The correct answer is 'GPS' which stands for Global Positioning System. GPS devices determine their position by triangulating signals from a group of satellites that are part of the Global Navigation Satellite System (GNSS). These signals provide location, vehicle speed, time data, and other information to GPS receivers located on or near the Earth's surface. 'E-readers' are incorrect as they are primarily used for reading digital books and don't require a geolocation network. 'Tablets' could have GPS capabilities but they are not defined by this function as they serve multiple purposes such as web browsing, media consumption, and productivity tasks. 'VR headsets' are also incorrect as their primary function is to provide an immersive virtual reality experience, and while they may track movement, they don't communicate with satellites for geolocation.