Modelling of the ionosphere by neural network for equatorial SBAS

The estimation of the ionosphere delay and associated confidence interval constitutes the major issue to reach APV1 availability performance level for single frequency SBAS above the equatorial area. The ionosphere is a complex physical system which dynamics is particularly disturbed at the Geomagnetic Equator while mid-latitude regions are quieter. Classical methods to compute ionosphere delays, such as those implemented in EGNOS and the WAAS, are specific to a smooth ionosphere behavior and are not really adapted to follow high spatial and temporal gradients, such as those observed in the equatorial area. Thus innovative methods, having flexible and reactivity qualities, shall be defined and adapted to propose efficient equatorial SBAS. Classically in SBAS concept, the knowledge of the ionosphere delay is obtained by a set of lines-of-sight between the network of ground stations and the navigation satellite constellation. Each line of sight intersects the ionosphere layer, assumed infinitely thin, and the dual-frequency combination allows to compute, at first order, the ionosphere delay that affects the GNSS measurements. From this set of heterogeneous information, locally sampled irregularly on the sphere and changing over time, we propose to build an interpolating method to calculate the ionosphere delay on a point of interest by an adaptive mesh, unlike fixed grids usually used. In this paper, we introduce an interpolation method based on the definition of a flexible network that can adapt to the spatial location of the data. It is therefore proposed to create self-organizing maps – as the Kohonen map – defined by a mesh that fits the data. This network ends up sticking to the data by "learning" in real time: the mesh becomes denser and denser in the presence of many measurements and relaxes otherwise. This technique increases the granularity of the ionosphere delay information to compute, in particular to be able to describe the local plasma bubbles or depletions, if they are observable. The adaptive array technology "learning" has been widely studied in the field of modeling neural networks. Their main advantage is to be able to reach an optimal state based on the information they process. The experimentation based on this technique shows a very good behavior in the case of strongly disturbed ionosphere conditions and the preliminary results are promising to bring the expected robustness to deploy SBAS in equatorial area

Robust EGNOS Availability Performances under Severe Ionospheric Conditions

The estimation of ionosphere delay and associated covariance is the key contributor to reach APV1 and LPV200 availability performance level for EGNOS above the ECAC area. To qualify the future EGNOS system release, ESA has defined a new synthetic data scenario representative of severe ionospheric conditions. In order to be consistent with the real ionosphere behavior, ESA has also defined a new criterion to characterize the dynamics of the gradients, the Along Arc TEC Rate (AATR), that represents the rms over one hour of the particular derivative of the TEC along a path. Typically the value of AATR that is present into the new synthetic scenario is around 2.5 mm per second for L1 at low latitude. To be able to follow such a gradient dynamics Thales Alenia Space France has developed a new Ionospheric Grid Computation module (IGC) to compute the EGNOS ionosphere corrections, in the scope of R&D activities. The IGC module is integrated into SPEED platform, the SBAS Operational Test-bed that fully represents EGNOS Performances in terms of accuracy, continuity, availability and integrity for Safety Of Life services. The outputs of this module is the set of GIVD, the vertical delay, and GIVE, the confidence interval of GIVD at 3.29 sigma, for each Ionospheric Grid Point (IGP). The analyses of the performances of this new ionospheric module have shown a real significant improvement of the APV1 availability performances map (more than 100% of availability performance improvements). These first results show that the compliance to APV1 and LPV200 availability performances can be reached with this solution under severe ionospheric condition

