SÉBASTIEN TRILLES, THIERRY AUTHIÉ, XAVIER VASSEUR, MARIE ABBAL, THALES ALENIA SPACE, TOULOUSE, FRANCE

To use GNSS systems for air navigation, various civil aviation organizations have defined an augmentation system capable of fulfilling two primary missions. The first is to calculate correction messages that allow aviation users to exploit GNSS data for precise positioning, even if the GNSS system incorporates intentional or unintentional degradations affecting geolocation. The second mission is to monitor all navigation data broadcast by GNSS systems in real time to detect any anomalies and alert aviation users within a timeframe compatible with their flight phase. Given civil aviation’s need to cover a large area, typically on the scale of a continent, the dissemination of these messages has naturally been directed toward geostationary satellites known as Satellite Based Augmentation Systems (SBAS).

The role of an SBAS is to decompose the various contributors to measurement errors and broadcast, through dedicated augmentation messages, corrections associated with each error contributor to users. These corrections are reassembled by the user receiver according to their geographical position, improving positioning accuracy and helping to mitigate error sources that affect distance information related to satellite clocks, their positioning, and ionospheric effects. All SBAS are interoperable and standardized [1].

The classic functional architecture of an SBAS is composed of a network of ground reference stations that collect GNSS navigation measurements and data, a set of central processing facilities that compute corrections and constructs augmentation messages, and a set of transmission stations that broadcast the radiofrequency signal toward the geostationary satellite.

Current SBAS systems are designed for single constellation GPS, single-frequency L1 users, using the L/NAV navigation message. The augmentation signal is broadcast on the L1 frequency band, modulated by a dedicated PRN, and contains orbital corrections, clock corrections, and a model to correct ionospheric elongation.

Future SBAS, called Dual Frequency Multiple Constellations (DFMC), are dedicated to dual-frequency L1/E1 and L5/E5a users, using L/NAV navigation messages for GPS and F/NAV for Galileo. The augmentation signal is broadcast on the L5 frequency band, modulated by a dedicated PRN, and contains orbital and clock corrections for satellites from different constellations.

The main limitation of SBAS accuracy and availability performance lies in the regional coverage of the ground reference stations network, which does not allow continuous monitoring of the satellites in the navigation constellation. As a result, SBAS must continuously manage satellite visibility losses for several hours, requiring complex strategies to detect any satellite event such as manoeuvres, clock anomalies and hardware bias as soon as measurements become available again. The strong coupling that exists between material biases and ionospheric elongation adds difficulty in the case of satellite raising because it is often difficult to separate a hardware bias jump and an ionospheric event at the edge of the zone.

Furthermore, a geographically restricted network of reference stations does not allow for the correct decoupling of satellite orbits and clocks. This limitation is not a problem for a small service area because the clock error partially compensates for the orbit error. However, clock error is a scalar while orbit error is a three-dimensional vector, so how good the compensation of one error by the other depends on the size of the geographical area to be covered and the geographical position of the user within it. Consequently, a wide service area needs good decoupling between orbit and clock, which a network of regional stations does not provide.

This article studies the possibility of spatializing all the components of a classic SBAS. In this approach, the three main steps of SBAS processing, collecting GNSS data, calculating augmentation messages and disseminating those messages to users, must be carried out by components in free fall around the Earth.

Global SBAS Architecture Overview 

The first step in the SBAS spatialization process involves taking fixed reference stations on Earth and placing them in orbit, under navigation constellations, i.e., in low Earth orbit (LEO). There is no point in flying the stations in cluster formation, as this would not solve the regional problem and, moreover, the service would only be intermittent during the cluster’s orbital period. We immediately assume a uniformly distributed constellation as the geometry for the station distribution.

By doing this, LEO flying stations (LFS) can see GNSS constellations permanently, which is an undeniable advantage for increasing the accuracy of corrections and detecting critical events. Another benefit is SBAS has the capacity to be a global service, representing a significant paradigm shift. The spatialization of the stations also avoids the difficulties of defining a terrestrial network, which must satisfy geopolitical conditions, not to mention that the Earth is 70% covered by oceans, limiting the possible terrestrial sites to emerged geographical areas.

On the other hand, GNSS reference stations are no longer fixed points on Earth; they evolve over time. However, their trajectories remain predictable as their movements are well known and correctly modeled in the short term because they are governed by the laws of space mechanics. It is necessary to have accurate orbits for LFS. Several solutions exist for performing this calculation. Three approaches naturally emerge:

1. The calculation of LFS orbits is performed simultaneously and in the same process as the MEO orbits of the constellation satellites; 

2. LFS orbits are estimated using GNSS measurements through a separate process;

3. LFS orbits are calculated using independent means and independent measurements.

The first approach raises several questions regarding the commonality: LFS are devoted to monitor the GNSS constellation satellite. Using GNSS measurements for both LEO and MEO positioning in the same process brings significant 
algorithmic complexity and risk on the impact of a feared MEO satellite event on LEO position and detection capabilities. Thus, this approach is not discussed. 

The second approach decouples orbit calculations but requires implementing GNSS fault detection and exclusion techniques such as RAIM or ARAIM to make the position of the reference stations insensitive to failures of the constellation satellites.

The last approach offers the greatest possible independence because it is achieved using measurements from a positioning technique that is completely decoupled from GNSS. The Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) system is an example of such an independent system. DORIS is a radio navigation and orbit determination system based on Doppler measurements of signals transmitted from the ground to satellites. It is developed and maintained by CNES, the French Space Agency, widely used for space geodesy, Earth observation, altimetry missions and achieving centimeter precision level. The onboard DORIS and GNSS receivers share the same clock. The clock is synchronized with System Network Time (SNT, the reference time of the globalized SBAS), so the orbit generated will be time tagged with respect to the SNT. We retain this for this framework.

At the planned altitude, the LFS are positioned above the area where the ionospheric plasma is most concentrated. The GNSS measurements collected on board shouldn’t be much affected by ionospheric delays. This also implies this type of system will not be able to develop an ionosphere model and calculate ionospheric corrections to single-frequency users. Thus, this framework is devoted for dual-frequencies users. According to this paradigm, the ionosphere model shall be elaborated by an external entity.

The local Earth environment or propagation effects (troposphere and ionosphere) no longer affect measurements collected by GNSS receivers. Therefore, the quality of the measurements is expected to be significantly improved compared to a ground-based system. This favorable environment, associated with a geodetic quality receiver, will improve the precision performance of augmentation navigation messages.

In this framework, the LFS move at a high speed, of the order of 7 km/s, which generates visibility durations for GNSS satellites of 30 minutes. These passage durations are much shorter than those observed from the ground by several hours, but they are long enough for floating ambiguity resolution. The rapid dynamic of the LFS generates high relative movement between LEO and MEO satellites, providing better decorrelation between orbits and clocks and improving SBAS augmentation message performance.

LFS Communicate Via Inter Satellite Links

The proposed architectural framework incorporates inter satellite links (ISL) between the LFS (Figure 1). Selecting optical or RF ISL is driven by the trade-off between ranging accuracy, security and volume of data to be transmitted versus satellite design complexity. Optical links are suitable for high bandwidth and security requirements but demand more advanced technology and precise alignment that affect satellite design. RF links represent a proven technology, simple to deploy and tolerant of inaccuracies, but limited in bandwidth and inherent security. 

ISL capability serves two functions:

• A communication function to share the information recorded by each satellite;

• A ranging measurement function to improve the algorithms for determining the orbits of LFS and to participate to generate the independent SNT.

The first is equivalent to the terrestrial network, the Wide Area Network (WAN), which ensures the transfer of information between SBAS elements. 

The second aims to improve the position calculation and prediction of LFS by feeding the precise orbit determination, initially based on the provision of DORIS measurements, with additional Inter Satellite Ranging (ISR) measurements. The geometry and the accuracy of these additional measurements will help, respectively:

• To improve accuracy positioning in normal and tangential directions;

• To precisely locate the phase center of GNSS signal reception;

• To cope with possible jamming or spoofing of the DORIS station by offering an independent set of measurements; 

• To connect LFS clocks between them to measure their desynchronization (Figure 1). 

Several approaches can be envisioned for forming the clock’s equations, in particular the classic method based on the dual one-way ranging that allows decoupling orbit and clock problems [4].

In this framework, ISL continuity is assumed to be maintained over time without interruption, which requires permanent precise pointing.

The Navigation Kernels are Decentralized

Navigation computations are no longer handled by a single element but are distributed. This distribution is either entrusted to an infrastructure external to the system, already in place and managed independently, or distributed among all LFS.

In the first option, the globalized SBAS has access to a space cloud that handles the entire computational load. The links between the LFS and the space cloud are provided by ISL.

In the second option, each satellite carries a shared computing capacity. The computations are decentralized: Each computing unit performs part of the workload and exchanges the results with each other. These results are assembled by each LFS to generate a common navigation context. 

Decoupling Differential Corrections Generation and Integrity Monitoring

According to the original SBAS architecture designed by Thales Alenia Space [2-3], the navigation processing facility is composed of two components to ensure the independence of integrity checks. The first one, the Processing Set (PS), calculates the SBAS corrections and generates the Navigation Overlay Frame (NOF) with respect to the message format and message sequence defined in the MOPS and SARPS. The Check Set (CS) is the second component responsible for checking the integrity of the corrections from the NOF received from the GEO satellite, using data from at least one other group of independent receivers from each RIMS. When needed, it generates alarms on satellites that are collected by the PS and injected inside the very next NOF in case an anomaly is detected. To ensure diversification, the set of RIMS is divided into two distinct groups: RIMS-A only feeds the PS and RIMS-B only feeds the CS. The rational of this “dual channels” architecture is to comply with the safety requirement stating no single or common mode of failure shall entail a non-integrity event.

