Abstract In December 1974 and September 1975 two SYMPHONIE satellites were launched and successfully placed at 11.5° west longitude in the geostationary orbit. These satellites operate in the 4–6 GHz frequency band. Four transmission channels, each 90 GHz wide, are available for the experimental telecommunications mission. The spacecraft transponders receive the telecommunications signals from the whole visible earth and transmit them to two transmitting zones, one American zone and one Europe-African zone. The satellites are 3-axis stabilized. New technologies which have been developed for the spacecraft subsystems and equipment which is now space qalified include a biliquid apogee motor and a biliquid hot gas system for orbit corrections. In addition, several earth stations have been developed for the earth segment.