Cell phone locator system holds promise for new 911 service

Researchers at MIT and TruePosition Inc. have contributed to an antenna design that helps locate cell phone users in an emergency.

Researchers at MIT and TruePosition Inc. have contributed to an antenna design that helps locate cell phone users in an emergency, but US government and industry delays in instituting enhanced emergency systems mean that it probably won’t be in use for some time.

TruePosition’s network-based system, as well as other existing technologies based on satellites in the global positioning system (GPS), would allow the long-awaited Enhanced 911 (E911) to become a reality.

As of October 2001, carriers were supposed to be capable of pinpointing cell phone callers to within a few hundred feet or better. The US Federal Communications Commission (FCC) has given the industry four more years to figure out the technology.

Residential phones are mapped to co-ordinates so emergency services can pinpoint them to the nearest intersection, but when a cell phone user makes a call, a radio signal is sent to the nearest cell tower, which could be up to 25 miles from the caller.

FCC rules adopted in 1996 were divided into two phases. Phase I, which has gone into effect, requires carriers to provide Public Safety Answering Points with the telephone number of the originator of a 911 call and the location of the cell site or base station receiving a 911 call.

Phase II, which must be complete by December 31, 2005, requires wireless carriers to provide more precise Automatic Location Identification. Each carrier’s method of achieving E911 capability varies depending on whether they are employing a handset or network-based system.

There are two possible ways to locate cell phone users. In the network-based system, a distress call from a wireless phone triggers modules at the three or more nearby cellular antennas. Each module records the time the caller’s signal reached the antenna, and the location of the caller is determined by ‘triangulating’ the caller’s distance from the receivers.

The second possibility requires embedding a GPS receiver inside each individual phone, or handset.

Working with scientist Kevin Kolodziejski of TruePosition Inc Rogers is testing antenna arrays for measuring the angle of arrival of the phone’s signal at the cell site.

The addition of the angle of arrival to the time of arrival helps improve the location accuracy.

‘The electrical characteristics of the antennas need to be reproducible between units, and they need to produce the same angle of arrival for a wide range of different signal polarisations, so we are assessing these characteristics in the anechoic chamber,’ Kolodziejski said.

TruePosition recently field-tested its wireless location system. Of the 2,300 calls made within the deployment area during a three-day period across a variety of calling scenarios, 67 percent were located with an accuracy of 80 metres or better, while 95 percent of the calls were located with an accuracy of 190 metres or better.

The FCC will require a caller’s location to be calculated to within 300 metres 95 percent of the time, and to within 100 metres 67 percent of the time.

The advantage of TruePosition’s network-based system is that it can be retrofitted onto existing towers and works with existing phones.

In wide-open places such as alongside interstate highways, cell phones can be located most accurately with the GPS system. In heavily populated cities, the network solution is better.