Sensor-driven systems are increasingly ubiquitous: they provide both data and information that can facilitate real-time decision-making and autonomous actuation, as well as enabling informed policy choices by service providers and regulators. But can we guarantee these system do what we expect, can their stake-holders ask deep questions and be confident of obtaining reliable answers? This is more than standard software engineering: uncertainty pervades not only sensors themselves, but the physical and digital environments in which these systems operate. While we cannot engineer this uncertainty away, through the use of models we can manage its impact in the design, development and deployment of sensor network software. Our contribution consists of two new concepts that improve the modelling process: frames of reference bringing together the different perspectives being modelled and their context; and the roles of different types of model in sensor-driven systems. In this position paper we develop these new concepts, illustrate their application to two example systems, and describe some of the new research challenges involved in modelling for assurance.