In over eight years of operation, the Chandra X-Ray Observatory flight software has been enhanced with a near flawless record resulting in the Observatory producing world-class science at a very high efficiency. However, upholding the as-launched flight software maintenance method poses significant challenges for the future. The Chandra Flight Operations Team initiated efforts to reduce the overall risk to the mission due to inadequacies in the flight software maintenance method in late-2003.The original expectation was that most flight software modifications would consist of patches to constants or stored command sequences and that the few source code patches that were required would be developed without recompiling the software. The general philosophy was that recompilation of the source code would not be done unless extensive or complex changes were required. Over time, the many modifications to the onboard computer flight software resulted in a divergence of the flight image from the originally compiled image. This eventually resulted in it not being possible to recreate the on-orbit image using compiled source code.Initially this approach was deemed acceptable; however, as the spacecraft evolved it became evident that this methodology would lead to additional risk as the mission progressed. More complex patches are envisioned as the spacecraft ages and attrition of flight software and other spacecraft engineering experts could lead to increases in the time to implement a source code modification as well as the probability of an error being included by the change. It was determined that the original flight software maintenance method could become impractical at some point in the future and that developing compiled versions of the code resulted in a more robust and lower risk approach to flight software maintenance.Due to these concerns, and the realistic possibility of a 15+ year mission, efforts were undertaken to incorporate the necessary changes in the source code files in order to bring them up to date with the on-orbit code functionality. This new version of the flight software was compiled and the resulting image was compared to the on-orbit image. All differences between the two images were explained on a bit-by-bit basis. Rigorous verification and validation testing is currently underway, repeating all relevant pre-launch tests, as well as adding additional tests to address areas that contain new or modified functionality.Many of the program risks have been mitigated by the activity of recompiling and testing the new source code. Uplink of the new compiled software isn't absolutely required, but it will reduce the risk of introducing an unintended error into the flight software if the current maintenance method is replaced by maintaining a single recompiled image. Uplink timing and methodology of the new code depends on many factors, including, but not limited to, personnel expertise, anticipated future patch complexity, time taken away from other high priority tasks, and spacecraft health and safety ...
Abstvuct -Many hture missions will be implemented with distributed spacecraft systems, which require formation flying capability. Here, formation flying means controlling the motion of one or more spacecraft relative to other nearby spacecraft to enable them to operate as a distributed sensor, in orbit about a planet or in deep space, at interspacecraft ranges fiom meters to many kilometers. This paper discusses BATC experience, modeling and study work in formation flying. We discuss our deep space formation flying work (StarLight, Terrestrial Planet Finder, and MAXIM Pathfinder), low Earth orbit formation flying work (Cloudsat), and rendezvous and docking work (Deep Impact and Orbital Express).
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