• At Lincoln Lab's Advanced Electromagnetic Systems Antenna Test Range, Alan Rogers (right) associate director and senior research scientist at Haystack, works with scientist Kevin Kolodziejski of TruePosition Inc. in a special chamber that dampens the echo of radio signals. They are testing a prototype cell phone antenna that measures the angle of arrival of the phone's signal, which helps pinpoint the caller's location in an emergency.

    Photo / Donna Coveney

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  • Lincoln Lab technician Tom Alosso monitors the results.

    Photo / Donna Coveney

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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, but 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 Federal Communications Commission (FCC) has given the industry four more years to figure out the technology.


Ironically, many of the nation's 65 million cell phone users first acquired the phones in case they needed to call for assistance on the road or other places lacking pay phones.

Residential phones are mapped to coordinates 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 anywhere from a few blocks 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 I information "is better than nothing, but it doesn't tell the emergency operator at all where you are," said Alan Rogers, associate director and senior research scientist at MIT's Haystack Observatory.

Phase II, which must be complete by Dec. 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.

Researchers at Haystack adapted radio astronomy and geodesy techniques they had developed for the National Science Foundation and NASA to the cellular phone system.

Working with scientist Kevin Kolodziejski of TruePosition Inc. in King of Prussia, Pa., 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 polarizations, so we are assessing these characteristics in the anechoic chamber," Kolodziejski said.

TruePosition recently field-tested its wireless location system in Wilmington, Del. 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 meters or better, while 95 percent of the calls were located with an accuracy of 190 meters or better.

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


"Much remains to be done to achieve the goal of having wireless 911 Phase II capabilities deployed throughout the country," the FCC said. "All participants--carriers, the public safety community, technology vendors, network equipment and handset vendors, local exchange carriers, and the FCC--must continue to work aggressively in the coming months and years to ensure the promise of this new life-saving technology becomes a reality."

Some industry observers speculate that E911 is not a priority within the industry because carriers would not make money on emergency calls. Others say that carriers are waiting to see how the technology pans out before they commit to purchasing and installing new equipment.

At Lincoln Laboratory's Advanced Electromagnetic Systems Antenna Test Range in Bedford, technician Tom Alosso, Rogers and Kolodziejski are testing the performance and reproducibility of the design of prototype antennas, preparing them to be manufactured commercially. Then they, like others, will have to wait and see what happens in government and industry.

A version of this article appeared in MIT Tech Talk on February 6, 2002.

Topics: Industry


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