Satellite Selection Methodology for Horizontal Navigation and Integrity Algorithms

With the new upcoming GNSS constellation in the future it might no longer be possible to use all satellites in view for navigation due to limited tracking channels. This is in particular true in the context of Advanced Receiver Autonomous Integrity Monitoring (ARAIM), where the use of dual frequency is favorable to mitigate ionospheric disturbances. To address the issues of limited channels we propose two different satellites selection strategies adapted for Horizontal ARAIM in this paper. First a bare geometric approach which comes with almost no additional computation effort at the cost of less stable results. And second a heuristic optimization which improves selection results significantly while adding additional computational effort. Both approaches are compared to brute force selected best sets in terms of resulting protection levels, computational cost and achieved ARAIM availability. Results show the general applicability of both presented selection methods in Horizontal ARAIM. Using limited sets instead of all satellites in view can still provide global availability. Depending on the method more or less satellites are necessary to ensure sufficiently small and stable protection levels

Standardization of New Airborne Multipath Models

In aeronautical navigation the use of Global Navigation Satellite Systems (GNSS) is becoming ever more important. GNSS are one of the cornerstones of the performance based navigation (PBN) concept. They are currently used for navigation en-route, as well as during arrival procedures and for lateral approach guidance. Together with satellite-based or ground-based augmentation systems (SBAS, GBAS) satellite navigation can provide precision approach guidance down to CAT-I minima. In order to ensure sufficient global availability of these services and enable new services, such as Advanced Receiver Autonomous Integrity Monitoring (ARAIM) for providing services with higher performance levels, including in regions with active ionospheric conditions, existing integrity concepts and augmentation systems are upgraded to incorporate not only GPS but multiple GNSS constellations and also navigation signals on a second frequency. On the side of GNSS, all GPS satellites since the Block IIF generation with currently 12 operational satellites provide signals in the L5 band (in addition to the most commonly used signals in the L1 band), a second frequency band usable for aeronautical applications. The Galileo constellation has currently 22 operational satellites in orbit that all provide signals on the E1 and E5a frequency bands. Other constellations, such as Glonass and BeiDou are also launching further satellites so that a large number of navigation satellites are available to users. The use of dual-frequency and multi constellation techniques will mitigate the impact of most ionosphere-related disturbances, significantly increasing service availability. All GNSS-based navigation methods have in common that they need appropriate integrity concepts safely bounding any residual errors that may prevail in the position solution. With the ionospheric errors largely addressed by dual-frequency and multi-constellation methods, the residual noise and multipath becomes the most significant contributor to the residual errors. In order to bound these errors, standardized error models are used. The existing multipath model was developed based on extensive data analysis, however, using only the legacy GPS signal in the L1 band. Galileo is using a different modulation for the E1 signals which is less susceptible to multipath. The GPS and Galileo signals in the L5/E5a band are using a 10-times higher chipping rate than the L1/E1 signal. Therefore, also for these signals, the multipath envelope is significantly smaller, potentially allowing to have smaller error models for these signals. When using dual-frequency methods to remove the ionospheric delay, the receiver tracking noise and multipath error from the signals on both frequencies are combined. For all these cases the existing model is not well suited for error modelling. Within the frame of the DUFMAN project funded by the European Commission new multipath models for the new signals are developed in order to be able to exploit the potential benefits for aviation users. Previous papers on the project were addressing the methodology, described the results of the studies and the influence of the antenna. This paper explains the standardization activities and discusses choices that were made in setting up the data collection campaign and the subsequent steps to standardized models. Regarding standardization, the International Civil Aviation Organization (ICAO) is producing Standards and Recommended Practices (SARPS) for DFMC SBAS which will make use of the DFMC multipath models. Further requirements on the hardware exist e.g. in form of Minimum Operational Performance Standards (MOPS) that specify performance of certain components, such as the airborne antenna. A variety of antennas differing significantly in performance is available on the market. Furthermore, the airborne receiver hardware may use different correlator spacing and receiver bandwidth settings which may also have an impact on the results. In the effort to characterize the multipath errors, hardware and processing choices had to be made taking into account all those requirements and the impact on the final models. The paper discusses the interdependency between different standards and explains the choices that were made in the project, as well as results in terms of standardization

Antenna Group Delay Variation Bias Effect on Advanced RAIM

This paper investigates the impact of the range error caused by the antenna group delay variations (noted as AGDV) on the advanced receiver autonomous integrity monitoring (ARAIM). The new multipath and AGDV error models for aviation use of new GPS and Galileo signals developed within the Dual Frequency Multipath Model for Aviation (DUFMAN) project and relevant AGDV measurements analyzed in DUFMAN [1], [2] are applied for the assessment of the impact. In this work, several approaches are taken to address the contribution of the AGDV error in the current ARAIM airborne algorithm: consideration of the user antenna bias error as a measurement bias term or as a random process sigma term. We performed ARAIM service volume simulations for Localizer Performance with Vertical guidance (LPV)-200 by applying the proposed error modeling methodologies and integrity support message (ISM) parameters in line with the current ARAIM framework. We compared availability performances as well as protection levels between the methods. It was found that 99.5% LPV-200 availability increased by approximately 5% when the newly derivedDUFMAN multipath and AGDV error models were applied. On the other hand, despite the maximum improvement of roughly one meter in the vertical protection level, the AGDV effect considered as the bias term in the worst-case sense appears to be marginal to the ARAIM availability performance

Initial results for dual constellation dual-frequency multipath models

This paper presents an update of the ongoing work to develop dual frequency dual constellation airborne multipath models for Galileo E1, E5a and GPS L1 and GPS L5 in the frame of the project DUFMAN (Dual Frequency Multipath Models for Aviation) funded by the European Commission. The goal of this activity is to support the development and implementation of airborne GNSS-based navigation solutions, such as Advanced Receiver Autonomous Integrity Monitoring (ARAIM), dual-frequency multiconstellation Satellite Based Augmentation System (SBAS) and dual-frequency multi-constellation Ground based Augmentation System (GBAS). Previous work described the methodology proposed to derive the airborne multipath models and presented preliminary multipath models obtained from an experimental installation. In this paper we present the initial results obtained from flight campaigns conducted within DUFMAN on Airbus commercial aircraft. The measurements are collected from prototypes of dual-frequency multi-constellation avionics receiver and the antenna installed on the aircraft has been selected to meet at best the current dual-frequency dual-constellation antenna requirements. In addition to the initial results obtained from avionics hardware, the impact of the different receiver correlator spacing and bandwidth is investigated and discussed

Final results on airborne multipath models for dualconstellation dual-frequency aviation applications

This paper proposes DFMC airborne multipath models and antenna error models derived from measurement and supported by simulations. Based on the data evaluated, new multipath models (including the contribution from the antenna) for Galileo E1 and GPS L1 and Galileo E5a and GPS L5 are discussed. Furthermore, a model for the Ionosphere-Free combination of the signals is proposed

Real Time Advanced Receiver Autonomous Integrity Monitoring in DLR’s Multi-Antenna GNSS Receiver

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

GNSS Inter-Constellation Phasing: Validation of the Worst-Case Assumption

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

A Robust and Effective GNSS/INS Integration Optimizing Cost and Effort

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