Trucking companies use GPS systems to track hazardous cargo and monitor drivers. Corrections authorities use them to monitor sex offenders. Hikers, boaters and motorists use GPS devices to keep from getting lost. GPS technology is also being built into cell phones to help emergency dispatchers find 911 callers. They're also being used to prevent car theft.

Southworth trains victims advocates, law enforcement and prosecutors on stalkers' use of the technology, which she says is only just beginning to be abused.