- Modelling EGNOS orbit and clock corrections residuals, Quentin Tessier, ENAC (France), Laurent Azoulai, Airbus Operations SAS (France), Carl Milner, ENAC (France), Christophe Macabiau, ENAC (France)
The Instrument Landing System (ILS) is used to guide aircraft on final approach and is by far the most important and frequently used guidance system for landings. However this system is both expensive and inflexible because it is only able to provide a straight-in approach trajectory for one runway end, so that multiple installations for one airport are required. The successor of this system is expected to be the Global Navigation Satellite Systems (GNSS) with augmentation systems such as SBAS (Satellite Based Augmentation System) or GBAS (Ground Based Augmentation System). Such system are able to provide safe and reliable guidance with greatly improved flexibility in the definition of approach tracks.
In a previous article we evaluated the nominal residual clock and ephemeris errors remaining after applying the corrections, reflecting SBAS accuracy. The methodology and results using 2016 data collections will also be described in this article. The main objective is the residual error modelling for a user of the EGNOS system in the ECAC (European Civil Aviation Conference) area. This study is conducted in the frame of a thesis that is co-financed by the ESA, AIRBUS OPERATION SAS and ENAC and follows the study of SBAS Autoland feasibility conducted by AIRBUS OPERATION SAS and ENAC. The long-term purpose is to model the SBAS Navigation System Error (NSE) to determine the feasibility of SBAS Autoland CAT-I and CAT-II. The autoland system under test should be fed with an SBAS NSE composed of the nominal residual SBAS clock and ephemeris position error that is modeled in this article, of the nominal residual SBAS ionospheric position error. Then, limit and faulty errors of the SBAS system should also be considered to demonstrate airworthiness requirement for Autoland CAT-I.
At first, the extracted residual range error will be analyzed as a function of the user position, the satellite position and the satellite ID. This representation will show that a significant bias exists in our estimate that is dependent of the satellite ID and position. Then we will investigate the possibility that the bias is an artefact of the methodology that is not reflected at the user level because our methodology only uses broadcast GPS ephemeris data, broadcast SBAS data, and NGA/IGS reference data, and does not use actual measurements. The final part of the paper presents the explanation of the residual biases that are observed in the final distribution. The biases will be described according to their behavior as a function of the user position, the satellite position and the satellite ID.
- SBAS DFMC performance analysis with the SBAS DFMC Service Volume software Prototype (DSVP), Daniel Salos, Egis Avia (France), Hugues Secretan, CNES (France), Catalina Rodriguez, CNES (France), Mikael Mabilleau, Egis Avia (France)
The deployment of new dual-frequency GNSS constellations (modernized GPS, Galileo, GLONASS, Beidou) will support in the coming years aeronautical navigation services with an improved positioning performance and robustness (increased number of available signals and improved geometry, ionospheric delay mitigation, etc.).
The definition and analysis of the GNSS augmentation systems evolution to operate in this new dual-frequency multi-constellation (DFMC) environment is on-going. In the case of the SBAS evolution, the SBAS Interoperability Working Group (IWG) has worked on the development of an SBAS DFMC L5 Interface Control Document (ICD) in order to initiate the standardisation of the future SBAS DFMC system and services. Other working groups such as the EUROCAE WG-62 and the RTCA SC 159 have also started the development of standards to support the introduction of future SBAS DFMC user receivers.
A SBAS DFMC Service Volume software Prototype (SBAS DSVP) has been developed by Egis Avia under a CNES contract as a tool to support the consolidation of the SBAS DFMC standardisation activities. The prototype is compliant with the most recent documentation produced by the IWG, and generates the sequence of broadcast SBAS DFMC messages and calculates the broadcast parameters required to analyse the availability and continuity of APV and CAT I operations in the SBAS service area. The prototype user module computes Horizontal and Vertical Protection Levels (HPL/VPL) from the SBAS L5 messages elaborated by the ground module.
An analysis of the SBAS DFMC performance under different system configurations has been carried out with the SBAS DSVP. The presented results include the impact of different number of constellations augmented simultaneously, different ground reference station networks and a potential single-frequency L5 SBAS back-up service.
- Integrity Risk Evaluation for GPS/GLONASS RAIM with Multiple Faults, Eugene Bang, ENAC (France), Carl Milner, ENAC (France), Christophe Macabiau, ENAC (France)
With the full deployment of the Russian Global Orbiting Navigation Satellite System (GLONASS), an increased number of redundant Global Navigation Satellite System (GNSS) measurements are available, which has recently drawn interest in the feasibility of GPS/GLONASS Receiver Autonomous Integrity Monitoring (RAIM). Accordingly, the design of a rigorous integrity test methodology for GPS/GLONASS receiver has been needed in developing the GPS/GLONASS Minimum Operational Performance Standards (MOPS) for GPS/GLONASS L1-only airborne equipment. These standards and test procedures must be validated in order to chow that they protect the user with respect to the higher level requirements of integrity and continuity relating to safety.
