Phone companies know where their customers' cellphones are, often within a radius of less than 100 feet. That tracking technology has rescued lost drivers, helped authorities find kidnap victims and let parents keep tabs on their kids.

But the technology isn't always used the way the phone company intends.

One morning last summer, Glenn Helwig threw his then-wife to the floor of their bedroom in Corpus Christi, Texas, she alleged in police reports. She packed her 1995 Hyundai and drove to a friend's home, she recalled recently. She didn't expect him to find her.

The day after she arrived, she says, her husband 'all of a sudden showed up.' According to police reports, he barged in and knocked her to the floor, then took off with her car.

The police say in a report that Mr. Helwig found his wife using a service offered by his cellular carrier, which enabled him to follow her movements through the global-positioning-system chip contained in her cellphone.

Mr. Helwig, in an interview, acknowledged using the service to track his wife on some occasions. He says he signed up for the tracking service last year. 'AT&T had this little deal where you could find your family member through her cellphone,' he says. But he didn't use it to find his wife that day, he says. Mr. Helwig, who is awaiting trial on relatedassault charges, declined to comment further about the matter. He has pleaded not guilty.

The allegations are a stark reminder of a largely hidden cost from the proliferation of sophisticated tracking technology in everyday life -- a loss of privacy.

Global-positioning systems, called GPS, and other technologies used by phone companies have unexpectedly made it easier for abusers to track their victims. A U.S. Justice Department report last year estimated that more than 25,000 adults in the U.S. are victims of GPS stalking annually, including by cellphone.

The cellphone industry says location-tracking programs are meant to provide a useful service to families, and that most providers take steps to prevent abuse. Mike Altschul, chief counsel for wireless-telecommunications trade group CTIA, says recommended 'best practices' for providers of such services include providing notification to the person being tracked.

Mr. Helwig's wife had received such a notification, by text message, from AT&T. A spokesman for AT&T Inc. says it notifies all phone users when tracking functions are activated. But users don't have the right to refuse to be tracked by the accountholder. Turning off the phone stops the tracking.

Cellphone companies will deactivate a tracking function if law-enforcement officials inform them it is being used for stalking. Mr. Altschul says authorities haven't asked carriers to change their programs. He adds that carriers have long supported programs to give untraceable cellphones to domestic-violence victims.

There are various technologies for tracking a person's phone, and with the fast growth in smartphones, new ones come along frequently. Earlier this year, researchers with iSec Partners, a cyber-security firm, described in a report how anyone could track a phone within a tight radius. All that is required is the target person's cellphone number, a computer and some knowledge of how cellular networks work, said the report, which aimed to spotlight a security vulnerability.

The result, says iSec researcher Don Bailey, is that 'guys like me, who shouldn't have access to your location, have it for very, very, very cheap.'

That is, in part, an unintended consequence of federal regulations that require cellphone makers to install GPS chips or other location technology in nearly all phones. The Federal Communications Commission required U.S. cellular providers to make at least 95% of the phones in their networks traceable by satellite or other technologies by the end of 2005. The agency's intention was to make it easier for people in emergencies to get help. GPS chips send signals to satellites that enable police and rescue workers to locate a person.

To a large extent, that potential has been fulfilled. Last year, for example, police in Athol, Mass., working with a cellphone carrier, were able to pinpoint the location of a 9-year-old girl who allegedly had been kidnapped and taken to Virginia by her grandmother. In December, police in Wickliffe, Ohio, tracked down and arrested a man who allegedly had robbed a Pizza Hut at gunpoint by tracking the location of a cellphone they say he had stolen.

Mr. Altschul, of the cellphone-industry trade group, says the tracking technology has been of great help to both law-enforcement officials and parents. 'The technology here is neutral,' he says. 'It's actually used for peace of mind.'

But as GPS phones proliferated, tech companies found other uses for the tracking data. Software called MobileSpy can 'silently record text messages, GPS locations and call details' on iPhones, BlackBerrys and Android phones, according to the program's maker, Retina-X Studios LLC. For $99.97 a year, a person can load MobileSpy onto someone's cellphone and track that phone's location.

