where A is the received signal strength at 1 m, n is a signal propagation constant depending mainly on the environment, and d is the distance from the sender [19]. The landmark that has the lowest value of d represents the user's current location. This information has a twofold function: (i) it is used locally to speed up the image processing algorithm, and (ii) it is sent to the service running on the processing center, which makes it available for any interested service. Fig. 2 shows the main steps of the user localization process.