacks take the same approach as meaconing but are used to target systems that rely on GPS-based time stamping. These include financial transactions and power grid synchronization.

A final category of note relates to data-level manipulation attacks, which are used to target the timing signals in financial networks and power grids. These provide false orbital data, clock corrections and GPS time in addition to the location data. It’s a particularly advanced technique that is harder to detect and causes slower changes. 

Jamming of plane carrying EU leader
In August 2025, a jamming attempt was successfully undertaken against the plane carrying Ursula von der Leyen, President of the European Commission, which was attempting to land in southern Bulgaria for meetings with the country’s president. According to reports, the pilots were forced to resort to paper maps to land. 

This follows similar interference affecting diplomatic missions, including a March 2024  jamming incident on a U.K. Royal Air Force plane. The plane, with the country’s then defense secretary, Grant Shapps, on board reported a jamming incident while flying near Kaliningrad.

Aid missions have also been hit. In 2025, the Indian Air Force reported multiple incidents of signal jamming when attempting to deliver food and other relief materials to Myanmar after the country suffered a 7.7 magnitude earthquake. 

Adalynn and Front Eagle oil tankers collide in Gulf of Oman
In June 2025, two oil tankers, the Adalynn and Front Eagle, collided and caught fire in the Gulf of Oman. All crew were evacuated and no injuries were reported but the blaze was large enough to be seen approximately 15 nautical miles away on the UAE coast.

The security firm Diaplous Group has issued a maritime security advisory in response to the incident, noting that GPS spoofing likely contributed to the collision.

This followed a May 2025 grounding by MSC Antonia just 100 nautical miles off the coast of Jeddah in Saudi Arabia, for which GPS jamming was blamed. 

Reports also suggest GPS jamming increased as a result the 2026 Iran conflict with up to 1,650 ships affected in a single day, representing a 55% increase in the week since the conflict began.

PNT GEO Maritime

Figure 3: The number of ships affected by GNSS attacks spiked significantly in 2025 according to an  analysis by GPS Patron

Azerbaijan Airlines flight J2-8243 downed by Russian air defense systems
On Christmas day 2024, Azerbaijan Airlines flight J2-8243 was shot down by Russian air defense systems, killing 38 on board. One theory is that the flight had been targeted by a spoofing attack, which resulted in it being several hundred miles off course.

 

As they are less easy to monitor than positional jamming/spoofing attacks, fewer data are available for timing attacks. The consequences of a timing attack can, however, be seen through the 2016 decommissioning of the SVN23 GPS satellite. A 13.7 µs anomaly caused by a software error created several issues including the knock-out of multiple digital radio broadcasts and communication networks.

The event is cited as a warning for the financial sector and high-frequency trading (HFT), where a 1 millisecond advantage has been calculated to be worth $100 million a year to a major brokerage firm.

Relying on a single source for PNT is no longer a viable strategy and developing a resilient PNT ecosystem that can function in D3SOE (denied, degraded, and disrupted space operational environments) has become essential.

Spatial filtering via CRPA antennas
Spatial filtering involves the analysis of the phase difference of incoming interference via a multi-element-controlled reception pattern antenna (CRPA). These have elements organized into a precise geometric pattern with signal processing techniques applied to calculate an angle of arrival (AoA) and distinguish between ground- and space-based signals. 

By adjusting the gain and phase of each element on the CRPA, it is possible to create a null or blind spot in the direction of the (horizon-based) jammer, with suppression that can exceed 40 dB depending on the design.

Image

Figure 4: CRPAs use an array of antennas to identify the angle of arrival of a frequency and enable a null/blind spot to be created

Receiver Autonomous Integrity Monitoring (RAIM)
The RAIM PNT attack detection technique relies on there being redundancy built into GNSS constellations, which gives access to data from additional satellites. 

GNSS receivers calculate location using signals from four satellites, with more typically available. By cycling through subsets of all available satellites, RAIM algorithms can calculate a series of positions for each. Should a subset give a significantly different result to the others, RAIM-based systems can infer that an attack has taken place and exclude the identified erroneous signal from its calculations.

This concept has also been extended to multi-constellation environments, and Advanced RAIM algorithms will also cross-reference a GPS position against Galileo and/or BeiDou.

Cryptographic authentication
While civilian PNT architectures were originally designed without authentication, public key cryptographic authentication is being looked to for a number of constellations and has been implemented experimentally on NTS-3. This is via the CHIMERA (Chips-Message Robust Authentication) signal authentication protocol, which has been developed to authenticate satellite orbit data while also measuring the distance between the satellite and user.

This introduction enables the source to be verified and makes it exceptionally difficult to data-level spoof these satellites. 

Military PNT, which implements encryption, uses a different cryptography technique, with the legacy P(Y) and modernized M-code using shared secret keys.

Cryptographic techniques will continue to evolve, and public-key infrastructure will likely soon become vulnerable to quantum-computing attacks – another potential risk to individuals and critical infrastructure alike.

