Avionics systems are becoming more powerful and more dependent upon data exchanged between instruments. These instruments and subsystems reside on a network and must share time-critical data to achieve their mission. For example, targeting systems require real-time input of aircraft speed and attitude, as well as position and velocity data of the target. At the same time, additional bandwidth is required for data from onboard systems, such as GPS, airspeed and directional gyro, flight control systems, and dozens of other instruments and subsystems. As a result, network traffic is high, and potential data interactions can be highly complex. This complexity makes real-time integration of the data from disparate instruments during operational missions a significant challenge. Furthermore, upgrades of avionics and software applications during the useful life of the airframe means that new subsystems must be seamlessly integrated with legacy subsystems. In other words, data paths, interactions, and integration are not fixed forever. Today, aircraft systems typically are constructed to provide point-to-point communications between instruments and control systems that require realtime data. This approach has a significant impact on the complexity of the system and its subsequent maintainability. If an instrument is upgraded or replaced, the interfaces between it and other directly connected devices have the potential to change, requiring significant recoding and retesting.

POSIX Conformity

Correct upfront design choices can minimize the technical issues and complexity of integrating time-critical data from disparate avionics systems and enable those systems to make decisions based on that data. A proven method is to use a design architecture based on a standards-based commercial RTOS and data distribution middleware that abstracts the need for specific knowledge about the devices generating or consuming data. Such a solution can ensure timely delivery of data across an avionics network, no matter what specific instruments are present on that network An RTOS that employs the POSIX (Portable Operating System Interface) standard provides a known and published application interface that can support changes in applications, instruments, and even the RTOS itself. Full POSIX conformance ensures that applications can be easily ported between conforming operating systems, making it possible to upgrade or change code without major changes and extensive testing to the application. Further, an RTOS that conforms to the POSIX standard can also be tested and certified according to the rigorous requirements of the POSIX standard, as defined by The Open Group, proving full API support and the most portability for customers. Avionics system integrators can have confidence in the integrity, performance, reliability, and flexibility of their software.

Publish-Subscribe Dynamics

In addition to designing the system avionics network can be architected to focus on data rather than devices. Systems that use point-to-point data transfer through message passing or other mechanisms tend to be brittle and can break when data, code, or devices change. Specifically, point-to-point data transfers mean each instrument has to have knowledge of another instrument’s specific needs, its data, and its interfaces. This inherent awareness of the data endpoints represents a significant issue in the maintenance of individual applications and the system in general over the life of the avionics platform. Instead, by focusing on data definition, it is possible to abstract both code and instruments away from the details of the network endpoints, providing the foundation of a robust and scalable system. This is done by employing a publish-subscribe architecture where a source publishes its data, and the data is delivered to any device subscribing to that data. This architecture enables a data-centric design approach, where each device decides the data it needs, rather than worrying about the instrument’s configuration that generated the data. Network usage can be reduced through the use of multicast data delivery and tuning of Quality of Services (QoS) parameters on the data streams — a necessity for critical avionics systems. The publish-subscribe framework provides highly efficient scalability, enabling avionics integrators to add additional instruments and subsystems without having to worry about modifying their communication code.

A publish-subscribe architecture can be implemented with middleware that complies with the Object Management Group’s Distributed Data Service (DDS) standard, which supports real-time response, high performance, and comprehensive control via QoS. By abstracting data producers and consumers from low-level communication details, DDS allows distributed systems to be easily designed, implemented, and maintained. A standards-based solution that combines a high-performance RTOS with data-centric distributed middleware has significant advantages for the life cycle of an avionics system. The result is an avionics network that is technically easier to create and maintain over time, while also providing a standards-based platform for high performance and reliability. It can deliver greater safety and responsiveness as aircraft mission needs evolve. The lower complexity of this standards- based RTOS and middleware solution minimizes the cost of maintenance over the airframe lifecycle, both because of the abstraction of the details of applications and instruments and because of the reduced requirements for systemlevel testing when adding or modifying devices. Because performance and reliability are maintained as the devices on the network increase, cost should prove predictable, even with an expected airframe life measured in decades.

This article was written by Dr. Heidi Schubert, Senior Applications Engineer, at Real Time Innovations in Santa Clara, CA; and Sonia Leal, Senior Product Manager, at LynuxWorks in San Jose, CA. For more information, contact Dr. Schubert at This email address is being protected from spambots. You need JavaScript enabled to view it., or visit http://info.hotims.com/10968-403 .