Tracking and Data Relay Satellite
The Tracking and Data Relay Satellite System is NASA's network of specialized communications satellites that orbit 22,300 miles above the Earth. As the name suggests, the satellites relay signals between spacecraft including the International Space Station and ground control stations on Earth. The spacecraft are a vast improvement over the string of ground stations that were used in the past to communicate with spacecraft for short periods of time as they passed over or near the stations. With the TDRS spacecraft in place, spacecraft including Earth-observing missions and NASA's Hubble Space Telescope have near-constant communication links to Earth.