In this paper, we propose a multiple hypothesis based approach to test the performance of the conventional RAIM fault detection and the corresponding integrity bound against multiple failures. In particular, we investigate the resistance of GPS/GLONASS RAIM against multiple faults, including GLONASS constellation failure, based on the proposed method. We first determine the worst case fault, which is the most difficult to detect whilst leading to a potential positioning failure. This corresponds, for each failure, to identifying both the worst fault direction and magnitude. Next, we evaluate the exact probability of missed detection (PMD) under each fault, and we compare this PMD to the requirement for single failure, as in lines with the proposed GPS/GLONASS MOPS test procedures. This is to check if the single-faliure protection bound based on the current RAIM could protect the users against multiple failures. Also, we compute total probability of hazardous misleading information (PHMI) by accounting for all possible fault hypothesis and compare it with the integrity risk requirement of 10-7.
This paper carries out integrity risk evaluations for GPS/GLONASS RAIM with several failure modes based on the newly proposed test method: GPS dual faults, GLONASS double faults, a single GPS fault and a single GLONASS fault, and GLONASS constellation fault. It is shown that the fault detection performance of the current residual based RAIM method for a pair of failures would not meet the corresponding PMD requirement of 10-4 throughout the world. We also show that the current single-bias RAIM fault detection could provide worse performance for multiple correlated failures than for a single failure. In the same way, PHMI analysis for the proposed multiple failure modes is performed. In particular, it is shown that frequent integrity fails take place under the GLONASS constellation fault. This work would help to design new integrity monitoring test procedures for GPS/GLONASS L1-only airborne equipment.
- From the pseudo - range overbounding to the integrity risk overbounding, Igor Nikiforov, Troyes University of Technology, UTT/ICD/LM2S, UMR 6281 CNRS (France)
The advantages of a navigation system that can monitor its own integrity are obvious. All GNSS integrity monitoring methods can be broadly divided into two classes: “active integrity methods" and “passive integrity methods". If an unbounded GNSS channel degradation occurs at an unknown time then the outputs of the least square algorithm are not an optimal solution and this unbounded degradation leads to an unbounded additional bias of the user’s fix. In this case, the only solution to preserve a high constant integrity level of GNSS positioning is the active integrity methods, like ARAIM. However, nowadays another type of problem arises due to reduced alarm limits (HAL and VAL): even bounded degradations of the pseudo-range measurements lead to an unacceptable additional bias of the user’s fix. Unfortunately, the probabilities of false alarm, missed detection and false isolation of ARAIM FDE algorithms (even in the case of optimal statistical tests) for such bounded degradations are unacceptable with respect to the advanced MOPS for GPS/Galileo for some safety-critical mode of flight. In such a case, a reasonable solution to the problem of integrity monitoring consists in the passive integrity method, which is called “overbounding”. The overbounding approach allows all pseudo-range measurements without rejection. The concept of overbounding suggests to define conservative bounds (overbounds) for the cumulative distribution function (CDF) of pseudo-range errors and to get a conservative bound for the integrity risk by using such overbounds for the pseudo-range errors. Three methods of overbounding for scalar random variables have been developed by DeCleene, by Rife, Pullen, Enge, and Pervan and by Rife, Walter, and Blanch. By using the above-mentioned methods, the instantaneous integrity risk overbounding of the vertical positioning is calculated. Unfortunately, these three methods cannot be directly used to calculate the instantaneous integrity risk overbounding for the horizontal positioning. In the latter case, it is necessary to find a conservative bound for the CDF of a nonlinear combination of several random variables. However, the paired overbounding theorem cannot be used in the general case. Moreover, the MOPS for GPS/Galileo requires calculating the integrity risk for a given time period. The integrity risk is defined “per approach” or “per hour”. Understanding that the pseudo-range errors are strongly auto-correlated, such calculation is not trivial. The original contribution of this paper is twofold. First, we propose two new methods of overbounding in the horizontal plane by using one or two Gaussian PDFs with an inflation coefficient. Next, these conservative Gaussian bounds for pseudo-range errors are transformed into the conservative bound for the integrity risk in the horizontal plane. Second, we estimate the impact of the pseudo-range errors autocorrelation on the conservative bound for the vertical/horizontal integrity risks. The proposed solution is reduced to the first-passage-problem for the autoregressive model AR(1). A method of numerical integration by using the Gaussian quadrature and the 5-point numerical derivative has been used to solve the above-mentioned problem.
