The 14th Multi-GNSS Asia Annual Conference (MGA2024) will be held From January 30^th to February 2^nd at Mae Fah Luang University, in Thailand. Included among the panels focusing on Latest Tech, Latest R&D and Latest Applications & Business, is a Lunar PNT Panel.

This panel will include the following invited speakers:

Lunar PNT invited panelists:

・Mr. JJ Miller (NASA, USA)

・Dr. Jung Min JOO (KARI, Korea)

・Dr. Javier Ventura-Traveset (ESA, Europe)

・TBD (GISTDA, Thailand)

・Prof. Xiongwen He (CAST, China)

・Dr. Masaya Murata (JAXA, Japan)

For more information, visit the conference website at https://www.mga-conference.com/

The post Lunar PNT Panel Session at the Multi-GNSS Asia Conference 2024 appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

A close look at several options that identify potential improvements at system level for a reasonable increase in complexity.

ÉLINE RENAZÉ, CHRISTOPHE BOURGA, MATTHIEU CLERGEAUD, THALES ALENIA SPACE

JARON SAMSON, EUROPEAN SPACE AGENCY (ESA)

The Satellite-Based Augmentation System (SBAS) integrity concept expects the system to alert the user of “out of tolerance conditions” within a required delay that’s compatible with intended operations. This delay is called the time to alert (TTA).

The SBAS TTA is the part of time to alert allocated to SBAS. It corresponds to the maximum allowed time elapsed between the “out of tolerance condition” and the reception of an alarm at the user level. When using precise differential corrections, an out-of-tolerance condition is defined as a horizontal error exceeding the Horizontal Protection Level (HPLSBAS) or a vertical error exceeding the Vertical Protection Level (VPLSBAS) [1] attachment B 3.5.7.5.1.

The European Geostationary Navigation Overlay Service (EGNOS) currently complies with the 6-second time to alert requirement compatible with Category I precision approach operations as defined in table 3.7.2.4.2-1 [1] (split into 5.2s for the SBAS system and 0.8s for the user display).

Early feedback from users indicates there may be an interest for future services offering a TTA budget smaller than the TTA budget of SBAS systems. The target for global TTA is between 1.5 and 5 seconds (allowing to achieve up to Category II approach TTA minima).

To prepare this evolution for further EGNOS versions, Thales Alenia Space has studied several evolutions to reduce the SBAS TTA. This study is system driven, identifying potential improvements at system level, for a reasonable increase of complexity. 

A SBAS computes corrections and integrity data based on GNSS observations that are performed by the ground reference station network (named Receiver Integrity Monitoring Stations (RIMS) in EGNOS). In the context of a Dual Frequency Multi Constellation (DFMC) service, the observations contain the GNSS and SBAS navigation messages received and carrier-code, carrier-phase measurements on L1, L5 (for GPS) and E1, E5a (for Galileo) frequencies. 

The SBAS also uses these observations to detect out of tolerance conditions. 

The corrections, integrity data and alerts are then sent through SBAS messages using a GEO satellite. Observations are computed at RIMS level at 1Hz frequency, which corresponds to the frequency at which SBAS messages are broadcast.

Figure 1 provides a first level decomposition of the time to alert.

The “out of tolerance condition” is detectable from the SBAS using the next observations done at ground station level. Knowing the observations are performed at 1Hz frequency, the “waiting for next observation” time is between 0 and 1,000 ms (1,000 ms in worst case).

The SBAS processing time corresponds to the time needed by the system to process the observations, generate the corresponding NOF (Navigation Overlay Frame) message and prepare it for broadcasting.

The NOF message is then sent at the next GPS second. This generates a “waiting for next broadcasting in NOF” time. This time is between 0 and 1,000 ms.

The SBAS processing time is spread over the SBAS systems. Figure 2 provides a high-level overview of the EGNOS V2 system.

The system is composed of the following subsystems:

• The Receiver Integrity Monitoring Stations (RIMS) that collect GNSS/GEO navigation messages and provide GNSS measurements to the Central Processing Facility (CPF). 

• The CPF computes the complete navigation context, provides corrections and integrity data based on the data provided by RIMS and generates the NOF message to be broadcast

• The Navigation Land Earth Stations (NLES) that broadcast the NOF message to the GEO satellite

• The GEO satellite that broadcasts the NOF Signal In Space (SIS) to users

• The EGNOS Wide Area Network (EWAN) that is responsible for the data transmission from one subsystem to another

Each of the previously defined subsystems contribute to the SBAS processing time included in the time to alert.

Figure 3 gives the EGNOS V2 TTA budget allocation.

This article presents the optimizations axis for each part of TTA, except the user processing part that cannot be improved by the SBAS.

Optimization of SBAS Processing Time

The SBAS processing time is the part of TTA that is completely under SBAS responsibility.

It is composed of observations processing at RIMS level, transmission of observations from RIMS to CPF through EWAN, CPF processing, transmission of NOF message from CPF to NLES through EWAN and NLES processing.

Figure 4 provides a high-level decomposition of SBAS processing time based on EGNOS V2 allocation.

An important part of the budget allocated to SBAS processing is dedicated to transit delay through EWAN (in orange in the previous graph).

The transit delays through EWAN constitute an important part of the TTA budget allocated to SBAS processing. 

The collocation of CPF and NLES makes it possible to avoid transit delay between CPF and NLES. Based on EGNOS V2 allocation, this reduces SBAS processing time to 200 ms.

The transit delay between RIMS and CPF is mainly driven by the technology used for the link.

There are two types of RIMS sites:

• Wired RIMS. RIMS connected to a terrestrial access line.

• VSAT RIMS. RIMS connected to a Hub VSAT (a teleport) via a satellite link.

VSAT links are used on less than 25% of the RIMS sites on EGNOS V2.

Even if the RIMS to CPF maximum transit delay requirement is the same for both types, the observed transit delay is higher for VSAT than for Wired RIMS as shown in the next analysis.

Observation of transit delay on EGNOS V2 over a 11-day period gives the following results for all available VSAT RIMS and wired RIMS:

A solution without VSAT links reduces the allocated time to about 500 ms without the need for further technology improvement.

750 ms are allocated to CPF processing. The CPF is composed of two components:

• Processing Set (PS) that ensures the correction and integrity data and constructs the navigation message.

• Check Set (CS) that ensures the system integrity and verifies the generated messages.

Both components shall be analyzed. The CPF execution time is driven by the maximum execution time of both components and by the exchanges between PS and CS and with other subsystems like NLES. The exchanges are taken into account in the analysis.

Two axes of improvements have been studied: An update of hardware and algorithms with their scheduling improvements.

A previous analysis of PS algorithms (legacy and new algorithms) response time was performed using EGNOS V2 implementation, keeping the same algorithm sequencing and repartition over boards on a Linux Platform with a 3.5GHz CPU. Following this experimentation, 32 ms are theoretically needed per cycle to perform the computation of the latest V2 algorithms. 

This leads us to consider an overall estimation of about 75 ms of total execution per cycle with a 1.5GHz CPU on nominal conditions (in single frequency context). Considering Dual Frequency Multi Constellation context (about 25% of additional computation time), about 25% of overhead for management of external interface (CS, NLES) and inter algorithm management, and an additional margin on about 30%, about 150 ms to 200 ms is needed for EGNOS V2 algorithms processing within the CPF allocated time. 

Requirements for available CPU margins also have been considered. The analysis shows considerable margins exist in terms of CPU load to achieve a reduction of TTA budget for the Processing Set. 

The analysis concludes a hardware composed of six boards with 1.5GHz CPU, reducing the CPF Procession Set execution time to 200 ms.

An equivalent analysis was performed with the CPF Check Set. The analysis was performed using EGNOS V2 implementation on a multi-board architecture based onVM6052 running at 2.2GHz.

This analysis concludes the processing time can be reduced to 200/300 ms. 

In the context of DFMC, as part of the algorithms (inter frequency bias) are not needed, we can conclude that CPF processing time can be easily reduced to 250 ms.

In addition to EGNOS V2, Thales Alenia Space has developed its own Processing Set with a single board architecture (VM6052 running at 2.2GHz). By using SW partitioning, all algorithms can be scheduled on a single board with CPU margins compliant with resource consumption requirements (CPU is only used at 30% with a 2.2GHz processor). 

Taking into account the CPU consumption on a single frequency implementation, and adaptation to DFMC (expandability factors for new constellation and non-required algorithms), the algorithms can be processed in less than 200 ms.

Taking into account these analysis, CPF processing can be reduced to 250 ms (4Hz).

Taking into account optimization of transit delays and CPF processing time, the SBAS processing time can be reduced to 905 ms as shown in Figure 5.

Optimization of waiting for next measurement time

As shown in Figure 1, part of the delay to alarm is due to the time elapsed between the “out of tolerance” event and the observation at RIMS level.

Several axes have been analyzed to reduce this time:

• Introduction of asynchronous processing. This makes it possible to detect “out of tolerance” events as soon as enough observations are available.

• Increase of RIMS output rate. Observations are performed more often at RIMS level, reducing the time between the “out of tolerance” event and the next observation.

Asynchronous Processing

In current EGNOS V2 implementation, observations are performed at the same time on all RIMS synchronized with GPS time. CPF algorithms are executed at 1Hz frequency as soon as almost all RIMS measurements are received.

In an asynchronous scheme, the RIMS produces measurements not synchronized with GPS time. The generation of measurements at the RIMS level would be done so the reception at CPF level would be uniformly distributed. 

The analysis shows that for satellites with good observability (20 RIMS in view) 700 ms are required (with an acceptable probability) to detect “out of tolerance” events. Even with this optimistic hypothesis on satellite observability, the maximum TTA reduction obtained with asynchronous measurements is 300 ms. 

Increase RIMS Output Rate

In this concept, all RIMS will generate raw measurements at the same time but with a frequency of higher than 1Hz. First, the quality of these measurements must be verified and the detectable “out of tolerance” events analyzed. Figure 6 shows adopting a frequency up to 10Hz loop bandwidth for the PR measurements (compared to the usual 1Hz) would not have a significant impact on processing performance.

To take advantage of the increased RIMS output rate, the CPF integrity check algorithms shall be executed at the same frequency. To be consistent with the computation capability studied at CPF level, the proposed frequency is 4Hz. 

