- The basic configuration of free space satellite optical communication includes a transmitter and a receiver. The transmitter satellite must point the optical information beam to the receiver satellite in order to establish communication. An important aspect in satellite optical communication is to obtain minimum bit error rate using minimum power. This aim can be achieved with very small transmitter beam divergence angles. The disadvantages of too narrow a divergence angle are that the transmitter beam may sometimes miss the receiver satellite due to pointing vibrations, and that the transmitter optics aperture required is large and expensive. The optimum value of the received power as a function of the pointing vibration displacement determines the optimum bean divergence angle. The performance of the tracking system determines the amplitude limits of the vibrations of the transmitter beam in the spatial domain. A mathematical model including the performance of the communication system as a function of the performance of the tracking system is derived. From this model we derive the optimum transmitter telescope gain. An example for a practical communication system between a low earth orbit satellite (LEO) and a geostationary earth orbit satellite (GEO) is presented. Using this model makes it possible to choose appropriate optics for the transmitter with reduced size and weight.