Future Traffic Tracking
You are in a shrinking minority of the American population if you don’t own at least one electronic communications device. There are more than 119 million cell-phone users in the United States as of July 2001. Each day, thousands more sign up. Millions more have two-way pagers. The radio signals emitted from these devices can reveal our location at anytime. This ability to locate cell-phone users will become a vital component of future traffic-management systems.
On a short stretch of highway in Calgary, Alberta, Cell-Loc is testing out its new cell-phone tracking technology. In July 2001, the company sent a known vehicle down a 1.25-mile (2-km) section of a major highway, through the heart of the town, to test the accuracy of its system. The truck carried a GPS receiver onboard to compare the system’s accuracy.
“We collected data from both the GPS receiver in the vehicle, and from our system that was monitoring the cell phone remotely, and we compared the two and found them to be, not identical, but close enough for our applications we’re talking about,” Andrew Hillson, Cell-Loc’s director of service technology, said.
Here’s how the Cellocate system will work, according to Hillson and company documents:
- Listening posts are placed throughout a city, either next to a cell-phone base station or in independent locations. Listening posts are comparable to half a base station: They can detect but not transmit radio signals.
- Three listening posts are needed to get a two-dimensional position of a cell-phone user.
- Listening posts detect cell-phone transmission, decode it and then time-stamp the arrival of a wavefront from the transmission.
- Once three towers have time-stamped a transmission, the information is quickly sent to a central computer that uses hyperbolic multilateration to determine the cell phone’s position on a highway.
“Hyperbolic multilateration” is just a fancy way of saying triangulation, Hillson said. A position is determined by locating the intersection of the hyperbolas from the radio waves detected by the listening posts. By analyzing how long it takes the radio wave to reach the listening post from the cell phone, a computer can calculate almost precisely where someone is located on the highway. If the person’s location on the map is shown as off the highway, the computer corrects for this and snaps the location to the road. The entire process of detecting a person’s position occurs in seconds.
Hillson said that Cellocate meets the FCC’s mandate and is accurate within 330 feet (100 m) 67 percent of the time. Within 990 feet (300 m), the system is accurate 95 percent of the time. It supports AMPS(Advanced Mobile Phone System) and CDMA (Code Division Multiple Access) air interfaces. Cell-Loc is pursuing partnerships with cell-phone service providers. The service, which would allow cell-phone users to receive instant, personalized traffic warnings, will likely be available in a year or two and cost about $4 or $5 per month.