%A Alexei Iliasov %A Elena Troubitsyna %A Linas Laibinis %A Alexander Romanovsky %A Kimmo Varpaaniemi %A Dubravka Ilic %A Timo Latvala %T Developing Mode-Rich Satellite Software by Refinement in Event B %X To ensure dependability of on-board satellite systems, the designers should, in particular, guarantee correct implementation of the mode transition scheme, i.e., ensure that the states of the system components are consistent with the global system mode. However, there is still a lack of scalable approaches to formal verification of correctness of complex mode transitions. In this paper we present a formal development of an Attitude and Orbit Control System (AOCS) undertaken within the ICT DEPLOY project. AOCS is a complex mode-rich system, which has an intricate mode-transition scheme. We show that re?finement in Event B provides the engineers with a scalable formal technique that enables both development of mode-rich systems and proof-based verification of their mode consistency. %C Antwerp, Belgium %D 2010 %L deploy240