Locating Cellular Users: Mandates and Solutions

By Dees, Tim | Law & Order, January 1999 | Go to article overview
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Locating Cellular Users: Mandates and Solutions


Dees, Tim, Law & Order


If your agency has a public safety communications center, the federal government has a surprise for you.

In 1996, the Federal Communications Commission released a "Report and Order" regarding enhanced 911 emergency calling systems. The document imposed a number of requirements on Public Safety Answering Points (PSAPs,) one of which was a mandate to be able to locate cellular telephones in communication with a PSAP by "identifying the latitude and longitude of a mobile unit making a 911 call, within a radius of no more than 125 meters in 67% of all cases." This particular requirement has a due date of five years, making it effective on October 1, 2001.

Necessary Evil

This mandate is of great concern for managers of chronically underfunded PSAPs - but there is good reason for the FCC's position. The Cellular Telephone Industry Association (CTIA) estimates that almost 18 million wireless calls were made to PSAPs in 1994 alone; for some jurisdictions almost half of their E-911 calls originate from cellular phones. This number is expected to grow dramatically as there are more cellular subscribers than ever before (over 53 million in the United States.) Over half of these cite "safety and security" as being their main reason for obtaining cellular telephone service.

With a landline 911 call, most modern police systems can immediately identify the number from which the call is originating, and enhanced 911 centers identify the address at the same moment. Even for less sophisticated centers, locating a hardwired telephone is a relatively simple matter. However, cellular telephones pose an entirely different problem.

Cell phones, being mobile, can place calls from anywhere, even outside their "home" exchange or area. While the typical cell phone transmits its electronic serial number (ESN), which can be used to identify the owner to police, the actual location of the phone is still a mystery.

Experienced PSAP call-takers are very familiar with the situation where the caller, even in an area familiar to them, cannot accurately state their location. Naming landmarks in sight, such as supermarkets or gas stations, is often misleading, because there can be more than one business with the same name in a PSAP's service area. When the caller is in a rural location, far from any man-made landmarks, the problem is aggravated. There are several methods to locate cellular signals (as well as any other radio signal,) but they are time-consuming, demanding on equipment and personnel and expensive.

How are communications centers going to be able to comply?

Handset or Network?

The easiest approach from the public sector perspective is to base cellular location strategy from the cellular phone itself. For this, hardware capable of receiving and encoding Global Positioning Satellite (GPS) data would be incorporated into every cellular telephone. The GPS location information would be transmitted along with the carrier signal from the cellular telephone, and would update continuously, providing not only location, but also direction of travel and altitude, if applicable. Typical GPS accuracy for a stand-alone GPS receiver is about 100 meters horizontal, which can be improved to a few meters if extra steps are taken.

GPS has limitations, however. When used in a urban environment, accuracy may be degraded by limited availability of satellites, and by signal degradation due to reflected signals (multipath.) Either way, it doesn't appear that GPS technology will be used to meet the requirements of the FCC mandate.

There are several reasons why this plan is probably not going to come together, even though GPS technology based companies would love it. Foremost, introducing this requirement would instantly obsolete the majority of all cellular telephones in use in the United States, most of which do not have GPS hardware installed. There are cellular telephones available with GPS capabilities, but these are not in common use.

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