The paper describes a platform for context-aware computing that enables applications to follow mobile users as they move around a building. The platform is designed for richly equipped, networked environments and requires users to carry only a small sensor tag, which identifies them and locates them accurately in three dimensions. The platform builds a dynamic model of the environment using location sensors and resource information gathered by telemetry software, presenting it in a form suitable for application programmers. The platform consists of five main components: a fine-grained location system, a detailed data model, a persistent distributed object system, resource monitors, and a spatial monitoring service. The paper also discusses the principles of indoor location sensing, the scalability of the sensor system, and the evaluation of the current location system. Additionally, it introduces a 3D model of the building and explains how Bat readings are interpreted to generate accurate object location information. The paper concludes with a case study of a Follow-me application called "Bat Teleporting," which allows a user's desktop to follow them as they move around the environment. The infrastructure supporting this application is robust, scalable, and can be used by multiple applications.The paper describes a platform for context-aware computing that enables applications to follow mobile users as they move around a building. The platform is designed for richly equipped, networked environments and requires users to carry only a small sensor tag, which identifies them and locates them accurately in three dimensions. The platform builds a dynamic model of the environment using location sensors and resource information gathered by telemetry software, presenting it in a form suitable for application programmers. The platform consists of five main components: a fine-grained location system, a detailed data model, a persistent distributed object system, resource monitors, and a spatial monitoring service. The paper also discusses the principles of indoor location sensing, the scalability of the sensor system, and the evaluation of the current location system. Additionally, it introduces a 3D model of the building and explains how Bat readings are interpreted to generate accurate object location information. The paper concludes with a case study of a Follow-me application called "Bat Teleporting," which allows a user's desktop to follow them as they move around the environment. The infrastructure supporting this application is robust, scalable, and can be used by multiple applications.