The solution studied proposes maintaining this distinction between the roles of the sets, PS on one side and CS on the other, and further extending independence by specifically allocating the measurements collected by a LFS to the PS or the CS functions exclusively. This leads to two separate LFS fleets: one dedicated to fulfilling the PS functions (denoted LFS-A, and acting as RIMS-A) and one dedicated to fulfilling the CS functions (denoted LFS-B, and acting as RIMS-B). The CS can communicate with the PS at the minimum level of integrity parameters to fulfil integrity checks. 

With such separation, the globalized SBAS architecture guarantees complete diversity in the measurement geometry to fulfil the PS and CS functions: the measurements from LFS-A will capture a very different observation geometry from that captured by the LFS-B measurements to perform integrity monitoring. This capability represents a significant advancement over previously developed ground-based architectures (EGNOS and KASS, for example) that co-locate RIMS A and B (in reality these two stations are separated by a few dozen meters to diversify the local environment. However, both RIMS capture the same observation geometry).

An even stricter independence step is to dedicate one batch of LFS to perform only the PS function and the other batch to perform only the CS function. The constellation is divided into two sub-constellations: partition A and B. The first calculates the navigation message (partition A allocated to the PS) and the other monitors the integrity of the message (partition B allocated to the CS). Each partition implements the distributed calculation of the PS and CS functions.

The two partitions communicate with each other via ISL to construct the message to be broadcast: The PS communicates the NOF ready to be sent to the CS, and the CS returns the results of the independent integrity checks to the PS. Different combinations are possible on the geometric distribution of the PS and the CS. This article focuses on two options: partitions (A and B) evolve at the same altitude (Option-1), or the partitions are positioned at two different altitudes, one specific for A and another for B, (Option-2). 

System Time Scale Generation

A SBAS must generate its own time reference, the SNT, which must be parallel (as much as possible) to the TAI. All clock corrections are computed relative to this system time reference. Because the bandwidth of NOF messages is 
limited (currently 250 bits per second), the SNT is steered to GNSS time to limit the magnitude of the corrections.

Several techniques are possible to achieve this internal time scale. Conventional SBAS only have RIMS-GNSS satellite links; the links between RIMS clocks are only accessible from a common satellite visibility by simple difference. Some SBAS develop the SNT using only a set of RIMS (EGNOS), which requires the construction of simple difference measurements; others (KASS) construct the SNT using all available clocks, including those of the RIMS and those of the GNSS satellites.

The globalized SBAS allows SNT construction based solely on the LFS clocks thanks to the direct links that connect them. This architecture makes it possible to construct a timescale independent of the GNSS constellations. The dual one-way ranging technique allows measurement of clock differences over time between two satellites connected by a laser link:

where h_i and h_j are the clock desynchronization of LFS clocks i and j, H_ij is the dual one-way ranging measurement corrected by relativity effects, hardware delays (in meter) relative to the ISL antenna on the receiving chain and on the transmitting chain, and phase centre offset relating on both emitter and receiver satellite [4].

It is therefore possible to construct the clock problem and solve it using various techniques [5-8]. The high quality of the dual one-way ranging measurements, combined with high-quality atomic clocks, allows the construction of a composite timescale whose expected qualities have phase continuity, frequency continuity and high stability (measured by the Allan variance). The SNT is aligned with the GNSS constellation timescale in a conventional manner, either by calculating a timescale difference, a posteriori, or directly during SNT generation by adding constraint equations. This steering will be performed using navigation messages from the GNSS constellations.

The timescale obtained is implicit; it is a paper time because it is calculated as a “well-constructed” average of all the clocks contributing to the calculation. The result of the composite clock algorithms provides biases that represent the advances or delays of each of the LFS clocks relative to the SNT timescale. Once these biases are applied, each clock is assumed to represent a realization of the SNT. This process, therefore, enables the global synchronization of all the LFS in the SBAS system. It then becomes possible to transmit a signal to the constellation every second of the SNT time.

TTA Reduction

The classic implementation of an SBAS (like that of the EGNOS V2 and KASS operational systems) is designed to be a Periodic (repetitive cycle of operations), Synchronous (each operation is performed according to its own timing), and Pipelined (all operations are performed in series) system. Specifically, observations are performed simultaneously at all ground RIMS stations at the second round of GPS time. Data are then transmitted to the navigation cores, where the algorithms are executed at a frequency of 1 Hz as soon as almost all RIMS measurements are received. Each operation has a specific execution time allocation, and the entire system is designed to complete a cycle in 5.2 s.

In the globalized SBAS concept, the LFS also perform measurements in a synchronous manner, meaning all stations observe GNSS events at the same coordinated moment. However, unlike the classic implementation, the specific timing of these measurements is optimized. The synchronization point is not arbitrarily fixed to the second round of system time, but is strategically chosen. This optimization takes several constraints into account: the requirement for the NOF to be available for broadcast starting at a specific round of system time, the estimated data transmission time between LFS, and the computational resources needed to generate the NOF. By aligning the measurement moment with these operational constraints, the system can maximize efficiency and ensure timely availability of the SBAS corrections for end users.

Assuming measurements time is optimized, the time allocations in the different elements of the system would be [9]:

• 1,000 ms to acquire new measurements, due to the 1Hz frequency of NOF broadcasting;

• 200 ms to generate the raw measurements (150 ms) and to format them (50 ms);

• 150 ms to disseminate the data to all SV through the ISLs;

• 350 ms to process data in the DPS (200 ms for computation and 150 ms for exchange data between satellites).

At the end, the NOF ready for broadcast is available in less than 1 second (Figure 2).

The time to alert (TTA) corresponds to the maximum time elapsed between the moment an anomaly likely to compromise user safety is detected and the moment the user receives the corresponding alert, informing them to no longer trust the service. In other words, it is the maximum time for any fault/error detected or suspected by the system to be reported to users by an alarm message transmitted via the SBAS signal. TTA is a central criterion for safety-of-life applications. For vertical guidance approach services (APV-I / LPV-200 type), the international ICAO SARPS standard sets the maximum TTA at 6 seconds. If the SBAS detects a loss of integrity, the alert must reach the user within this time. A short TTA ensures users will be quickly informed of a loss of performance or an anomaly, and can react or interrupt critical procedures when service reliability cannot be guaranteed.

The TTA takes the duration of NOF transmission (1 s) into account and the time allocated to user processing (800 ms). The time of SBAS signal propagation from LEO to user is neglected in this first apportionment (around 3 ms). The complete SBAS cycle is completed in between 3 and 4 s (compared to 5.2 s in classic case). The TTA is then below 3.5 seconds, representing a reduction factor of two with respect to classic ground SBAS (Figure 3). 

Finally, the concept of fast alert, [9] would be enabled. Fast alert messages are broadcast using the Q-channel and contain the alert flags (alarm/no alarm) for all satellites set in the PRN mask. The complete SBAS cycle is completed in 1.5 s (compared to 5.2 s in a classic case). This operational flexibility would allow a TTA of 2.3 seconds (Figure 4). 

LFS Broadcast the NOF

In a conventional SBAS architecture, the NOF is transmitted to users by one or more geostationary satellites. This message is generated on the ground, so it must be encoded into a signal and transmitted by a dedicated RF ground up-link station to the GEO. Because the first bit of the NOF must be sent synchronously at the second round of SNT time (close to GNSS time by specification) at the phase center of the GEO satellite, conventional SBAS systems implement a long loop that controls the signal transmission time to the ground. The SBAS is also endowed with an “integrity box” that checks the NOF return link to ensure the NOF received by the user is the same as the one computed by the system. In most cases, both NOF are strictly equal; if a corruption is detected the integrity box cuts the emission at ground.

In the case of a space-based SBAS system, LFS are capable of transmitting the NOF. To be properly processed in the GNSS receiver computing chains, this signal must be modulated by a PRN code that will spread the carrier spectrum using the Code Division Multiple Access (CDMA) technique. For this signal to be correctly processed within GNSS receiver chains, it must be modulated using a Pseudo-Random Noise (PRN) code, effectively spreading the carrier spectrum via CDMA. There are two primary approaches for assigning PRN codes to the LFSs:

• All LFSs transmit using the same PRN code;

• Each LFS transmits using a dedicated PRN code.

In the first case, the globalized SBAS broadcast the NOF with a single, unique PRN for all LFS transmissions. When multiple LFS are within the receiver’s field of view, the receiver can typically differentiate between transmissions by exploiting distinct Doppler shifts, which result in separate correlation peaks in the time-frequency domain. However, the probability of collision between the two correlation peaks is significant. Assuming a Doppler shift of 50 kHz, a loop bandwidth of 5 MHz and a PRN code of length 1,023 chips, the probability of a collision between two peaks can be estimated as (5.10³/5.10⁵)×1/1,023≈10^-5 per millisecond, corresponding to roughly one collision per 100 seconds. If three satellites are in view, the likelihood of simultaneous collision among all three signals becomes negligible. Therefore, using a unique PRN for all LFS requires continuous visibility of at least three LFS. However, this approach implies LFS signals cannot be used for ranging: While the NOF message can be received, the receiver cannot distinguish which LFS transmitted it.

In the second approach, each LFS is assigned a distinct PRN code. Currently, GNSS receivers store the navigation contexts of NOF messages received from each GEO SBAS, identified by its dedicated PRN. Out of all recorded contexts, the user applies only one; when the receiver switches PRNs, it replaces the navigation context accordingly and the old one is purged. In the context of globalized SBAS, however, the visibility time of each LFS is very short, about a dozen minutes, which is insufficient for a receiver to fully update its navigation context. In this situation, the user must retain the navigation context when switching PRNs instead of purging it. This ensures seamless continuity for the user; the navigation solution remains coherent regardless of the current LFS because the integrity and accuracy information provided by each NOF is consistent across all LFS. This adaptation necessitates an evolution of SARPS and MOPS standards to accommodate the new PRN allocation schemes envisaged for global SBAS. This approach allows the ranging function to be achieved even if it involves a significant increase in the number of PRNs required. 

