## 1 Introduction  
Indoor localization is an ongoing field of research with new realizations of hardware, position estimation algorithms, calibration methods, and applications being developed every year. In the past decade, local positioning systems (LPS) based on ultrasound (US) have been established which are mostly intended for locating moving devices and persons. However, locating of numerous quasi-static devices integrated in a network is also important where US locating with high accuracy offers significant advantages for these systems. A wireless sensor network (WSN) with numerous nodes can be used as an example. Many applications dedicated to WSN can be significantly improved by node locating, e.g., network integration of nodes, supply node locations to application programs, supervising locations with respect to accidentally dislocating, detecting faking of node locations, and automatic device setup and security aspects in building networks [1,2]. Several design concepts for US locating systems have been presented. The concepts of these systems are influenced by a wide range of parameters, which will be briefly discussed in the following. For a more thorough discussion of this topic, the interested reader is referred to [3] where the most important properties are summarized in this section.  
In order to find device locations, systems perform time-of-flight (ToF) or time-difference-of-arrival (TDoA) measurements with well-known beacon positions. The benefit of using a TDoA system is that synchronization between the transmitting and receiving devices is not required thus reducing system costs by avoiding a second communication channel, e.g., by radio frequency transmission or using an infrared link. The ToF and TDoA measurements can then be related to distance and distance-difference measurements using the speed of sound which is 343.2 m/s in dry air at 20 \(\,\)C (68 \(\,\)F). Locations can then be obtained by tri- or multilateration for the ToF and TDoA case respectively. As the speed of sound changes with ~0.6 m/s/\(\,\)C a LPS aiming for high accuracy needs some method for compensation of changes in the speed of sound, e.g., a temperature measurement or the use of a reference distance.  
Another very basic design decision is whether the located device is transmitting an US signal and the beacons are receiving the signal or vice-versa. This is a crucial design decision when considering the locating of numerous devices. If the localized devices are actively transmitting US pulses, a media access protocol is required where code division multiple access method (CDMA) and time division multiple access method (TDMA) are the most common ones. On average scalability of such systems is rather limited. If the localized devices are passive and the beacons are transmitting, a single locating sequence allows locating of all devices in parallel. The locating rate depends only on how fast the sequence can be repeated where collision avoidance of the US signals again can be implemented by CDMA or TDMA. An advantage of passive locating is privacy of located devices which is sometimes claimed and which is given in a passive system because located devices do not emit any information which could be traced.  
Common to all LPS systems is that the positions of the beacons have to be determined. This process is called calibration of the LPS and can either be performed manually by establishing a system of coordinates and measuring the beacon positions or by an automatic calibration algorithm. Final accuracy of an LPS is always a combination of the accuracy of the localization and the accuracy of the beacon positions. If a quantitative judgment on the accuracy of the system is desired care must be taken that the individual systematic and random components can be assessed individually.