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International audience; Advanced Receiver Autonomous Integrity Monitoring (ARAIM) is a concept that extends RAIM to other constellations beyond GPS. ARAIM will enable theintegration in the position solution of the newer GNSS core constellations that may have different properties (in particular with higher failure rates than GPS). This inclusionwill provide better levels of performance of horizontal guidance than RAIM with GPS alone. In addition, when sufficient satellites have dual frequency (L1-L5) signals,ARAIM could enable aviation safety of life operations, including approaches with vertical guidance.The European Union (EU) and the United States (US) have an agreement establishing cooperation between GPS and Europe's Galileo system. As part of this cooperativeagreement a technical subgroup was formed to investigate the benefits of Advanced Receiver Autonomous Integrity Monitoring (ARAIM) [1]. This EU-US ARAIMsubgroup has developed a reference airborne algorithm [2] and two potential ARAIM architectures [3, 4, 5]:Offline ARAIM to support horizontal and vertical navigation based on a low latency (days to months) ISM.Online ARAIM to support horizontal and vertical navigation based on an hourly ISM from the ground, which would include clock and ephemeris corrections.We will describe the current status of ARAIM as a concept. We will start by discussing the relationships between SBAS and ARAIM, and among the different architectures.This is important, as ARAIM aims to provide globally the services that SBAS already provides regionally. Then, we will present preliminary Integrity Support Messageformats adapted for a GNSS broadcast, both for the offline architecture (which are the same for horizontal and vertical), and the online architecture (which include clock andephemeris corrections). Finally, we will describe a notional ARAIM roadmap for the deployment of a global ARAIM service.

This paper addresses the integration between Global Navigation Satellite System (GNSS) and Inertial Navigation System (INS) from an integrity perspective. In order to achieve this, we analyze a low-complex coupling scheme, where a GNSS layer resets the INS strapdown algorithm with position, velocity and their respective protection levels. In particular, RAIM and Velocity RAIM algorithms are used here as the integrity-based core system. Special focus is dedicated to the propagation of uncertainties in terms of covariances through the strapdown inertial algorithm, as well as the impact of the reset uncertainties and accelerometer noise on the final computed protection levels. Finally, it is shown how our integrated system may solve the continuity and availability problems of GNSS by coasting not only the position solution but also the protection levels.

This paper presents a flight performance analysis of next generation ARAIM based on GPS and GLONASS dual-frequency data collected during flight trials using DLR’s research aircraft ATTAS. The test results are based on joint PVT and ARAIM post-processing and investigate potential differences between simulated ARAIM performance levels and actual operational performance. The PVT and ARAIM performance is evaluated with respect to positioning accuracy, position error bounding (integrity) and availability of LPV200 (Localizer Performance with Vertical Guidance) service during typical arrival and approach maneuvers. This evaluation takes into account operational constraints such as signal outages due to banking. These constraints are not included in analyzes based on static user grids with fixed elevation masks and thus constitute the characteristic feature of this study. To ensure that the performance metrics obtained can be compared with performance simulation results, the error model targeted at future GNSS with L1 and L5 signals is adapted to represent the error model for the data collected, L1 and L2. Compliance with integrity requirements is demonstrated conditioned on a fault-free scenario, which is ensured by post-processing and analyzing the airborne data together with ground reference data. Flight test results are compared with results from a predictive study that estimates ARAIM performance with a simulation for a static user in the vicinity of the airport. The comparability of these results is made possible by adjusting the dual frequency user range error model to the scenario at hand and applying the same SIS error parameters both to the real-data ARAIM processing chain and the simulated scenario.

