fed to two digital computers (nominal and redundant), as well to a back-up analog computer. It was also possible to orient the satellite by using radio-frequency sensors. Actuators were three momentum wheels, and hot gas (hydrazine mono-propellant) thrusters. One of the momentum wheels having failed in July 1975, an alternative scheme was developed, allowing station-keeping with the two remaining wheels and thrusters. Electric propulsion ATS-6 was equipped with two electric thrusters based on the acceleration of cesium ions, that were to be used for North-South Station Keeping. This subsystem development followed earlier failed attempts on the previous ATS spacecraft. Each of the thrusters had a mass of , used 150 W of electric power, and produced a thrust of , with a specific impulse of 2500s. The on-board supply of cesium would have been sufficient for 4400 hours of thrust. Unfortunately, both thrusters failed prematurely, one after 1 hour of operation, one after 95 hours. However, some of the experiments objectives could be met, such as the measurement of the effective thrust, the absence of any interference with the radio-frequency payloads (from 150 MHz to 6 GHz), no cesium redeposition on the critical parts of the payload (such as the radiometer), and the correct neutralisation of the spacecraft versus its environment. Payload Radiometer A radiometer was on board ATS-6, mounted on the earth-facing panel. This instrument was (for the time) of very high resolution. It operated on two channels: infra-red (10.5 to 12.5 µm) and visible light (0.55 to 0.75 µm). Images taken with the radiometer covered the whole earth disk, with a resolution of 1,200 lines of 2,400 pixels each ( square pixel in infra-red, and square in visible light). The IR detector was passively cooled at 115K, and the visible light detector maintained at 300K. A complete image of earth's disk was transmitted to ground every 25 minutes. Several hundreds images were taken and transmitted, until a mechanical component of the radiometer failed, two and a half months after launch. Telecommunication experiments The main mission of ATS-6 was to demonstrate the feasibility of direct-to-home (DTH) television broadcasting. To this end, in addition to the high-gain antenna, the spacecraft payload was able to receive in any of the VHF, C, S and L-bands, and to transmit in S-band (2 GHz) through a 20-W solid state transmitter, in L-band (1650 MHz) at 40W, in UHF (860 MHz) at 80W (which