Abstract
Indoor location and positioning systems (ILPS) are used to locate and track people, as well as mobile and/or connected targets, such as robots or smartphones, not only inside buildings with a lack of global navigation satellite systems (GNSS) signals but also in constrained outdoor situations with reduced coverage. Indoor positioning applications and their interest are growing in certain environments, such as commercial centers, airports, hospitals or factories. Several sensory technologies have already been applied to indoor positioning systems, where ultrasounds are a common solution due to its low cost and simplicity. This work proposes a 3D ultrasonic local positioning system (ULPS), based on a set of three asynchronous ultrasonic beacon units, capable of transmitting coded signals independently, and on a 3D mobile receiver prototype. The proposal is based on the aforementioned beacon unit, which consists of five ultrasonic transmitters oriented towards the same coverage area and has already been proven in 2D positioning by applying hyperbolic trilateration. Since there are three beacon units available, the final position is obtained by merging the partial results from each unit, implementing a minimum likelihood estimation (MLE) fusion algorithm. The approach has been characterized, and experimentally verified, trying to maximize the coverage zone, at least for typical sizes in most common public rooms and halls. The proposal has achieved a positioning accuracy below decimeters for 90% of the cases in the zone where the three ultrasonic beacon units are available, whereas these accuracies can degrade above decimeters according to whether the coverage from one or more beacon units is missing. The experimental workspace covers a large volume, where tests have been carried out at points placed in two different horizontal planes.
Highlights
In recent decades, indoor positioning systems have spread worldwide due to the huge number of contexts and smart applications where they can be applied, such as personalized healthcare, safety care, intelligent monitoring, tracking, context-aware and location-based services, etc
Since only the beacons from one ultrasonic local positioning system (ULPS) are available, the results present some limitations on coverage and accuracy, for the perpendicular direction to the plane where the beacons are installed
It is worth noting that, in general terms, acoustic local positioning systems (LPSs) present a performance that may strongly depend on the environment complexity, where multi-path effect or non line-of-sight (NLoS) situations can rapidly degrade the correct estimation of TOAs or TDOAs, implying higher positioning errors
Summary
Indoor positioning systems have spread worldwide due to the huge number of contexts and smart applications where they can be applied, such as personalized healthcare, safety care, intelligent monitoring, tracking, context-aware and location-based services, etc. The technology choice depends on some parameters: the requested accuracy, the cost of the system, the final application, or the positioning algorithm involved, among others [4,5] In this way, recently, some commercial systems based on Ultra-Wide Band (UWB) signals have successfully achieved positioning errors in the range of decimeters [6,7], with an operation mode based on the determination of times-of-arrival and measuring distances from the LoS path. This definition of this ULPS becomes suitable for 3D environments, and it is partially based on a previous system called LOCATE-US [11,24] It proposes a deployment scheme of the beacons to minimize positioning errors in the common coverage area, whereas the encoding applied to transmissions allow the simultaneous determination of difference-times-of-arrival without synchronization between beacons and receivers.
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