The NOF is transmitted synchronously at the second round of SNT time. In other words, all LFS transmit the NOF to users at the same second of SNT time. This approach removes the necessity and the complexity of the long loop.

Every second, the system broadcasts a single common NOF according to a fixed and predictive message-sequencing scheme, compliant with the requirements of the SARPS standard. This augmentation message is broadcast in L5-I signal frequency.

The constellation is designed so at least two LFS are visible beyond 5° elevations of any user on Earth. The LFS are assumed to be able to receive GNSS signals in the Radio Navigation Satellite Service (RNSS) band and emitting the Aeronautical Radio Navigation Service (ARNS) band without jamming between emission and reception.

Whereas in conventional systems two or three GEOs are active, the possible loss of a GEO has a direct and immediate impact on availability and service continuity performance over a sometimes large geographical area. This new approach multiplies the number of NOF emission points, which greatly increases the resilience of system performance to this type of failure, reducing the impact to only a few users. Different combinations are possible on the geometric distribution of NOF emission points: in Option-1 the two partitions broadcast the NOF, in Option-2 only one partition broadcasts the NOF (Figure 5).

In Option-1, the two fleets broadcast the NOF. A standard functional allocation would consist in apportioning the same number of satellites to partitions A and B. As the LFS A and B are placed at the same altitude, the LFS-B cannot treat the NOF received as the user will; LFS-B only checks the NOF the system is ready to send the user. As all LFS emit the NOF, the return link function is not possible. Detecting possible NOF corruption (a LFS emits a message not conformed to it specification) is then allocated to the user. If the NOFs are different, SBAS can no longer be used. 

In Option-2, partition A is placed above partition B: altitude of the PS function is higher altitude of the CS function and only partition A broadcasts the NOF. The two partitions remain connected by ISL. The difference is now the CS can receive and monitor the NOF messages being sent by partition A and identify whether corruption is possible. The CS identifies the LFS-A responsible for this corruption and sends it a command requiring it to stop sending the NOF. At the next second, this specific LFS-A will cease the emission. The redundancy of LFS-A is designed to limit the impact of this corruption at user level and to maximize the level of performance of availability and continuity.

A global SBAS should bring permanent continuity in GNSS satellite visibility so it can monitor, at any time, all GNSS satellites configured in the PRN mask. 

LEO Ranging Function

The realization of this function assume seach LFS transmits using a dedicated PRN code. LFS have an independent orbit estimate and are synchronized with each other, giving them the information necessary to fulfill the LEO ranging function (Figure 6). This consists of considering LFS as an additional ranging signal. Consequently, the globalized SBAS can naturally act as a LEO PNT system:

1. The satellite precisely synchronizes the start of a PRN code sequence transmitted in the signal with the start one second of SNT time, 

2. The satellite synchronizes the first bit of the navigation message with the second round of SNT time, 

3. The satellite maintains synchronization of the start of a navigation bit with the start of a PRN code sequence, 

4. The satellite maintains the code-carrier consistency.

The internal navigator of the LFS provides an orbit and a clock synchronization bias relative to the SNT. Orbitography algorithms also provide a variance-covariance matrix that can be used to provide URA data. A suitable ARAIM concept could provide the integrity of LEO satellite navigation data, allowing LEO ranging measurements to be incorporated into a safety-of-life solution. The LFS navigation message shall be encoded in the signal broadcast to the user.

In the classic SBAS paradigm, GEO-Ranging function is possible and GEO data navigation takes place inside the NOF itself (MT9 dedicated for GEO SBAS L1 ephemeris). In the spatialized SBAS paradigm, the number of transmitting satellites is increasing considerably and inserting LEO navigation data into the NOF would congest the available bandwidth. It is better to transmit the SIS ranging data in a dedicated message rather than the NOF. In this aspect, two options are envisioned: either LEO ephemeris are encoded in L5-Q signal frequency or L1-I signal frequency. The first is the most energy-efficient because one signal is generated on L5, which modulates the NOF on the I channel and the ephemeris on the Q channel. The second option requires generating and transmitting two signals on two different frequency bands, which consumes more energy and adds complexity. Users can leverage these two frequencies to form the iono-free combination, as is done with GNSS satellites, and improve their positioning.

Monitoring and Control

Classic SBAS provides system monitoring and control, which involves overseeing and managing the ground segment subsystems, supporting maintenance tasks—including configuration management—offering data archiving capabilities for offline activities, and enabling communication with external entities.

In spatialized SBAS, the need for the system monitoring and control function is still present: it is even mandatory to operate the system. The operator ensures operational management of the system, maintenance of the infrastructure and supervision of the service provided to users. Several Mission Control Centers (MCC) on ground are necessary for this. The MCC and the LFS constellation communicate with each other by classic TM/TC.

LEO Constellation Infrastructure 

Achieving a worldwide SBAS solely through ground stations is impractical due to the necessity of comprehensive global coverage, which would require an immense and continuously maintained network of ground infrastructure spread across the entire Earth’s surface. This makes it difficult to provide consistent, reliable augmentation signals everywhere. 

Deploying space-based stations in LEO orbits offers a more efficient and effective solution. LEO satellites can cover vast areas of the planet from orbit, overcoming environmental masking faced by ground stations. This space-based approach ensures continuous, global augmentation service with improved scalability, making it preferable for establishing a worldwide SBAS.

Another benefit of the LEO constellation, and a consequence of global coverage, is its unique capability to receive measurements from GNSS satellites throughout their entire orbits, regardless of the satellites’ position relative to the Earth’s surface. This comprehensive 
visibility enables LEO satellites to monitor GNSS signals continuously, eliminating geometrical blind spots that can occur when relying on ground stations. The estimation of GNSS satellite orbits and clock errors becomes more accurate and robust, leading to improved navigation performance. By providing consistent and diverse observational data from multiple vantage points in space, LEO constellations significantly expand the precision and reliability of GNSS orbit and clock determination. This visibility is the main driver of constellation size: ensuring at least one LEO satellite is visible at all times everywhere on Earth is required to receive the NOF message. Given the safety-of-life nature of the system, safety guidelines further recommend a minimum of two LEO satellites be visible at all times everywhere to cover a single satellite failure. Visibilities are considered when the satellite is above 5° of elevation above the horizon.

The characteristics of a LEO constellation satisfying this constraint mainly depends on altitude. For this study, two different altitudes are considered: 

• 750 km of altitude. This leads to a constellation made of 96 satellites.

• 1,200 km of altitude. This leads to a constellation made of 57 satellites.

These constellations are designed to ensure a minimum number of satellites to respect geometric constraints. They do not constitute the real constellation that will necessarily be sized to take into account failures, redundancy, etc.

These two LEO constellations are configured and represented in Figure 7. 

For each altitude considered, the constellation is minimal in terms of number of satellites. Any constellation with fewer satellites does not ensure at least two satellites in visibility at all times.

The visibility constraint can be verified by using a simple simulation. Visibilities are calculated for a grid of users on Earth. However, as the LEO satellites have an orbital period between an hour and a half and two hours, they make around 15 orbits on a single day. This means globally, the visibility statistics are the same for all users on a same latitude. Therefore, the results in Figure 8 show the minimum, average and maximum number of satellites in visibility as a function of latitude. 

The constraint of at least two satellites in visibility is satisfied everywhere. The statistics do not differ significantly between the two constellations, with visibilities being minimum at the equator, increasing for higher latitudes and decreasing near the poles.

A LEO satellite used for a safety-of-life system will have associated constraints in terms of certification and will be costly. So, the number of satellites should be minimized, making the constellation at 1,200 km the seemingly preferred option. However, this criterion should also be in balance with criteria related to the satellite payload, onboard available power, antenna design, launching constraints and end-of-life constraints. A higher altitude means satellite signals will have to be transmitted with a higher power. The launchers will need to reach a higher altitude, and deorbiting the satellites when they reach their end of life will require more manoeuvring capabilities. This all must be taken into account, but it is also very dependent on satellite platforms, payloads and launching capabilities.

Distributed Processing Facility 

Space-based computing

Space-based computing has been increasingly taken into consideration over the past few years. Among others, we mention the innovative EU-funded study Advanced Space Cloud for European Net zero emission and Data sovereignty (ASCEND) [10], which focuses on the feasibility of deploying space-based data centers and relying on space-based cloud based solutions [11].

In May 2025, China launched the first 12 satellites of a planned 2,800-strong orbital supercomputer satellite network. These satellites aim at performing calculations in space without relying on any ground-based computing facility.

Computing in space offers three major advantages compared to traditional ground-based systems:

• Reduced data transmission costs: Processing data locally in space reduces the need to downlink large volumes of data to Earth, saving bandwidth and costs;

• Low latency: The mutual proximity of satellites in the constellation reduces communication delays, enabling faster data processing;

• Scalability: Space-based cloud computing can potentially scale by deploying additional resources (later named spare satellites) and can be reconfigured dynamically.

A centralized space-based solution 

A straightforward route to designing an efficient space-based processing facility is to rely on a centralized space-based solution, where an independent and already in place infrastructure located in space hosts the entire calculation. This solution exploits already existing algorithms and relies on computing infrastructures currently used on ground-based systems. Nevertheless, this easy to follow route faces the following three main challenges:

• Single point of failure: The entire computing system relies on a single dedicated platform. This means any hardware or software failure can disable the entire computing capability, unless duplicate/diversified computing capabilities are incorporated into the constellation. This expensive solution makes the global infrastructure rather complex.

