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Chair of Space TechnologyCommunication System

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Communication System

BEESAT-2 communicates with the ground station using the ultra high frequency (UHF) band at 435.9500MHz, which is dedicated to amateur radio satellites. The modulation used is Gaussian minimum shift keying (GMSK) with a modulation index of 0.3. The nominal data rate is 4800 bits per second. Half-duplex communication is used. On demand, the data rate can be increased up to 9600 bits per second. The communication link is secured by forward-error-correction (FEC), checksums (CRC), interleaving and scrambling. Transmitted data is unencrypted.

The communication system is designed fully redundant as well. Each unit consits of an 8 bit micro controller, a modem, a transceiver, and an antenna made of a trimmed spring-steel strip. The antenna is safely wound around the satellite in launch configuration and is released after a programmable time delay once the satellite is deployed from the launch container.

The two monopole antennas are arranged in such a way that in the case of unfavorable orientation of one antenna with respect to the ground station, the adjacent second antenna is just perfectly aligned. Both transceivers are in receive mode at the same time. However, signal transmission is only performed by the transceiver that yields the better communication link, i. e. by the one that is oriented best towards the ground station. Nevertheless, each communication link can be used selectively by the operator.

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