Integrity Based on MT28 for EGNOS: New Algorithm Formulation & Results

The purpose of a Satellite Based Augmentation System, such as EGNOS or WAAS, is to decompose all range error sources and to distribute them to the civil aviation user community with reliable navigation services for different flight phases. Integrity refers to the notion of trust that the user may have in the positioning. Integrity includes the ability of the system to provide confidence thresholds as well as Alarms in case of anomalies. Considering satellite integrity, two ways are possible for an SBAS. The first is to broadcast per satellite the same UDRE value applicable for every user located inside the service area defined by the message type 27 content. As these UDREs are dimensioned to protect the worst user location – generally at the border zone – it penalizes automatically the users having lower residual errors. The second is to broadcast per satellite a covariance matrix of residual errors through the message type 28. This matrix contains all the structure of the orbitography and synchronization residual errors. It can be seen as a protective ellipsoid around the computed satellite position and clock containing the true – but unknown – satellite position and clock at a certain level of confidence. Using the MT28 message each user is able to reconstruct the integrity value by projecting this protective ellipsoid along its line of sight. EGNOS operational system is implementing the MT27 solution whilst the WAAS is based on the MT28 one. The MT28 approach is the current baseline for EGNOS V3. Starting from recent R&D activities, Thales Alenia Space together with CNES (French Space Agency) has developed a dedicated module providing the message type 28 based on orbitography and fast synchronization variance-covariance matrices combined with satellites residual measurement errors. The MT28 formulation has been designed in collaboration with Statistics and Probabilities laboratories (Paris Descartes and Toulouse Paul Sabatier) in France. The algorithm contains also a mechanism that reacts immediately to orbit or clock satellite feared events such as a clock jump. This new concept for EGNOS provides a drastic improvement with respect to the first studies provided during EGNOS V3 phase A & B and HISTB V2 by the Thales consortium. The new MT28 module is integrated into a SPEED platform, the SBAS Operational Test-bed that fully represents EGNOS performances in terms of accuracy, continuity, availability and integrity for Safety Of Life services. The performance evaluation shows a very good level of EGNOS LPV200 availability with respect to the MT27 current approach. The integrity is constantly maintained on the Geostationary Broadcast Area with a good level of integrity margin. This paper provides a high level architecture description of this new EGNOS algorithm as well as a set of performance figures showing the achieved improvements

Performances Monitoring and Analysis for KASS

The Korea Augmentation Satellite System (KASS) is the future SBAS of the Republic of Korea. It is developed by the Korea Aerospace research Institute (KARI) for the government of the Republic of Korea, and Thales Alenia Space is the industry prime contractor of this development. The function of the KASS is to decompose all possible range error sources and to distribute corrections and/or alerts to its users by means of geostationary satellites. The KASS Processing Station (KPS) is the component of KASS in charge of computing the orbit, clock and ionosphere correction and alert information (below ‘Navigation Overlay Frame’, NOF) using data from a set of reference stations. The KPS is composed of two independent elements: the Processing Set (PS) and the Check Set (CS). The first element is responsible of computing the complete navigation context for the GNSS constellation (orbits and clock) and the ionosphere model, then to prepare and send the NOF to be broadcast to the users. The second element acts as a super user by applying the NOF to the GPS messages checking that this is consistent with an independent set of measurement to control and insure the integrity. The KPS-PS component plays a key role in the KASS performance achievement where the APV-1 service level is required. To feed the KPS, the KASS has specific KASS Reference Station (KRS) located on the Rep. of Korea land masses. Compared to other SBAS, this leads to a very concentrated station network. This particularity makes a specific algorithm adaptation of the KPS-PS necessary, as compared to the EGNOS solution, to provide the desired APV-1 performance. These adaptations regard both orbit determination and all the more ionosphere corrections due to the very low number of Ionosphere Grid Points (IPG) that need be modeled and monitored. To cope with these KASS specificities, Thales Alenia Space has designed, developed and qualified a new complete real time navigation algorithm chain that provides MOPS-compliant NOF messages. The ionosphere model is different from the EGNOS one that favors a local analysis counter to a global approach as the TRIN model [2] used in EGNOS. This new algorithm chain provides the specified APV-1 performance, particularly in the case of strong ionosphere activity, with a very good level of integrity margin. This paper presents the overall KASS system architecture as well as the results obtained using this new algorithm chain under different ionosphere contexts. The APV-1 service availability level is presented and the maximum of safety index on each monitored IGP and satellite is discussed

