A review of techniques for ultrasonic indoor localization systems

Accurate localization in indoor environments is crucial for the correct operation of location-aware and augmented reality applications, indoor navigation, and inventory management, among others. Magnetic, radiofrequency and inertial navigation systems typically provide room or meter-level accuracy. Despite being the most widely used, they are affected by error drifts or changes in the environment where they operate, and their accuracy is a drawback for certain applications, such as navigation. Optical-based systems provide better accuracy, but they can be expensive, and they are not privacy-oriented. Ultrasonic positioning systems can also give room-level accuracy, as acoustic propagation is contained within the room walls, helping resolve room or floor-level ambiguities of radio systems. They can even achieve centimeter-level accuracy and ensure privacy, while being low cost. These properties highlight acoustics as a versatile technology for different indoor localization applications, as stated by the research published almost over the last three decades. In this work, the operating principles of the different techniques employed by acoustic positioning systems are reviewed, covering narrowband and wideband systems (including the differences between coded and uncoded transmissions), fingerprinting, and the most recent systems based on machine learning.