Gaia is the new astrometric mission of the European Space Agency. It will measure the positions and proper motions of more than one billion stars and other objects with unprecedented accuracy, providing a sample of more than 1% of the stellar content of our Galaxy. Such a mission implies large technological and design efforts, since it will have to detect, select and measure hundreds of stars every second, sending their data to the Earth – more than 1.5 million kilometers away (1). Thus, the data transmission system must be highly optimized in order to make an efficient use of the downlink. We have focused the master thesis on this aspect; more specifically, we have revised and optimised the existing precompressing algorithms of the different instruments. Also different compression methods are tested in order to increase the final compression ratio. Our main goal is to guarantee the correct transmission of the highest amount of instrument data to the ground station. Therefore, the final ratio is the key factor that shall be analysed here, but CPU consumption and transmission reliability shall be taken into account as well