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This paper describes the results of evaluation tests of an aeronautical satellite communications experiments system in the Ka-band using Japan's communications and broadcasting engineering test satellite (COMETS). An active phased-array antenna with an open-loop satellite tracking capability operating in the Ka-band was installed just behind the passenger window of an airplane to receive a satellite signal. The propagation measurements showed that the received signal power was stable within a signal-power variation of 1 dBp-p while the airplane was cruising at 600 km/h. At specific directions of the antenna, the radiowave reflected from the wing of the airplane was measured to be 18.5 dB lower in power compared with the line-of-sight signal. To confirm the designed link budget, the multibeam antenna pattern of COMETS' on-board antenna was measured while the airplane flew across two antenna beams. The antenna pattern matched the one measured in a ground test before launch. Two binary phase-shift keying (BPSK) modems, one for pilot-assisted coherent detection and the other for multiple-symbol differential detection, were used to measure the bit-error rate (BER). The measured received signal energy per bit per noise power density (Eb/N0) at a BER of 10-4 showed only a small degradation of about 1 dB from that measured in a translator loop-back test.