Spatial localizers provide a reference coordinate system and make the tracking of various objects in 3D space feasible. A number of different spatial localizers are currently available. Several factors that determine the suitability of a position sensor for a specific clinical application are accuracy, ease of use, and robustness of performance when used in a clinical environment. In this paper, we present a new and low-cost sensor with performance unaffected by a the materials present in the operating environment. This new spatial localizer consists of a flexible tape with a number of fiber optic sensor along its length. The main idea is that we can obtain the position and orientation of the end of the tape with respect to its base. The end and base of the tape are locations along its length determined by the physical location of the fiber optic sensors. Using this tape, we tracked an ultrasound probe and formed 3D US data sets. In order to validate the geometric accuracy of those 3D data sets, we measured known volumes of water-filled balloons. Our results indicate that we can measure volumes with accuracy between 2-16 percent. Given the fact that the sensor is under further development and refinement, we expect that this sensor could be an accurate, cost-effective and robust alternative in many medical applications, e.g., image-guided surgery and 3D ultrasound imaging.