When Albert Belle was arrested in February for stalking his former girlfriend, it wasn’t the ­technology he used that got him into the papers—it was his fame as a retired major league baseball player [see photo, “Ball’s Belle”]. But if convicted, the five-time All-Star outfielder will enter a growing rogues’ gallery of those who have used Global Positioning System (GPS) devices to track and torment former loved ones.

According to the Scottsdale, Ariz., police report on Belle, in late January an object fell off the Mercedes-Benz sedan of his ex-girlfriend after she hit a bump. When she stopped to see what it was, she found a small black box with two magnets attached and a phonelike device inside. She told police that Belle had frequently shown up at unusual times and in odd places, and she believed he was following her. After she recorded phone conversations in which Belle seemed to be threatening her, he was arrested and indicted on a felony charge of stalking using a GPS device.

The case, just one of many, is a dramatic illustration of how an other­wise benign technology with many wonderful applications can turn into a real threat in the wrong hands.

GPS, originally developed by the U.S. Department of Defense, has been widely adapted for ­civilian uses in the last 10 years. One of the best-known applications is General Motors Corp.’s OnStar, which directs drivers to their destinations. Aftermarket versions that offer only tracking sell for less than US $1000 and can be wired directly into a car’s 12-volt electrical system.

The mobile GPS tracking equipment used in systems like OnStar rely on a matchbook-size sensor that receives signals from satellites. Processing logic triangulates the signals to locate the device to a resolution of roughly 2 meters. Early versions of such aftermarket devices had to be removed from the vehicle and connected to a computer to download their data, but newer models include a cellular transmitter that uploads the location data to a server. Users can track the vehicle via constantly updated Web pages or have text messages with the location data sent to mobile phones—a big temptation for those inclined to stalk estranged lovers, given the patchiness of existing laws.

The rules governing malicious uses of the technology vary widely from state to state in the United States. Many states have general laws against stalking or harassment, but far fewer specifically address GPS tracking, mobile phones, Internet spyware, and other recent developments in consumer technology.

The first recorded prosecution for GPS stalking was in Boulder, Colo., in October 2000. Robert Sullivan was convicted of harassment for having his teenaged sons install a TravelEyes Tracking Unit, sold by Bluewater Security, in Ann Arbor, Mich., in his estranged wife’s Oldsmobile. He was sentenced to three years in jail after the court ruled that the phrase “under surveillance” in that state’s stalking law included electronic monitoring. Though his jail time was subsequently reduced, he was later rearrested and jailed for multiple parole violations.

In 2003, Paul Seidler of Kenosha, Wis., was sentenced to nine months after he was convicted of felony stalking of his ex‑girlfriend. His innovation was to use a newly available real-time tracking service that depended on a GPS device he hid in her car that sent mobile text messages informing him where she was traveling so that he could pull up alongside her several times a day.

Harassment and stalking are no laughing matter. Domestic violence is responsible for 2 million injuries and nearly 1300 deaths among U.S. women age 18 and older, according to a division of the U.S. Centers for Disease Control and Prevention. (No solid data on the number of stalking cases are available, says Cindy Southworth, technology director of the National Network to End Domestic Violence, in Washington, D.C., because no standard reporting system exists among law-enforcement agencies.)

For anyone suspecting that she is a victim of GPS stalking, advice is surprisingly low-tech: first, trust your instincts, says Southworth; if someone repeatedly knows your travel patterns, be suspicious. Second, if you fear your vehicle is being tracked, talk to local law enforcement officers or a service station manager. Either the police or a mechanic can search your vehicle—although if any suspicious device is found, a mechanic must leave it undisturbed so it can be inspected as police evidence.

Southworth worries that GPS stalking will become more common as sensors and transmitters grow ever smaller and consume less power. What’s more, with all new U.S. mobile phones on track to provide GPS positioning data to the emergency 911 system, any phone can be tracked if the carrier chooses to offer such a service. No national law requires that a cellphone customer be notified that location data is being tracked, nor is the customer’s consent needed to record such data.

Though phone companies almost certainly would not intentionally provide tracking data to third parties without the customer’s permission, Southworth says she has seen cases in which a person managed to obtain tracking data for an estranged spouse. To avoid becoming a victim, people are advised to check with their cellphone service providers to find out if tracking is on and where tracking data are being sent, and to change all passwords giving access to their accounts.

If you consider yourself to be at risk, remember that stalking behavior is often a deep and intractable obsession. Baseball player Belle was rearrested on 17 May for continuing to harass the same woman. He had reportedly made dozens of hang-up calls and harassing comments, despite being under court order to refrain from any contact with her. He will now remain in jail until his case is heard.

Belle is learning the hard way that what goes around comes around: the electronic monitoring device he had to wear around his ankle before his second arrest also used GPS data to calculate the position of its wearer.