Equatorial Ionosphere Characterization for Sub-Saharan Africa SBAS

Performance Based Navigation (PBN) is a concept developed by ICAO (International Civil Aviation Organization) that specifies the operational performance required in an airspace, route or approach procedure. A Satellite Based Augmentation System (SBAS) enhances the performances of the existing satellite navigation system. It is used to deploy Global Navigation Satellite System (GNSS) approach for PBN procedures. The required performance level for vertical guidance is directly linked to approach category criteria. The real performance provided by an SBAS for a single-frequency user depends on the physical characteristics of the ionospheric layer. As Sub-Saharan Africa corresponds to geomagnetic equator region, the question of ionosphere dynamics characterization in equatorial zone is central to gauge what SBAS performance level can be achievable. In the equatorial zone the dynamics of the ionosphere is subject to complex physical phenomena, involving rapid recombination of ion-electron pairs. Moreover these phenomena are transient with high local spatial and temporal gradients. These zones promote the occurrence of scintillation phenomena, bubbles (strong local fall of TEC (Total Electron Content)), and small scale gradients, which must be evaluated for the ionosphere modeling and integrity data generation. Based on a large volume of GNSS measurements covering more than four years of data collected, Thales Alenia Space associated with IRAP (Astrophysics and Planetology Research Institute, Toulouse, France), present a panorama of observed physical events through the ionosphere in Sub-Saharan Africa zone. The main purpose of this study is to establish a clear view on the physical mechanisms that drive the equatorial ionosphere dynamics and the effects on GNSS measurements. This study is supported by information coming from TEC values, TEC gradients amplitudes, and the nature of scintillation events as intensity, impact area and occurrence in time. Conclusion of these activities is to highlight that ionosphere conditions above sub-Saharan area are consistent with the performances level of SBAS approach with vertical guidance. Indeed scientific analyses show that a precise service level is possible on this zone with a very good level of availability above the main airports

New Orbit Determination and Clock synchronisation modules for EGNOS

The purpose of a Satellite Based Augmentation System (SBAS), such as EGNOS or WAAS, is to identify all range error sources and to distribute the corresponding corrections to the civil aviation user community with reliable navigation services for different flight phases. The effect of a satellite location error depends on the user location while a satellite clock error with respect to a reference time scale directly translates into a common pseudorange error to all the users. Therefore the SBAS shall broadcast a 3D vector that represents the satellite orbit error and a satellite clock correction. To achieve this objective the SBAS shall internally estimate the orbits and clocks for all the navigation satellites in view of the service area. The orbit determination function is in charge of computing the satellite ephemerides. The synchronization function computes the corresponding clock bias for each epoch and each satellite. Then the corrections are constructed from the differences between these orbits and clocks and the corresponding ones broadcasted inside the GNSS navigation messages. Starting from R&D activities, Thales Alenia Space has developed new orbit determination and synchronization modules that are part of the Thales Algorithm Navigation Chain. These modules have been designed in collaboration with the orbit determination team at CNES (the French Space Agency). The new proposed orbit determination module is based on real time processing using code carrier measurement only. This module provides a stable and metric GPS orbit performance using an SBAS set of receivers corresponding to the EGNOS service area. The new synchronization module solves clock errors directly steered to GPS reference time scale, for the stations and satellites. It uses both code carrier and phase carrier measurements as well as the orbits estimated by the orbit determination process. The clocks solution error Allan’s deviation is around 10-12 at 120s leading to 7cm of possible deviation for a prediction up to 120s. This performance is fully compatible with the needs of the SBAS mission. These modules are now fully integrated into the SPEED platform, the SBAS Operational Test-bed that fully represents EGNOS Performances in terms of accuracy, continuity, availability and integrity for Safety Of Life services. The performance evaluation shows a real improvement over the current EGNOS algorithms, particularly in terms of the distribution of the Satellite Residual Error for the Worst user location (SREW). This paper provides a high level architecture description of this new Thales solution. A set of performance figures showing the achieved improvements is also presented

Altimetry for the future: Building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the ‘‘Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Altimetry for the future: building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the “Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Analysis Software for Interplanetary Trajectories using Three Body Problem

International audienceThe aim of this paper is to present the PRIAM Toolbox(Poincare Resolution of Interplanetary and Astrodynamic Missions), designed to study interplanetary missions and particularly suited to the dynamics analysis near collinear libration points defined as isolated zeros of a smooth gravitational vector field. Orbits computation and optimal transfers constitute the core of the toolbox

Study of Integration Schemes Suited for the Long-Term Extrapolation

International audienceThis paper is focused on the integration methods for the orbit extrapolation on the long term time. This issue is of importance when it comes to choosing the graveyard orbit during the orbit design process for a space mission. In this paper, classical and geometric integration schemes will be discussed and compared on LEO (Low Earth Orbit) cases. We will give a particular interest to the variational scheme, in the last chapter, since this geometric integrator has the property of preserving the geometry of the problem like symplectic schemes but, contrary to symplectic integrators, it can be also used in case of non conservative dynamics