The decision to include a third spacecraft, the UKS, in the AMPTE mission was made in 1981. The reasons for this are presented, together with a description of the spacecraft, its subsystems, and a summary of its early orbit performance. The UKS scientific instruments, and early results from them, are described in companion papers in this issue.

Anextreme ultraviolet spectrometer experiment by University College London for observation of whole-Sun spectrum -line fluxes, and flown on Orbiting Solar Observatory-6, is described. Results for the four lines with absolute calibration, are show n for the first 6 months of flight, from August 1969 to February 1970. The mean fluxes for the 30.4, 53.7, 58.4 and 121.6 nm lines are 8.1 x 10 9 ± 35 % ph cm-2 s-1 , 3.2 x 10 8 + 40% ph cm -2 s-1 , 2 .5 x 10 9 + 40% ph cm -2 s-1 , and 3 x 10 11 ± 50% ph cm -2 s -1 respectively. Relations between these fluxes and the 10.7 cm radio flux and Zurich sunspot number are presented.