GPS devices read signals transmitted by a network of satellites. Individual devices calculate their locations based on their distances from each satellite. The system allows law enforcement agency personnel to monitor vehicles equipped with such devices by reading their movements on a map displayed on a laptop computer or on a computer screen in a dispatch center. Thirty percent of the 40 survey respondents reported having GPS systems in their vehicles. Some agencies have had their devices installed for as long as 9 years. Eighty percent of the agencies were satisfied with their GPS systems; the remaining 20 percent reported that their devices have not been installed long enough for an evaluation. The most popular use of GPS devices is for tracking the locations of patrol cars, followed by the capture of historical data (e.g., travel speeds and locations of cars at specific times), the quick location of officers in need of assistance, improving response times, rerouting cars to calls when customary routes are blocked, improving officers' efficiency, and keeping track of officers' activities. The most frequent reason given for not using GPS systems was the cost and the higher priority given to other needed equipment and systems. The most sophisticated GPS devices can indicate an officer's activities at a scene; however, this has raised issues of officers' privacy.