Building Confidence in the Quality and Reliability of Critical Software

Software in critical civilian and military aerospace applications, including avionics and other systems in which quality and reliability are imperative, continues to become both more common and more complex. The embedded software development organizations that build these systems must meet stringent quality objectives that are mandated by their organizations or required by customers or governments. For engineering teams to meet these objectives, and to ideally deliver high quality software, state of the art testing and verification solutions are needed. This article examines formal methods based software verification and testing approaches that have been applied to critical software projects in civil and military aerospace and defense projects. Examples are provided to illustrate how these verification techniques can be deployed in practice to improve the quality and reliability of complex avionics systems. Building Confidence in the Quality and Reliability of Critical Software 1. The Components of Avionics Embedded Software Avionics software implemented in critical aerospace applications consists of special purpose embedded software. This software often operates in real time and is responsible for critical operations. Examples include digital flight control systems, full authority digital engine control, guidance navigation control, and similar systems. The embedded software responsible for these systems will consist of multiple components, including automatically generated, handwritten, and third-party code as well as libraries (see Figure 1). Generated code: Generated code is synthesized from models that are used to describe and analyze the behavior of complex systems and algorithms. Handwritten code: Handwritten code may include interfaces to hardware (for example, driver software for a cockpit display system, airspeed sensor, or another hardware subsystem), or it may be translated manually from specification documents or models. Third-party code: Third-party code may be delivered by suppliers or it may be required as part of larger software system (for example, to interface with the real-time operating system). Libraries: Object code is part of the application code that exists as a library or as compiled legacy code. By definition this software is delivered or is only available in the form of object code (binary files). Figure 1: Components of embedded software. Figure 2: Embedded software design, implementation, and verification (V Diagram). Even with robust verification processes, complex systems can fail. Causes of failure include insufficient specification, design errors, software coding errors or defects, and other issues unrelated to software. Ideally design and coding errors should be detected on the right-hand side of the V diagram during software, hardware, and aircraft testing and integration processes.