- In depth characterization of EGNOS ground stations response to space weather disturbances, Ridha Chaggara, ESSP (France), Bernard Duparc, ESSP (France), Ulrich Ngayap, Abbia GNSS technologies (France), Claudia Paparini, ESSP (France)
GNSS/SBAS systems are subject to different kind of signal degradations including multipath, jamming and ionosphere disturbances. The latter has a non-negligible impact on GNSS systems and users especially at high and low latitudes even if during severe space weather incidents also mid-latitudes may experience a substantial impact on performance.
It is well known that ionospheric scintillations can have an important effect on radio signals traveling through the atmosphere and they remain one of the main contributors to GNSS performance perturbations. The occurrence of scintillation and irregularities at high latitudes is related to the several phenomena such as aurora oval, cusp, and polar cap patches, through the formation of small-scale plasma structures due to particle precipitation or plasma instabilities. It has been observed that scintillation is more common on geomagnetically disturbed days in the aurora oval region and close to noon and midnight. Generally, with increasing levels of geomagnetic activity, the aurora oval will expand toward the equator. Thus, the range of latitudes affected will depend on the level of activity.
In this context, EGNOS ground stations archived data combined with broadcast SiS (Signal in Space) during two years period (2014 and 2015) have been deeply analyzed and processed. In this work a picture of how EGNOS respond to different ionosphere events has been investigated.
More precisely, the impact and the peculiarities of aurora scintillations experienced over the northern part of ECAC have been characterized considering both ground station tracking capabilities and signal in space performance in terms of IGP and GPS monitoring capabilities.
Relevant ionosphere indicators such as the AATR (Along ARC TEC Rate) and ROTI (Rate of change of TEC index) are provided. These parameters are based on TEC evaluation through the TECCalibration Techniques developed by ICTP/ ICT4D laboratory.
AATR inter-correlation level between different ground stations and during different ionosphere conditions has been assessed in terms of spatial and temporal variability. Taken into account the nature of the ionosphere process which is a combination of time dependent component (linked to the time of the day) and a kind of un-deterministic process reflecting the level of the disturbance; an average cross correlation function has been considered. More precisely the final cross correlation is obtained as the mean of individual function estimated over a short period of time (typically one hour).
Preliminary results revealed that during quiet days the level of AATR cross-correlation between close RIMS is quite high. Meanwhile, during disturbed days the level of correlation remains relatively high. Nevertheless we noticed a different cross-correlation shape in that the inter-correlation level remains high even when large time shift is considered. This behavior suggests that the disturbance has lasted for several hours. This latter aspect is currently under studies for a more complete characterization.
Deeper investigations of the results obtained covering long period of time and relative to different ground stations are currently ongoing to provide a better understanding of the ionosphere process related methodology.
- A demonstrator to prove ARAIM concept, Daniel Salos, Egis Avia (France), Juan Pablo Boyero, European Comission (Belgium), Nidhal Dahman, Airbus Defense and Space (France), Mikael Mabilleau, Egis Avia (France)
Receiver Autonomous Integrity Monitoring (RAIM) relies on GNSS measurement redundancy to provide positioning integrity. RAIM can benefit from future dual frequency multi-constellation (DFMC) GNSS. On that basis, the EU/US Working Group C (WGC) has recently developed the Advanced RAIM (ARAIM) concept, and has quantified, in theory, its potential in terms of navigation performance. Such potential can in turn lead to operational and environmental benefits for the aviation community.
The WGC proposes a concept based on an Integrity Support Message (ISM) and a corresponding user algorithm designed to take benefit of the DFMC advantages. Different architectures have been proposed to provide integrity for en-route (Horizontal ARAIM) and precision approach operations (Vertical ARAIM) down to 200ft decision height.
The aviation community now foresees to include in their plans standards evolution for the ARAIM technology, starting with H-ARAIM. For example, ICAO NSP, EUROCAE WG62 and RTCA SC 159 have included H-ARAIM in their respective work plans as part of their short term standard productions (between 2018 and 2020).
This paper introduces the overall project scope of activity of the ARAIM Demonstrator project launched by the European Comission and will further detail the design retained for the demonstrator, highlighting its capabilities as a tool for the ARAIM proof of concept. The algorithm structure will be also be described. The project will comprise a nine-month experimentation period with both real and simulated measurements. Towards the end of the experimentation period, real flight trials implementing ARAIM will be conducted. The experimentation plan and logic will be developed and initial prototype results in simulated and real environments will be presented.
Results and lessons learnt throughout the project are expected to provide a major contribution to the preparation of the aviation standards, such as the ICAO CONOPS, as well as to provide solid recommendations aimed at future ISM providers.