Robust FDIR approach for future hybrid navigation system for VEGA-C
Creators: name:  S. RAMIREZ NAVIDAD 
name:   R. POLONIO 
name:   S. DE LA RIVA 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLIGHT DYNAMICS
GNC
AVIONICS
DYNAMICS ANALYSIS
DEGRADED MODES

description: Typical hybrid navigation units combine high frequency but drifting measurements, such as the ones provided by an IMU, with low frequency but bounded measurements, such as the ones provided by a GNSS receiver. This architecture allows to obtain a high frequency and bounded navigation solution throughout the trajectory. However, it requires a robust mechanism able to detect different failures and to avoid their propagation, which could lead to the mission loss. This mechanism is the Fault Detection and Recovery (FDIR) system. In the context of space navigation systems, such as the new navigation unit of the VEGA-C, the FDIR takes an even more important role given the criticality of maintaining a continuous and accurate navigation solution. The first step of the work was to gather the different sensor errors, in particular de GNSS and IMU, and to trace them to the final navigation unit errors, i.e. a fault tree analysis. The outcome of this analysis was the identification of the necessary FDIR functions to cut the single point failures branches of the fault tree. Without considering the redundancy of the navigation unit, the proposed FDIR approach can divided into two different approaches, one for each type of sensor. The approach for the IMU sensor is to try to recover its measurements in case of failure detected by the sensor, and to consider at the Kalman Filter level (which hybridises the data coming from the two sensors) higher process noises for the propagated states that uses the IMU data. On the other hand, for the GNSS sensor, the proposed approach is to try to detect any failure, detected or undetected, and to discard it use within the Kalman Filter. Thus, in case of GNSS failure, the navigation unit could still provide the navigation data using the IMU propagation feature. The complex part of this approach, apart from its specific tuning, is the necessity of detecting GNSS failures that are not identified by the sensor itself. Several FDIR techniques can be found in the literature, in particular, a subset of these techniques have been retained to perform a dedicated trade-off for this application. After an initial trade of the different algorithms, the Cumulative Sum (CUSUM) algorithm has turned to be the most effective FDIR algorithm in terms of detection rate and false alarm rate. The CUSUM is a multi-epoch test method able to track slowly growing failures and based on a whiteness innovation test. It does so by comparing the innovation with the innovation covariance. Large values of this fraction will lead to the discard of the GNSS measurement. In conclusion, this paper presents the different steps followed (i.e. fault tree analyses, literature review, trade-off) to finally obtain the proposed approach. Finally, it is also presented the preliminary results for the validation of this FDIR approach within a real product.