• Communication: All data must be routed to and processed by the centralized unit, which represents a major bottleneck in terms of communication and elapsed times before calculation. High communication loads may significantly reduce real-time responsiveness;

• Energy inefficiency: A centralized computing solution may require high power consumption for processing large data movement within the single dedicated infrastructure. This may create additional energy constraints.

A fully distributed space-based solution

For all these reasons, an alternative solution must be proposed. In a fully distributed space solution, each satellite in the LFS constellation corresponds to a specific node of the distributed computing facility. Each satellite of each subconstellation acts as a computational unit and communication between the different nodes is handled by ISL links. 

With at least two LFS visible beyond 5° elevations of any user on Earth, the estimated total number of satellites of the global constellation is bounded by 96. Because half of the LFS are attributed to the augmentation message, the total number of nodes of the computing facility is bounded by 48. This moderate network size for a distributed computing facility allows state-of-the-art parallel algorithms to be employed to compute and broadcast messages [12] [13]. At first sight, distributed computing offers immediate advantages over a centralized solution: 

• Efficiency: Multiple nodes can handle different computations concurrently. This speeds up the overall computation of navigation messages with respect to a centralized solution, where communications may become a significant bottleneck;

• Fault tolerance: Because processing is spread across multiple nodes, random failure of one node does not necessarily harm the entire navigation system. Other existing nodes of the LFS-A subconstellation may take over degraded tasks. This flexibility is one of the main advantages of the distributed computing solution. Additional spare satellites also may be incorporated into the LFS-A subconstellation to improve fault tolerance.

• Redundancy: Distributed systems can implement both hardware and software redundancy more naturally by duplicating specific critical functions across multiple nodes, reducing potential single points of failure resulting from random failure.

The distributed computing facility relies on the core idea that each node performs a specific part of the computational workload and exchanges messages (possibly with each other) through ISL links. At the end of the procedure, the results are gathered by each LFS to generate a common navigation context. This may induce a potentially high volume of point-to-point or collective communications between the nodes of the LFS-A constellation. Therefore, it is of outmost importance to rely on algorithms that minimize the global volume of communication. The computation of the state vector during the filtering process in the PS function provides an instructional example in this regard. Of interest is a parallel algorithm for the solution of least-squares problems that requires a low volume of communication. Does this orthogonal factorization distributed algorithm exist at all?

The answer to this question is positive if we rely on advanced numerical linear algebra methods for the solution of least-squares problems. In our context, a parallel algorithm named Communication-Avoiding QR (CAQR) is worth considering [14]. CAQR is a class of QR orthogonal factorization algorithms designed to minimize (and not avoid) the costly communication between nodes in distributed systems. Because data movement often dominates the energy consumption and runtime of numerical algorithms, CAQR aims at improving both performance and energy efficiency by reducing the communication overhead. Communication in our context includes data transfers, which are often more expensive (in time and energy) than arithmetic operations [12]. This reduction of communication overhead is obtained through a specific factorization: CAQR typically divides the matrix to be factorized into different panels (i.e. blocks of columns). Instead of applying the classical Householder QR method [15], [16] on the panel, CAQR applies Tall-Skinny QR (TSQR) [17] instead, a specific QR factorization. TSQR minimizes communication by recursively factorizing smaller blocks, using a reduction tree structure (e.g., a binary tree of partial QR factorizations) to combine results efficiently with limited data movement [18] [19]. In short, communication costs are reduced by organizing QR operations as tree-structured reductions rather than linear sequences. This algorithmic feature enables parallel processing of independent blocks, combining partial results with minimal communication steps. The number of communication steps is logarithmic in the number of panels [20].

By significantly reducing the volume of communication, CAQR delivers a reduced energy consumption while providing improved overall runtime [14]. CAQR maintains the numerical stability properties of classical Householder QR factorizations, ensuring accurate and reliable factorization results despite the communication optimizations. At the end of the algorithm, the solution of the least-squares problem is known on a leaf of the reduction tree and a single collective communication is used to share this information on the other nodes of the LFS-A subconstellation. This shows a distributed algorithm with a low overhead in terms of communications can be applied during the filtering process. 

Space-based distributed computing is a doable approach that introduces additional constraints to satisfy during the design of the architecture of the spatialized SBAS: 

• Limited computational resources: Each node has a specific limited CPU (or GPU) power, memory and energy compared to Earth-based computing centers. A key point is to optimize the global hardware resource efficiency with respect to the properties of the LFS-A subconstellation (number of satellites and total volume of communication essentially);

• Physical and environmental constraints: Space-based computing often meets challenging physical conditions (such as temperature extremes, vibration, radiation in space) that may affect hardware reliability. Hardware must be resilient against environmental factors. A key point is to overestimate the number of satellites in the LFS-A subconstellation to provide redundant calculations.

These additional constraints must be carefully considered when designing the global constellation and when performing the safety analysis.

Safety Dimensioning in New SBAS Architecture Concepts

When analyzing novel SBAS architectural concepts from a safety standpoint, it is imperative to recall the overarching safety dimensioning principles to guide the assessment of their compliance and the identification of associated constraints.

This analysis is framed within the context of civil aviation. At the system level, the primary safety-feared events and their corresponding severity classifications are defined as:

• Integrity is customarily established as the measure of the trust that can be placed in the correctness of the information supplied by a navigation system. Integrity includes the system’s ability to provide timely warnings to users when it should not be used for navigation. A failure in integrity, termed a “non-integrity event,” is linked to a hazardous severity classification [21].

• Continuity is the ability of the total system (comprising all elements necessary to maintain craft position within the defined area) to perform its function without interruption during the intended operation. More specifically, continuity is the probability the specified system performance will be maintained for the duration of a phase of operation, presuming the system was available at the beginning of that phase. A “non-continuity event” corresponds to a major severity classification [21].

Foundational Safety Engineering and Safety Assurance Principles

The applicable European Cooperation for Space Standardization (ECSS) standards in Europe stipulate that “no single system failure or single operator error (SPOF) shall have critical (i.e. hazardous) or catastrophic consequences.” This has profound architectural implications; it requires that any function whose failure could result in critical/hazardous consequences must be underpinned by a minimum of two independent components.

Development Assurance Level (DAL)

Given the safety-critical nature of civil aviation, software development is governed by rigorous standards. Safety analyses underpin the allocation of Development Assurance Levels (DAL) to various items in accordance with the architecture.

Development Assurance involves specific planned and systematic actions providing confidence that errors or omissions in requirements have been identified and corrected to the degree the system implemented satisfies the applicable safety requirements. System/sub-systems and products are assigned DALs based on failure condition classifications associated with system level functions implemented in the sub-systems and products. The rigor and discipline needed in performing the supporting processes vary corresponding to the assigned development assurance level.

The initial software DAL determined can be mitigated when considering the different kinds of protections or alternate potential design implemented into the architecture, with provision that evidence of full independence between involved software functions is provided. Finally, the DAL allocation is a consequence of the implemented architecture: The redundancy, independence, and segregation embedded within the architecture dictate the refinement of DAL assignments. DAL levels play a pivotal role in component selection and exert a significant influence on project costs. It is prudent to iteratively assess candidate architectures to converge upon an optimal solution.

Software failures with potential hazardous implications (e.g., non-integrity events in SBAS) necessitate DAL B [22]/SWAL 2 [23].

Software failures leading to major events (e.g., non-continuity events and Accuracy Major event in European SBAS) require DAL C [22] / SWAL 3 [23].

In typical SBAS architectures, functions contributing directly to the integrity check of augmentation messages and certain critical data dissemination tasks—those that guarantee the non-corruption of broadcast messages—are assigned DAL B, in recognition of their integrity-related criticality. Conversely, functions related to data collection and non-critical dissemination generally carry a DAL C assignment in Europe, reflecting their continuity focus.

Emitted SBAS Signal Monitoring

For any safety-critical system intended for safety-of-life applications, the following principle remains salient: Wherever possible, the SBAS system should internally monitor its own transmitted signal, permitting real-time awareness of failures (primarily those in the dissemination chain) and take adequate actions instead of relying on open-loop operation. While not a formal requirement provided other safety principles (in particular the SPOF principle) are observed, this best practice is inherent to the present concept.

Implementation of Safety Principles in Operational European SBAS EGNOS V2

These foundational safety principles are stringently applied in the operational European EGNOS system, with their fulfillment evidenced across the following major functions:

Data Collection: EGNOS V2 employs physically and logically separated RIMS A and B chains, both developed according to DAL C1.

• Augmentation Message Calculation and Integrity Checking: The Central Processing Facility (CPF), assigned DAL B1, comprises two independent units fed by independent data: the PS fed by RIMS-A and the CS fed by RIMS-B. In accordance with [22], the PS is allocated DAL C1 and the CS receives DAL B1. This dual-channel design directly supports enforcement of the SPOF principle.

• User Dissemination: The operational SBAS in Europe relies on NLES and GEO segments. Safety-critical integrity related functions—such as CPF selection and Integrity Check— are segregated and allocated DAL B. Functions that contribute primarily to continuity rather than integrity are assigned to DAL C1.

The integrity check function—which continuously verifies the fidelity of the broadcast NOF via the Integrity Box—effectively upholds the SPOF principle by precluding integrity events stemming from a single failure or corruption of the NOF within the dissemination chain. This mechanism ensures continuous monitoring of the emitted SBAS signal, empowering the system to respond appropriately in the event of any dissemination anomaly.

Complementing this capability, GEO signals as received at the RIMS, are relayed to the CPF, facilitating the prompt issuance of alarms or corrective actions whenever discrepancies are identified. Should a failure—specifically, NOF corruption—arise within the dissemination chain (in cases where the chain does not broadcast the information as instructed by the CPF), it is possible that, even if the CPF detects the anomaly and generates alarms, these messages might not be transmitted due to the compromised dissemination chain. The integrity check function is designed to address this scenario.

