SBAS TTA compensation based on GNSS/IMU sensor fusion user algorithms

Abstract

Current Satellite Based Augmentation Systems (SBAS) enable precision approach operations with Global Navigation Satellite Systems (GNSS) up to CAT-1. A definitive agreement on CAT-2 requirements has not been achieved yet. However, it is foreseeable that the current SBAS Time-To-Alert (TTA) performances of 6 seconds could be a limiting factor for the enabling of GNSS-based CAT-2 operations. Unfortunately, the improvement of such performance looks very challenging and expensive. Therefore, alternative solutions should also be investigated, where the SBAS TTA is, at least partially, compensated by the user algorithms. On-board sensors like Inertial Measurement Units (IMU) could be the ideal aiding technology for the compensation of the SBAS TTA. Therefore, an Extended Kalman Filter (EKF) was designed in this work for implementing an SBAS-GNSS/IMU sensor fusion framework. The application of SBAS-augmentation to an EKF-based algorithm, as well as the countermeasures proposed to solve the critical issues that this leads to, represented one of the most innovative aspects of the present work. Integrity and continuity fault trees were derived for the proposed system, with correspondent risk budget allocation and Protection Levels (PL) formulas. A simple TTA compensation scheme was then described where all the GNSS and IMU measurements in the last X seconds (X smaller or equal to TTA) are buffered and the EKF runs X seconds in the past. Each estimate is then propagated up to the current time instant through the strapdown integration of the inertial measurements only. The compensation window X represents a trade-off between the amount of TTA compensated and the accuracy degradation/PLs inflation due to the strapdown integration. Numerical evaluations of such trade-off were performed in this work by considering different IMU grades in simulated scenario based on a real landing trajectory. As expected, the results showed that the accuracy degradation depends on both the amount of TTA compensation and the IMU grade. In particular, for the high IMU grade no significant impact was observed when compensating 4 seconds out of SBAS TTA (overall TTA equal to 2 seconds).