Commission 4 'Positioning & Applications' of the International Association of Geodesy WPS is a 100% pure software-based indoor positioning system which uses the signal strength of WLAN (Wireless Local Area Networks) infrastructure transmission from/to WLAN access points to determine the position of the user equipped with mobile computing devices such as Personal Digital Assistants (PDA), laptop computers and so on. In a previous paper (Wang, 2003), the authors gave a detailed description of their WPS system, including hardware, software and system architecture. Meanwhile, based on the WPS test bed, several basic experiments were conducted: (a) to determine the stability of the 2.4GHz WLAN infrastructure radio signal strength, (b) reliability experiment of the 2.4 GHz WLAN infrastructure radio Ssgnal, (c) verification of the empirical model and the effect of Geometry of Distribution (GOD), and (d) wall penetration loss experiment. From the results of these experiments and data analysis it was concluded that a wireless access point-based indoor positioning system is feasible. Experimental results show a positioning accuracy of 1-3m. In order to improve the stability and reliability of the WPS system the authors propose two new methods. Thedifferential approach requires setting up a fixed differential correction base station in the same environment to compensate for the influence of susceptible radio frequency on the user device. In addition, from an analysis of positioning measurements, the authors determined that, to some extent, signal strength at one fixed position is having a trend of weakness, which means that the signal strength value always weakens when it is affected by a different environment factor. So, based on this observation, the authors propose a minimal signal strength value algorithm.
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