Several aspects of testing the universality of free fall (UFF) with spaceborne atom interferometers are discussed. Theoretical effects that could lead to an (apparent) violation of the UFF are reviewed and the requirements for test mass material choice with respect to ultra cold atoms are discussed. Different orbit geometries for the STE-QUEST (Space Time Explorer and QUantum Equivalence principle Space Test) mission are analysed with respect to the integrated sensitivity of the UFF measurement. A reference setup of an optical phase-locked loop (OPLL) for atom interferometry is demonstrated and a theoretical model of the OPLL for phase noise assessment and optimisation is developed using Laplace transforms.