The IEEE P1952 draft standard for mitigating GNSS attacks
IEEE P1952 is a draft standard that has been developed to help foster the creation and implementation of resilient PNT user equipment. The standard defines five levels of resilience. 

Level 1 is the lowest level and dictates that systems be able to detect potential attacks and alert if one is detected. A level 1 system does not need to mitigate against an attack.

Level 2 builds on Level 1 and requires that a system be able to recover after an attack. 

Level 3 is the first to require a level of holdover capability in order to not just recover but actively resist a PNT attack and have the ability to maintain an “acceptable” level of PNT performance after the attack. 

Level 4 requires the maintenance of resilience to be indefinite to allow continuation in fully denied, degraded, and disrupted space operational environments (D3SOE) situations. While a number of methods can be deployed, multi-sensor fusion techniques are among the more common techniques in use to achieve level 4.

Level 5 demands not only indefinite resilience, but also the ability to verify that information from a PNT source is accurate. 

IEEE P1952 is still in the draft phase. While a final vote date has not been set, the working group actively collaborated throughout 2025 and has been supported by the U.S. Department of Homeland Security. 

Once ratified, it is predicted to become at least a military procurement requirement, and many systems are therefore coming to the market that promise to help meet these levels. 

Alternative satellite constellations for positioning
In addition to the MEO satellite constellations that are used by GPS, Galileo and GLONASS, LEO satellites can also be used as a backup to provide positioning data.  Examples of these include Iridium, which transmits L-band signals that includes time-and-location data since 2016. 

This is delivered as a licensed service, with the supporting hardware available as individual receiver modules and as integrated systems for different platforms and applications, including indoor operation.

The Iridium constellation sits at approximately 780 km from the earth’s surface, which is 95% closer than the MEO PNT constellations. This makes jamming attacks significantly harder as its signals are approximately 1000-times as strong (c.30 dB) as those from GNSS satellites when they reach the earth. LEO satellites also fly across the sky more quickly and have a c.7-fold increase in angular velocity versus MEO satellites. This creates a larger Doppler shift that can more easily be used to distinguish from ground-transmitted signals and act as a check against GPS spoofing.

IMU navigation
To enable Levels 3 and 4 P1952 resilience requires the ability to operate when PNT signals are fully denied. The use of inertial measurement units (IMUs) based on accelerometers and gyroscopes enables this continuation in D3SOE conditions. 

IMU accuracy is significantly lower than a satellite signal, and these devices suffer from sensor bias, with the output deviating consistently in a given direction even when the physical input is zero. Tactical- and military-grade equipment, which suffer less sensor bias, are therefore required. However, even when implementing these most-accurate of IMU grades, a build-up of errors over time will still take place without recalibration. 

Sensor fusion can mitigate this bias, pooling data from as many sensors as possible to average out the error from each individual sensor node. 

As such, multi-element IMU systems based on tactical-grade MEMS-based sensors can achieve the level of resilience described by P1952 Level 3, enabling a navigation system to enter a holdover mode when an attack is detected. After this detection, position will be calculated by the navigation system via IMU only until the detection systems determine that a jamming or spoofing signal is no longer in range and the GNSS signals can, once again, be trusted. And by combining inertial sensors with a LEO-based aiding source, position, velocity and timing can continue through GNSS denial and be recalibrated when the inertial solution drifts.

The use of IMUs also enables an additional source to verify GNSS integrity, giving the ability to alert should a large position jump or velocity change be detected. 

Vision-based navigation systems
Vision-based navigation systems have also been developed. Such systems provide an additional method to maintain accurate positioning in the complete absence of GNSS signals and can provide recalibration data to counter bias when relying on inertial navigation in D3SOE. This technique, which uses a 3D vision-based positioning algorithm to compare visual patterns from the onboard camera with pre-loaded, satellite-imagery-derived 3D maps to track against known landmarks, was developed in 2025 by VIAVI’s Inertial Labs division. The Visual-Aided Inertial Navigation System (VINS) can use both visible-light and infrared images to allow for both day and night operation. 

In a GNSS-denied environment, a VINS system can maintain a horizontal position to within 35 meters, a vertical position within 5 meters, and a desired velocity within 0.9 m/s.

Figure 5: VIAVI’s Visual-Inertial Navigation System (VINS) combines 3D vision aided mapping with inertial accelerometers to enable positioning in D3SOE environments – shown in prototyping stage

The ITU-T G.8272.1 standard for holdover in primary reference time clock
The ITU-T G.8272.1 recommendation defines the architectural and performance requirements for enhanced primary reference time clock (ePRTC), mandating a significantly higher level of frequency stability and time accuracy than standard PRTCs. The ePRTC integrates not just a GNSS signal, but also a method to allow an autonomous holdover capability. Based on the standard, the maximum permissible time deviation is less than 30 ns from UTC when entering holdover. The holdover requirement scales with the prior locked-mode period, extending to within 100 ns of UTC for up to 40 days.

Cesium clocks and GEO constellation timing
There are two key techniques to enable continuity and meet ITU-T G.8272.1 when it comes to timing in the event that GNSS signals are denied, degraded or disrupted.

