time. Finally, hybrid positioning systems combine different methods in an attempt to overcome each individual method's shortcomings. Types of technologies used There is a wide variety of systems concepts and designs to provide real-time locating. Active radio frequency identification (Active RFID) Active radio frequency identification - infrared hybrid (Active RFID-IR) Infrared (IR) Optical locating Low-frequency signpost identification Semi-active radio frequency identification (semi-active RFID) Passive RFID RTLS locating via steerable phased array antennae Radio beacon Ultrasound Identification (US-ID) Ultrasonic ranging (US-RTLS) Ultra-wideband (UWB) Wide-over-narrow band Wireless local area network (WLAN, Wi-Fi) Bluetooth Clustering in noisy ambience Bivalent systems A general model for selection of the best solution for a locating problem has been constructed at the Radboud University of Nijmegen. Many of these references do not comply with the definitions given in international standardization with ISO/IEC 19762-5 and ISO/IEC 24730-1. However, some aspects of real-time performance are served and aspects of locating are addressed in context of absolute coordinates. Ranging and angulating Depending on the physical technology used, at least one and often some combination of ranging and/or angulating methods are used to determine location: Angle of arrival (AoA) Angle of departure (AoD) (e.g., Bluetooth direction finding features a mobile-centric RTLS architecture - see US 7376428 B1) Line of sight (LoS) Time of arrival (ToA) Multilateration (Time difference of arrival) (TDoA) Time of flight (ToF) Two-way ranging (TWR) according to Nanotron's patents Symmetrical double-sided two-way ranging (SDS-TWR) Near-field electromagnetic ranging (NFER) Errors and accuracy Real-time locating is affected by a variety of errors. Many of the major reasons relate to the physics of the locating system, and may not be reduced by improving the technical equipment. None or no direct response Many RTLS systems require direct and clear line of sight visibility. For those systems, where there is no visibility from mobile tags to fixed nodes there will be no result or a non valid result from locating engine. This applies to satellite locating as well as other RTLS systems such as angle of arrival and time of arrival. Fingerprinting is a way to overcome the visibility issue: If the locations in the tracking area contain distinct measurement fingerprints, line of sight is not necessarily needed. For example, if each location contains a unique combination of signal strength readings from transmitters, the location system will function properly. This is true, for example, with some Wi-Fi based RTLS solutions. However, having distinct