speed of the vehicle: Position-based systems Radio beacons Optical recognition Dead reckoning Position-based systems GPS is based on a network of satellites that constantly transmit radio signals. GPS receivers pick up these transmissions and compare the signals from several satellites in order to pinpoint the receiver's location to within a few meters. This is done by comparing the time at which the signal was sent from the satellite to when it was picked up by the receiver. Because the orbital paths of the satellites are known, the receiver can perform a calculation based on its distance to several of the orbiting satellites and therefore obtain its position. There are currently 31 satellites making up the GPS network, and their orbits are configured so that a minimum of five satellites are available at any one time for terrestrial users. Radio beacons Roadside radio beacons, or bollards, work by transmitting data to a receiver in the car. The beacons constantly transmit data that the car-mounted receiver picks up as it passes each beacon. This data could include local speed limits, school zones, variable speed limits, or traffic warnings. If sufficient numbers of beacons were placed at regular intervals, they could calculate vehicle speed based on how many beacons the vehicle passed per second. Beacons could be placed in/on speed signs, utility poles, other roadside fixtures, or in the road itself. Mobile beacons could be deployed in order to override fixed beacons for use around accident scenes, during poor weather, or during special events. Beacons could be linked to a main computer so that quick changes could be made. The use of radio beacons is common when ISA systems are used to control vehicle speeds in off-road situations, such as factory sites, logistics and storage centres, where occupational health and safety requirements mean that very low vehicle speeds are required in the vicinity of workers, and in situations of limited or obscured visibility. Optical recognition systems Optical recognition technology has focused on recognizing speed signs, road markings and roadside objects such as "cat's eyes". This system requires the vehicle to pass a speed sign or similar indicator and for data about the sign or indicator to be registered by a scanner or a camera system. As the system recognizes a sign, the speed limit data is obtained and compared to the vehicle's speed. The system uses that speed limit until it detects a