radar, the Doppler shift is also taken into account, determining velocity rather than location (though it helps determine future location). Real Time Location Systems RTLS using calibrated RTLS, and TDOA, are commercially available. The widely used Global Positioning System (GPS) is based on TOA of signals from satellites at known positions. Mobile phones Radiolocation is also used in cellular telephony via base stations. Most often, this is done through trilateration between radio towers. The location of the Caller or handset can be determined several ways: angle of arrival (AOA) requires at least two towers, locating the caller at the point where the lines along the angles from each tower intersect time difference of arrival (TDOA) resp. time of arrival (TOA) works using multilateration, except that it is the networks that determine the time difference and therefore distance from each tower (as with seismometers) location signature uses "fingerprinting" to store and recall patterns (such as multipath) which mobile phone signals are known to exhibit at different locations in each cell The first two depend on a line-of-sight, which can be difficult or impossible in mountainous terrain or around skyscrapers. Location signatures actually work better in these conditions however. TDMA and GSM networks such as Cingular and T-Mobile use TDOA. CDMA networks such as Verizon Wireless and Sprint PCS tend to use handset-based radiolocation technologies, which are technically more similar to radionavigation. GPS is one of those technologies. Composite solutions, needing both the handset and the network include: assisted GPS (wireless or TV) allows use of GPS even indoors Advanced Forward Link Trilateration (A-FLT) Timing Advance/Network Measurement Report (TA/NMR) Enhanced Observed Time Difference (E-OTD) Initially, the purpose of any of these in mobile phones is so that the public safety answering point (PSAP) which answers calls to an emergency telephone number can know where the caller is and exactly where to send emergency services. This ability is known within the NANP (North America) as wireless enhanced 911. Mobile phone users may have the option to permit the location information gathered to be sent to other phone numbers or data networks, so that it can help people who are simply lost or want other location-based services. By default, this selection is usually turned off, to protect privacy. International regulation Radiolocation service (short: RLS) is – according to Article 1.48 of the International Telecommunication Union's (ITU) Radio Regulations (RR) – defined as "A radiodetermination