FamilyWhere ® uses a number of techniques to determine a phone's location. T-Mobile phones that have A-GPS (Assisted GPS) return the most accurate locations when they have a clear line of sight to GPS satellites. For example, the phone is outside, in a car, or near a window. For phones where A-GPS is not available, FamilyWhere ® uses cell tower information to provide the most accurate location possible, which is usually within a few hundred yards to a few miles of the phone's actual location. This includes non A-GPS phones, as well as A-GPS phones that are not in a clear line of sight to GPS satellites.