Boubeker Belabbas

Non-Gaussian Error Modeling for GBAS Integrity Assessment

Four basic error sources exist for residual pseudo-range errors in a single frequency differential GPS system for ground based augmentation (GBAS): signal multipath, increased receiver noise (carrier-to-noise density ratios (C/N0)) due to interference, residual differential troposphere error, and the error induced by ionosphere gradients. Without restricting ourselves to classical Gaussian overbounding, we combine their probability density functions (pdfs) to a total pseudo-range error distribution. This distribution is propagated through the GBAS Hatch filter and then mapped into the position domain using a worst case (selected by maximum vertical dilution of precision (VDOP)) of a full 31 satellite constellation with the two most critical satellites failed observed at Braunschweig Airport, Germany. Our calculations yield a significant reduction amounting to 46% of the position domain error at the 1.5 × 10-7 integrity risk level when compared with the classical Gaussian overbounding approach.

GBAS ionospheric threat analysis using DLRs hardware signal simulator

Ionospheric fronts pose a significant threat to single frequency ground based augmentation systems (GBAS) for airplane precision approach. Here, we show our implementation of two ionospheric threat models in the DLR MASTER GNSS Simulator, a modified Spirent GSS7780/779. These threat models include the standard GBAS ”front” threat model as required for CAT-1 certification and a simplified plasma bubble model intended to represent equatorial conditions. These models can now easily be simulated at the German Aerospace Centers facilities. As an example, we simulate a GBAS ground facility (with a code carrier divergence monitor) affected by both a plasma bubble with a constant differential delay of −2 m and fronts of various gradients and speeds. Results confirm that a plasma bubble can lead to worst case position biases that may cause misleading and hazardously misleading information to be used by an approaching aircraft. Moreover, we confirm that ionospheric fronts at a speeds close to the airplane approach speed and propagating into the approach direction cause the most severe range biases. These, mapped to the position domain, can also cause hazardously misleading information.

Use of High Altitude Platform Systems to augment ground based APNT systems

An Alternative Position Navigation and Timing (APNT) service is needed to backup satellite navigation for civil aviation in case of a radio frequency interference. Current APNT services rely on legacy systems and range sources based on ground stations and therefore, present limitations in terms of geometric diversity due to poor visibility. Moreover, there is still no straight forward solution how to maintain the correct synchronization of the ranging sources in case of GNSS outage. In this work, we propose the potential use of High Altitude Platform Systems (HAPS) first as pseudolites to enhance a ground APNT system. We evaluate the coverage of the platform and the enhancement in terms of dilution of precision. This lead to a ground-stratospheric APNT system with increased navigation service area. Besides, we propose these platforms for time synchronization and for integrity monitoring of the ground stations.

Parametric study of loosely coupled INS/GNSS integrity performance

Low-cost inertial sensors have been in the focus of ongoing research since they enable a low priced and low complex way to increase the robustness and reliability of navigation systems, especially GNSS. In this paper, we analyze a loosely coupled INS/GNSS including evaluation of the inertial error propagation and robustness of the corresponding navigation integrity concept with respect to the inertial system parameters. Moreover, we analyze the system integrity performance in terms of overall system protection level dependent on the inertial sensor error model. We discuss and compare the expected performance of using either typical high-end, middle-grad, low-cost or a network of redundant low-cost IMUs in terms of integrity and GNSS coasting time. We show that a network of four independent and identical distributed and orthogonally mounted low-cost sensors boosts the integrity performance significantly and even outperforms the one of a higher grade sensor. Additionally we discuss the trade-off between inertial networks and increased system complexity with respect to integrity performance in this paper.

Redundant inertial-aided GBAS for civil aviation

In this article, we investigate the impact of redundant MEMS-IMU on a loosely coupled INS/GBAS system. Therefore, we considered and analyze the combined performance of three identically and orthogonally mounted low-cost IMUs. It is shown that due to the redundancy, the INS based position error is reduced by an average factor of two. In conventional GBAS, no position solution can be obtain while GPS outages which lead to a full system failure. It is demonstrated that our redundant INS/GBAS integration sufficiently bridges GPS outages up to 10 seconds. Hence, the system availability is significantly improved. Additionally, an integrity concept is introduced providing integrity information even in the case where no GPS solution is available. This is essential for all safety-of-life applications such as civil aviation. We analyze our concept both, analytically and by Monte-Carlo Simulations. Our investigations clearly show that the robustness of the GBAS can be dramatically increased while maintaining low-costs and low-complexity.