Taking into account the increased RIMS output rate of 4Hz and a CPF integrity check algorithm execution time of 250 ms, the SBAS message, including the alert, is ready to be broadcasted after 1,155 ms. Figure 7 shows the alert availability for broadcast.

The “waiting for next measurement” time is then reduced to 250 ms in the worst case, instead of the 1,000 ms for 1Hz measurement sampling. However, if the alerts are sent only through NOF with a 1Hz frequency, even if the system can detect alert conditions at a 4Hz frequency the alerts will be sent at 1Hz frequency. To take advantage of this improvement, the alerts shall be sent with the same frequency, 4Hz. 

Optimization of waiting for next broadcasting and broadcasting time

The alerts are sent in the NOF, which is broadcast with a 1Hz frequency synchronized with GPS time.

This generates a delay up to 1 second. Note the NOF is broadcast over a period of 1 second. Users can decode the alarm only after the complete reception. A delay of 1 second shall be added to the TTA.

This delay (up to 2 seconds) is directly linked to the alarm broadcasting solution.

To reduce this time, an alternative solution “fast alert message” has been analyzed to broadcast the alarm to users.

The aim is to send alerts to the user as soon as possible without waiting for the next SBAS message broadcasting, which is done one time per second synchronized with GPS time.

Fast alert messages contain the alert flags (alarm/no alarm) for all satellites set in the PRN mask. 

Management of fast alert messages implies a re-design of the user receiver that would require a MOPS standard update.

The user receiver shall consolidate the status of each satellite using both alert messages and NOF.

Figure 8 shows how the user receiver can incorporate fast alert messages to his NOF context.

“Construct NOF SIS” module constructs context using the NOF messages. After each alert message reception, the “extract alarms” module extracts the list of satellite alarms. The NOF context is then updated with the extracted alarms: a satellite is considered in alarm if an alarm is raised in fast alert message or in NOF SIS. The updated NOF context is then used like before.

Alarm hysteresis and repetition are ensured by the 1Hz chain in the NOF message.

There is no obligation to use the fast alert message for a receiver. There may be classes of users who can ignore the fast alert messages.

Fast Alert Message

SBAS alert messages, such as the Message Type 34, are transmitted today using a 250-bit long frame, including signalling and control fields (headers, CRC) at a transmission bit rate of 250 bits/s. As a consequence, a minimum 1-second latency is necessary to demodulate an alert message before processing the useful data. As an alternative, this article considers using the Q-component of the L1 and L5 frequency bands, with the strong assumption that the signal power available on the Q-component equals the I-component, without 
taking into account that other potential services could be broadcast on the Q-channel in the future. To reduce the alert message duration, and hence the TTA, two complementary approaches are considered: a bit rate increase requiring alternative signal waveforms and a shortened frame, at least for alert message types.

An analysis of the bitrate increase approach is provided in [2], where several potential candidate signal waveforms were proposed. In [2], the use of Code Shift Keying (CSK) modulation is particularly emphasized and allows for reaching an increased bit rate up to a factor 4, i.e., 1 kbit/s. The CSK was used in conjunction with specially designed Low-Density Parity-Check (LDPC) 
coding schemes within a Bit-Interleaved Coded Modulation, with Iterative Decoding (BICM-ID) architecture. The construction of GNSS waveforms bring additional arithmetical constraints on the sizes of LDPC uncoded words and coded words when taking into account the PRN sequence length, the number of code periods, and the size of the MT frame. In [2] the objective of the bit rate increase was reached, and the pros and cons of the candidate signals were proposed. However, using these alternative signals would constrain the alert message duration over 500 ms.

We preferred to use a more conservative approach, BPSK (because use of CSK would induce a heavy re-design of the receivers as it introduces a new modulation scheme) only as the Option 2B mentioned in [2], but with the perspective of a possible shorter alert message of 125 bits instead of 250 bits. It is foreseen that short-block LDPC codes have degraded performances with respect to larger block codes. Nevertheless, we envisaged to assess the Word Error Rate (WER) performance of an (125,250) LDPC code, concatenated with BPSK modulation, so as to enable 250 ms-latency alert messages. This proposal is presented in Table 2.

Some LDPC codes were proposed in an Experimental Specification published by CCSDS for short-block LDPC codes [3]. More specifically, we investigated the (128, 256) code and found a C/N₀
performance of ~30.7 dBHz for a WER=10-3, which is quite a bit over the 30 dBHz target.

An alternative to the CCSDS codes was proposed in [4], and the new parity-check matrix by Medova in the (128, 256) code has shown a 0.5 dB improvement, reaching a WER=10-3 performance for a C/N₀ of 30.25 dBHz, considered as an acceptable performance. Further improvements were also made to adapt the parity-check matrices to a (125, 250) code size without degradation of the C/N₀ performance. Even better LDPC codes can be designed to reach the required 30 dBHz, opening the path to 250 ms latency (alert) messages and the necessity to construct a 125-bit long message.

The new proposed message structure is based on MT34 from DFMC standard [5]. It is described in Table 3.

This message is used to transmit the Integrity Information through DFRECIs and DFREIs, for all the Augmented Slot Indices derived from the Satellite Mask. The payload is composed of 216 bits comprising: 92 DFRECIs (Change indicators), 7 DFREIs, Issue of Data Mask (IODM), 2 additional bits are reserved. The signalling and control (overhead) are composed of 34 bits: a preamble field, a Message Type ID field, and a Cyclic Redundancy Check (CRC) of 24 bits.

In the context of a shortened version of integrity information, we assume it shall have the lowest impact on other existing message types already transmitted on the I-component. This implies the satellite mask (MT 31), allowing to set up to 92 satellites, is maintained in its current definition. Hence, our main objective is to obtain a minimal alert information (1 bit for alert flag) for all 92 satellites within a total of 125 bits, which leaves only 33 bits.

After taking into account the IODM using 2 bits, the Message Type ID field over 6 bits, and the preamble over 4 bits, there remains only 21 bits for CRC. The standard CRC-24Q currently used in EGNOS is thus too long for this new potential framing. However, because the message frame length also has decreased, the choice for a shorter CRC was proposed. Our study shows it is possible to reduce the number of bits for the CRC with equivalent bit protection level (Hamming Distance), or without loss of integrity by replacing the CRC-24Q with a CRC-16 (0x9EB2).

Finally, we propose a new 125-bit message structure in Table 4 using a well performing CRC-16, as described in Table 3, compatible with the candidate signal Option 2D at either R_b=250 bits/s (500ms latency) or R_b=500 bits/s (250ms latency).

Fast Alert Message Broadcast

To shorten the “waiting for next broadcasting” time, the fast alert message can be sent as soon as an alert is available. However, if alert detection is based on 4Hz RIMS measurements, the fast alert message broadcasting can be done with a 4Hz frequency synchronized with alert availability at CPF output.

Fast alert message insertion makes it possible to eliminate the « waiting for next broadcasting » and to reduce the broadcasting time to 250 ms. The global delay is 500 ms instead of 2 s.

Conclusion 

This article identifies several options to improve the TTA for SBAS. 

It defines a first level of improvement that avoids any modification of the SBAS user standard and considers the benefit of optimizing CPF processing along with using measurements coming from wired RIMS only and/or additional RIMS measurements. These features allow us to reach a 4 second target compared with the current 6 second budget. 

A second level of improvement is obtained with a modification of the standard and the introduction of fast alert messages. Fast alert messages are broadcasted using the SBAS Q-channel and backward compatibility is maintained (Fast Alert messages could be ignored). This way, a target close to 2.5 seconds is met, as depicted in Figure 9. More details can be found in [6]. 

Acknowledgements

This work has been performed and funded under a contract of the European Space Agency in the frame of the EU Horizon 2020 Framework Program for Research and Innovation in Satellite Navigation.

Disclaimer 

The views presented represent the authors’ opinions. They should be considered R&D results and not taken to reflect the official opinion of the EU and/or the European Space Agency, in particular for what relates to present and future EGNOS system designs.

References 

(1) ICAO SARPs Annex 10 Volume 1 amendment 9

(2) Axel Javier Garcia Peña, Rémi Chauvat, Christophe Macabiau, Jaron Samson, Ivan Lapin, et al., Potential candidates for new SBAS signals. PLANS 2020 IEEE/ION Position, Location and Navigation Symposium, Apr 2020, Portland, United States

(3) CCSDS 231.1-O-1Short Block Length LDPC Codes for TC Synchronization and Channel Coding. Experimental Specification. Issue 1. Recommendation for Space Data System Standards (Orange Book)Book), CCSDS 231.1-O-1, Washington, D.C.: CCSDS, April 2015.

(4) L. R. Medova, P. S. Rybin and I. V. Filatov, “Short Length LDPC Code-Candidate for Satellite Control Channel,” 2018 Engineering and Telecommunication (EnT-MIPT), Moscow, Russia, 2018, pp. 163-166, doi: 10.1109/EnT-MIPT.2018.00044.

(5) ED259A (draft 21-06-2023) MOPS for Galileo/GPS/SBAS airborne equipment 

(6) C. Renazé, C. Bourga, M. Clergeaud Thales Alenia Space, Toulouse, France, J. Samson ESA. Reduction of system time to alert on SBAS. NAVITEC 2022 

Authors

Céline Renazé is a specialist in safety architecture at the Algorithms Design and Performance department of Navigation Domain, Thales Alenia Space. She received her M.S. in software engineering from the Paul Sabatier University, Toulouse, France, in 2003. She currently works on SBAS ASECNA as a System Architect.

Christophe Bourga is the head of the Navigation Systems Architecture department at Thales Alenia Space. Since 1997, he has worked on various topics in the navigation domain: GNSS mission, payload design, Galileo signal, formation flying, Galileo algorithms and performance, EGNOS architecture and evolutions, and ARAIM.

Matthieu Clergeaud is a system engineer at the Radionavigation department at Thales Alenia Space. He received his Engineering degree in Space Communications Systems from Telecom INT (Institut National des Télécommunications) in 2003. He now works on EGNOS-related projects.

Jaron Samson works as a Navigation Engineer at ESA’s Technical Centre (ESTEC) and has been involved in EGNOS, GNSS Evolutions, and R&D related to navigation. From 2012 until 2021 he worked as a System Engineer at ESA’s EGNOS Project Office in Toulouse, France. Jaron holds an M.Sc-degree in Physical and Mathematical Geodesy from the Delft University of Technology, The Netherlands.

