2.65 Gbps Downlink Communications With Polarization Multiplexing In X-Band For Small Earth Observation Satellite

This paper reports our new communication components and downlink tests for realizing 2.65 Gbps by utilizing two circular polar-izations. We have developed an on-board X-band transmitter, an on-board dual circularly polarized-wave antenna, and a ground station. In the onboard transmitter, we optimized the bias conditions of GaN High Power Amplifier (HPA) to linearize AM-AM performance. We have also designed and fabricated a dual circularly polarized-wave antenna for low-crosstalk polarization multiplexing. The antenna is composed of a corrugated horn antenna and a septum-type polarizer. The antenna achieves Cross Polarization Discrimination (XPD) of 37 - 43 dB in the target X-band. We also modify an existing 10 m ground station antenna by replacing its primary radiator and adding a polarizer. We put the polarizer and Low Noise Amplifiers (LNAs) in a cryogenic chamber to reduce thermal noise. Total system noise temperature of the antenna is 58 K (maximum) for 18 K physical temperature when the angle of elevation is 90 degrees on a fine winter day. The dual circularly polarized-wave ground station antenna has 39.0 dB/K of Gain - system-noise Temperature ratio (G/T) and an XPD higher than 37 dB. The downlinked signals are stored in a data recorder at the antenna site. Afterwards, we decoded the signals by using our non-real-time software demodulator. Our system has high frequency efficiency with a roll-off factor alpha = 0.05 and polarization multiplexing of 64APSK. The communication bits per hertz corresponds to 8.41 bit/Hz (2.65 Gbit/315 MHz). The system is demonstrated in orbit on board the RAPid Innovative payload demonstration Satellite (RAPIS-1). RAPIS-1 was launched from Uchinoura Space Center on January 19th, 2019. We decoded 10 10 bits of downlinked Rand L-channel signals and found that the downlinked binary data was error free. Consequently, we have achieved 2.65 Gbps communication speed in the X-band for earth observation satellites at 300 Mega symbols per second (Msps) and polarization multiplexing of 64APSK (coding rate: 4/5) for right- and left-hand circular polarizations.