GPS + RFID

March 1, 2009 By: Guenther Retscher, Qing Fu GPS World

Another Look Indoors

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).

We have employed three different methods in our research: cell-based positioning, trilateration using ranges to the surrounding RFID tags deduced from received signal-strength measurements, and RFID location fingerprinting. In our experiment for the positioning of a pedestrian from the subway station Karlsplatz to one of the offices in a building of the Vienna University of Technology, we combined all three methods.

The results showed that these approaches using RFID are suitable to navigate the user. Currently, the user must manually select the positioning method when moving from one environment to another. To automate the selection process, we propose to integrate the three methods of positioning into one intelligent software package and to employ a knowledge-based system for automatic selection. This will enable continuous and automatic user positioning in complex environments.

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. The position is a function f as shown by:

position = f (ID, RSSI, known location) at a certain time t. 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 enviroment.

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 the test shown in FIGURE 1 along the path from subway station Karlsplatz to the university building, 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 university building (TU Vienna), we installed seven tags, with three more tags at the building's entrance, a transition zone. Each circle indicates a different cell.