Japan's CS spacecraft was launched successfully in December 1977 and placed at 135°E longitude in a geostationary orbit. On-orbit performance of the satellite was verified quite well with the exception of two sets ofK-band transponders. Substantial communication experiments have been conducted for more than half a year starting in May, 1978. In the satellite loop-back configuration at the Main Fixed Earth Station, 1872 and 972 channels of FDM-FM telephony signals can be transmitted through anyK- andC-band transponder, respectively, with signal quality better than 50 dB inS/(N + D). The quality of FM-TV signals is also good (more than 59 dB in unweightedS/NatK-band). Basic transmission characteristics of PCM-PSK signals with various modulation parameters were measured with good results. However, it became clear that spin modulation effects must be considered for estimating the BER performances, especially atK-band. DPCM-TV signal transmission tests were also conducted with satisfactory results. PCM-TDMA systems were verified to work well atK-band, including functions such as frame and burst synchronization, initial acquisition and so forth. Diversity switching functions will be tested in a later experimental phase. In propagation experiments, rainfall attenuation, excess noise temperature due to rain, and effects of the ionosphere have been studied. Although insufficient data have been accumulated, some estimation of the necessary link margin against the rainfall attenuation atK-band was obtained. In general, confidence has been attained that some CS-like system withK-band links will be suitable for practical use. The experiments are now being conducted and will continue through the nominal mission period of 3 years.