Testing of location systems using WiFi in indoor environments
GUENTHER RETSCHER, ESMOND MOK
A further interesting result was that the positioning accuracy was quite good in the lobby of staircases (see Figure 4) where no access points are located and the average signal strength values are quite low. Generally, more than 10 access points could be seen and the average signal strength was around -40 to -80 dB along the corridor. However, if the notebook computer was placed at the lobby, then the signal strength dropped quite significantly by more than 20 dB. The achieved positioning accuracy, however, was in the range of ± 1.3 to 3.2 m.
In conclusion it can be said that an average positioning accuracy of around 3.9 m could be achieved in core BC and about 5.3 m in core PQ in indoor area in the performance tests at the Hong Kong Polytechnic University. Thereby the minimum number of used access points was always 5. Basically the results could confirm the achievable positioning accuracies using the Ekahau system obatined by other tests reported in the literature (see e.g. Teuber and Eisfeller, 2006).
Tests of the WiFi Positioning System ‘ipos’ at the Vienna University of Technology
It is a software platform as a basis for the realization of LBS applications. It consists of an efficient, freely parameterizable framework, which is suitable for multiple application architectures. Thereby signal strength measurements are performed on user terminals, while evaluations and visualizations can take place if necessary on user terminals. The developed positioning system “ipos” makes use of a standard WiFi infrastructure and no modification of the hardware is required. In a study the performance and the achievable positioning accuracies of the positioning system “ipos” have been tested. This study was conducted in cooperation between the Vienna University of Technology and IMST GmbH. The tests were performed in a localization test bed in an office building of IMST (see Retscher et al., 2006). With seven access points an area of over 1500 m2 is covered and the tests have been performed in an area half of the total covered size. It could be seen that it is possible to localize a user in the test bed with an accuracy of around 3 metres.
Further system testing was performed at an office building of the Vienna University of Technology where our institute is located. For this purpose a cooperation with the German company IMST GmbH was established and they provided the indoor location system ‘ipos’. First test results are shown in Figure 5. Figure 5 on the top shows the location of the calibrated points for a first system test on the 3rd floor of our office building and Figure 5 bottom the location test performed by students in our Practical Course on Location-based services moving along the corridor. Due to the small number of calibrated points and the location of the access points the trajectory of the moving user could be obtained with a standard deviation of about ± 3 to 5 m. Using the knowledge of the building model the trajectory can be matched to the corridor in a post processing step.
As the WiFi positioning systems usually provide only location capability in two dimensions, the augmentation of the indoor location system with a barometric pressure sensor for direct observation of the altitude of the user was also investigated. For that purpose the Vaisala PTB 220 pressure sensor was employed for the direct observation of the altitude of the user. This study was conducted in our research project NAVIO (Pedestrian Navigation for Combined Indoor/outdoor Environments) and the results are presented in Retscher (2005) and Retscher and Thienelt (2006). In the study it could be seen that we are able to determine the correct floor of a user in a multi-storey building. The maximum deviation of the determined height in our office building was less than ± 1 m for over 90 % of the oberservations.