Compliance of the Fully Space-Based SBAS Concept with Safety Requirements

At the system level, from a safety perspective, the high-level architectural proposal is summarized in Figure 9. 

At the system level, the architecture preserves the logic of maintaining two independent channels—extending from data collection through correction computations and integrity checks. This dual-channel strategy ensures adherence to the SPOF principle at the highest level. Specifically, LFS-A is dedicated to feeding the PS, whereas LFS-B supplies the CS. The strict separation between LFS-A and LFS-B guarantees the independence of input data for each critical process. 

The correction PS, sourced from LFS-A, is entrusted with generating corrections and the associated integrity bounds. In parallel, the CS leverages independently sourced measurements from LFS-B to validate the corrections and their integrity parameters. This rigorous, independent, dual-channel design ensures a single fault or failure cannot compromise overall system integrity.

For data collection, several considerations stem from safety recommendations. Positioning GNSS data collection stations is critical for the calculations performed by the SBAS processing system. Leveraging mobile GNSS data collection stations introduces the necessity to strictly ensure the accuracy of their geospatial coordinates. To safeguard against error or bias propagation, the positioning solution for LFS stations should be established using means and data independent from those employed by the SBAS system itself. This mitigates the risk that systematic biases or errors could be inadvertently transmitted into the final positions computed by the SBAS. Solution 3 (“LFS orbits are calculated using independent means and independent measurements”) directly fulfills this requirement for independence. In addition, leveraging ISL connectivity with ranging capabilities further increases and consolidates the accuracy of LFS location estimates.

Deploying two distinct fleets—LFS-A and LFS-B—allows separation between correction computation and integrity verification channels. With their positioning, LFS-A and LFS-B will achieve substantially different observation geometries; the system hence ensures data streams used for corrections and integrity bound computations and those for integrity checks remain independent, enhancing the robustness of integrity check.

Analogously to the RIMS DAL C allocation within “terrestrial” SBAS systems—attributed for their respective contributions to continuity—the data collection function is designated a DAL C1.

With respect to data processing and integrity verification, the principle underpinning this architecture is to preserve complete independence between the PS and the CS, upholding the SPOF criterion. To this end, the PS and CS are provisioned with independent inputs from LFS-A and LFS-B respectively, each implementing diversified algorithms purposed to detect and mitigate feared events, initiate appropriate alarms when required, and compute/verify corrections and associated integrity bounds.

Drawing upon a safety monitoring principle [22] that’s applied within operational EGNOS V2, the PS is allocated DAL C1, whereas the CS receives a DAL B1 allocation. These designations impose considerable constraints on software development for the LEO satellite segment.

Both the PS and the CS are proposed under the paradigm of a “fully distributed space-based solution,” whereby the PS function (and likewise the CS function) is performed by an “active sub-pool” of LFS-A (and, correspondingly, of LFS-B). Owing to the permanent communication links established among all LFS units, any failure occurring within one of the active sub-pool LFS nodes is instantaneously propagated. This enables the swift activation and integration of a replacement LFS into the active sub-pool for a given PS/CS sub-function. Thanks to the scale of the constellation, the computational resources available to each LFS unit, and—critically—the capability afforded by the ISL that ensures all LFS nodes maintain an identical level of information, the system can exploit “hot redundancy” among LFS nodes for PS and CS sub-function. This design enhances the overall availability and continuity of the global system.

The concept’s reliance on transmitting a singular, uniquely defined NOF stream simplifies redundancy management across both user receivers and within the system’s own infrastructure.

It is noteworthy that the alternative logic of a “centralized space-based solution” is not inherently prohibitive from a safety perspective. While such an approach does introduce a central point of failure from a RAMS standpoint, this vulnerability can be mitigated by implementing robust redundancy architectures or, if needed, diversified processing chains. Such design adaptations could render the centralized solution sufficiently resilient, thereby restricting its adverse impact on system availability and continuity.

The dissemination of the NOF concept entrusts the LFS with dissemination responsibilities, diverging from the conventional reliance on GEO satellites typical of SBAS. 

Broadly, a failure within the dissemination chain may precipitate:

• A continuity event, triggered by loss of functional capability;

• An integrity event, arising from corruption of the NOF by the LFS.

In the first scenario, loss of a single LFS impacts availability and continuity, but these consequences are geographically constrained and limited to a small subset of users (in marked contrast to the loss of a GEO satellite), rendering such events generally acceptable.

Conversely, in the event of NOF corruption by an LFS, a potentially significant integrity event may ensue. Owing to the density of the LEO constellation, comprehensive real-time monitoring of all NOF transmissions from all LFS assets is unfeasible for the SBAS system. As a consequence, a single undetected failure could compromise system integrity. The safety concept herein articulated recommends users monitor at least two independent LFS sources and cease using the service if discrepancies are detected between the NOF received from these sources.

This mitigation is not considered fully satisfactory from a safety standpoint. First, it does not necessarily protect against all types of dissemination failures, such as systematic software faults affecting LFS-A, which could lead to correlated failures across seemingly independent units. Secondly, it places the burden of integrity monitoring on the user, exposing a fundamental limitation in the system’s intrinsic ability to autonomously detect and respond to dissemination failures—which is not optimal from a safety perspective.

Option-2 offers a different approach, whereby the NOF broadcast from LFS-A is subject to independent monitoring by a separate LFS-B asset, typically operating at a lower orbital altitude. In this arrangement, LFS-B would possess the authority to inhibit or terminate transmissions from LFS-A if an inconsistency or corruption in the NOF is detected. This monitoring of the LFS-A by the LFS-B would be DAL B allocated. 

Additional Safety Considerations and Way Forward

These safety considerations do not identify any fundamental showstoppers to the global SBAS concept using a fully space-based infrastructure. This concept eliminates de facto classic local ground effects such as multipath, interference, tropospheric delays, and tidal effects, improving performance. Nevertheless, several broader points must be explored: 

Applicable SBAS Regulatory Framework 

The safety reference framework and associated requirements considered are currently in force for SBAS within Europe. One major consequence of this regulatory baseline is the requirement for dual, fully independent “A” and “B” chains—most notably, the need for PS and CS functions to be separated and developed respectively to DAL C and DAL B. In particular, the imposition of DAL B on software development for LEO satellites may result in very significant development costs.

The SPOF principle for critical/hazardous events, as inherited mainly from ECSS, appears to be more stringent than those applied in the aeronautical domain. A review of [21] reveals:

• No explicit “no SPOF” criterion for hazardous failure conditions;

• No requirement that no combination of two independent system failures or operator errors should lead to catastrophic consequences (required by the ECSS). 

Notably, aeronautical standards demand the absence of SPOF only in the case of catastrophic consequences. In Europe for a SBAS, the requirement for no SPOF in critical/hazardous systems may be justified by the large number of aircraft potentially affected by any failure—a rationale that arguably holds even greater weight for a global system of this nature.

Applying the SPOF principle at the critical/hazardous level mandates the implementation of two independent chains for the CS and PS. It would be relevant to analyse this architecture and corresponding DAL allocation in light of the [24] guidelines (which are not part of the European baseline for SBAS). According to [24], if a hazardous failure condition could result from a combination of possible development errors between two items, either one should be allocated at least DAL B, or both should be assigned DAL C. This latter approach could offer a more balanced allocation of development assurance levels and potentially alleviate some of the stringent constraints currently imposed on LEO satellite software development. 

With regard to implementing the approach outlined in [24], the current concept involves exchanges between the PS and CS chains. In particular, the function responsible for generating corrections and integrity bounds is not fully duplicated across both PS and CS chains. Should the current level of independence between PS and CS be insufficient to comply with the principles set forth in [24], minor modifications to the concept could be considered. For instance, the PS functions could be integrated within the LFS-B, with dissemination of information also performed by the LFS-B (as in Option 2). In this case, two NOFs would be distributed, and a voting mechanism at the user level would help identify an erroneous NOF. However, this scenario would lack monitoring of the NOF broadcasted by the LFS-B to the user.

Identification of New Feared Events Arising from Spatialization 

Introducing “fully based” elements—specifically the implementation of ISL, using the DORIS system, and the spatialization of equipment that is traditionally ground-based in an SBAS—should lead to identifying new internal feared events to address in the system-level analysis.

Global System Considerations

The core strength, innovation and advantage of this concept lie in its potential to provide truly global coverage for integrity services. The positive implications of such an advancement would be substantial, but it’s necessary to address questions regarding responsibilities and roles among different countries, particularly given the safety-critical nature of the service on a worldwide scale.

Conclusions 

This article explores a groundbreaking shift in SBAS architecture by proposing the spatialization of its core components—data collection, augmentation message computation and dissemination—within a distributed network of LEO satellites. By moving reference stations into orbit as LFS, the system achieves global GNSS visibility, eliminates the constraints imposed by terrestrial station distribution, offers a worldwide service, and significantly enhances the accuracy and resilience of navigation augmentation data.

The architecture leverages advanced technologies like inter-satellite links and space-based distributed computing, enabling real-time data sharing, independent time scale generation, and robust integrity monitoring. The proposed partitioning of the constellation further meets stringent safety-of-life requirements, ensuring redundancy, diversity of observations, and fail-safe operations.

Simulation results demonstrate that appropriately sized LEO constellations can guarantee continuous visibility and redundancy for service availability, while the distributed processing facility uses state-of-the-art parallel algorithms to minimize communication overhead and maximize computational efficiency. While the technical feasibility is affirmed, the design must also accommodate the unique constraints of space infrastructure—including hardware resilience, energy consumption and operational safety.