The first and traditional approach is an oscillator-based system. While the drift of rubidium oscillators and OCXOs in holdover can exceed ITU-T G.8272.1 limits within hours, cesium clocks exhibit very low long-term drift because their frequency is referenced to the cesium atomic resonance, which is what allows them to meet ePRTC holdover requirements.

However, cesium-based atomic clocks have several issues that limit their use in ITU-T G.8272.1 systems. Cost is a primary issue, with those meeting the base level priced in the region of $45,000. High-performance clocks capable of delivering a 100 ns-holdover performance for 100 days are considerably more expensive. Such clocks are also highly sensitive to shock and require a lengthy setup period subject to tight ECCN 3A001.i export licenses, which can take months to process. 

A second approach to mitigating GNSS timing disruptions is via alternative satellite constellations, including GEO-L based services. The first such cesium-less ePRTC was unveiled in 2026 at Mobile World Congress (MWC) Barcelona by VIAVI.

For this, VIAVI used an altGNSS GEO-L service that enables 100 ns accuracy in GNSS-denied environments indefinitely. 

VIAVI is the global leader in resilient PNT and provides a wide range of systems to enable a zero-trust ecosystem and to secure critical infrastructure and military assets against D3SOE threats. 

The company has also made a range of strategic acquisitions, including IMU expert Inertial Labs in 2024[LP1]  and assured PNT specialists Jackson Labs in 2022.  As a result of this expertise, VIAVI offers a comprehensive portfolio of systems that enable the transition of operations from total GNSS-dependence to Assured PNT (APNT).

The company works with government, military and civilian organizations to ensure the resilience of critical infrastructure and received a 2025 award from the U.S. Department of Transportation to advance complementary PNT for protecting cellular networks. In the same year, the company took first prize  in the U.S. Government’s Challenge.gov Electronic Warfare Battlefield Realism Prize Challenge. 

SecureTime GEO and LEO services
VIAVI offers two multi-orbit GPS/GNSS-independent (altGNSS) satellite timing services to ensure PNT services are fully resilient to attack. 

Its SecureTime altGNSS GEO services deliver a highly accurate, encrypted and resilient alternate timing source that uses Inmarsat’s GEO satellite constellation and offers both eGNSS and GEO-L timing source options.

SecureTime altGNSS LEO services offer robust positioning and timing options that can also be used indoors. This service uses an encrypted connection to Iridium’s LEO satellite constellation and delivers highly resilient alternate PNT capabilities

SecurePNT™ 6200
VIAVI SecurePNT™ 6200 series is a next-generation resilient timing reference that combines VIAVI’s SecureTime GEO and LEO services alongside technologies such as its ePRTC360+ platform and TrustedPNT™ to create a truly zero-trust platform for reliable PNT data in D3SOE situations. 

The SecurePNT 6200 units are targeted to a wide range of timing applications including cellular networks, data centers, financial transactions, deep-indoor and power systems as well as R&D labs requiring a GNSS-independent timing and positioning reference.

Visual-Inertial Navigation System (VINS)
VIAVI’s VINS navigation technology combines tactical-grade MEMS sensors with robust 3D vision-based positioning to enable UAVs to accomplish very long-range missions in the most GNSS-challenged environments. 

Launched in 2025, the system is designed for operation at low altitudes and uses a comprehensive modular design that incorporates processing and sensor modules, a GNSS or CRPA antenna plus an air-data computer and digital windspeed sensor alongside MEMS accelerometers and gyroscopes. It can be used for operation with both fixed-wing and multi-rotor UAVs.

Subhead: RSR Transcoders
VIAVI’s second generation RSR Transcoder can be used to upgrade legacy systems and deliver assured PNT with a wide range of outputs. The transcoder includes a GPS constellation simulator to translate a wide range of trustable sources – M-code, SAASM, IMU/INS and other signals of opportunity – into universal GPS L1 and L2 signals with C/A-code and P-code output allowing its use with any GPS equipment.

IRINS
VIAVI’s IRINS is a LEO-aided inertial navigation system that integrates state-of-the-art positioning, navigation and timing technologies to enable full D3SOE operation on land, air and sea.

Launched in 2026, the system combines inertial navigation systems (based on a tactical-grade MEMS 3-axis accelerometer, gyroscope and clock plus embedded barometers and magnetometers) with an attitude and heading reference system (AHRS) and air-data computer, as well as an STL-2600 LEO Iridium receiver module. Through these, it is able to calculate altitude, position, velocity and time data with minimal drift. 

The system also integrates a GNSS receiver with a CRPA port and is capable of detecting and mitigating spoofing and jamming attack signals.

ePRTC360+
In 2026, VIAVI unveiled its cesium-less ePRTC360+. At time of launch, the clock is the only alternative to cesium clocks capable of meeting ITU-T G.8272.1 and significantly lowers the cost of implementing holdover clocks to enable resilient timing in D3SOE attacks.  VIAVI also supplies a range of GNSS disciplined oscillators.