The design of the Cassini Remote Terminal Interface Units (RTIUs) had to change to meet the requirements for support of the Cassini tour of the Saturnian system. The science instrument teams used the RTIUs, purchased in the early 1990s, both before and after launch. Since the Cassini prime mission does not end until 2008, the original RTIUs would have been required to last at least fifteen years. When the requirements on the RTIUs changed after launch, the design of the RTIU had to change as well. This paper will describe the history of the RTIUs on the Cassini project, how the need for an upgrade was identified, why management approved the task, why the Cassini instrument teams agreed to upgrade their systems, how the design of the software was improved and why these improved RTIU capabilities should be designed, built and integrated into the flight Mission Operations System before launch.In their simplest form, the RTIUs are used in the science instrument test beds to connect the engineering model (EM) of the instrument to its Ground Support Equipment (GSE) computers. They send commands from the GSE computers to the EM and return telemetry. The RTIUs appear to the instrument to be the Cassini spacecraft's Command and Data Subsystem (CDS) and to the GSE computer to be the tail end of the Cassini Ground Data System (GDS).Before launch, the RTIUs were used to build and test the science instruments, to integrate the instruments onto the spacecraft and, to write and test both instrument flight and ground software. During flight, the RTIUs have been used for science planning, sequence development, instrument flight software development, instrument flight software testing and anomaly resolution.There were originally three different implementations of RTIU used on the project. Two of the designs were made at the JPL. The third was a public domain implementation from outside the National Aeronautics and Space Administration (NASA). In addition, some of the instruments had constructed their own special purpose hardware that was integrated with their RTIU. The initial motivation for an upgrade post-launch was simply to replenish the RTIU hardware. At that time, the majority of Cassini's ground computers were being replenished, so the RTIUs were included in that task. We could not simply replace the RTIU hardware with new components because neither of the two RTIUs from JPL had viable software development environments. A hardware replenishment usually requires a change in operating system version, interface cards and drivers. So, the software would have needed to be recompiled if not modified. This would have been difficult for the JPL-built RTIUs. The RTIU software from outside NASA was still being actively developed, so that was, at least initially, an option as a replacement. Downloaded by 41.251.71.197 on June 21, 2016 | http://arc.aiaa.org |
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