Earth-observing satellites travel in low-Earth orbit (LEO) gathering data and making a full circuit of Earth about every 100 minutes. They must wait to be over their respective ground stations to downlink the information they have acquired, however. Generally, this leaves only 10 minutes out of every 100 to transfer their information to the users on the ground. Any data gathered between these windows must be stored until the next downlink opportunity.
The European Data Relay System solves the downlink delay by connecting to the LEO satellites as they orbit past. EDRS collects the LEO data from its higher position in GEO and relays it down to its European ground stations, avoiding the time delay that occurs when the LEO satellite has to wait for ‘line of sight’ with its ground station. In fact, just one of the EDRS nodes can quadruple the Earth observer’s contact time with its ground segment.