Advanced Receiver Autonomous Integrity Monitoring (ARAIM) uses satellite range measurements from multiple GNSS constellations to determine navigation integrity. Providing robustness against multiple simultaneous satellite faults, ARAIM needs a much higher number of available measurements than a classical RAIM [1]. Studies [2, 3, 18] expect that the performance of ARAIM based integrity will be sufficient to allow for LPV-200 based operations only if two complete constellations are present. Performance simulations using GPS and Galileo constellations use an arbitrarily selected definition of the relative positioning of orbital planes of these two constellations. In reality however, the orbital plane phasing between Galileo and GPS is a determined parameter varying very slowly due to orbit perturbation. Because the RAAN (Right Ascension of the Ascending Node) parameter of all orbits drifts slowly and this drift rate depends on the orbital altitude of the spaces vehicle, Galileo and GPS have different RAAN drift rates. As a result, identical RAAN phasing between the two constellations reappears at a period of 11 years, and a potential worst case would persist for significant time, i.e. several years. Identification of such a worst case constellation phasing is thus important to avoid too optimistic performance estimates in simulations. Most previous performance studies assume that the worst case constellation phasing exists when three of the GPS planes have identical RAAN parameters to the Galileo planes as this setup fosters weak geometries where satellites from GPS and Galileo appear to be close together. This worst case assumption has been confirmed in DOP-based studies such as [4] for navigation accuracy, but not yet for ARAIM performance. Because ARAIM based navigation is much more susceptible to small and weak geometries it is necessary to review the validity of the worst case assumption with respect to ARAIM. Moreover, past work on inter-constellation phasing effects has only compared the "full alignment" scenario with the "most separated" scenario where Galileo planes are distributed exactly in the middle between the GPS planes. This paper analyses ARAIM performance for a more detailed range of RAAN phasing scenarios, and determines the worst case for ARAIM based navigation integrity. Furthermore we demonstrate the projected performance for the Galileo mission under the assumption that the recently launched Galileo SVs already define the inter-constellation phasing. By extrapolation of available orbit data to a full Galileo constellation the ARAIM performance at Galileo FOC and during the first years of operation is predicted. The results obtained from the simulations demonstrate that the constellation phasing does impact the ARAIM performance, but the magnitude of this change is small. The minor characteristic of this effect is also confirmed for a combined constellations based on current GPS configuration and Galileo at FOC. The individual impact on the ARAIM VPLs for specific users however is rather large and can be observed in both directions, i.e. the change from a "best case" to a "worst case" constellation phasing has a positive impact for the performance of some users, and a negative impact for other users.

The present paper introduces a real-time implementation of Advanced Receiver Autonomous Integrity Monitoring (ARAIM) implemented in DLR’s array antenna receiver “GALANT”. The receiver is capable of tracking multiple frequency measurements both from GPS and Galileo signals. The position, velocity and time (PVT) unit and the Multi- Hypothesis Solution Separation (MHSS) based ARAIM unit are closely coupled and operate in a real-time processing environment within the receiver demonstrator. The presented MHSS based RAIM is extended with Fault Detection and Exclusion (FDE) functionality that can identify large measurement faults in one or multiple range measurements, and adapt its measurement model to mitigate the effects of such biases. Different concepts to obtain such FDE functionality are discussed and compared. A series of measurements using the Galileo Testbed GATE has been recorded and processed in real time using the presented hardware/software platform. Feared event scenarios were generated by introducing biases to one or multiple pseudorange observations on signal level, i.e. at the GATE processing facility. The PVT and integrity results for nominal and feared event scenarios are presented. The ability of MHSS ARAIM to provide a robust navigation solution in the presence of such large faults is assessed and the improvement of positioning accuracy introduced through the application of FDE methods is demonstrated.

