Requirements-Driven Design of Small Satellites: TET and AsteroidFinder

Abstract

The design and implementation of a satellite mission is divided into several different phases, normally ranging from phase A (feasibility) to phase E (utilization). Parallel to these phases an evolution of requirements will take place starting with the user requirements. The whole development process has to be traceable back to requirements until the system engineers can achieve the final state, which resides in the actual specification of the system. Therefore, the implementation of a satellite can be defined as a continuous process with the requirements at its center. One of the biggest problems faced is the handling of the requirements, which are edited mainly with WYSIWYG word processors (such as MS Word or OpenOffice). During the process of manually copying the actual requirements (or references to them) between documents (e.g. from the functional to the technical specification) a lot of errors may arise while attempting to achieve backwards traceability, if not in the initial stages of requirement creation or tracing, then certainly in the later project phases when the possibility exists of and increase in the number of inconsistencies. Such critical points in time can occur after a review process like a detailed/critical/… design review, or after major changes in one of the subsystems of the satellite. In this paper we present the approach which we are using for both the TET and AsteroidFinder missions, where the design of the satellite missions is driven by a tool- based requirements management process. Using Telelogic DOORS as the tool of choice, all requirements will be stored in a central database which is accessible by all project partners regardless of their location. Using features such as baseline management and change requests the software supports the typical development phases of a satellite in an ideal way, also making the time-consuming process of requirements reviewing and RID-processing a lot easier. The usage of the web interface DOORSnet and e-mail notification completes the projects' goal of allowing internal and external users to stay informed of all changes in the satellite's database. In our paper we will show that, especially for small satellite projects, the usage of a requirements management tool can save valuable time and money, enhancing the verification and validation process and eliminating many errors during the process of requirement definition.