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SouthPAN calling OEMs to take part in survey to integrate SBAS precise positioning down under.

The Australian and New Zealand governments, with support from FrontierSI, are conducting a survey with Original Equipment Manufacturers (OEMs) to understand the opportunities and barriers for integrating SouthPAN signal support in your GNSS chips, devices and equipment.

SouthPAN is the first Satellite-Based Augmentation System (SBAS) in the Southern Hemisphere and provides improved positioning and navigation services in Australia, New Zealand and maritime regions. Depending on the device, precise positioning from SouthPAN will offer improved accuracies down to as little as 10 centimeters. SouthPAN provides augmented and corrected satellite navigation signals directly from the satellite rather than through a mobile phone, providing accuracy that overcomes gaps in mobile internet and radio communications.

SouthPAN early Open Services have been live since September 2022, with aviation safety-of-life certified SouthPAN services planned in 2028. Safety-of-life certified services will support end users engaging in operations where life can be at risk, like landing an aircraft at an airport. SouthPAN is bringing next-generation SBAS to Australia and New Zealand, creating new market opportunities for OEMs and end-users. An independent economic benefits analysis estimated SouthPAN will provide more than $AUD 7.6 billion of benefits for Australia and New Zealand over 30 years.

As an OEM of equipment providing positioning and/or navigation services, we would value your insights on the support of SouthPAN’s three services into chips, devices and equipment. In particular, we would be keen to hear your views on barriers and opportunities for support of the L1, Dual Frequency Multi-constellation (DFMC) and Precise Point Positioning Via SouthPAN (PVS) services.

The information you provide will assist Geoscience Australia (GA) and Toitū Te Whenua Land Information New Zealand (LINZ) to understand your needs and realize the full potential and benefits offered by SouthPAN. Information about SouthPAN compatibility will be published on GA and LINZ websites to help end-users identify what GNSS devices can access SouthPAN signals. All other information will be aggregated so no individual or business information is identified. The survey should take approximately 10 minutes. If you would like to speak with us about SouthPAN, the survey, or other opportunities, please contact southpan@frontiersi.com.au.

We are seeking your expertise and input into the opportunities and barriers to integrate SouthPAN precise positioning by September 30.

Images courtesy of Geoscience Australia.

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An overview of the Korean Augmentation Satellite System’s first deployment.

CAROLLE HOULLIER, CÉLINE BENASSY FOCH, THIERRY AUTHIÉ, GUILLAUME COMELLI, THALES ALENIA SPACE, TOULOUSE, FRANCE

BYUNGSEOK LEE, EUNSUNG LEE, YOUNGSUN YUN, KOREA AEROSPACE RESEARCH INSTITUTE, REPUBLIC OF KOREA

CHEON SIG SIN, ELECTRONICS AND TELECOMMUNICATION RESEARCH INSTITUTE, REPUBLIC OF KOREA

The Korea Augmentation Satellite System (KASS) is the future satellite-based augmentation system (SBAS) for the Republic of Korea. It is currently developed by the Korea Aerospace Research Institute (KARI) for the government of the Republic of Korea. Thales Alenia Space is the industry prime contractor for this development.

The purpose of KASS is to provide SBAS service compliant with ICAO SARPS Annex 10 [1] over the South Korea area with service level up to APVI.

The KASS system will comprise of the following segments obtained from different manufacturers or service providers:

• A ground segment including network of KASS Reference Stations (KRSs), the redundant KASS Processing Stations (KPSs), the KASS Control Stations (KCSs), the KASS Uplink Stations (KUSs) and an external data interface.

• A network segment ensuring the communication network between all subsystems distributed across Korea (WAN) and the WAN Network Monitoring (WNM).

• A space segment including the Geostationary Earth Orbiting (GEO) satellites and the navigation payloads on-board them. 

KASS system deployment began at the end of 2020 with the onsite installation of the system reference stations network. Figures 1 and 2 show the reference station installation details with the station shelter and the reference station hardware (KRS cabinet and antenna).

In parallel, the wide area network has been extensively tested and verified to allow continuous data transmission.

Since this first deployment step, data has been collected and Thales Alenia Space has undertaken the first performance analysis of real data.

This article presents the overall KASS system architecture as well as the first performance results using the deployed system under real conditions. These results are obtained through KRS real data collection and a replay on KASS test bench hosting fully representative KPS algorithms.

KASS Design and Architecture

The KASS system is the SBAS in Korea. A SBAS is a Global Navigation Satellite System (GNSS) augmentation system standardized under the International Convention on Civil Aviation SARPS Annex 10 [1], Volume 1, a document published and maintained by the International Civil Aviation Organization (ICAO). KASS will provide safety-critical services for civil aviation as well as an open service, usable by other forms of transportation and possibly other position, navigation and timing (PNT) applications.

The KASS system will provide improved GNSS navigation services for suitably equipped users in the defined service areas of the Republic of Korea by broadcasting a signal augmenting the U.S. Global Positioning System (GPS) Standard Positioning Service (SPS). The augmentation signal provides corrections of GPS satellite orbits and clocks and integrity bounds of orbit/clock residual errors, as well as corrections and integrity bounds for ionosphere delays. The augmentation signal will be broadcast by two GEO satellites and will be used by GPS/SBAS user equipment to compute a navigation solution. 

The KASS system is designed to ensure four safety-critical service levels:

• En-Route over Incheon FIR area, flight segments after arrival at initial cruise altitude until the start of descent to the destination. 

• Terminal over Incheon FIR area for descent from cruise to initial approach fix.

• NPA over Incheon FIR area, for non-precision approaches (NPA) in aviation, an instrument approach and landing that uses lateral guidance but does not use vertical guidance.

• APV-I over South Korea land masses (including Jeju Island), for precision approaches with vertical guidance. KASS will also provide open service over the Incheon FIR area. 

Figure 3 shows KASS service areas.

The KASS system is designed to be a system-of-systems that performs these main functions:

• Collects GPS data at various locations in the Republic of Korea (and possibly in other states in the future).

• Computes corrections and associated integrity bounds from ranging measurements of GPS satellites in view of the KASS service area.

• Formats messages compliant with the SBAS user interface standardized in ICAO SARPS Annex 10 [1] and the RTCA MOPS 229 [2].

• Uplinks a signal carrying these messages to navigation payloads on the KASS GEOs.

• Broadcasts the signal to users after frequency-conversion to the L1 band.

Additional support functions are implemented:

• Wide area network between the KASS sites.

• Monitoring and control of the KASS elements. 

• Support to KASS operations.

Main elements of the KASS architecture are:

• 7 KRS (reference stations) deployed on seven KRS sites.

• 2 KPS (processing units) and 2 KCS (monitoring and control units) deployed on the two main control center sites.

• 3 KUS (uplink stations) deployed on 2 KUS satellite broadcast sites.

• A wide area network ensuring inter-sites data exchanges.

Figure 4 shows the overall KASS architecture.

The KASS reference stations network was deployed at the end of 2020. Figure 5 shows the final KRS network locations over Korea land masses.

The KRS detailed site implementations are outlined in Table 1.

The KRS subsystem is composed of two independent channels, each one providing independent measurements to KPS-PS and KPS-CS. The KRS are based on NovAtel WAAS GIII receivers.

The KPS subsystem includes one processing set and one check set. The KPS processing set (KPS-PS) determines the user data information (corrections, confidence intervals, etc.) using GPS raw measurements from one channel of each KRS. The integrity of the information is checked by the KPS check set (KPS-CS) using independent data from the second KRS channel (independent receiver and antenna). The KPS transmits safety status information to the KUS to allow a possible switch by selecting a KPS to be used as the user data provider.

Key Performance Parameters 

In the frame of the analysis, KASS system performances are evaluated to verify the main SBAS requirements.

Performance of a satellite navigation system are expressed through five criteria: accuracy, integrity, continuity, availability and time-to-alert (TTA).

The accuracy feature is the difference between the computed value and the actual value of the user position and time.

The integrity risk is the probability that an error, whatever the source, during the period of operation might result in a computed position error exceeding a maximum allowed value, called the alert limit, where the user is not informed of the error within the specific time-to-alert.

The alert limit is, for a given parameter measurement, the error tolerance not to be exceeded without issuing an alert. It is dependent on the flight phase, and each user is responsible for determining its own integrity in regard to this limit for a given operation phase following the information provided by the SBAS SIS.

The system TTA is the time an alarm condition occurs to the time the alarm is displayed in the cockpit. Time to detect the alarm condition is included as a component of integrity.

The continuity of service of a system is the capability of the system to perform its function without unscheduled interruptions during the intended operation (for example the landing phase of an aircraft). It is evaluated as the probability that from the moment the criteria of precision and integrity are completed at the beginning of an operation, they remain so for the duration.

Finally, the availability feature is the percentage of time when, over a certain geographical area, the criteria of accuracy, integrity and continuity are met.

The detailed KASS performance specifications are outlined in Table 2.

It must be noted that performance requirements include:

• The system performance in fault free conditions

• The system performance in front

of threats/feared events (FE) (GPS FE, KRS FE, Hardware FE, etc.)

This article only presents the KASS peformances in fault free conditions, in front of the first real data collection.

Performance Test Bench Description 

The performance assessment is based on an engineering test bench that hosts the KASS KPS algorithms (both PS and CS). It is able to simulate end-to-end behavior of the KASS system. 

The KASS test bench solution is an evolution of the legacy EGNOS test bench (SPEED), using an optimized hardware architecture and virtual machines. This architecture is based on a single server that ensures:

• The use of KASS Processing set algorithms (KPS-PS prototype) based on Thales Alenia Space development, allowing the ability to monitor any internal data.

• The use of KASS Check set algorithms (KPS-CS prototype) based on IFEN company development, allowing the ability to monitor any internal data.

• The real time monitoring and control (RTMC) behavior of the KPS components that is managed by a hosting structure.

• The KASS network and uplink station emulation.

• The emulated reception of data from KRS and GEO satellites.

• The ability to process data in a fast mode (fast synchronization loop).

Figure 8 shows the SW component architecture overview. The KASS CS and PS algorithms are hosted respectively in TNKICAF and TNKPS SW components. The SIDF, TSWAN, SCCM SW and Bastion SW components manage the external interfaces and ensure the data flow exchange/control to simulate the system close loop. 

Results and Discussion

KASS is characterized by relatively small service areas and a concentrated KRS network over South Korea land masses, which constitute some challenging conditions in terms of satellite and ionosphere observability as shown in Figure 9.

