Finding those elusive cell phones
There are two major kinds of cell phone location technologies - the US FCC has different accuracy requirements for them . For “network-based” technologies, accuracy must be within 100 metres for 67% of calls, 300 metres for 95% of calls; for “handset-based” technologies, within 50 metres for 67% of calls, 150 metres for 95% of calls. The remaining 5% of calls are handled on a “best efforts” basis.
“Network-based” refers to, usually, Time Difference of Arrival (TDOA) – a carrier has receivers located at cell towers and determines a cell phone location by processing the signals received from that cell phone, which locates wireless phones by comparing the time it takes a mobile station's radio signal to reach several location measurement units (LMUs) installed at an operator's base stations. One advantage is that the technology works with legacy handsets, but the drawback is that it doesn't always work well in rural areas where there are fewer base stations and towers to measure a signal.
“Handset-based” generally refers to a cell phone with built-in Global Positioning System (GPS) receiver – it calculates its own location by receiving transmissions from at least three GPS satellites.
CDMA operators opted for handset-based location technology based on global position system (GPS) technology using satellites to determine the location of a handset. This offers greater accuracy but can't pinpoint callers indoors — because the signal is too weak — and is ill suited in the urban canyons common to cities.
Most of the GPS technologies currently deployed include a fall back to Advanced Forward Lateral Triangulation (AFLT) or Assisted GPS (A-GPS) technology to achieve better accuracy. Accuracy rules require these handset-based systems to provide accuracy within 150 metres for 95% of the calls and 50 metres for 67% of calls. A-GPS also is used on WCDMA networks.
There is also another technology from Rosum Corporation (www.rosum.com) who is exploring TV-GPS, a system that combines television signals with global positioning satellite (GPS) technology for tracking assets/people right to places where GPS alone can't go, such as in the high-rise “canyons” of urban centres and even inside buildings and garages.
Location Technologies include:
E-CID - Enhanced Cell-ID (500-1000 metres depending on cell-site density)
Derives additional timing advance and power measurements from the wireless network.
A-GPS - Assisted Global Positioning System (>30 metres)
Uses modified handsets that contain a GPS receiver and a special network server to assist in location calculation.
Uplink Time Difference of Arrival (>50 metres)
Uses low-cost location measurement units installed in the operators' base stations to precisely calculate location using trilateration (aka Lateral Triangulation).
AOA - Angle of Arrival (100-500 metres)
Uses two or more antennas with multiple element arrays, allowing the exact location of each AOA element to be calculated precisely.
Links: < www.gps-practice-and-fun.com/a-gps.html >, < www.911dispatch.com/911/wireless911.html >.
In the States the FBI can also use credit card transactions in REAL TIME. Cash remains king (or queen).
Suspicious people can always have their cell GPS receiver disconnected - a -minute operation at any cell service centre. Emergency calls can also be made when the SIM is removed or a cell is not in their providers area.