Wireless 9-1-1

Shelby County 9-1-1
Wireless 9-1-1

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Cell phone calls to 9-1-1 have come a long way since the days when we had no information about you or your actual location.

As of May 2005 we have Phase II 9-1-1 service with all cell phone companies (carriers).

Scroll down for a description of how your location is determined.

Want to Learn More?

Federal Communications Commission - Many good links and resources

General Overview of 9-1-1 - at FCC site

Alabama Wireless Board
This group administers Wireless 9-1-1 in Alabama

Alabama Chapter of NENA
Our professional association, many good links. ALNENA Wireless Page

National NENA - Wireless Section

With Phase II phones we get the latitude - longitude of the caller within the requirements of the FCC rules.

Accuracy Standards: The FCC adopted the following revised standards for Phase II location accuracy and reliability:

  • For handset-based solutions: 50 meters for 67 percent of calls, 150 meters for 95 percent of calls;

  • For network-based solutions: 100 meters for 67 percent of calls, 300 meters for 95 percent of calls.

Phase II is is not available in all areas when you travel...

There will be errors in locations as noted above...

Be prepared to give a location to the 9-1-1 Dispatcher!
Look around for landmarks, street signs, mile markers, addresses on mailboxes, store names, or anything else that will help us locate the emergency.

Simplified Description of Location Determination Technologies

Handset or GPS Based Location Network or Triangulation Location
The GPS system consists of 24 earth-orbiting satellites. A chip embedded in the wireless phone receives signals from three or more satellites and calculates the location as described below. 
1 2 3
A Chip-equipped phone measures the amount of time it takes for a satellite's radio signal to reach the phone.

Since the speed of radio signals is known, the distance from the satellite can be calculated.

The phone then measures the time for a second satellite's signal to reach the phone.

The phone is at one of the two points where the first and second circle overlap.

When the phone can receive a signal from three or more satellites the location becomes precise.

The location must then be transmitted to the 9-1-1 center and displayed on a map so the Dispatcher can determine the proper responders.


The towers can be existing cell-phone towers, water tanks, tall buildings, or other locations where receiving antennas can be mounted.
Using the known speed of radio signals, the distance from receivers can be calculated. It takes at least three and preferably four to get a good location.

Shelby County 9-1-1, 1004 County Services Dr, Pelham, Alabama 35124 (205)439-6911
Copyright 2012