The performance assessment was performed from March 20 to 24, 2021 (5 full days). This assessment is not fully representative of the final KASS performance because the real data collection for March 2021 has been limited to GPS raw measurements at high elevations (≥ 15°) defined by the KRS masking angle, whereas the KASS system is designed to allow GPS satellite tracking from 5° of elevation. KRS masking optimization activities will be handled in the frame of the system qualification.

The monitoring performance is mainly driven by the geographic surroundings of the antenna sites that are different for each station. The mountainous terrain constitutes challenging conditions for KASS, in terms of multipath and “clear horizon” obstacles.

Performance results are as follows:

• For March 22, 2021 when daily figures are considered. This is the middle day of the performance assessment period, and daily performances remain equivalent over the overall observation period.

• For the average of the five days when performance results are assessed from the user point of view.

Satellite Monitoring

Figure 10 shows the monitoring of the GPS satellites by KASS on March 22, 2021. 

The global monitoring performance is expected to represent about 85% of the maximal observability period (impact of data collection limited to elevation ≥15°).

When displaying the number of GPS satellites monitored at the same time, the analysis shows that five to 11 satellites can be monitored simultaneously (Figure 11).

The KASS system ensures an average of more than seven monitored GPS satellites (around 7.8 satellites).

IGP Monitoring

The IGP Mask is defined by 86 IGPs, more or less close to Korea land masses. The average number of 43 monitored IGPs is achieved on March 22, 2021. The monitoring of IGPs depends directly on the KRS locations and indirectly on IPP locations (Figure 13). The map presents the monitoring ratio for each IGP. Red squares correspond to 100% of monitoring.

The IGP monitoring is satisfactory with respect to the KRS network geometry (Figure 12), in particular at North where the IPP density is weaker (Figure 9). The GIVDs monitoring for the two IGPs at latitude 45° and longitudes 125° and 130° is ensured thanks to the internal model of the ionosphere of KPS-PS. The dynamic of the ionosphere captured at neighboring observation points(IPP) monitored by the system allows this monitoring with a sufficient confidence index. 

Integrity from Satellite Corrections

To display the integrity of the satellite corrections for each GPS satellite, the notion of the UDRE Safety Index is used to assess the integrity margin. The UDRE Safety Index is defined as the ratio

The satellite residual error for the worst user location (SREW) is computed as the pseudorange error projection due to the remaining satellite ephemeris and clock errors after KASS corrections have been applied, for the worst user location of the relevant service area. The relevant service area corresponds to the intersection of the service area and of the monitored satellite footprint. The “true” SREW has been computed with the help of the precise GPS ephemerides provided by the International GNSS Service in sp3 format.

The user differential range error (UDRE) is defined in section A.4 of MOPS [2] and provided by the KPS Subsystem. For a given satellite, the user integrity is ensured as long as the UDRE SFI remains lower than 5.33.

The satellite SFI remains under the value of 1 for all GPS satellites on this day (Figure 14), which demonstrates the large integrity margin of the UDRE corrections broadcast by KASS.

Integrity of Ionosphere Corrections

To display the integrity of the ionosphere corrections for each IGP, the notion of the GIVE Safety Index is used. The GIVE Safety Index is defined as the ratio 

The GIVD Error is defined as the vertical pseudorange error at the considered IGP location, due to the remaining ionospheric delay after applying the GIVD corrections. The “true” GIVD error is computed thanks to precise real ionosphere conditions provided by the International GNSS Service in IONEX format.

The ionosphere SFI remains under the value of 2.0 for all IGPs on this day (Figure 15), which demonstrate the large integrity margin of the ionosphere corrections that KASS broadcasts. 

Given the integrity assessment of satellites corrections (C) and of ionosphere corrections (D), the contribution of fault free conditions to the integrity risk is negligible with real data and is similar with the results obtained from synthetic scenarios used prior to the system deployment. Therefore, the integrity risk for APVI and the other KASS services is mainly induced from the impact probabilities of feared events.

Accuracy at the User Level

The location of users is defined on a grid of longitudes from 124° to 134° East, and of latitudes from 30° to 39° North, with steps of 1°. The presentation is limited to the APVI Service level. The performances requirements for the other services (en route, terminal, NPA) are also fulfilled.

The horizontal and vertical navigation service errors (HNSE and VNSE) are presented in meters for each user position, for the APVI Service (at 95 percentile).

The accuracy performance demonstrates very good behavior of the KPS algorithms, correcting the GPS orbits, clocks and ionosphere with efficiency and high accuracy in vertical as well as horizontal axis (Figures 16 and 17).

APVI Protection Levels at User Level

Figure 18 shows the APVI protection levels (HPL and VPL) achieved for a user located at [128° East in longitude; 36° North in latitude] (South Korea landmass center). The vertical alert limit (VAL) is 50 m and the horizontal alert limit (HAL) is 40 m for the APVI Service.

On March 22, 2021, protection levels show very good margin versus the APVI alert limits (HAL and VAL). 

As expected, a high correlation exists between the rising/setting of GPS satellites and the HPL/VPL usable by safety-of-life users.

APVI Availability 

The APVI service availability (Figure 19)is fully ensured over the complete service area. Some availability degradation is limited to the extreme South, outside any KASS service area, and without impact on performance. 

APVI Continuity Risk

The service continuity is fully ensured over the APVI service area (Korea Land Masses), when using real data from March 2021, in fault free conditions (Figure 20). Continuity degradations are only raised at south edges, with no impact on APVI performance.

The main contribution to the continuity risk in front of APVI and other KASS services correspond to the impact probabilities of feared events.

Conclusions

With a real data collection from March 20 to 24 2021, the KASS system performances in fault-free conditions and for APVI service level show very good levels of accuracy, continuity, integrity and availability. 

These performances are achievedwith a data collection of GPS raw measurements being limited to satellite elevations higher than 15°.

The specific geometry of the KRS concentrated network, and the clear horizon around KRS sites (limited by the KRS masking angle), are well handled by the KPS navigation algorithms that ensure service performance during the five day period. 

The post KASS: The Future of SBAS in Korea appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

GMV and Lockheed Martin have plans to develop a global satellite based augmentation system (SBAS), changing the model for how navigation and precise point positioning (PPP) services are received.

Rather than each region developing and maintaining separate services, the goal is to have just one independent company providing the fee-based service worldwide. Discussions about the possibility began in 2015, with an initial focus on the Asian Pacific area.

The companies have taken the first step toward that goal with their participation in the new Southern Positioning Augmentation Network system, or SouthPAN, which will bring SBAS for navigation and PPP services to Australia and New Zealand. They join the list of countries and regions that already have SBAS systems, including the U.S. (WAAS), Europe (EGNOS), India (GAGAN) and Japan (MSAS).

The project is a joint initiative of the Australian and New Zealand governments, with SouthPAN becoming the first such system available in the Southern Hemisphere. GMV recently signed an agreement with Lockheed Martin to develop the processing and control centers as well as to provide ongoing maintenance for the system.

“We made some noise about the global SBAS concept at different conferences,” GMV Director of Navigation Augmentation Systems and Services José Caro said. “It was in 2017 that the Australian and New Zealand governments organized a test bed project to assess the benefit of having an SBAS system complemented with PPP in that region.”

The Australian government prepared a cost benefit assessment to estimate the benefit of having such a system in the region—and it turned out to be $7.6 billion Australian dollars over 30 years, Caro said. The Australian government decided to move forward with the project, launching a call for tenders in 2020 just before the pandemic lockdowns. A consortium led by Lockheed Martin was granted the contract in September 2022.

“This is the first stone on the way to this global SBAS,” Caro said, “so the project is of great importance for Lockheed Martin and GMV. It goes beyond having the infrastructure working successfully. You can imagine the commitment that goes with this project is total. We are really seeking the global SBAS concept.”

GMV’s role
In the late 1990s, GMV was charged with developing the corrections processing facility as well as other components for EGNOS, which is now in its second generation. GMV then went on to develop its own suite of SBAS products in 2005, investing in tools and prototypes the company uses in demonstrations in different regions of the world, such as South America, Central America and South Africa, to show what an SBAS system could achieve.

Much like with EGNOS, GMV will develop the Corrections Processing Facility (CPF) and the Ground Control Centre (GCC) for SouthPAN, Caro said.

The corrections subsystem for SouthPAN will collect information from the reference stations to determine if any satellite information is wrong and should be excluded, he said. The facility also provides additional corrections to estimate orbit and clock bias and the delay the GNSS signal experiences when going through the ionosphere.

More important than correcting or estimating these delays or errors that contribute to accuracy, Caro said, is the level of confidence users can put in the corrections.

“When users apply the corrections information generated by the SBAS system on top of the information received from the GNSS satellite,” Caro said, “they are able to build a cylinder around the estimated position so that the likelihood of the user location being outside the cylinder is very small, in the range of 10 to minus 7.”

This cylinder is what allows the information generated from SBAS to be used in civil aviation, he said. Such safety-of-life services should be available via SouthPAN in 2028 approximately. The system also generates PPP information, a new feature for SBAS, that with some convergence time can provide very good accuracy. This service targets other user communities beyond aviation.

GMV is charged with developing the ground control center as well, which will monitor the SouthPAN system’s entire infrastructure. It will operate 24 hours a day, 7 days a week, performing all the functions needed to control SouthPAN. The facility will conduct maintenance activities, assist in planning those activities and ensure services are not interrupted. It will also provide information to users about the system’s operation and availability of its services.

The timespan of the entire project is about 19 years, Caro said, and it’s critical to ensure the stability of the services.

“We are in charge of assessing the performance of the system and computing the associated KPIs,” Caro said. “We also have to maintain the infrastructure that we are deploying in the system, essentially the two subsystems, the corrections processing facility and the ground control center. We’ll provide general engineering support as well.”

A significant step forward
With SouthPAN, Australia and New Zealand will contribute to improved global coverage and interoperability for navigation and PPP services. SouthPAN will boost the economy for both countries, providing a digital infrastructure for the future.

Beyond the critical safety of life applications, SouthPAN will benefit users in a variety of applications, including agriculture, road, air, maritime, construction and rail transportation.

