Editing Enhanced 911 (section)

===Wireless location===
The billing address associated with a [[cell phone]] is not necessarily considered the location to which emergency responders should be sent, since the device is portable. This means that locating the caller is more difficult, which resulted in the second phase of the Enhanced 911 service (E911 Phase 2), which relates to locating wireless or [[mobile telephone]] devices.

To locate a mobile telephone geographically, there are two general approaches. One is to use some form of [[radiolocation]] from the [[cellular network]]; the other is to use a [[Global Positioning System]] receiver built into the phone itself. Both approaches are described by the [[Radio resource location services protocol]] (LCS protocol).

Radiolocation in [[cellular telephony]] uses [[base station]]s. Most often, this is done through [[triangulation]] between [[radio masts and towers|radio tower]]s. The location of the [[Caller (telecommunications)|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 [[Line-line intersection|intersect]].
*[[Multilateration|Time difference of arrival]] (TDOA) works like [[GPS]] using [[multilateration]], except that it is the networks that determine the time difference and therefore distance from each tower (as with [[seismometer]]s).
*Location signature uses "[[fingerprint]]ing" to store and recall patterns (such as [[Multipath propagation|multipath]]) which mobile phone signals are known to exhibit at different locations in each cell.

The first two depend on a [[Line-of-sight propagation|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 [[T-Mobile US|T-Mobile]] 2G use TDOA.<ref name="fcc_report">{{cite web|url=http://hraunfoss.fcc.gov/edocs_public/attachmatch/DOC-257964A1.pdf|title=FCC Report to Congress on the Deployment of E-911 Phase II Services by Tier III Service Providers|date=April 1, 2005|publisher=[[Federal Communications Commission]]|access-date=2010-11-18}}</ref> [[AT&T Mobility]] initially advocated TDOA, but changed to embedded GPS in 2006 for every GSM or [[UMTS]] voice-capable device due to improved accuracy.

[[Code division multiple access]] (CDMA) networks tend to use handset-based radiolocation technologies, which are technically more similar to radionavigation. GPS is one of those technologies. [[Alltel]], [[Verizon Wireless]], T-Mobile 3G, and [[Sprint PCS]] use [[Assisted GPS]].<ref name="fcc_report" />

Hybrid solutions, needing both the handset and the network include:

*[[Assisted GPS]] ([[wireless]] or television) 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)

Mobile phone users may also have a selection to permit location information to be sent to non-emergency [[phone number]]s or [[data]] networks, so that it can help people who are simply lost or want other [[location-based service]]s. By default, this selection is usually turned off, to protect privacy. In areas such as tunnels and buildings, or anywhere else that GPS is not available or reliable, wireless carriers can deploy enhanced location determination solutions such as Co-Pilot Beacon for CDMA networks and LMU's for GSM networks.

The [[3GPP]] specified protocol for handset geolocation in GSM networks is called [[RRLP|Radio Resource Location Protocol]].