Tag-compass: Determining the spatial direction of an object with small dimensions

Identifying an object's spatial direction (or orientation) plays a fundamental role in a variety of applications, such as automatic assembly, indoor navigation, and robot driving. In this paper, we design a fine-grained direction finding system called Tag-Compass that attaches a single tag to an object (whose size may be small) and identifies the tagged object's orientation by determining the spatial direction of the tag. We exploit the polarization properties of the RF waves used in the communications between an RFID reader and the tag on the object. Polarization mismatch between the tag and the reader's antenna affects the received signal strength at the reader. From the measured signal strength values, we are able to deduce the tag's direction through a series of transformations and deviation minimization. We propose a system design for Tag-Compass and implement a prototype. We evaluate the performance of TagCompass through extensive experiments using the prototype. The experimental results show that Tag-Compass provides accurate direction estimates with a median error of just 2.5° when the tag's position is known and a median error of 3.8° when the tag's position is unknown.