GPS, Galileo and GLONASS are preparing for the transmission of signals in two protected frequency bands (L1, and L5). The combined use of such signals and the verification of their consistency significantly improve the autonomous integrity of position estimates. Aviation applications benefit from this increase in performance, and the use of GNSS as primary means of navigation for precision approach, e.g. in the LPV- 200 category, comes into reach. Previously considered integrity architectures heavily depend on augmentation, and have to respond within the time-toalert. Autonomous methods, such as the Advanced Receiver Autonomous Integrity Monitoring (ARAIM) considered here, do not suffer from the latter constraint. However the threat space is significantly inflated: Simultaneous satellite faults as well as constellation faults need to be considered. This can be addressed by an ARAIM algorithm employing the principle of Multiple Hypothesis Solution Separation (MHSS), and limiting the threat space to faults with large probability. The present paper assesses the performance of this algorithm using combinations of future GNSS under different assumptions of satellite fault probability and constellation fault probability. Combinations of GPS, Galileo and GLONASS are investigated to determine the worldwide availability coverage using the integrity requirements defined in LPV-200: Vertical Protection Level (VPL), Effective Monitoring Threshold (EMT) and vertical positioning accuracy (Accv). Based on the simulation results presented here, combinations of GPS and Galileo are shown to provide sufficient VPL-based performance for LPV-200 precision approaches at all runways worldwide. However there exist limitations on meeting the EMT requirement if a single satellite is unavailable. Minimal dependence on the probability of satellite fault is observed for both VPL and EMT. With fault probabilities higher than 10^-3/approach, the LPV-200 availability requirement is not met. This probability threshold corresponds to approximately 40 observed faults per year for a nominal 27- SV constellation, with a 6-hour latency after fault onset. When using a triple GPS-Galileo-GLONASS constellation, the simulated performance becomes generally robust enough to handle even worst case scenarios without any degradation of the relevant performance metrics. However it is shown that assuming independent satellite fault events at high probabilities leads to high computational complexity with potential implications for the real-time capability of ARAIM. Thus, the current research results recommend a limitation on the total number of satellites used for processing integrity limits with MHSS ARAIM, possibly by employing a selection process to visible satellites from all available constellations.

The development of new monitoring algorithms needs to have access to a platform that offers flexibility, operational evaluation facilities, test and validation capacities. DLR’s GBAS test bed offers all these features with additional simulation capability. This paper illustrates these properties by showing an application of the recently developed absolute ionosphere gradient monitor by taking advantage of a network of three reference receivers located in the DLR’s research airport at Braunschweig. This paper presents a first draft of an absolute ionosphere gradient monitor capable of detecting gradients from 300 mm/km to 2000 mm/km in order to fulfill the GAST-D requirements in terms of ionospheric gradient monitoring for the ground subsystem. The three receivers give the possibility to build two independent Absolute Slant Ionosphere Gradient Monitors (ASIGM). The performances achieved depend on the performances of the receivers, their relative location and the orientation of their baselines with respect to the runway direction. ASIGM with baseline in the direction of the runway provide the best observability of an ionsphere gradient that a GBAS user can experience during the approach phase (for straight-in approaches). The smaller the angle between the runway and the considered monitor baseline is, the lower the uncertainty of the gradient in the direction of the runway. Distribution of receivers parallel to the runway provides the best observability conditions. This paper gives the optimal relative location of receivers in order to achieve 100% detectability in the 300 to 2000 mm/km range of the absolute slant ionosphere gradient. Two configurations were investigated: one with 3 and another one with 4 receivers linearly distributed. The results obtained are extremely promising for the GAST-D requirements fulfillment and the corresponding architectures can easily be implemented.

Meeting all requirements for ying approaches in bad weather conditions is one of the most demanding and challenging aspects of present day airborne navigation. Stand-alone satellite navigation has not yet reached the point of being suciently robust and accurate in order to reach certication level. Therefore, in this work the performance of an integrated satellite/inertial navigation system (GNSS/INS) is investigated in order to cope with short term losses of GNSS signals. We consider a low-cost Micro Electronic Mechanical System (MEMS) INS which is constantly reinitialized with information coming solely from GNSS. It takes over navigational responsibility when a loss of signal occurs or other failures in the satellite navigation system are detected. For the GNSS to provide all information necessary to initialize an INS, a minimum of three antennas is needed to measure the aircraft's attitude along with its speed and position. Error models for positioning, speed and attitude estimation are used to create a model for initialization uncertainties. Together with error models for the accelerometers and gyros in the Inertial Measurement Unit (IMU), the behavior of the whole proposed architecture is determined via performance simulations. As a maximum allowable error 15.3 meters (which corresponds to the CAT III horizontal alert limit for GNSS approaches) are taken. Our simulations show that this limit is not exceeded for at least 14 seconds after the take-over of navigational responsibility by the INS.