Finding your way inside Part 1: GPS + RFID

13 Jul 2009

Most of the developed systems for pedestrian navigation, and guidance services rely on GNSS, sometimes in combination with other sensors and positioning methods. Our approach in the Ubiquitous Cartography for Pedestrian Navigation (UCPNAVI) project uses active radio-frequency identification (RFID) in combination with GNSS and inertial navigation systems (INS) to navigate areas such as urban canyons and indoors, where satellite positioning does not fulfill pedestrian requirements.

RFID transmits data via radio waves without line-of-sight contact. Nowadays, RFID can also be used for positioning, because the location estimation can be based on received signal strength indication (RSSI), which measures the power present in a received radio signal. The receiver can compute its position using various methods based on RSSI (for example, a range-based positioning system based on trilateration, modeling of indoor positioning systems based on location fingerprinting).

Positioning with Active RFID

Through RFID technology, data can be transmitted from RFID tags to a reader via radio waves without line-of-sight contact. The transmitted data might include the ID and the information of the position of the RFID tags. Our approach uses only one reader and a large number of active tags. The moving user or object carries a portable RFID reader (a reader in the form of a PCMCIA card that can be plugged into a laptop), and the stationary objects in the surrounding environment have RFID tags. If the user moves inside the surrounding of an RFID tag, the tag\'s ID, signal strength RSSI, known location, and time can be obtained. User position can be calculated with various methods based on RSSI and known location of the tag with a certain ID, and the measured data recorded in a database. Cell-based positioning, trilateration, or RFID location fingerprinting will be employed, depending on user environment.

Cell-based positioning algorithms determine the location of the user in a cell around the RFID tag with a size defined by the maximum range of the RFID signals. The achievable positioning accuracies depend on the size of the cell, - up to 20 meters using our long-range RFID equipment. Therefore, this method only works well in areas where accuracy is not that important, such as the urban outdoor environment.

In a test along a path from the Karls platz subway station to the one of the offices in a building of the Vienna University of Technology, we applied cell-based positioning in outdoor areas as an alternative to GPS positioning. Three RFID tags were installed at the entrance of the subway station, an indoor area. Along the outdoor road area between the station and the office building, we installed seven tags, with three more tags at the building\'s entrance, a transition zone.

Cell-Based. As the accuracy of cell-based positioning generally depends on the size of the distinguishable cells, the achievable positioning accuracies might not be sufficient for indoor areas. Ranges from the RFID tag locations have been achieved up to around 20 meters. Indoor positioning, for the most part, requires higher positioning accuracies. Therefore we installed several RFID tags in the transition zones between outdoor and indoor, to locate the user with a higher precision using trilateration.

Trilateration. The second method of determining the relative positions of moving objects (for example, a user with a portable RFID reader) uses the known location of two or more stationary objects or tags, and the range between the mobile user and the stationary objects.

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