“We are essentially providing three different kinds of services with SouthPAN. So there is the L1 SBAS service that is the legacy SBAS currently being provided in the USA or Europe, also in India or in Japan. This service would be provided in two different ways, safety of life and open service.” Caro said. “We’re also providing PPP corrections that will be broadcast in the same band used in GNSS. What that means is users will be able to use all the services without the need for accessing any land communication system, so all these corrections can be accessed in remote areas without a mobile telephone or ground infrastructure. So, in general, all the users of satellite navigation will take benefit of SouthPAN.”

What’s next
GMV is currently working on system development and has already provided preliminary services, Caro said. Several steps must be taken over the next several years for all the services to be declared operational, and the company is currently ramping up its team to make that happen.

With more than 600 GNSS engineers onboard and with the company having previous experience with SBAS, the team at GMV is up to the task. They will work in close communication with partner Lockheed Martin, with the main goal still to eventually develop a global SBAS service.

“Lockheed Martin and its subsidiary Zeta have experience with WAAS and development of GPS satellites and GMV has the experience of having participated in EGNOS,” Caro said. “We are collecting all this experience to propose a system concept that takes the best of these systems.”

The post GMV, Lockheed Martin Partner on SouthPAN, Working Toward a Global SBAS Concept appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

The SAGAIE network was deployed in West-Africa in 2013 to assess the feasibility of an equatorial for ASECNA SBAS by studying ionospheric scintillation. This study of the network’s measurements, taken between 2013 and 2016, analyzes the characteristics of the scintillation recorded by five receivers that cover Sub-Saharan West-Africa.

By AURÉLIEN GALMICHE, SÉBASTIEN TRILLES, SÉBASTIEN ROUGERIE, THALES ALENIA SPACE

VINCENT FABBRO, ONERA/DEMR

LAURENT FÉRAL, LABORATOIRE D’AÉROLOGIE, UMR 5560

JULIEN LAPIE, LOUIS BAKIENON, ASECNA

The Satellite Based Augmentation System ASECNA for Africa & Indian Ocean (A-SBAS), led by the Agency for Air Navigation Safety in Africa and Madagascar (ASECNA), an international organization of 18 Member States in charge of air navigation services provision in 16 million square kilometers, is under development in view of services declaration in the next years. A-SBAS is recognized by the International Civil Aviation Organisation (ICAO) as the part of the Global Navigation Satellite Systems (GNSS) infrastructure. It is designed to deliver a safe positioning accuracy by augmenting the GPS satellite navigation constellation with differential corrections and integrity data, in order to enable advanced navigation operations down to precision approaches in all runway ends in Africa. The core of the mission consists of decomposing all possible range error sources and distributing corrections and/or alerts to its users by means of geostationary satellites.

The major range error source affecting the A-SBAS system performances is the ionosphere dynamic because the magnetic equator, as a main driver, crosses the Western African sector. This particular latitudinal situation is one of the factors leading to the apparition of the ionospheric scintillation at night and close to the equinoxes in this area. 

The SAGAIE network of GNSS stations was deployed in Western Africa in 2013 by CNES and ASENCA to assess the feasibility of an equatorial SBAS for ASECNA area by studying the phenomenon, which has a strong negative influence on GNSS signals, among others. 

This study of the SAGAIE network’s measurements aims to analyze the characteristics of the scintillation recorded by the five receivers covering Sub-Saharan West-Africa between 2013 and 2016. This period covers almost half a solar cycle, with the scintillation highly varying. It is shown that scintillation has a temporal global pattern well marked in the five SAGAIE stations: an increase after local sunset, a peak between 8 and 11 p.m. and a decrease overnight. The amplitude of the phenomenon increases close to the equinoxes, where scintillation is at its maximum. However, variability in temporal occurrence or magnitude with longitude and latitude has also been observed in the different stations. 

The Importance of Studying Scintillation

The ionospheric scintillation phenomenon is characterized by a strong fluctuation of the phase and amplitude of the radio wave received on Earth. It is due to small-scale irregularities of the electronic concentration of the plasma crossing the beam of the incident signal. 

Close to the magnetic equator, for instance, scintillation has been linked with the equatorial anomaly [1]. Waves with frequencies up to 12 GHz can be affected by ionospheric scintillation [2]. Therefore, scintillation is of great concern for applications with high service availability such as GNSS. In addition to increasing the noise and reducing the accuracy of the estimated position of a user, it can lead to cycle slips, and even to the loss of the satellite-to-Earth link. 

Hence, the study of scintillation is needed to understand the features of the phenomenon, linked with the atmosphere and the sun’s physics. Mention is made of different behaviors depending on latitude and longitude of the observation, but literature lacks global studies made in the African continent. However, Africa is the continent where civil aviation will experience the most important growth in the next 10 years. The emergence of satellite augmentation GNSS technologies, providing Safety-Of-Life aeronautic operations as they exist in North-America (WAAS), Europe (EGNOS), India (GAGAN) and Japan (MSAS) to assist users must take this destructive phenomenon into account.

Among the different patterns followed by scintillation, different studies have shown a clear dependence of frequency and intensity of scintillation with the sun’s activity (that can be quantified by Sun Spot’s Number) [3]. The cycle of the Sun’s activity, n°24, which was rather weak compared to the cycles 22 and 23, had its activity peak between 2013 and 2014, and ended in 2020. The next solar cycle’s maximum shall not happen before 2025, according to studies [4]. The SAGAIE stations that observed the Sub-Saharan West-African ionosphere from 2013 until present are hence a powerful tool to study the ionospheric scintillation in the scope of a characterization of the phenomenon. 

Presentation of the SAGAIE Database

A network of GNSS stations deployed in the African equatorial region and measuring the ionospheric scintillation is an interesting way to measure the spatial and temporal dynamic of the ionospheric plasma. Indeed, the L-band is subject to the ionosphere’s variations while always crossing the medium. Therefore, the SAGAIE network seems perfectly adapted to analyze the turbulent ionosphere in this special area of the world. 

SAGAIE is an acronym corresponding to Stations ASECNA GNSS pour l’Analyse de l’Ionosphère Equatoriale, or in English, GNSS ASECNA Stations for the Analysis of the Equatorial Ionosphere, where ASECNA stands for Agence pour la Sécurité de la Navigation Aérienne en Afrique et à Madagascar, or Agency for Air Navigation Safety in Africa and Madagascar. SAGAIE network is a cooperation between the French Space Agency CNES and ASECNA, financed by CNES and hosted by ASECNA. It was operated and maintained by Thales Alenia Space until 2021. It began its service in 2013. It will be further extended in the coming years and is operated by CNES since 2021. The accumulation of data since 2013 by SAGAIE enables a wide characterization of the ionospheric conditions observed since the peak of solar cycle 24. The aim of this network is also to challenge the robustness of GNSS algorithms, especially the ones embarked on aircraft equipped with SBAS (Satellite-Based Augmentation System) technology [5].

The SAGAIE network is composed of five stations in Western and Central Africa. The stations are located in Dakar (Senegal), Ouagadougou (Burkina Faso), N’Djamena (Chad), Douala (Cameroon), and Lome (Togo). Their positions are shown in Figure 1.

ASECNA premises host the stations. The use of such quality facilities prevents power outages and enables an internet link for real-time ionospheric analysis. The stations’ installation was preceded by a study phase that aimed to minimize the data corruption by undesired propagation effects such as ground multipath. The five SAGAIE stations were compared to the IGS (International GNSS Service) sites through a GPS bi-frequency signal quality check. The five stations were ranked among the best IGS stations in the world in terms of multipath [5].

Two kinds of receivers were used to collect SAGAIE GNSS data: 

• A multi-frequency (whole L band), multi-constellation (GPS, GLONASS, Galileo, SBAS) scintillation monitor PolaRxS Pro ISM manufactured by Septentrio 

• A multi-frequency (L1, L5, L2), multi-constellation (GPS, GLONASS, Galileo, SBAS) GNSS receiver FlexPak 6 OEM628 manufactured by NovAtel. 

The Dakar and Lome stations are equipped with both receivers and a “choke ring” Septentrio antenna, whereas the other three stations are equipped only with the FlexPak 6 OEM628 NovAtel receiver. Ouagadougou, Douala and N’Djamena receive signals through a NovAtel A703 antenna.

The first observations made by the network show a strong link between the medium-to-strong scintillation events, and the satellites’ elevation observed from the stations, the equinoxes period and the hour of the day [5]. These observations are in line with previous results on the equatorial scintillation from the literature [6], [7].

In this paper, ionospheric scintillation is studied through the amplitude scintillation index S₄ [8]:

where < > refers to the average operator, performed on one minute of data. The signal intensity here is supposed to be given by the signal to noise ratio C/N₀ [8]. C/N₀ is here a vector, sampled at 1 Hz. It is obtained from the SAGAIE GNSS receivers through a Thales Alenia Space designed processor. Because the average operator is performed on one minute, for each S₄ calculated using expression (1), 60 values of C/N₀ are used in a nominal case. If the link is temporarily unavailable, and 20 values of C/N₀ or less are processed, the S₄ index status is set to unavailable so it does not fool the measurements. 

An elevation mask of 30° is usually applied in such studies to avoid ground multipath [9]. However, the favorable environment of the SAGAIE stations permitted a 15° elevation mask without being troubled by multipath or other undesired propagation effects, as shown by measurement quality analysis performed by Thales Alenia Space algorithmic core [10]. More line-of-sight can therefore be observed to characterize the medium. The period during which data were observed in this paper ranges from August 2013 to August 2016. As presented in Figure 2, this period was chosen to cover the maximum and decrease of solar cycle 24. The Sun Spot Number is used as a proxy for solar activity, [4] to be able to observe a possible variability of scintillation over the intensity of the solar activity [3].

In total, more than 60 million S₄ values have been calculated over the time frame selected. The repartition of those values between the five stations is presented in Table 1, as well as the number of S₄ above a threshold of 0.5. There is a good balance of the number of S₄ indices observed between the five stations; this will permit interesting comparative studies, which are conducted in the following sections.

The number of S₄>0.5 is quite significant: between 14,000 and 37,000, depending on the stations, (S₄>0.5 is characteristic of medium-to-strong scintillation events [9], [2]). This denotes a variable state of the ionosphere during the time of observation between steadiness and turbulence. The next sections will also focus on describing those variations.

