  Accountability is the property of a system that enables the uncovering of
causes for events and helps understand who or what is responsible for these
events. Definitions and interpretations of accountability differ; however, they
are typically expressed in natural language that obscures design decisions and
the impact on the overall system. This paper presents a formal model to express
the accountability properties of cyber-physical systems. To illustrate the
usefulness of our approach, we demonstrate how three different interpretations
of accountability can be expressed using the proposed model and describe the
implementation implications through a case study. This formal model can be used
to highlight context specific-elements of accountability mechanisms, define
their capabilities, and express different notions of accountability. In
addition, it makes design decisions explicit and facilitates discussion,
analysis and comparison of different approaches.
