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Meeting	ACGS Committee Meeting 94 - Reno - November 2004
Agenda Location	4 GENERAL COMMITTEE TECHNICAL SESSION 4.1 Government Agencies Summary Reports 4.1.2 NASA 4.1.2.1 NASA Langley
Title	NASA Langley
Presenter	Celeste Belcastro
Available Downloads*	presentation
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Abstract	This paper describes a Single Event Upset (SEU) experiment performed at the Los Alamos National Laboratory. A closed-loop control system consisting of a Recoverable Flight Control Computer (FCC) and a Boeing 737 simulator was operated while the FCC was exposed to a neutron beam. The purpose of this test was to analyze the effects of neutron bombardment on avionics control systems operating at altitudes where the occurrence of neutron effects is 100 or more times more probable than at sea level. The neutron energy spectrum produced at the Los Alamos National Laboratory is similar in shape to the spectrum of atmospheric neutrons but much more (at flight altitudes up to a million times more) intense. The higher intensity results in accelerated life tests that are representative of the actual neutron radiation that a FCC may receive over a period of years. The objectives of this research are to: (1) investigate the effects of SEU on closed-loop systems, (2) develop SEU closed-loop system test methods and capability, (3) develop design and analysis methods to mitigate and assess SEU closed-loop system effects, and (4) investigate the applicability of recoverable computing techniques to mitigating SEU effects. The expected benefit of this research will be the development of design and validation guidelines for the achievement of aircraft system performance in the atmospheric neutron environment that provides the degree of safety needed. Particular focus in on aircraft functions critical to safe flight. The Recoverable FCC is a prototype computer based on the Airplane Information Management System (AIMS) technology which is used in the 777 to handle several avionic tasks such as flight management, displays, airplane condition monitoring, thrust management, digital flight data, and engine data interface. AIMS uses a reliable fault-tolerant architecture based on dual lock-step processors with high speed comparison. To execute multiple tasks in a single fault-tolerant computing unit, Application-Specific Integrated Circuits (ASICs) with robust software partitioning were implemented. Most of theses ASICs have been previously neutron tested at the chip level therefore providing background data for the susceptibility analysis of the FCC. The Recoverable FCC adds automatic rapid correction of soft faults to the AIMS architecture. The correction is done using rollback recovery. The information management system software was replaced with basic 737 aileron and elevator control algorithms. For closed-loop testing in the high intensity radiated fields chambers at NASA Langley Research Center, the AIMS ARINC 629 I/O board was replaced with an optical ARINC 429 interface. For the test in Los Alamos, the power supply Circuit Card Assembly (CCA) was replaced with four external current-limited power supplies. These power supplies were connected to the RCS via a 20ft cable and a printed wiring board. The purpose of the current-limited power supplies was to avoid damage to components that might latch-up during the experiment. If current exceeds the limiting threshold, the power supplies would shut down before component damage occurred in the Recoverable FCC. Approximately 100 baseline runs were performed prior to neutron exposure and between exposure runs. Approximately 100 exposure runs were completed. For each target there were recoverable fault events. For most targets there were also unrecoverable faults. Overall, the number of recoverable faults was significantly greater than that for unrecoverable faults. The recoverable flight control computer tested in this experiment demonstrated an ability to be robust in an adverse particle environment. The recovery scheme resident with AIMS technology provides a measure of tolerance to single event effects. Dual lock-step processors with the additional architecture elements for rapid rollback recovery provide a fault tolerant methodology for flight critical systems in harsh particle environments. When neutron effects assessments are performed on electronic systems, it is the "complex"/highly integrated electronic devices (microprocessors, ASCIs, FPGAs, etc) upon which attention is focused. Never the less, it is suspected that most of the unrecoverable failures observed during testing were due to strike counters (normally not part of a flight control system fault response) disabling the recoverable computer processing. There were, however, instances were neutron induced phenomenon did cause failures which were both unrecoverable and can not be attributed to the action of resident AIMS legacy software. Future research will involve further analysis of the effects of rollback recovery on closed loop stability as well a further test on the recoverable FCC to isolate suspected chips and chip sets. This identification of neutron susceptible components and subsystems could have implications for fault tolerant design strategies if similar components are currently being used, or will be used, in flight critical systems.