Localization and Exploration for Intelligent Systems (LOPSI)

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RF-based Local Positioning Systems The LOPSI group uses the propagation of radiofrequency (RF) signals for localization in large indoor spaces (like a whole building). RF-LPS might be built with wireless communication technology (Wifi, Bluetooth or Zigbee), or by using dedicated beacons, notably ultrawideband radio (UWB) or radio-frequency identification (RFID) tags, placed in previously known locations. Unlike ultrasound, RF signals propagate through solid obstacles (walls), so a wide area in a building can be covered with a moderate number of radio beacons. Conceptual design of an RF-based LPS, showing the beacon emissions with yellow spheres. In most cases, direct range-based multilateration positioning is not viable, given the high propagation speed of RF and the inability of the hardware for accurate time-of-flight measurements. Instead, location has to be inferred indirectly from the received signal strength (RSS). The experimental dependence of the RF signal strength with the distance from beacon to receiver has a large variance (blue points: individual measurements; red lines: mean and standard deviation of the RSS). In indoor environments, the RF signal strength depends not only on the range from emitter to receiver, but is also disturbed by phenomena like multipath propagation, interference, blocking caused by obstacles and people, etc. To achieve high accuracy positioning in spite of this difficulty, we use Bayesian methods to combine the inherently noisy RSS measurements. We have set up an RFID-based localization system with 70 RFID tags distributed in our building floor, about 1600 square meters. The error we obtain in static localization is under 3 m for 90% of position estimations, and the median error was 1.5 m. RFID tags (left) and reader (right) manufactured by RFCode, used in experimentation for RF-LPS by the LOPSI group. Experimental positioning results obtained with our RFID-LPS system: individual position estimations are shown as red dots, along with the confidence ellipses for 90% of the position estimations. Besides RFID technnology, the LOPSI group also utilizes ultrawideband radio (UWB) for indoor localization.