Cell Phones: The Drama of Tracing a Phone Call
April 12, 2010 - By Jason Hamilton
So you’re watching the next suspense-filled, albeit repetitive, episode of 24. Everyone has a cell phone. Even the bad guys. Every now and then one of the CTU agents will pull up a screen on their workstation to triangulate the location of someone’s cell phone. And within less than a minute, they’ve located their suspect. Welcome to the 21st century.
In my last article, Landlines: The Drama of Tracing a Phone Call, I discussed how tracing a call over a land line didn’t quite match up with what Hollywood likes to show you. However, with cell phone technology there are quite a few differences, but you’ll be pleased to know that Hollywood isn’t far off the mark. Herein, we will cover the basics of a cellular network and discover how a cell phone call is traced and how the phone can be tracked.
How cellular calls actually work
The medium for cell phones is radio signals that are transmitted and received by multiple radio towers in a given area. Each tower covers a small area called a cell (hence cellular network). The cell tower with its transmission equipment is called a cell site. Each cell site’s precise location is known via GPS. Most cell radio towers today are usually equipped with three directional antenna arrays. Each array usually of three transceivers. This configuration aids with multilateration, which we will discuss later on. More towers within a given area ensures better cell phone coverage and lower power consumption by each cell site, as opposed to one site covering a larger area which requires the tower to transmit a stronger signal
Cell sites are constantly transmitting on predetermined frequencies. No two adjacent towers will transmit the same frequencies so that one call doesn’t override another. When you power on your cell phone, it registers with the closest tower. When you are moving between cell sites, your phone will register with the next site as you get closer to it and receive instructions from the new cell site to switch to a new and available frequency from that site. This happens within milliseconds. And if you are on a call, it prevents your call from being dropped. This process is called a handoff or a handover.
When you place a call on your cell phone, its radio signals are received by the tower at the cell site and are converted to electrical impulses. These impulses are transmitted over copper and/or fiber optic cable to the switching center for cell phone traffic. From here similar protocols apply to those we discussed in the last article, concerning the termination of phone call into a switch; each number has a termination in the switch and Call Return information is logged. At any time your cell phone number is known to the switch, and whether or not a call is in place. This information can be viewed by the cell provider. What is also known, by the switch, is the cell site to which you are currently connected.
How tracing is accomplished
Are you a bad guy running from the law? Don’t carry a cell phone. Your cell phone is always communicating with the nearest tower, which is ready to complete a call as soon you dial. With landlines you don’t rove around town. Your phone stays put. The switch never needs to know when you’re moving from house to house. But the cellular network is different in this regard. Your cell phone registers with each tower with which you come into range. This is necessary so that when you move out of your local area, the switches know how to route your call. The cellular network ensures your phone and the closest tower are always in communication.
In response to the terrorist attacks on the U.S. on September 11, 2001, the FCC now requires that cellular carriers provide a means to track each cell phone via GPS. Most providers began equipping their phones with GPS receivers, while few adapted a technology to measure the time and direction of a cell phones signal to calculate where the phone is in relation to the cell site. Most of the cell phones equipped with GPS are not GPS enabled, meaning the subscriber (you) cannot access that information on your phone. But what happens is in the event that a 911 call is placed from that phone, the GPS information is sent to the carrier and to emergency services.
Whether or not your cell phone is GPS equipped, the signal from your cell phone is still measured and calculated by cell sites within range; your position can still be determined by this process, although it is debatable whether it is any more accurate than using a GPS measurement. The cell phone signal is measured in a process called multilateration, which is similar to triangulation. Where triangulation uses a minimum of two fixed points to determine the direction of incoming signals, multilateration uses measurements from a minimum of three cell sites to provide a more accurate point on the map. Again, when a call is placed, this information is available in the cellular network switch.
How it works in real life
So now that the basics are covered, let’s put all this together in a potential, real life scenario:
You’re visiting a city to which you have never been, but fate and circumstance are not on your side because you have collided with another vehicle. The driver of the other car is unconscious and bleeding profusely. You become hysterical as you pull out your cell phone and dial 911. The dispatcher asks for directions to the scene and your cell phone dies before you can finish. But don’t worry, you placed a call and your Call Return information has already been registered in the switch serving the local cellular network; along with the tower to which you connected. Before your cell phone died, it got pinpointed by several cell sites; the sites have measured the distance from the tower to your cell phone and your location. Your position is now known within a few feet. Depending on the local protocols, the 911 dispatcher is being fed this information before you feel the tingle in your fingers from hyperventilating. The EMS crews are already on their way to you.
In summary, we can look at cellular call tracing from two viewpoints, the positive and the negative. In the event of an emergency, the tracing and mulitlateration will aid in your rescue. If, on the other hand, you are a criminal. Every step you take is tracked. While no one may be watching right now, once law enforcement officials know for whom they are looking, you can be certain the potential exists to find you. The important thing for you to know is that for wrong or right, carrying a cellular phone makes you trackable. It is often said that convenience comes at a price, and, as we now know, cellular telephones are no exception.