Overall, this study shows that a globalized, space-based SBAS could offer transformative improvements in augmentation accuracy, reliability and scalability—paving the way for a next-generation system capable of meeting the demanding needs of civil aviation navigation on a truly worldwide scale. Future work will focus on refining the constellation design, optimizing system safety, and addressing the operational and certification challenges inherent to spaceborne navigation augmentation. 

Acknowledgment 

The authors thank Michel Monnerat for discussions regarding receiver signal processing and Celine Renazé for her useful advice and recommendations. 

References

[1] ICAO Standard and Recommended Practices (SARPs), Annex 10, Volume 1, up to Amendment 93

[2] D. Flament, J. Poumailloux, J-L. Damidaux, S. Lannelongue, J. Ventura-Traveset, P. Michel, C. Montefusco, “The EGNOS System Architecture Explained”, May 2011.

[3] User Guide for EGNOS application developers, Ed. 2.0, 15/12/2011, ISBN 978-92-79-20335-0 ESA.

[4] M. Laurenti, L. Maisonobe, P. Roldan, J. Anton, P. Guerin, S. Trilles, “Improving GNSS Navigation Messages Performance using Inter Satellite Links Technology”. Inside GNSS May/June 2024, pp 36-42

[5] Brown, K. R. (1992). The Theory of the GPS Composite Clocks, Proceedings of ION GPS-91, 11-13 September 1991, pp. 223-242.

[6] Greenhall, C. A. (2007). A Kalman filter clock ensemble algorithm that admits measurement noise: corrections and update, Metrologia, 44, 491-494, doi:10.1088/0026-1394/44/6/008

[7] Senior, K. L., & Coleman, M. J. (2017), The Next Generation GPS Time, NAVIGATION: Journal of The Institute of Navigation

[8] Roldan, P., Trilles, S., Serena, X., Tajdine, A., “Novel Composite Clock Algorithm for the Generation of Galileo Robust Timescale,” Proceedings of ION GNSS 2022, September 2022, pp. 2790-2799. https://doi.org/10.33012/2022.18521

[9] C. Renazé, C. Bourga, M. Clergeaud, J. Samson, “Reduction of system time to alert on SBAS”.  Inside GNSS, November-December 2023, pp 28-36

[10] https://ascend-horizon.eu/

[11] https://www.thalesaleniaspace.com/en/press-releases/thales-alenia-space-reveals-results-ascend-feasibility-study-space-data-centers-0

[12] G. Hager and G. Wellein, “Introduction to High Performance Computing for Scientists and Engineers”, CRC Press, 2011.

[13] P. Pacheco and M. Malensek, “An Introduction to Parallel Programming”, 2nd Edition, Morgan Kaufmann, 2021.

[14] J. Demmel, L. Grigori, M. F. Hoemmen, and J. Langou, “Communication-optimal parallel and sequential QR and LU factorizations”, SIAM Journal on Scientific Computing, Vol. 34, 1, pp. A206-A239, 2012, https://doi.org/10.1137/080731992

[15] Å. Björck, “Numerical Methods for Least Squares Problems”, 2nd Edition, SIAM, 2024.

[16] G. H. Golub and C. F. Van Loan, “Matrix Computations”, 4th Edition, Johns Hopkins University Press, 2013.

[17] M. F. Hoemmen, “Communication-avoiding Krylov subspace methods”, PhD thesis, University of California at Berkeley, 2010.

[18] E. Agullo, C. Coti, J. Dongarra, T. Hérault and J. Langou, “QR factorization of tall and skinny matrices in a grid computing environment,” 2010 IEEE International Symposium on Parallel & Distributed Processing (IPDPS), Atlanta, GA, USA, 2010, pp. 1-11,  https://doi.org/10.1109/IPDPS.2010.5470475.

[19] G. Ballard, J. Demmel, L. Grigori, M. Jacquelin, N. Knight, H.D. Nguyen, “Reconstructing Householder vectors from Tall-Skinny QR”, Journal of Parallel and Distributed Computing, Volume 85, pp. 3-31, 2015. https://doi.org/10.1016/j.jpdc.2015.06.003.

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[21] CS-25 – European Union Aviation Safety Agency Certification Specification for Large Aeroplanes.

[22] ED-12B/DO-178B – Software Considerations in Airborne Systems and Equipment Certification

[23] ED-109A/DO-278A Software Integrity Assurance Considerations for Communication, Navigation, Surveillance and Air Traffic Management (CNS/ATM) Systems

[24] ARP4754B – Guidelines for Development of Civil Aircraft and Systems

Authors

Sébastien Trilles is an expert in navigation algorithms and performances. He received his Ph.D. degree in Pure Mathematics from the Paul Sabatier University and an Advanced M.S.in Space Technology from ISAE-SUPAERO. He heads the Performance and Processing Department where high precise navigation algorithms are designed as orbitography, system reference time generation, clock synchronization and time transfer, integrity and ionosphere modeling.

Thierry Authié is a specialist in navigation algorithms at the Performance and Processing Department of Navigation Domain, Thales Alenia Space. He received his M.S in Applied Mathematics from the Institut National des Sciences Appliquées (INSA), Toulouse (France). He currently works as navigation specialist in Advanced Projects.

Xavier Vasseur is a specialist in scientific computing at the Performance and Processing Department of Navigation Domain, Thales Alenia Space. He received his M.Sc degree from Ecole Centrale de Nantes (France) and his Ph. D. degree in Computational Fluid Dynamics from University of Nantes.

Marie Abbal is safety manager of Advanced Projects in Navigation Domain. She received her M.Sc degree from Ecole des Mines de Paris. She worked from 2009 to 2016 at Electricité de France company (EDF), particularly in nuclear safety. She joined Thales Alenia Space in 2016 as a safety specialist in complex and critical space system

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Institutional Escalation: ICAO and ITU Act

The most significant development for the GNSS community may be regulatory rather than technical. In October 2025, the International Civil Aviation Organization passed a resolution condemning GNSS interference originating from both Russia and North Korea as violations of the 1944 Convention on International Civil Aviation. The following month, the ITU’s Radio Regulations Board, at its 100th meeting, again urged Russia to “immediately cease any source of harmful interference” to safety services in the Radio Navigation Satellite Service — specifically interference affecting receivers in Estonia, Finland, Latvia, and Lithuania originating from Russian territory.

The Baltic situation had intensified steadily through the year. Lithuania coordinated a letter signed by 17 EU transport and digital ministers in June 2025 calling for a coordinated European Commission response. The European Council’s own data showed aircraft GNSS interference cases in Poland rising from 1,908 in October 2024 to 2,732 by January 2025. Estonia announced in July 2025 that Russia had moved jamming equipment to a site at Kingissepp, 20 kilometers from its border, and reported that GPS jamming had caused over €500,000 in damage in the preceding three months alone. Sweden’s Department of Transport stated that interference over the Baltic was occurring “almost daily” and had spread “both geographically and in scope.”

Active Conflict: Iran, India-Pakistan, Israel

The SWF report also documents the operational deployment of GNSS interference in three distinct conflict contexts in 2025.

During Iran’s 12-day war with Israel in June 2025, Iran jammed GPS over multiple metropolitan areas to counter drone and missile threats. Iran’s Deputy Communications Minister publicly acknowledged the disruptions were “for military and security purposes.” The Maritime Information Cooperation and Awareness Center estimated that 970 ships per day experienced GPS jamming in the Strait of Hormuz during this period, causing traffic through the Strait to drop by 20 percent as vessels limited transits to daylight hours. The report adds a technically notable January 2026 data point: during protests in Iran, Starlink ground terminals were found to have had their GPS units spoofed, causing packet losses of 30 to 80 percent. Users who switched to Starlink’s internal position estimates restored connectivity; SpaceX subsequently pushed a software update to mitigate the interference.

In South Asia, during India’s Operation Sindoor against Pakistan in May 2025, Indian electronic warfare forces were deployed specifically to interfere with GNSS signals to hamper Pakistani military aircraft navigation. The report notes that GPS spoofing has since migrated from the border zone into civilian airspace: more than 10 percent of flights in the Delhi region have reported spoofing incidents, and in November 2025 interference around Indira Gandhi International Airport was severe enough to divert flights to alternate airports.

Israel, for its part, entered into a formal commitment at the ITU in late 2025 to limit RNSS-interfering transmissions to situations involving imminent threats to life or critical infrastructure, capped at 15 minutes per incident — following a July 2025 meeting with Jordan and Egypt convened under ITU auspices.

The LEO Dimension

Perhaps the most technically striking finding for GNSS engineers: the report cites Aerospace Corporation research from July 2025 indicating that GPS jamming over Ukraine has created what researchers described as “a giant hole” in GPS coverage for small LEO satellites carrying onboard GPS receivers for position, navigation, and timing. The jamming environment over a conflict zone is now affecting space-segment PNT — not just ground users.

The Secure World Foundation’s Global Counterspace Capabilities 2026 is available here.

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The satellite had reached its 10-year design life on March 10. ISRO indicated the spacecraft will remain operational for limited services, including one-way messaging, but it will no longer support standard navigation functions that depend on precise onboard timing.

Timing loss directly impacts PNT performance

Atomic clocks are the foundational element of any GNSS or regional PNT system. Navigation signals rely on nanosecond-level synchronization between satellites and user receivers; without a stable onboard clock, a satellite cannot provide accurate ranging. In practical terms, the loss of IRNSS-1F as a timing node reduces usable signal geometry and degrades overall system robustness rather than simply removing a satellite from inventory.

IRNSS-1F was launched in March 2016 as part of the original IRNSS constellation, now branded as NavIC. Its loss for navigation further compresses a system already operating with limited redundancy. Public disclosures and recent reporting indicate that only a subset of NavIC satellites are currently fully usable for positioning services, leaving less margin for fault tolerance across the regional coverage area.

