## 3 Indoor Positioning Systems  
The technologies used in current positioning systems can be organized into two main categories: outdoor and indoor. In outdoor environments, several navigation systems are already well established and widely used, providing precise location and navigation services with error margins getting smaller and closer to zero [40]. These include Global Positioning System (GPS), Global Navigation Satellite System (GLONASS), and BeiDou Navigation Satellite System (BDS). However, in the indoor environment, the accuracy of satellite-based positioning decreases markedly due to signal losses when colliding with building structures, which causes the effects of multiple paths and delays in information delivery, thus not meeting the requirements of a reliable service location [41]. Indoor environments are where people spend about 80% of their lives and need to have reliable localization and navigation services for their occupants. Traditionally, indoor positioning systems (IPSs) can be considered systems that work continuously and in real time to provide locations of humans in indoor areas [19]. IPSs can be used for many activities, for example, detecting and tracking people, assisting visually impaired people in their daily activities, and facilitating medical monitoring. Some public places, such as parks, museums, and shopping centers, need indoor positioning services, for example, to provide indoor navigation for visually impaired people, and to assist tourists and visitors in finding their destination [19; 42]. In recent years, indoor positioning technologies and techniques have gained prominence, allowing the construction of systems for countless model scenarios [9; 42; 43]. A wide variety of technologies have been proposed, which is why IPSs can be categorized in several ways according to various criteria. For example, it is possible to classify them based on their architecture or on the way they determine the positions of indoor environments [11]. Indoor positioning systems classified based on their architecture can be divided into three classes: (1) self-positioning, (2) infrastructure positioning, and (3) assisted by self-directed infrastructure. In self-positioning architecture, devices determine their locations by themselves. In infrastructure positioning architecture, device positions are estimated using the locations of devices launched in the environment. In architecture assisted by self-directed infrastructure, an external system calculates the position and sends it to the tracked user in response to a request. Indoor positioning systems classified based on the way they determine their most prevalent technology identifies the positions of indoor environments. IPSs mainly employ (1) radio frequency (RF), (2) inertial, (3) audible and non-audible sound, (4) light-based, and (5) vision-based technologies. In this work, the indoor positioning systems were initially classified into two groups: based or not based on radio frequency signals. From these two classes, the systems follow an organization based on the technologies used to determine positions in indoor environments. Five subgroups were defined to indicate the predominant technologies used: radio frequency-based, inertial, sound, light-based, and computer vision-based systems. There is a sixth type, hybrid, which unites two or more technological approaches. A complete classification of IPSs is shown in Figure 1. Hybrid systems are not represented in this figure, as there are many possible combinations of IPSs that can form a hybrid system, making a general classification unfeasible. In this work, we focus only on hybrid systems that can be scaled for navigation and indoor location applications for the visually impaired. It is essential to know the performance and limitations of various options to select a suitable IPS, specifically for the visually impaired. Accuracy (i.e., position error) is usually a critical performance criterion for IPSs.