Distributed sensor networks are quickly gaining recognition as viable embedded computing platforms. Current techniques for programming sensor networks are cumbersome, inflexible, and low-level. EnviroTrack, as an object-based distributed middleware system, raises the level of programming abstraction by providing a convenient and powerful interface to the application developer geared towards tracking the physical environment. EnviroTrack is novel in its seamless integration of objects that live in physical time and space into the computational environment of the application. It contains run-time mechanisms that efficiently abstract groups of sensors by logical objects which maintain aggregate environmental state. Such objects may be logically attached to moving entities in the physical environment, in order to monitor the state of the tracked entity.

The EnviroTrack middleware library implements a set of protocols that off-load from an application developer the details of inter-object communication, object mobility, as well as the maintenance of tracking objects and their state. It abstracts away the fact that computation associated with the object may be distributed and performed by all sensor nodes in the vicinity of the tracked physical entity. As the

tracked entity moves, the identity and location of the sensor nodes in its neighborhood change, but the tracking object representing it remains the same. The programmer thus interacts with a changing group of sensor nodes through a simple, uniquely addressable, object interface.

The programmer's view of an application written in Enviro-Track is depicted in Figure 1. Sensors which detect certain user-defined entities in the physical environment form groups, one around each entity. A network abstraction layer associates a context label with each such group to represent the corresponding tracked entity in the computing system. Context labels can be thought of as logical addresses of virtual hosts (contexts) which follow the external tracked entity around in the physical environment. In the following, we use contexts and context labels interchangeably. Objects can be attached to context labels to perform context-specific computation. These attached objects are called tracking objects. They are executed on the sensor group of the context label. Since the actual location of the tracking object is the nodes in the physical vicinity of the target, the object can perform local sensing and actuation to interact directly with the target's locale. For completeness, EnviroTrack also supports conventional static objects that are not attached to context labels.