basically when you dial 911 your phne becomes a gps beacon... 911 dispatch can pick up the signal and find out where you are.

If they use triangulation, then they just need to receive your cell phone's transmission on 3 cell phone towers and based on the strength of the signal, they can calculate your near exact location

They get what they call a ping, which is a signal bouncing off of one of the towers which is near by where you are. and then they triangulate the signal to locate you.

There is a small GPS unit built into the phone, This unit uses the GPS system to determine you location. When you call the cellular 911 operator this information is sent to the 911 system. Below is an Excerpt from the Nov 19th 2005 Milwaukee Journal that dedscribes the system and it's limitations. Even in counties that will be able to locate cell phone callers to 911, a variety of factors, including the location of wireless towers, tall buildings, bad weather and even sunspots, can affect the system's accuracy, said Richard Tuma, director of emergency preparedness for Waukesha County, the first county in the state to implement the technology. Two technologies, which work together in most areas, are used to notify 911 call centers of the location of cell phone callers. One uses global positioning system technology, in which GPS-equipped phones receive satellite signals and relay the location coordinates to the 911 center receiving the call. The other technology uses three cell towers to "triangulate" the coordinates of the 911 caller. The GPS system is more accurate, Tuma said, and is capable of locating a caller within 50 meters (about 165 feet) 67% of the time. The triangulation technology can locate a caller within 150 meters 67% of the time, he said. Those standards have been mandated by the Federal Communications Commission. But each system has its flaws, Tuma said. To get an accurate GPS location, a caller needs to have a properly equipped cell phone and be standing outside. Being inside any structure, to some degree even a car, can skew the coordinates being picked up from the satellites, he said. "The GPS system needs a line of sight to the satellites," he said. Cloudy, rainy and snowy weather also can interfere with both technologies, and nearby tall buildings can throw off the triangulation system, Tuma said.