Temporal Study of the S₄ Index

This refers to the temporal variability of the S₄ index on the West-African Sub-Saharan region. Figures 3 through 7 show the percentage of occurrence for S₄ to be observed above the threshold 0.5, against the hour of the day in local time (x-axis) and the day of the year (y-axis) for the five SAGAIE stations. In Figure 8, the same quantity is represented, but averaged on the five SAGAIE stations. The value of S₄>0.5 usually denotes a scintillation that can severely impact the GNSS systems [11],[12]. This study refers to medium-to-strong scintillation events. 

A global tendency is observed and common to all stations: ionospheric scintillation appears around 8 p.m. solar time (i.e. 20h), and disappears around 1 a.m. solar time (i.e. 1h). The peaks of scintillation occurrence are observed during or close to the equinoxes, in agreement with what was previously reported in the literature [6],[7]. In Figure 8 (an average calculation of Figures 3 through 7), we remark that scintillation extracted from the SAGAIE measurements seems more frequent and more intense close to the spring equinox than close to the fall equinox. The dissymmetry between the two equinoxes is particularly well marked in Figure 4 from Douala and Figure 5 from Lome and can be distinguished in Figure 6 in N’Djamena and Figure 7 from Ouagadougou.

However, scintillation characteristics linked with the latitude and the longitude seem to differ for the five stations. In N’Djamena in Figure 6 and Ouagadougou in Figure 7, ionospheric scintillation appears before 8 p.m. and ends slightly after 1 a.m., whereas in Lome in Figure 5 and in Douala in Figure 4, scintillation ends before 1 a.m. In Dakar in Figure 3, scintillation starts earlier, around 7 p.m., and ends after 1 a.m. Hence, the phenomenon lasts globally longer than in the other stations. 

The phenomenon seems to last less in the two stations closer to the magnetic equator (Ouagadougou and N’Djamena), while the duration is the longest in Dakar. Ionospheric scintillation observed through SAGAIE therefore shows more frequent and intense amplitude scintillation from the stations close to the equatorial crests, between 5 and 15 degrees of latitude north and south of the magnetic equator.

These conclusions can be partially retrieved through the analysis of Table 1. The number of S₄ measured is similar for all stations (it is not exactly the same due to both the visibility of the satellites in the sky and the punctual loss of internet network resulting in minor data loss), the N’Djamena and Ouagadougou stations have less S₄>0.5 than the others. It confirms the conclusions previously stated, showing that the scintillation in those places close to the magnetic equator is less intense than in Douala, where there are twice as many events for which S₄>0.5. 

Figure 4, referring to Douala, shows the percentage of occurrence of S₄>0.5 is close to 20% during equinoxes, with a great duration in time, something neither observed in Ouagadougou nor in N’Djamena. The number of scintillation events for which S₄>0.5 is also greater in Dakar and Lome than in N’Djamena and Ouagadougou, showing a higher presence of medium-to-strong scintillation events in those places. Those are some hints of the geographical characteristics of the equatorial scintillation. 

Spatial Study of the S₄ Index

The mean IPP (Ionospheric Pierce Point, calculated at a height of 400 km) of the events for which S₄ has been measured over the thresholds 0.5 and 0.7 are shown in Figures 9 and 10, respectively. The color of the point shows the measured value of S₄ with respect to the adjacent color bar. The position of the geomagnetic equator is represented in a dotted red line in the two figures. The locations of the SAGAIE stations are highlighted by black crosses. More than 125,000 events are displayed in Figure 9, and 25,000 are displayed in Figure 10, showing the significant number of medium-to-strong scintillation events. Those two figures show the geographical positions of the places where medium-to-strong (threshold on S₄ of 0.5) and strong (threshold on S₄ of 0.7) ionospheric scintillation has been observed. 

In Figure 9, it is common to notice events for which S₄>0.5 is close to the geomagnetic equator, even though most of the events are north and south of this line. In Figure 10, the vast majority of the events are located north and south of the geomagnetic equator. The location of these strong scintillation events shows the presence of well-marked equatorial crests between latitudes 15° and 20° (north crest) and -5° and 5° (south crest). As discussed earlier, the Douala station contributes well to the observation of strong scintillation; this station is the furthest away from the geomagnetic equator, and the majority of the events for which S₄>0.7 have been observed from there.

These equatorial crests observations are in agreement with the common description of scintillation apparition, linked with the phenomenon of equatorial fountains [13], [14].

Events of moderate-to-strong scintillation appear east and west of SAGAIE stations Dakar, Ouagadougou and N’Djamena in Figure 9. Those places do not belong to the equatorial crests and should therefore not be subject to strong scintillation. However, it is known the S₄ index is inversely proportional to the elevation angle [15], [16]. This effect then contributes to increasing the S₄ indices at low elevations. However, a partial decorrelation of the scintillation (particularly the S₄ index) and the elevation angle under which the signal is received can be seen. 

Indeed, there are only a few strong scintillation events located east and west of the SAGAIE stations in Figure 10, whereas many events are seen north and south of the stations (i.e. located on the equatorial crests), though they are seen with the same elevation as the east and west events. 

Focusing only on the Dakar station, Figure 11 shows the number of S₄ measurements against their elevation seen from Dakar (x-axis) and the latitude of their IPP (y-axis), while Figure 12 represents the same quantity, but only for events having S₄>0.5, respectively. 

Figure 12 clearly shows, between elevations 15° and 30°, more events at high latitudes (15° to 23° north) than at low latitude (below 10°N), whereas Figure 11 shows the number of IPP in sight is approximately the same for low and high latitude under those elevations. The north equatorial crest is roughly located between 15° and 25° north in Dakar, whereas no crest is present between 5° and 15° north. This clearly shows a dependence of scintillation to the equatorial physics, and not only in the elevation.

Scintillation and Solar Cycle

Another major feature of the scintillation impacting both the geographical distribution and temporal occurrence of the event was observed through SAGAIE. This is the dependence of the scintillation with the solar cycle.

From Figure 2, it is possible to notice that the highest peak of solar cycle 24 occurs at the beginning of the year 2014, while 2016 is a year counting a Sun Spot Number three times lower in average and is therefore a weaker year.

The mean IPP of the events for which S₄ has been measured over the thresholds 0.7 are shown in Figure 13 for March 2014 and in Figure 14 for March 2016. The color of the point shows the measured value of S₄ with respect to the adjacent color bar. The position of the geomagnetic equator is represented in the red dashed line. The locations of the SAGAIE stations are highlighted by black crosses. As shown earlier, March is a month of strong scintillation for the SAGAIE network. 

The conclusions of the previous section on the elevation and geographical position of the IPP also can be inferred from Figures 13 and 14. Clearly, fewer strong scintillation events (i.e. S₄>0.7) were observed in March 2016 (337) than in March 2014 (2,804). March 2014 had over 13 times more events than March 2016, in fact. Ionospheric scintillation, and in particular events apt to severely influence GNSS signals, clearly depend on solar activity.

It is then useful to recall that SAGAIE data measurements began in 2013; the study of scintillation presented in this paper hence globally refers to a weak solar cycle (24) when it is compared to solar cycles 22 or 23 [4]. Therefore, the conclusions of the frequency and intensity of ionospheric scintillation events might be under-estimated compared to the two previous solar cycles. The levels of ionospheric scintillation observed could have been drastically increased if the data had been measured during solar cycle 22, for instance, when the peak counted Sun Spot Numbers two times higher in average than the peak of solar cycle 24.

Conclusion and Look to the Future

The SAGAIE database is a useful tool to study the turbulent equatorial ionosphere in Western and Central Africa, a region of the world where the scintillation phenomenon has not been as well characterized as in South America or South-East Asia ([7], [17] among others), although it appears to have specificities.

After presenting the SAGAIE network and the context in which the SAGAIE database was collected, the impact of the ionospheric scintillation on the GNSS signal’s amplitude characterized through the S₄ index has been studied in detail. The temporal variability of this index indicates a scintillation apparition cycle depending on the hour of day and the day of year. 

Those findings are similar to what was observed in the literature in South Asia or South America: scintillation is mainly present close to equinoxes (especially the spring one), and mainly absent during solstices for the peak of solar cycle 24. Some events are, however, reckoned close to the winter solstice. In terms of hourly variations, scintillation appears at the end of the afternoon (between 6:30 and 8 p.m. UTC), weakens, and then disappears in the middle of the night (between 11:30 p.m. and 2 a.m. UTC).

The geographical variability of the medium and strong scintillation draws crests between 5° and 15° north and south of the geomagnetic equator, typical of the equatorial phenomena, especially in terms of plasma dynamic. It has been shown that low elevation, although it is naturally a factor of S₄, is not sufficient to observe strong scintillations; they can only be physically explained through the process giving birth to the equatorial crests.

Finally, the impact of solar activity on scintillation is observed. SAGAIE covers the peak and the decreasing part of a weak solar cycle. The scintillation strength as well as its occurrence probability may increase in the case of a strong solar cycle observation.

The objective of this work was to make a stenography of ionosphere turbulence in the sub-Saharan zone and to assess whether the scintillation phenomenon could be a blocking point to developing the A-SBAS. Experimental results had indicated this is not the case. Following this, between 2019 and 2021, ASECNA managed the definition and preliminary design phase of its program awarded to Thales Alenia Space. This phase addressed technical and technological feasibility issues related to System Architecture, System Performances, RAMS, IVV, Safety, deployment, operation and maintenance topics. On this occasion, the engineering team of Thales Alenia Space developed an Equatorial Navigation Kernel (NACA solution) that provides satisfying performances. These performances were validated on the field in 2020 using a real-time demonstrator broadcasting in Western African an SBAS augmentation signal compliant with the ICAO SARPs and RTCA MOPS standard.

This demonstrator, whose architecture is illustrated in Figure 15, collects GNSS data through a set of stations in Western and Central Africa encompassing the SAGAIE and MONITOR network, augmented by some other stations coming from the REGINA network, and feeds the navigation kernel that processes in real-time conditions. The navigation message is thus sent to an uplink station that broadcasts the signal to the Nigcomsat 1-R payload, the Nigerian GEO telecommunication satellite. System performances are continuously monitored using TheOwl GNSS monitoring service developed by PildoLabs. 

In January 2021, ASECNA conducted several demonstration flights at Lome airport (Togo) to showcase the performance of the real-time A-SBAS signal to perform SBAS precision approaches and landing procedures. These demonstration flights have proven the efficiency of the A-SBAS augmentation message and its great added-value for air navigation, in particular for approaches to main and secondary African airports that are not served by a conventioanl Instrument Landing System (ILS). 

