Читать книгу Position, Navigation, and Timing Technologies in the 21st Century - Группа авторов - Страница 60

The wireless‐assisted GPS (A‐GPS) was pioneered by SnapTrack (now part of Qualcomm) and can be used for indoor locales. The approach leverages the cellular network together with GPS signals. Many cellular network towers have GPS receivers (or a base station nearby), and those receivers often constantly collect satellite information to detect the same satellites as cellular phones. This data is sent to the cellular phone (when requested), speeding up the time to first fix (TTFF; to acquire the orbit and clock data of relevant GPS satellites), which on a mobile device without assistance can take a long time (minutes) in some cases. Not only does the TTFF get reduced, but the approach can enable localization in indoor environments, where the GPS signals detected by the cellular phone are often very weak, with accuracies ranging from 5–50 m.