Integrated Inertial Navigation System with multiple APNT ranges: Expected performance and considerations

Inertial sensors constitute an essential part in civil avionics systems. It has also been identified as a required system to meet the foreseen availability requirements in the context of Alternative Position Navigation and Timing (APNT). For example, DME/DME/Inertial (DDI) is currently considered for RNAV 1.0 operations. In this work, we look beyond DDI and investigate the possibility of an integrated inertial system with multiple APNT ranges. Thanks to our simulation framework, we evaluate in different scenarios the integration of inertial system with ranging sources using real DME locations. Based on the results, we finally analyze and comment on the limitations, implications and issues of using tactical grade instead of navigation grade inertial sensors.

GBAS based autoland system: A bottom up approach for GAST-D requirements

Ground Based Augmentation Systems (GBAS) for the precision approach of aircraft have never been so close to support automatic approach and landings under CAT IIIc conditions as they are today. However, as one key requirement for certification, it has to be demonstrated that the required integrity, continuity and availability allocations for GBAS are met, even under the most unfavorable circumstances. As for ILS/MLS, the performance of the total system (GBAS, the aircraft and its automatic landing system) is the key to eventual certification. Contrary to ILS/MLS-based guidance, at this moment no standard models and parameters for navigation errors and disturbances have been defined yet for assessing automatic landing performance based on GBAS. This raises the questions (1) how well does the integrated system perform with state of the art GBAS error models, and (2) for a given performance level reached with ILS/MLS, how much error could be tolerated when using GBAS instead? In this paper we have made a preliminary step to find an answer to the first question. To this end we have taken an existing, flight tested automatic landing system, tuned for use with DLRs fly-by-wire test bed ATTAS (Advanced Technologies Testing Aircraft System), which is representative for EASA Part 25-class aircraft. The landing system was adapted for use with GBAS and an advanced GBAS error model was incorporated in the simulation model. With this set-up, we performed the same Monte-Carlo simulations as were originally done with ILS. Initial results indicate that, based in the adopted error models, CS-AWO criteria can be met with more ease than with ILS/MLS-based guidance. It is expected that performance of GBAS-autoland systems can be further improved by exploiting new features that GBAS has on offer, tolerating larger NSE errors or possibly allowing operational autoland limitations, like maximum crosswind, to be relaxed

'EGNOS Performance Monitoring using the DLR's Experimental Verification Network','','','','','','','','494','505',

The Experimental Verification Network (EVNet) of DLR is an observation and verification platform for GPS, SBAS and in a close future for Galileo. This network has 4 stations regularly distributed over Europe and connected to a processing center located at DLR- Neustrelitz near Berlin. Its modular architecture is suitably designed for verification, testing and monitoring new receiver algorithms and navigation systems with respect to specific application performance requirements. A Network Integrity Monitoring Platform coupled with the EVNet will give the user the possibility to monitor the performances of specific services. EGNOS the European Geostationary Overlay Service is for the moment under validation process. A need has been identified to monitor EGNOS independently by comparing availability of protection levels with respect to standard RAIM protection levels. The results obtained using PRN120 (Actually still used for ESTB the EGNOS System Test Bed but planed to be upgraded for EGNOS) for a 4 day measurement campaign with 4 EVNet stations show lower availability with respect to a standard solution with RAIM protection levels. Nevertheless, degraded performances during an approval phase of a system might be observed and justifies the necessity for continuous monitoring of these services. The DLR’s EVNet with the Network Integrity Monitoring Platform is perfectly designed to fulfill this task as explained in this paper.

Issues in Implementing the Galileo Safety-of-Life Service

To understand the issues of implementing the Galileo Safety-of-Life service, it is necessary to explain the underlying motivation and the technical challenges induced by the targeted performance.