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Position and Orientation for Distributed Sensors: The PODIS Network

In order for the information provided by networks of unattended ground sensors (UGS) to be of use to the tactical planner, the location of each sensor must be obtained. Sensor localization is typically achieved by careful hand-emplacement, or facilitated by anchor nodes whose positions are precisely known. This paper describes a new sensor node implementation designed to meet the growing need for UGS networks that can self-localize without the use of anchor nodes in a GPS-denied environment. The Position and Orientation for Distributed Sensors (PODIS) application facilitates traditional threat identification and tracking by self-localiz ing a network of senor nodes. The PODIS node hardware consists of a COTS acoustic signal processor, a four element microphone array and a low power, networked radio for remote sensing with multi-hop routing capability. PODIS utilizes a messaging protocol that facilitates system calibration. While the PODIS application is independent of the ranging medium, the prototype network is implemented using time of arrival estimates of acoustic ranging signals. In this paper we describe the applicability of electronically generated pseudonoise calibration signals. We describe a two-step localization procedure that partitions the network calibration into relative and absolute solutions. We describe the typical anchor nodes as a special case of aimpoints, prior location information about a subset of nodes with associated uncertainty. We first compute relative node locations and orientations using time and direction of arrival estimates, then use aimpoint information provided by the user at deployment to rotate and translate the network onto an absolute frame of reference. We provide a simple channel model to exercise the system in the simulation domain, with results for self-localizing networks of various geometries and levels of connectivity. Finally, simulations of typical network deployments predict the PODIS localization accuracy.