Craig Thompson, Retina-X's operations director, says the software is meant to allow parents to track their kids and companies to keep tabs on phones their employees use. He says the company has sold 60,000 copies of MobileSpy. The company sometimes gets calls from people who complain they are being improperly tracked, he says, but it hasn't been able to verify any of the complaints.

Installing such programs requires a person to physically get hold of the phone to download software onto it.

GPS-tracking systems provided by cellular carriers such as AT&T and Verizon Communications Inc. are activated remotely, by the carriers.

Justin Scheck

• interview [´intəvju:] n.&vt.接见；会见；交谈 (初中英语单词)
• assault [ə´sɔ:lt] vt.袭击；殴打 n.攻击 (初中英语单词)
• comment [´kɔment] n.&v.评论；评注；注意 (初中英语单词)
• hidden [´hid(ə)n] hide 的过去分词 (初中英语单词)
• counsel [´kaunsəl] n.商议；劝告；律师 (初中英语单词)
• account [ə´kaunt] vi.说明 vt.认为 n.帐目 (初中英语单词)
• function [´fʌŋkʃən] n.机能；职责 vi.活动 (初中英语单词)
• computer [kəm´pju:tə] n.计算机；电子计算器 (初中英语单词)
• security [si´kjuəriti] n.安全；证券；抵押品 (初中英语单词)
• consequence [´kɔnsikwəns] n.结果；后果；推断 (初中英语单词)
• federal [´fedərəl] a.联邦的，联邦制的 (初中英语单词)
• intention [in´tenʃən] n.意图；打算；意义 (初中英语单词)
• enable [i´neibəl] vt.使能够；赋予权力 (初中英语单词)
• rescue [´reskju:] vt.&n.救援；挽救 (初中英语单词)
• extent [ik´stent] n.长度；程度；范围 (初中英语单词)
• working [´wə:kiŋ] a.工人的；劳动的 (初中英语单词)
• virginia [və´dʒinjə] n.佛吉尼亚(州) (初中英语单词)
• grandmother [´græn,mʌðə] n.(外)祖母 (初中英语单词)
• actually [´æktʃuəli] ad.事实上；实际上 (初中英语单词)
• director [di´rektə] n.指导者；....长；导演 (初中英语单词)
• complain [kəm´plein] vi.抱怨，叫屈；控诉 (初中英语单词)
• carrier [´kæriə] n.搬运工人；托架 (高中英语单词)
• related [ri´leitid] a.叙述的；有联系的 (高中英语单词)
• everyday [´evridei] a.每日的，日常的 (高中英语单词)
• annually [´ænjuəli] ad.每年；按年计算 (高中英语单词)
• holder [´həuldə] n.所有物；土地；股份 (高中英语单词)
• access [´ækses] n.接近；通路；进入 (高中英语单词)
• location [ləu´keiʃən] n.定位；位置；地方 (高中英语单词)
• install [in´stɔ:l] vt.任命；装置；安置 (高中英语单词)
• commission [kə´miʃən] n.委任(状) vt.委任 (高中英语单词)
• potential [pə´tenʃəl] n.&a.潜在的；可能的 (高中英语单词)
• unexpectedly [´ʌniks´pektidli] ad.意外地；突然地 (英语四级单词)
• verify [´verifai] vt.检验；查对；证明 (英语四级单词)
• physically [´fizikəli] ad.按照自然规律 (英语四级单词)
• radius [´reidiəs] n.辐射光线；半径范围 (英语六级单词)
• kidnap [´kidnæp] vt.拐骗；绑架 (英语六级单词)
• reminder [ri´maində] n.提醒物；纪念品；暗示 (英语六级单词)
• spokesman [´spəuksmən] n.发言人 (英语六级单词)
• satellite [´sætəlait] n.(人造)卫星；随从 (英语六级单词)