To illustrate these performances, Tables 2 and 3 provide the worst user pseudorange projected error (SREW) during the four days of experimentations. The SREW contains the unmodeled satellite and clock error of the augmented navigation message.

In comparison to EGNOS, over the same period, the performance provided by the ASECNA aviation demonstration is very satisfying.

Beside these activities, Thales Alenia Space has developed a new Navigation Kernel compliant to dual-frequency multi-constellation (DFMC) standard messages. Based on these new algorithms, whose performance in fault-free and feared event present a very good behavior in terms of not modeled residual error distribution [18], and a proven operational demonstrator capability already available for single frequency SBAS service, Thales Alenia Space has been awarded a contract by CNES to develop a DFMC satellite-based augmentation system (SBAS) prototype with the objective to demonstrate the service in Africa with a L5/E5 augmented signal. 

Acknowledgments

The authors would like to thank Damien Serant and Xavier Berenguer from Thales Alenia Space for the help they brought to the data computing. They also thank ASECNA for hosting the SAGAIE stations that produced the raw measurement used in this paper and CNES for making this particularly valuable GNSS data collection possible.

References

(1) Rishbeth, H., 1997, The ionospheric E-layer and F-layer dynamos–a tutorial review, Journal of Atmos and Solar-terrestrial physics, vol.59, no 15, pp 1873-1880, DOI: 10.1016/S1364-6826(97)00005-9.

(2) ITU recommendation, Ionospheric propagation data and prediction methods required for the design of satellite networks and systems, P series radiowave propagation, August 2019.

(3) Fejer, B. G., de Paula, E . R., Gonzalez, S. A., Woodman, R . F., 1991, Average vertical and zonal F region plasma drifts over Jicamarca , Journal of geophysical research, vol.96, no A8, pp 13901-13906, DOI: 10.1029/91JA01171.

(4) Bhomik, P., Nandy, D., 2018, Prediction of the strength and timing of sunspot cycle 25 reveal decadal-scale space environmental conditions, Nat Commun 9, 5209 (2018), DOI: 10.1038/s41467-018-07690-0.

(5) Secretan, H., Rougerie, S., Ries, L., Monnerat, M., Giraud, J., Kameni, R., 2014, SAGAIE a GNSS network for investigating ionospheric behavior in sub-saharan regions, InsideGNSS pp 46-58, issue September/October 2014.

(6) Aarons, J., 1993, The longitudinal morphology of equatorial F-layer irregularities relevant to their occurrence, Space science reviews, vol.63, issue 3-4, pp 209-243, DOI: 10.1007/BF00750769.

(7) Moraes, A. O., Costa, E., Abdu, M. A., Rodrigues, F. S., de Paula, E. R., Oliveira, K. and Perrellan, W. J., 2017, The variability of low-latitude ionospheric amplitude and phase scintillation detected by a triple-frequency GPS receiver, Radio Sci., 52, 439–460, DOI: 10.1002/2016RS006165.

(8) Van Dierendonck, A. J., Klobuchar, J., Quyen, H., 1993, Ionospheric Scintillation Monitoring Using Commercial Single Frequency C/A Code Receivers, Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993), Salt Lake City, UT, pp. 1333-1342. 

(9) Mushini, S. C., 2012, Characteristics of scintillating GPS signals at high latitudes during solar minima, Ph.D. thesis from University of New Brunswick.

(10) Foucault, E, Blelly, P-L, Marchaudon, A., Serant, D., Trilles, S., 2018, Equatorial Ionosphere Characterization for Sub-Saharan Africa SBAS, 31st ION GNSS+ 2018, 24 September 2018 – 28 September 2018, Miami, United States pp.2222-2240. hal-01915098.

(11) Rama Rao P. V. S., Gopi Krishna S., Niranjan, K., Prasad, D. S. V. V. D., 2006. Study of spatial and temporal characteristics of L-band scintillations over the Indian low-latitude region and their possible effects on GPS navigation, Annales Geophysicae, European Geosciences Union, 2006, 24 (6), pp.1567-1580, DOI: 10.5194/angeo-24-1567-2006.

(12) Guo, K., Aquino, M., Vadakke Veettil, S., 2019, Ionospheric scintillation intensity fading characteristics and GPS receiver tracking performance at low latitudes, GPS Solutions (2019) 23:43, DOI: 10.1007/s10291-019-0835-1.

(13) Klobuchar, J. A., Anderson, D. N., Doherty, P. H., 1991, Model studies of the latitudinal extent of the equatorial anomaly during equinoctial conditions, Radio Science, vol.26, no4, pp 1025-1047, DOI: 10.1029/91RS00799.

(14) Blelly, P.-L., Alcaydé, D., 2007, Ionosphere, in Kamide, Y., Chian, A., 2007, Handbook of the Solar-Terrestrial Environment, Springer-Verlag Berlin Heidelberg, pp. 189-220.

(15) Rino, C. L., 1979, A power law phase screen model of ionospheric scintillation, 1. Weak scatter, Radio Science, vol. 14, n°6, pp. 1135-1143, DOI: 10.1029/RS014i006p01135.

(16) Galiègue, H., L Féral, and V. Fabbro, 2016, Validity of 2-D electromagnetic approaches to estimate log-amplitude and phase variances due to 3-D ionospheric irregularities, J. Geophys. Res. Space Physics, 121, DOI: 10.1002/2016JA023233.

(17) Ji, S., Chen, W., Weng, D., Wang, Z., 2015, Characteristics of equatorial plasma bubble zonal drift velocity and tilt based on Hong Kong GPS CORS network: from 2001 to 2012, Journal of geophysical research space physics, vol. 120, pp 7021-7029, DOI: 10.1002/2015JA021493-T.

(18) T. Authié, C. Bourga, J. Samson, M. Dall’Orso, 2022, Analysis of the Position and Pseudorange error distributions and autocorrelation functions of a DFMC SBAS, Navitec 2022. 

Authors

Aurélien Galmiche graduated from l’École Nationale de l’Aviation Civile in Toulouse (M.Sc.Eng., 2015), and obtained his Ph.D. from Université de Toulouse III (2019). He was hired in 2018 as a Navigation Engineer at Thales Alenia Space, where his research topics focus on ionosphere and integrity monitoring.

Vincent Fabbro is a doctor in microwave propagation. He is currently a research engineer at ONERA, Toulouse. His areas of interest are focused on the modeling of propagation over the sea, interactions of micro waves in the troposphere and the associated attenuations. He also worked on transionospheric propagation with quantification of scintillation effects, including modeling and prediction, especially on GNSS signals.

Laurent Féral received the Ph.D. degree in propagation and active remote sensing of the atmosphere from the University of Toulouse (UT3), France, in 2002. Since 2005, he is Assistant Professor with UT3. In 2020, he joined the Laboratoire d’Aérologie (Observatoire Midi-Pyrénées, Toulouse, France). His main research interests are the modelling of the micro-wave propagation in the atmosphere for telecommunication, GNSS and remote sensing applications. 

Sébastien Trilles is a doctor in mathematics, expert engineer in navigation at Thales Alenia Space and teaches space mechanics at the Federal University of Toulouse Midi Pyrénée. He heads the Performance and Processing department where high precise algorithms are designed as the navigation, integrity and ionosphere modelling algorithms of SBAS systems, the orbitography solutions and the composite time reference generation for Galileo.

Sebastien Rougerie graduated from the French national civil aviation school ENAC with an engineer diploma and obtained a master of science degree from the University of Toulouse. He received a Ph.D. degree in array processing from ISAE Toulouse. From 2012 to 2014, he worked for AUSY as a research engineer on behalf of ONERA and Thales Alenia Space. From May 2014 to September 2021, he lead the propagation department at the French national space agency CNES. His research concerned propagation channels measurements and modeling (ionosphere, troposphere, multipath, optic), for space transmissions including GNSS. Since September 2021, he works at Thales Alenia Space as an expert engineer on signal and array processing for space link transmission.

Julien Lapie is a civil aviation engineer, graduated from the French Civil Aviation University (ENAC), Toulouse (France), in 2001. He currently serves at ASECNA as Advisor to the Director of Air Navigation Services and as A-SBAS Programme Manager. He is the ICAO Navigation Systems Panel (NSP) member nominated by ASECNA.

Louis Bakienon is a civil aviation engineer, graduated from the African University of Civil Aviation and Meteorology (EAMAC) in Niamey (Niger) in 1989. He also received a MBA degree from Paris Dauphine University in 2008. He currently serves as the Director of Air Navigation Services of this Agency, directing the SBAS programme.

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The European Geostationary Navigation Overlay Service (EGNOS) will soon launch a new maritime solution that will make legacy and costly coastal ground-based augmentation systems redundant. Meanwhile, the next generation EGNOS V3, featuring dual-frequency, multi-constellation (DFMC) services, is set to come online by 2028, once GPS L5 is declared operational.

Maritime transportation re-mains the backbone of worldwide economic activity, representing 80% of global merchant traffic. Europe is one of the world’s leading maritime hubs, with 329 key seaports and control of about 30% of the world’s merchant fleet.

Shipping around the extensive European coastline relies on differential GNSS (DGNSS) signals for navigation and safety. These signals depend on a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by GNSS and known fixed positions. Today, almost all commercial GPS receivers, even hand-held units, allow DGNSS data inputs, and DGNSS is commonly used in maritime settings. However, the cost of maintaining this aging infrastructure is high, and the long-standing debate over what to do with this service is unresolved.

Jean-Marc Piéplu, European Geostationary Navigation Overlay Service (EGNOS) Exploitation Program Manager at the EU Agency for the Space Program (EUSPA), told Inside GNSS, “We are developing a new EGNOS service dedicated to Maritime users, which will complement and serve as an alternative to the local DGNSS network currently deployed along the European coasts.”

EGNOS Knows Maritime Transport 

Piéplu said EUSPA is targeting 2023 for declaration of the initial EGNOS maritime service, which will then evolve in steps. First, users will be able to access EGNOS corrections via existing aids to navigation (AtoN), using existing EGNOS signal in space (SiS) and/or via the EGNOS Data Access Service (EDAS). Next, standardized, certified receivers will directly access the EGNOS maritime service, using the existing L1 SiS, with some service guarantees. The final service will include a maritime safety message in SiS, if needed, and a multi-system, shipborne, radionavigation receiver (MSR), leveraging the DFMC capability of EGNOS V3, that is, augmented GPS L1/L5 and Galileo E1/E5.

