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G. Retscher, Q. Fu:
"RFID Trilateration for Indoor and Outdoor Positioning";
Talk: European Navigation Conference - ENC-GNSS 2008 Toulouse, Toulouse, France; 2008-04-22 - 2008-04-25; in: "Toulouse Space Show ´08 - European Navigation Conference ENC-GNSS 2008", (2008), 10 pages.

Alternative location methods for absolute positioning in areas where no GNSS position determination is possible due to obstruction of the satellite signals are needed in mobile positioning. Active RFID (Radio Frequency Identification) can be used also for position determination, although the system was not only developed for positioning and tracking but mainly for identification of objects. Using RFID in positioning, different approaches can be distinguished, i.e., cell-based positioning if the RFID tags are installed at active landmarks (i.e., known locations) in the surroundings, trilateration if ranges to the RFID tags are deducted from received signal strength (RSS in RFID terms) values and location fingerprinting where the measured signal power levels are used directly to obtain a position fix. Using Cell of Origin (CoO) the achievable positioning accuracy depends on the size of the cell and is therefore usually several metres up to 10´s of metres using long range RFID equipment. Higher positioning accuracies can be obtained using trilateration and fingerprinting. In this paper the use of trilateration is investigated in an office building of the Vienna University of Technology and its surroundings.

The received signal strength from the RFID tag at the reader depends also on the distance between the tag and the reader. To convert the measured signal strength into a range a conversion model has to be deduced. The conversion of the signal strength to the distance can be performed using a radio wave propagation model. That is an empirical mathematical formulation for the characterization of radio wave propagation as a function of frequency, distance and other conditions. Such models typically predict the path loss along a link or the effective coverage area of a transmitter. For that purpose signal strength observations have been performed along a known baseline and at a regular grid in a building and in urban environment. For the signal strength to distance conversion two different models are introduced and described in the paper. In the first model a logarithmic relationship between the signal strength and the distance is formulated, whereas in the second model a linear regression using a polynomial function is used. The models are employed for a signal strenght to distance conversion in short range from the tags in indoor environment and its surroundings.

In practical tests performed at the Vienna University of Technology the conversion of the signal strength to a distance has been tested. The tests have shown that active RFID is a meaningful method for indoor positioning. It can be also employed for positioning in the transistion zones between indoor to outdoor and in urban environment as well as in combination with other methods such as WiFi, UWB or GNSS. From the test results it could be seen that the mean of the residuals is larger using the logarithmic model than for the simple polynomial fit for the signal strength to distance conversion in trilateration. For this reason, the simple polynomial model provides a more accurate fit to the distance data as the logarithmic model. The range to the RFID tag is then obtained with a standard deviation of bettter than half a metre and therefore the location of the mobile user can be determined with a standard deviation of around 2 m.

Project Head Georg Gartner:
Ubiquitäre Kartographie für Fußgängernavigation