Читать книгу Position, Navigation, and Timing Technologies in the 21st Century - Группа авторов - Страница 47

Sonic waves are mechanical vibrations transmitted over a solid, liquid, or gaseous medium. The sonic waves produced by vibrations below and above the threshold of human hearing are known as infrasonic and ultrasonic waves, respectively. There are indoor localization solutions that propose the use of ultrasonic range finders and sonars [21]. The relative distance between two devices can be estimated from ToA measurements (see Section 37.5.1.2) of ultrasound pulses from the emitter to the receiver, and thus ultrasound signals can be used to estimate the position of the emitter tags from the receivers. Typical ultrasound systems operate in the low‐frequency band compared to the other RF signaling technologies. In contrast to RF waves, the ultrasound ToA operating range is 10 m or less due to the specific decay profile of the airborne acoustic channel. Doubling the distance causes the signal’s sound pressure level to attenuate by 6 dB due to radial intensity attenuation and absorption, which translates to an inverse quadratic attenuation in 3D space. In general, ultrasound signals are unable to penetrate walls, and they reflect off most indoor obstructions (furniture, people), resulting in echoes that can lead to localization inaccuracies. It has also been observed that high levels of ambient noise prevent accurate detection of the sonic signal; co‐interference caused by the presence of multiple sonic emitters in the environment also leads to errors. Variations in the speed of sound over air are another challenge: for instance, temperature variations are known to affect the speed of sound in air [22]. Therefore, sonic‐wave‐based systems cannot be used in environments with frequent and drastic temperature changes.