Figure 3.1 shows a reusable software architecture for satellite control systems. The realtime executive provides an infrastructure that is independent of the particular system requirements and can be reused across multiple spacecraft. It connects spacecraft specific device drivers and functional modules. These modules perform such functions as rotating solar arrays, moving momentum wheels, determine position based on various sensors and so on. The code of each module is executed sequentially at an appropriate clock rate. For each clock cycle the module performs the appropriate computation which includes reading ground commands and sending telemetry information. Modules communicate by shared variables which require no synchronization if the reader and writer modules run at the same clock rate.
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During the design process aerospace engineers (AE) develop the control laws for each functional module. Typically a single engineer works on a module. The control laws are coded and evaluated in MatLab to determine proper behavior. The result of these tests are plots that show various responses to control inputs.
Figure 3.1 show the development process for an individual module. A software engineer (SE) takes the design document produced by the aerospace engineer and develops appropriate Ada code. This code is unit-tested and later integrated into the system. Separate software documentation is produced for the hand-written Ada code. The design document is also used as a basis for developing ground software that needs to interpret telemetry information and generate commands.
