Satellites designed, built and launched by humans orbit the earth to carry out a myriad of tasks, friendly and hostile, commercial and scientific. Many of these missions supply critical data to model, mitigate and predict planet-scale processes such as El Nino events, sea level rise, plate tectonics and the earthquake cycle. Spacecraft move at 4 - 8 kilometres per second, and are between 500 and 20,000 km above the earth’s surface but for scientific purposes we need to know where they are to within a few centimetres, and we need to know the time they transmit their signals at the nano-second level. This lecture explains how this is achieved using concepts from fundamental physics.