ry, that are operational today or will be in the near future are for example, GPS, Galileo, Satellite Based Augmentation Systems (SBAS), Modernized GPS, Quasi Zenith Satellite System (QZSS) and GLONASS (Global Navigation Satellite System). A mobile station with GNSS measurement capability can operate in an autonomous mode or in an assisted mode, for example, terminal-assisted or terminal-based mode. In autonomous mode a mobile station determines its position based on signals received from GNSS without assistance from the network. In assisted mode, the mobile station receives assistance data from the network. Mobile station may support one or several GNSSs and the assistance data content may vary depending on this capability. The assistance data enables combined usage of satellite signals belonging to different GNSS or simple usage of one GNSS system independently from the other. T-DOA and GNSS techniques are the leading solutions for the communication systems, offering best performance and positioning accuracy [35]. 9.12.3.2 Network Architecture 3GPP specifies a concept called Location Service (LCS) but does not specify any LBS except locating the emergency calls. LCS specifies all the essential elements, functions, interfaces, messages that are deemed necessary to implement position functionality and offer location based services via cellular networks. LCS (Location Services) can be implemented on the GSM/UMTS structure through the addition of network nodes, such as Mobile Location Center (MLC), Location Measurement Unit (LMU), and a number of new interfaces. The term LBS should be distinguished from the LCS, as LCS cover the software and hardware such as network elements and entities necessary to provide LBS, without specifying any location based value added service except emergency call locations [35]. A generic LCS architecture is shown in Figure 9.26. Some brief details of the new element and interfaces are as follows [38,3 39]: GMLC: Gateway Mobile Location Center is the first node in a GSM PLMN (Public Lan