Joint estimation of indoor position and orientation from RF signal strength measurements

In personal indoor guidance and navigation applications, it is desirable to determine both the user´s position and spatial orientation, since this enables to provide him with directions in a more natural way. The common method for estimation of orientation uses the magnetic compass included in numerous portable devices; this approach becomes unreliable when the device´s own orientation relative to the user is unknown, or if the environment contains metallic structures which disturb the magnetic field. In this work we propose an alternative approach for estimation of personal position and orientation which requires no additional sensors and is based solely in the attenuation of radiofrequency signal strength (RSS) introduced by the user´s own body. The physical system consists in a set of emitting RF nodes located at known positions in the environment, and two RF receivers placed in the front and back parts of the user´s body (alternatively, a single receiver with two antennas). We show how the position of the user can be inferred from the average received signal strength, while his orientation is estimated from the difference of signal strengths from the two receivers, by adapting two commonly used techniques in indoor localization: least-squares minimization and Bayesian filters. We have validated experimentally both techniques with an RFID-based localization system arranged in a typical building, showing that orientation can be determined with a mean error of 0.3 radians, sufficient for most applications.