Indoor localization using ultrasonic time difference of arrival

Indoor environments like those found in mining and firefighting operations are extremely dynamic and Immediately Dangerous to Life and Health (IDLH), requiring Incident Commanders and supervisors to constantly be aware of the location of their personnel. This paper presents a graduate student project to develop, in the span of one semester, a prototype for indoor localization using readily-available ultrasonic sensors and Time Difference of Arrival (TDOA) techniques to showcase the feasibility and accuracy of such a system. TDOA employs measurements of difference in time between signals arriving at receiver nodes, with the absolute position of the nodes being known. A time difference of arrival between two receiver nodes defines a hyperbola on which the transmitter must be located, and three or more TDOA measurements provide a location fix where the hyperbolas intersect. A system was implemented using one transmitter circuit with six ultrasonic transducers spaced in such a manner as to create omnidirectional coverage, and four receiver circuits converting the presence of an ultrasonic broadcast to a high output. The four receiver outputs were then fed into a microcontroller that time-stamped the signal and solved the issue of needing a common clock. The resulting data, even when using simplified equations, yielded position location accuracy within a foot in both X and Y directions. Some issues were encountered in and around the center lines of the test area. Overall, this prototype proved the feasibility of implementing a simple localization system that provides relatively accurate location results.