Our self made communication unit is designed around an SI4461 FSK transreceivier integrated circuit produced by Silicon Laboratories. It could be widely configured to different solutions. The desired frequency, data rate, modulation and fequency deviation can be set. As for the test flight, the future goal is to involve the amateur radio community. Therefore, appropriet settings is choosen so they could receive data from the balloon. Our transmit frequency is in the ISM range near the 70cm band which is popular among amateur radio operators. An avarege HAM radi ohas a maximal bandwith up to 3kHz, so the data rate, frequency deviation and bandwith had to be chosen that fit into this range. So we used the 625bps data rate and 625kHz deviation. To increase the spectral efficiency, Gaussian FSK modulation is used. Since the communication is done via an additive noisy channel , it is necessary to filter out the erroneous packets , so they are supplemented with CRC error code.
The orientation of the capsule is changeing continuously, so an antenna is integrated into the system, which radiate in all directions as far as possible. Thus, a monopole antenna is connected the probe. Because, according to the ISM standard limited radiated power, the resulting SLOS is compensated by the antenna unit of ground station.
Our main ground station is the control centre of the Masat-1(the first Hungarian CubeSat). On the station four cross-Yagi antenna is located. In order to avoid cross-attenuation they are used in circularly polarized mode.
The ground station equipped with an RF circuit similar to the one in the COM unit. The control software is running on a Raspberry PI minicomputer which processed the data direct through SPI-BUS, so we got the flight information in real-time. The control commands have also been sent through this interface. Setting the directions of the antennas is processed by the GPS coordinates sent back to optimize the data transfer.