Ultrasound positioning based on time-of-flight and signal strength

Ultrasound positioning systems for indoor use can be distinguished by what kind of information they extract from the received signal. Time-of-flight (TOF) is measured with reference to a radio signal to get ultrasound time-of- arrival (TOA). Line-of-sight to three or more nodes is required for 3D positioning. Accuracy is in the cm or sub-cm range. A radio-free alternative can be made if time-difference-of-arrival (TDOA) is measured instead. Coding of the pulse is often used in order to allow simultaneous transmission from multiple transmitters. The simplest received signal strength (RSS) systems are binary and will just determine if the ultrasound signal can be detected or not. This is used on its own for room-level positioning. Another important application is in assisting RSS-based RF-systems such as WLAN positioning. The ultrasound RSS-system helps reduce the number of large errors (5-10 m) of the WLAN-system. Such systems have recently been deployed world-wide today by companies like Sonitor and Aeroscout. It has also just been demonstrated that RSS-based ultrasound positioning can be done with accuracies in the 10 cm range. This parallels the ubiquitous RF-based RSS systems and requires a propagation model. For ultrasound the model involves spherical spreading and absorption. There are also hybrid systems where the entire echo structure in the time history of the received signal is analyzed. Both the amplitude and time information are used in order to obtain a position of the node using only a single transmitter.