Persistent clock reliability challenges

The latest failure reinforces a long-standing issue within the NavIC program: the reliability of space-qualified rubidium atomic clocks. Earlier IRNSS satellites experienced similar failures, forcing ISRO to operate the constellation conservatively and accelerate plans for replacement spacecraft. Across GNSS architectures, clock reliability is a critical determinant of system availability—satellites can remain on orbit yet become functionally irrelevant for navigation if timing degrades.

ISRO has been pursuing second-generation NavIC satellites to restore and expand capability, but progress has been uneven. The NVS-02 satellite, part of this next-generation effort, encountered issues following launch despite successful orbital insertion. At the same time, ISRO continues to invest in the broader timing infrastructure supporting NavIC, including international metrology collaborations aimed at strengthening reference time systems.

Strategic implications for sovereign PNT

The immediate effect is not a loss of NavIC service, but a reduction in resilience at a time when sovereign PNT capability is increasingly treated as critical infrastructure. NavIC was designed to reduce India’s reliance on foreign GNSS for both civilian and government applications. As jamming, spoofing and geopolitical risk reshape the PNT landscape, constellation health—particularly onboard timing integrity—becomes a primary measure of operational capability.

Timing integrity remains the system’s linchpin

The IRNSS-1F failure underscores a fundamental point: end-to-end resilience begins with stable space-segment timing. Advances in signal structure, augmentation and receiver design cannot compensate for degraded clocks in orbit. Restoring NavIC’s full capability will depend not only on replenishment launches, but on achieving durable, next-generation clock performance across the constellation.

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Over the last several days, maritime analytics providers have documented interference events affecting more than 1,000 ships in the Middle East Gulf, alongside a growing pattern of AIS anomalies and “dark” operations. At the same time, tanker and container traffic has slowed or stopped near the Strait of Hormuz, and leading war-risk insurers are withdrawing cover for the region. 

The episode illustrates in practical terms what a contested RF environment means for ships that still rely heavily on satellite-derived position for navigation, tracking and compliance.

Interference profile: GPS jamming and AIS spoofing on a regional scale

Maritime intelligence firm Windward reports that more than 1,100 vessels experienced GPS and AIS interference across the Middle East Gulf within a single 24-hour period following the outbreak of hostilities between Iran, the United States and Israel. Ships’ reported positions were displaced onto airports, inland locations in Iran and the Gulf states, and even over a nuclear power plant, producing track histories that are clearly inconsistent with physical reality. 

A parallel assessment reported by Wired, based on analysis of satellite navigation attacks since the start of the air campaign against Iran, arrives at a similar figure of roughly 1,100 ships affected, underscoring that interference is not limited to a small subset of vessels or a single narrow area. 

Dryad Global notes “heightened risk of GPS jamming and AIS spoofing” in the Gulf of Oman and Strait of Hormuz, explicitly linking recent anomalies to Iranian naval exercises and electronic warfare activity. 

Taken together, the data suggests:

• GNSS-derived position can become systematically biased over wide areas, not only momentarily lost.
• AIS tracks based on those positions may show vessels apparently transiting over land, clustered around inland targets, or moving in circular or jagged patterns that reflect repeated loss and reacquisition of signal.
• Some operators respond by switching AIS off altogether, which protects them from misinterpretation of spoofed positions but reduces visibility for collision-avoidance and traffic management.

From a PNT standpoint, this is a textbook case of how GNSS jamming and spoofing propagate through downstream systems that treat satellite position as authoritative.

Shipping, insurance and security advisories

The interference is occurring against the backdrop of a broader shipping disruption centered on the Strait of Hormuz.

Reuters reports that around 150 ships, including oil and LNG tankers, are currently stranded near the Strait of Hormuz, with at least five tankers damaged and crew casualties following drone and missile attacks. In the wake of US and Israeli strikes, Iran has announced that it is closing the strait, and many market participants now characterize conditions as a “de facto” closure of a route that normally carries about one-fifth of global oil exports and substantial volumes of gas.

In response to the increased risk:

• Major war-risk underwriters, including Gard, Skuld, NorthStandard, the American Club and others, are cancelling war-risk cover for ships operating in Gulf and Iranian waters from early March, with premiums for any residual cover rising sharply. 
• Container carriers such as Maersk and CMA CGM have begun rerouting or suspending services that would normally pass through Hormuz, adding to the reduction in commercial traffic through the area. 

On the governmental side, a recent advisory from the US Maritime Administration designates the Strait of Hormuz, Persian Gulf, Gulf of Oman and parts of the Arabian Sea as an area of active military operations and potential retaliatory strikes by Iranian forces. The advisory highlights the risk of hailing, boarding or detention of commercial vessels and directs operators to closely monitor updates and guidance from US Naval Forces Central Command. 

Although these notices are primarily focused on kinetic threats, several security circulars from P&I clubs and risk advisers now explicitly call out the likelihood of GPS interference and AIS anomalies in the region and recommend that ships treat GNSS-based position with caution when operating there. 

Implications for PNT resilience

The current pattern of events around Hormuz reinforces several points that have been discussed in standards bodies and industry forums for some time:

• GNSS reliability is not uniform. In certain strategic waterways, including parts of the Gulf and Strait of Hormuz, interference can reach a level where satellite-based positioning should be treated as advisory rather than authoritative. 
• Spoofed or displaced positions can have regulatory and commercial consequences, not just navigational ones, when automated compliance systems interpret false AIS tracks as evidence of port calls or territorial incursions. 
• “Going dark” on AIS reduces exposure to mis-located tracks but increases dependence on radar and visual watchkeeping, especially in confined waters.

For PNT system designers and policy-makers, the current situation underscores the value of alternative and complementary positioning sources, whether that means terrestrial systems, inertial aids, or hardened multi-constellation receivers, and the need to assume that in some regions, GNSS degradation will not be an exception but a recurring operating condition.

In that sense, the developments around Hormuz are less an isolated crisis than another data point in an evolving pattern: satellite navigation has become a routine instrument in regional competition, and maritime navigation practices are having to adjust accordingly.

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According to Divirod’s announcement, the deployment targeted mountainous terrain exposed to extreme rainfall and seismic activity and collected “continuous, all-weather” measurements across three areas of interest during the monitoring period. Divirod said its algorithms analyzed daily GNSS-R measurements to detect subtle changes in the ground surface, then classified observed changes into three categories: slope failure events, creep/slow-moving landslides, and temporary terrain changes often linked to rainfall or ground-moisture variations. Divirod reported detecting “hundreds” of terrain changes and correlating them with rainfall measurements and earthquake events, producing risk mapping and identification of active zones. 

A highlighted event in the release was a landslide at Hakikoga in the city of Asakura. Divirod said imagery taken on Aug. 10 and 11 showed visible slope changes during daylight, while its GNSS-R terrain change maps indicated the slope movement occurred overnight when on-site cameras could not observe the event due to darkness; Divirod reported its sensors registered a spike in ground movement associated with the terrain shift. 

GNSS-R, sometimes described as “bistatic radar of opportunity,” is a passive remote-sensing technique that uses reflected GNSS signals to infer properties of the reflecting surface. In spaceborne applications it has been used for ocean and land remote sensing (including soil moisture and other geophysical parameters), with the “passive” characteristic often cited as a differentiator versus active radar systems. Divirod positions its approach as GNSS-R sensing plus algorithms for environmental intelligence and deformation/anomaly detection in support of risk monitoring and early warning. 

Divirod CEO Javier Marti said the work demonstrates how “advanced GNSS-R solutions” can enhance early detection and situational awareness for geohazards in Japan and that the company aims to expand deployments in Japan and globally. OKI CTO and head of technology division Yoichi Kato said the joint demonstration contributes to early response to natural disasters and strengthening regional resilience, and that OKI expects to continue the partnership to enhance disaster-prevention capabilities. 

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SES, a space solutions company, and the European Union Agency for the Space Programme (EUSPA) today announced an extension of the European Geostationary Navigation Overlay Service (EGNOS) GEO-1 satellite service agreement through 2030, with an option to extend until 2032, helping maintain high-precision navigation services for aviation and other critical users across Europe.

By improving the accuracy and integrity of satellite positioning signals, EGNOS supports aircraft in landing in low-visibility conditions, as well as planning more efficient routes, reducing fuel burn and CO₂ emissions. At the core of the EGNOS service is Europe’s regional Satellite-Based Augmentation System (SBAS) that improves the accuracy and reliability of Global Navigation Satellite System (GNSS) signals, such as GPS. Beyond aviation, EGNOS supports maritime navigation and precision-driven agriculture, contributing to efficient operations and sustainability by reducing fuel consumption and emissions.

Under the extended GEO-1 contract, SES will continue operating an EGNOS hosted payload on its SES-5 satellite, as well as the ground segment from its facilities in Europe.

“This extension ensures a robust EGNOS space segment, ready for the transition towards its next version and the development of new services, while safeguarding high-precision navigation for aviation and other critical users across Europe,” said Rodrigo da Costa, EUSPA Executive Director.

“EGNOS is a cornerstone of Europe’s aviation and broader navigation applications. The agreement underscores SES’ and EUSPA’s joint commitment to advancing satellite-based services that enable secure, reliable, and sustainable navigation solutions,” said Philippe Glaesener, Senior Vice President, Global Government at SES. “Thanks to the service, millions of users and operators will benefit from efficient and more reliable air transportation services across all of Europe. This commitment reflects our broader mission of delivering resilient satellite solutions for critical infrastructures.”

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As SBAS services continue to evolve and new satellite platforms emerge, payload designers must integrate augmentation functionality within ever tighter constraints on mass, volume, power and interface compatibility.

