# Evaluation of Multi-Sensor Fusion Methods for Ultrasonic Indoor Positioning  
[PERSON]  
1 EuE Laboratory, Faculty of Sciences of Monastir, National Engineer School of Tunis, University of Tunis El Manar, B.P. 37, Le Belevedere, Tunis 1002, Tunisia; [EMAIL_ADDRESS]  
2 Electronics Department, University of Alcala, E-28805 Alcala de Henares, Spain;  
[EMAIL_ADDRESS] (A.H.); [EMAIL_ADDRESS] (J.M.V.) 3 EuE Laboratory, Faculty of Sciences of Monastir, University of Monastir, Monastir 5019, Tunisia;  
[EMAIL_ADDRESS]  
Tunis 1002, Tunisia;  
4 Correspondence: [EMAIL_ADDRESS]  
[PERSON]  
2 Electronics Department, University of Alcala, E-28805 Alcala de Henares, Spain;  
[EMAIL_ADDRESS] (A.H.); [EMAIL_ADDRESS] (J.M.V.) tunis 1002, Tunisia;  
supervision, public safety, etc. [2]. Different previous works have already addressed these tasks in extended indoor environments, such as train/bus stations, airports, hospitals, universities, or commercial centres. In this context, ultrasonic positioning systems have been considered as an interesting technology for indoor applications, mainly due to certain advantages, such as suitable accuracy, low power and low cost [3,4]. Thus, some 3D ultrasonic positioning systems have been developed using diverse configurations, as reviewed in [5,6]. They are based on two main approaches: emitters are located at fixed positions while receivers are moving in the environment, and vice-versa. They are often based on trilateration or multi-lateration [3,7, 8, 9, 10], consisting of the determination of times of arrival (TOA) [5] or time differences of arrival (TDOA) [3]; however, it is also possible to find other options, such as those based on angles of arrival (AOA) [9], or hybrid techniques [10]. In certain cases, the availability of multiple position estimates also involves applying a merging technique to obtain a final position [6,10,11]. Concerning the first approach, where the emitters are located at known positions and the receivers are moving in the environment, 3D positioning systems have been developed with diverse configurations, positioning techniques, and arrays of beacons. Some configurations consist of beacons fixed at ceiling corners [11], where three autonomous beacons, synchronized with the receiver, are used for a hybrid method based on AOAs and TOAs; also, the deployment of four beacons at the ceiling corners was introduced in [12]. Other configurations placed a set of beacons in the ceiling, as in [7]. It presented a set of six synchronized beacons in the ceiling, pointing to the centre of the room to measure TDOAs. Moreover, four beacons were synchronized with a fixed microphone in [5], in order to estimate the distances between the emitters and the receiver. In addition, a set of four beacons could be placed in only one plane (or slightly out of the plane to avoid coplanarity) to point to the desired workspace [3,13]. The main constraint in most coplanar structures is the measure of the position in the perpendicular axis to this specific plane. To overcome this constraint, some previous works have deployed beacons in different or parallel planes [5,10,11]. Concerning the second approach, a 3D positioning system was proposed in [14], based on five ultrasonic emitters installed on a mobile user, and five ultrasonic receivers fixed at known locations. This system utilizes a trilateration positioning technique and the extended phase accordance method as a tracking algorithm to measure the distance to the mobile target, as well as a time division multiplexing access (TDMA) communication link, so that a trigger pulse synchronizes the emitters and the receivers. In [15], this proposal was also developed by setting a receiver with four coplanar beacons located perpendicularly to the mobile emitters.