Satellite-based augmentation systems (SBAS), and EGNOS in particular, are already used widely in maritime applications. SBAS corrections have been retransmitted through International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) beacons in key use cases in France, Estonia and Germany, and most portable pilot units (PPUs) now on the market are EGNOS-enabled. Ninety-three percent of navigation devices used in SOLAS vessels (those that conform to the International Convention for the Safety of Life at Sea) and 89% of navigation devices used in non-SOLAS vessels are EGNOS capable.

Challenges for maritime GNSS and SBAS include signal obstruction and complex multipath effects, especially when other vessels are nearby, and within close, dynamic and varied harbor environments. Intentional interference and spoofing also pose problems.

EGNOS V3

Looking further ahead, over the coming years,” Piéplu said, “the particular challenge for EGNOS is the transition to EGNOS V3, which will replace EGNOS V2, continuing to deliver the current ‘legacy’ safety-of-life services as well as offering new dual-frequency, dual-constellation services. So, it will augment both GPS L1/L5 and Galileo E1/E5. All this we are doing in the frame of a combined safety certification and security accreditation scheme. The entire EGNOS program is gearing up for this transition to the next generation.” The new version will also feature a completely revamped ground infrastructure.

“The equilibrium of the program schedule and budget relies on a successful operations and services transition, from EGNOS V2 to V3,” Piéplu said. “The efforts of the program shall therefore be well balanced and regularly adjusted between preparing the transition to V3 while still investing on V2, to extend its lifetime and improve its capabilities until EGNOS V3 enters into service.”

“The EGNOS V2 system will also be refreshed with more robust ionospheric correction algorithms in 2023, as we launch the maritime service,” Piéplu said. To fulfill its mission, he added, EUSPA is engaging with industry and operators to renew and improve the current infrastructure, to renovate the GEO space segment and to address issues of technical obsolescence within sub-systems. 

“We are continually working to make the performance of the system even more robust. This is essential in particular when it comes to ionospheric disturbances, as we are quickly approaching the next solar cycle peak.

“In 2024, Inmarsat 4F2 will be replaced by a new payload on board the Eutelsat E5WB satellite (GEO-3) also improving EGNOS robustness. An additional payload is also expected to be launched in 2022, GEO-4, which will be dedicated to EGNOS V3. DFMC services, that is L5 frequency and augmentation to Galileo, will be provided after GPS L5 FOC.” 

Piéplu said the full EGNOS V3 transition should start in 2026–27, first with version 3.1, which will ensure improved performance with the legacy service, and then version 3.2, delivering DFMC service in 2028.

New Governance Arrangements

The recent transition from GSA to EUSPA coincides with the transition to the new EU Space Programme Regulation and the new Financial Framework Partnership Agreement (FFPA) between the Commission, the European Space Agency (ESA) and EUSPA.

“This does not bring major changes in the governance of EGNOS,’ Piéplu explained, “but EUSPA is entrusted with more direct responsibility in the management of operational security and in the direct management of industry for the improvements to the system in operations, currently EGNOS V2. Our relationship with the European Commission and ESA is similar to what it was in the previous phase, but when looking more into details, the new governance arrangements established by the FFPA brings noticeable improvements based on lessons learnt. For instance, we are implementing new ways of working through integrated teams with ESA, and we’ve shortened the decision loops between the exploitation manager (EUSPA) and the industrial team maintaining and improving the operational system.”

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Speaking at a special EGNOS Workshop, Fiametta Diani, head of marketing at the EU Agency for the Space Program (EUSPA), described where Europe’s GNSS augmentation system is going, in the air, on land and at sea.

The European Geostationary Navigation Overlay Service (EGNOS) is Europe’s regional satellite-based augmentation system (SBAS). In 2011, EUSPA (then GSA) officially launched the EGNOS Safety-of-Life Service with aviation as a major priority area of application. Ten years on, EUSPA Marketing Head Fiametta Diani said, “In aviation, we have reached the amazing situation where we have almost 800 EGNOS approaches available, and we have EGNOS embedded in the avionics of almost 40% of our EU fleet.”

Safe and Clean Aviation 

“We now have 46 EGNOS approaches for helicopters,” Diani announced, “especially in emergency and medical flights. But we want to go a step forward. We are now working very hard to bring EGNOS into the small aerodromes, the ones that are not equipped with other tools for air navigation. We have more than 2,000 of these small airports in Europe that have no or limited ground infrastructure.”

EUSPA’s recently published safety assessment guidelines for small airports represent a new, important achievement, Diani said. “We created these guidelines working together with EASA [European Union Aviation Safety Agency] and with the national air service providers, and we are aiming at the first implementation of EGNOS in small aerodromes, already next year [2022].”

The EU Green Deal is a clear expression of the Union’s prioritization of environmental performance at all levels of activity. “We believe EGNOS in aviation is contributing and will contribute even more to the Green Deal,” Diani said, “and we see this in at least four specific areas. First, EGNOS permits the possibility to reduce the distance flown, that means shorter trajectories, and accordingly fuel and CO₂ emissions savings. Second, we can help to reduce the number of missed approaches. Third, we have more options for closer alternates, in case we have to select another aerodrome for landing.” 

Finally, Diani said, EGNOS supports more rationalized conventional radar operations. “We are now also setting up tools that permit us and users and airlines to estimate in a more precise way the fuel and CO₂ savings.”

Drones Surge

Diani singled out drone operations as the most promising area for EGNOS development: “I think unmanned vehicles is a steeply growing market, in terms of numbers but also in terms of use cases. More and more, drones are used today in professional domains. A traditional surveyor or geometre once used to do their work with handheld tools, to inspect bridges, for example, and now they use drones. But UAVs are also coming for urban mobility.”

‘REALITY’ is a EUSPA-funded R&D project working to equip drones with EGNOS capabilities. “We are co-investing in this project to prepare drones for urban mobility. Here, the challenge we think is to adapt the manned aviation RNP [required navigation performance] and integrity concept, especially to define the narrow air corridors that are needed for drones,” Diani said.

Sea and Land

Turning to maritime matters, Diani said, “We already have EGNOS in our non‐SOLAS vessels, that is our leisure boats.” SOLAS (Safety of Life at Sea) is an important convention governing merchant ships. “And we also have EGNOS transmission by the current AIS and IALA navigation beacons. To go farther, we want to work for the maritime service in SOLAS vessel navigation, and also in autonomous vessels. EGNOS is complimenting IALA differential GNSS infrastructure. We are on track on our ambition to have our service for maritime, directly from the signal in space. And we have finalized the receiver guidelines, so now the receiver can be prepared.”

EGNOS is widely used in agriculture. Indeed, it is the leading technology used today in precision farming in Europe, Diani said, and also in the traditional surveying segment. “EGNOS and EGNSS in general are also widely used for wagon tracking in rail transport,” she said, “but here we also want to take a step forward, in the near future, we want to see EGNSS and EGNOS in train signaling.”

EUSPA is working to include EGNOS services within the European Rail Traffic Management System (ERTMS). The agency is supporting two ongoing mission studies aimed at defining EGNOS service for rail. “We also have two projects in our ‘Fundamental Elements’ program, that is the program for the development of receiver technologies, and we have also updated our rail signaling road map to include EGNOS, working with our main rail stakeholder associations.”

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[Imag above: NAVAREAs and METEAREAs with Inmarsat C / Mini C coverage. Courtesy Inmarsat. The METAREA coordinators support the Worldwide Met-Ocean Information and Warning Service (WWMIWS) while the NAVAREA coordinators support the Worldwide Navigation Warning Service. Both provide forecast and warning products to mariners via SafetyNet and Navtex as part of the WWMIWS.]

The combination will create a global communications innovator with enhanced scale and scope to affordably, securely and reliably connect the world. The complementary assets and resources of the new organization will enable the availability of advanced new services in mobile and fixed segments, driving greater customer choice in broadband communications and narrowband services, including the Internet of Things (IoT).

The combined company intends to integrate the spectrum, satellite and terrestrial assets of both companies into a global high-capacity hybrid space and terrestrial network, capable of delivering superior services in fast-growing commercial and government sectors. This advanced architecture will create a framework incorporating the most favorable characteristics of multi-band, multi-orbit satellites and terrestrial air-to-ground systems that can deliver higher speeds, more bandwidth, greater density of bandwidth at high demand locations like airport and shipping hubs and lower latency at lower cost than either company could provide alone.

The combined company will be able to offer:

• A broad portfolio of spectrum licenses across the Ka-, L- and S-bands and a fleet of 19 satellites in service with an additional 10 spacecraft under construction and planned for launch within the next three years.
• A global Ka-band footprint, including planned polar coverage, to support bandwidth-intensive applications, augmented by L-band assets that support all-weather resilience and highly reliable, narrowband and IoT connectivity.
• The ability to unlock greater value from Inmarsat’s L-band spectrum and existing space assets by incorporating Viasat’s beamforming, end-user terminal and payload technologies and its hybrid multi-orbit space-terrestrial networking capabilities.
• Viasat’s highly vertically-integrated technology and service offerings, along with Inmarsat’s extensive eco-system of technology, manufacturing and service distribution.

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QZSS functions as a regional satellite-based augmentation system (SBAS0, improving GPS and Galileo accuracy for Japanese users in urban areas. It uses one geostationary satellite and three satellites in highly inclined, slightly elliptical, geosynchronous orbits. Each orbit is 120° apart from the other two and their ground traces are asymmetrical figure-8 patterns designed so that one is almost directly overhead (elevation 60° or more) over Japan at all times.

In December of last year, QZSS inaugurated a Centimeter Level Augmentation Service (CLAS), broadcasting a signal for nationwide open PPP-RTK service in Japan and providing centimeter positioning accuracy in a minute. This increased the number of available GNSS satellites up to 17: GPS, Galileo and Quasi-Zenith Satellite System (QZS) satellites all-in-view are corrected by the QZS L6 signal.

Designed for a 15-year lifetime, QZS-1R becomes one of four in the current QZSS system, which began offering services in November 2018. The Japan Aerospace Exploration Agency (JAXA) plans for a total of seven QZSS satellites by 2023. That satnav system will function independently of GPS.

Photo courtesy Mitsubishi Heavy Industries Launch Services.

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