Enhanced 911: Encyclopedia II - Enhanced 911 - Wireless Enhanced 911

Enhanced 911 - Wireless Enhanced 911

A second phase of Enhanced 911 service is to allow a wireless or mobile telephone to be located geographically using some form of radiolocation from the cellular network, or by using a Global Positioning System built into the phone itself.

Radiolocation in cellular telephony uses base stations. Most often, this is done through triangulation 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) works like GPS 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.

Hybrid 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)

The purpose of any of these in mobile phones is twofold: first, the wireless system must know to which PSAP it should route the call, and second, the PSAP that answers the call should know where the caller is and exactly where to send emergency services.

Mobile phone users may also have a selection 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.

Adapted from the Wikipedia article "Wireless Enhanced 911", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki