Autonomous Fault Detection Research Articles

FDI Algorithms for GPS Signals: A Different Perspective of a Known Problem

Autonomous fault detection and isolation capability is an essential figure for GPS based sole means of Navigation systems. Integrity is as important as accuracy for aeronautical applications. Thus the defmition of a new perspective for the analysis of GPS satellite malfunctions, and the synthesis of RAIM algorithms is discussed. New features and parameters for failure detection and isolations are introduced. The new approach is based upon the joint exploitation of all the possible LS solutions applicable to a given linear estimation problem, e.g. GPS estimation, and the analysIs IS performed m the position domain. Preliminary results show accordance with the predicted outcome and the effectiveness of the proposed implementation.

Adaptive neural control for space structure vibration suppression

Despite recent advances in efficiency, current methodologies for space structure control design still engage significant human resources for engineering development and routine maintenance. The adaptive neural control (ANC) program is part of an effort to develop neural network based controllers capable of self-optimization, on-line adaptation and autonomous fault detection and control recovery. This development in addition supports the long-term space exploration objectives for which autonomous spacecraft involving self-reliant control systems are a necessity. The ANC program comprises two phases. The first, basic phase focused on the development of efficient and completely autonomous neural network feedforward control for the case of broadband disturbances. Algorithms were developed that work with no prior modeling information about the system to be controlled and adapt to changing conditions, while minimizing or eliminating the introduction of extraneous training signals. The algorithms were demonstrated experimentally on an optical structure testbed at Harris. The second phase of the program demonstrated a more complex neural controller on the advanced space structures technology research experiments (ASTREX) test facility at the Air Force Research Laboratory capable of the fault-tolerant adaptive control of multiple sensors and actuators. This system used six actuation channels of the existing ACESA struts on the ASTREX structure to simultaneously cancel three independent tonal disturbances in the 10-15 Hz band, measured at non-collocated sensors on the secondary tower of the structure. The system demonstrated impressive fault-recovery performance, maintaining good cancellation performance with successive actuators disabled. Cancellation of individual tones was between 25 and 55 dB, with over 27 dB attenuation realized root mean square. The algorithm required very low computational throughput, operating at a sample rate of 1/20 Hz. The results of the ANC program show that adaptive cancellation systems can reduce vibrations in precision structures without prior modeling information and can adapt successfully to certain failures in actuators or sensors, optimally reconfiguring themselves without human intervention. These capabilities should significantly reduce the expense of designing and maintaining vibration control systems for spacecraft.

Autonomous fault detection and removal using GPS carrier phase

This paper is focused on the use of carrier phase measurements and parity space methodology to investigate the limits of high-integrity and high-continuity satellite-based navigation performance. In this regard, a new algorithm for receiver autonomous fault detection and removal is developed with the specific goal of attaining the high levels of integrity and continuity required for aircraft precision approach and landing applications. Fault detection and removal algorithm performance is demonstrated through analysis and simulation, and the results of tests using deliberately induced failures in raw night data are presented.

Combining Pseudoranges from GPS and Loran-C for Air Navigation

User equipment that combines pseudoranges from GPS and Loran would be more reliable than an unaided GPS receiver or a Loran-only receiver. It could remove coverage outages caused by satellite shutdowns and/or bad satellite geometry. Similarly, it could remove most of the coverage outages caused by Loran station shutdowns, high atmospheric noise levels, or precipitation static. A hybrid GPS/Loran receiver could also reliably provide self-contained fault detection and isolation (otherwise known as receiver autonomous integrity monitoring, or RAIM). This paper treats position fixing, receiver autonomous fault detection, and receiver autonomous fault isolation as system services, and computes the probability of outage for these three services. Our analysis computes the outage probability for the level of service required for nonprecision approach and en route navigation in the National Airspace System. It assumes that GPS is configured in the Optimal 21 Satellite Constellation, and includes the effects of satellite as well as Loran station shutdowns. Finally, our analysis examines whether it is important to synchronize Loran transmissions to GPS time.

Optimum Skewed Redundant Inertial Navigators

The problem of proper placement of the inertial sensors to optimize system performance is important to system designers. This is especially true of redundant strapdown systems that employ more sensors than the conventional, mutually orthogonal sets of three. In systems designed for a free fall environment and with no preferred direction, such as for spacecraft attitude reference, the sensor input axes should divide the three-space equally, and can thus be viewed as being normal to the faces of regular polyhedra.' In Earth-bound inertial navigators, the effect of gravity-dependent sensor errors and the generally reduced effect of azimuth errors on navigation accuracy significantly alter the situation. Both effects tend to decrease sensor elevation angles in an optimized system. Formulas are derived and curves are drawn for optimum sensor elevation and azimuth angles vs a 0-sensitive sensor error parameter, and a mission relative azimuth error parameter. Tetrad, pentad, and hexad arrays are analyzed, affording a dramatic improvement in accuracy as well as autonomous fault isolation and/or detection capability.