Kongsberg Space Electronics has developed a compact, versatile SBAS onboard down converter with support from the European Space Agency (ESA). “An SBAS onboard down converter receives correction and integrity signals uplinked from ground stations to the satellite in C-band, converts them to the navigation L-band, amplifies the signal, and broadcasts it to users over a wide service area.”

Although this function is well proven, many existing designs offer limited adaptability when confronted with new frequency plans, platform interfaces or multi-channel requirements. The Kongsberg project focused on the design, and development of an engineering qualification model (EQM) suitable for SBAS payloads on geostationary satellites, with potential applicability to future medium Earth orbit (MEO) platforms.

Addressing a real need

The project results were presented at a recent ESA-hosted event by Kongsberg Product Responsible and Radio Frequency Design Lead Angelica Viola Marini, and R&D Project Manager Grunde Joheim. Their new solution employs a highly integrated yet modular architecture that enables efficient SBAS down-conversion while maintaining design flexibility. It supports both single-channel (L1) and dual-channel (L5 and E5b) SBAS configurations, with adaptable uplink and downlink frequency plans.

Modular building blocks, including C-to-L band converter hybrids, SAW filter modules, flexible frequency generation, and configurable DC/DC power conversion, allow the design to be tailored to different satellite buses without extensive redesign.

Technical performance was demonstrated through a comprehensive qualification campaign, including vibration, shock, thermal-vacuum, EMC and electrical testing. Results showed stable gain and output power over temperature, low noise figure, strong spurious suppression and good return loss, confirming suitability for operational SBAS payloads.

Importantly, the compact unit achieves this performance with a mass below 1.6 kg and a reduced envelope, directly supporting more efficient payload integration. For end users of SBAS, ranging from aviation to maritime and emerging autonomous applications, this work delivers space segment robustness, which directly translates into service availability and integrity on the ground.

The project ‘Compact versatile SBAS down converter’, was funded under ESA’s NAVISP program, aimed at strengthening the competitiveness of the European positioning, navigation and timing (PNT) industry.

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Established under a bilateral partnership between Australia’s Geoscience Australia (GA) and New Zealand’s Toitū Te Whenua Land Information NZ (LINZ), SouthPAN is the first satellite-based augmentation system (SBAS) in the Southern Hemisphere, designed to deliver next-generation PNT services across Australasia and its maritime zones.

From the outset, SouthPAN has embraced a dual-frequency multi-constellation (DFMC) SBAS architecture, explicitly augmenting both GPS and European Galileo signals (E1/E5a). This DFMC capability, when operational, along with precise point positioning via SouthPAN (PVS), will place SouthPAN in the vanguard of regional SBAS programs, offering sub-meter and potentially decimeter-level precision across land and sea without reliance on terrestrial networks.

2025 benchmarks

Late 2025 has seen the successful completion of SouthPAN’s critical design review, a pivotal systems engineering milestone validating the technical maturity of mission design and subsystem integration ahead of full scale deployment. The review was led by Lockheed Martin Australia with strategic contributions from European partner GMV, signaling SouthPAN is on track for integration and testing phases towards safety-of-life SBAS certification by 2028.

On the occasion of the completion of the review, Myra Sefton, Head of the SouthPAN Branch at Geoscience Australia, stated, “This milestone underlines our commitment to providing navigation solutions that significantly enhance safety, efficiency, and innovation across Australia, New Zealand, and beyond. SouthPAN exemplifies effective international collaboration, setting a global standard in satellite navigation infrastructure.”

Also in 2025, Australia and New Zealand expanded SouthPAN’s space segment through a A$252 million contract with Viasat. The agreement extends earlier arrangements originally signed with Inmarsat, which has since been acquired by Viasat, and ensures long-term access to geostationary satellite payloads needed to broadcast SouthPAN correction signals.

DFMC born and bred

SouthPAN’s DFMC service augments both GPS and Galileo, closely aligning with emerging dual-frequency, multi-GNSS standards promoted in Europe and through ICAO.

While it has remained largely in the background of global GNSS discourse, SouthPAN stands apart because it was designed from the outset as a DFMC system, not an upgrade of a single-frequency, single constellation, legacy architecture. By natively integrating both constellations for Southern Hemisphere conditions and supporting SBAS and PPP services, it serves as an early testbed for next-generation augmentation.

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Although the second iteration of the EU-Latin America and Caribbean Aviation Partnership Project (EU-LAC APP II) formally concluded in September 2025, participating governments and regional organizations have expressed a clear intention to continue cooperation under a prospective EU-LAC APP III, identifying SBAS as a priority area for sustained technical and institutional engagement.

Ongoing progress has built on momentum generated by an EASA-supported regional workshop held in Lima in April 2025, where participants established a permanent SBAS working group tasked with coordinating planning, technical alignment, and implementation efforts across participating states and air navigation service providers.

Activities under EU-LAC APP are organized into defined work streams, structured around six practical pillars: governance and institutional arrangements; ionospheric monitoring and modeling; demonstrator and validation testbeds; cost-benefit and financing analysis; capacity building and training; and an implementation roadmap.

EASA has highlighted the initiative’s broader impact on regulatory and operational practices, including close coordination with the International Civil Aviation Organization (ICAO). At an EU-LAC APP forum, ICAO Secretary General Juan Carlos Salazar said “The cooperation established by the project with ICAO regional offices has been unique in the world,” underscoring the depth of institutional involvement supporting continuing SBAS activities.

As of late 2025, participating stakeholders have outlined next steps focused on phased demonstrator concepts and validation testbeds, including testing of SBAS corrections and localizer performance with vertical guidance (LPV) procedures.

Why it matters

A South American SBAS would materially improve vertical guidance and precision-approach availability across regional airports, especially in terrain-challenged or remote locations. EU-LAC APP also creates a mechanism for European technical support, including sharing of expertise, standards and training, while keeping governance and ownership regional.

Industry contributors, including Spanish technology and consulting company Indra, regional air navigation service providers and European SBAS experts. A provisional timetable has demonstrator design and initial field trials ramping up by mid-2026.

Participants have also examined the potential for future regional data-sharing hubs and identified candidate pilot airports capable of supporting SBAS demonstrators, including Lima, Bogota, and Brasilia. In parallel, discussions are underway on targeted training cohorts to upskill air navigation service provider engineers and regulators.

A follow-up working group meeting is expected in early 2026 to confirm demonstrator sites, timelines, and financing approaches, and to initiate coordinated lab-to-field test campaigns at the national level.

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The ROK’s Loran-C system was installed by U.S. Forces in 1979. The ROK took over the Loran-C system from U.S. Forces in 1989 and has continually maintained the transmitters, recently upgrading them to the eLoran system. Initial Operational Capability (IOC) for eLoran was achieved in 2023.

The U.K. and France cooperated with other nations in a Loran-C network until the end of 2015. The system was dis-established in deference to the developing Galileo GNSS, which would become operational at the end of 2016. While other nations decommissioned their stations, the U.K. maintained its single transmitter for use as a timing source. The U.K. is in the process of establishing its own sovereign eLoran network of six transmitters and has allocated a first funding tranche of $93 million. French President Emmanuel Macron announced In July that his nation would join with the U.K. in providing eLoran.

As the first nation allied with the West to establish an eLoran network, the ROK has taken the initiative to begin international discussions to ensure sovereign eLoran systems are standardized to support global transportation and trade. We reached out to the ROK’s Ministry of Oceans and Fisheries, the department responsible for the nation’s eLoran system, to learn more about the country’s motivations and plans.

Q: The ROK has operated Loran for decades. What was the motivation behind the recent upgrade from Loran-C to eLoran?

A: Since 2010, ROK has intermittently experienced GPS jamming in the West Sea. In such cases, vessels become unable to receive GPS position information through their navigation equipment. Therefore, to provide resilient and robust positioning, navigation and timing (PNT) services even under GPS jamming conditions, we developed the eLoran system and built the necessary infrastructure. A pilot service is currently being conducted in the West Sea.

Q: Is the eLoran standards meeting with the U.K. and France the first of its kind?

A: Yes. We held the meeting to share each country’s policy direction and technology development status related to eLoran, and to discuss future mutual development plans.

Q: Do the ROK, U.K. and France have a shared vision for the future of eLoran?

A: In order to respond to GPS jamming incidents occurring worldwide, we shared information on eLoran technologies and policies, and discussed potential future cooperation. Through this process, we believe a consensus was built on the importance of mutual collaboration.

Q: The ROK is on the opposite side of the globe from the U.K. and France and the countries’ eLoran systems won’t interact. Why is it important to establish shared standards?

A: Due to the recent increase in radio jamming incidents, IMO, ICAO and ITU issued a joint statement in March recommending measures to strengthen resilient and robust PNT systems for the safety of vessels, aircraft and timing systems.

With the goal of advancing eLoran as a resilient and robust alternative navigation system, we discussed technical standards such as signal specifications, data formats and receiver performance.

Even if the three countries’ systems are not directly interconnected at the moment, we believe that if more countries adopt eLoran standards in the future, gradual interconnection across regions such as Europe and Asia will become possible.

Q: Will the three countries be building on the existing eLoran standards set by SAE, or proposing something new?

A: We are reviewing areas where existing technical standards developed by international organizations—such as SAE, IMO, IALA, RTCM and ITU—may need to be supplemented or expanded. Through this process, we plan to continue discussions on standardization to promote the activation of eLoran services and to facilitate a smooth user environment.

Q: Does the Far East Radio Navigation Service organization still exist and meet? Will there be eLoran standards meetings that include China and Russia?

A: ROK, China and Russia operate the Far East Radio Navigation Service [FERNS] to promote cooperation and the development of maritime safety and radio navigation aids, and this cooperation continues.

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