In this paper we present a wearable positioning system that requires less than 10 mW to operate. The system consists of an infrastructure of ultrasonic transmitting devices, and a receiver device on the wearable. The receiver comprises an ultrasonic pick-up, an op-amp, and a PIC. The PIC implements a particle filter for estimating X and Y positions, which is robust in an environment with limited precision. We have modified the traditional particle filter in order to operate on a micro-controller with limited memory capabilities. The number of particles is only limited by the amount of memory on the PIC; we only use 5% of the PIC's computational power.