Part of the operational concept for laser communication networks such as the European data relay system is planning constraints. In addition to hard constraints such as angular range of the actuators, there is also the atmosphere, which has a gradual increasing impact as the distance between laser beam path and earth decreases. The shortest distance between the laser beam path and the surface of the earth is called “grazing altitude.” The atmosphere impacts spatial acquisition as well as communication performance. In-orbit measurement data of geostationary orbit–low-Earth orbit (GEO-LEO) links between two laser communication terminals (LCT) developed by TESAT-Spacecom, Germany, in the frame of Alphasat TDP1 optical GEO-relay demonstration program are presented. Planning constraint guidelines are formulated based on the observed influence. TESAT-Spacecom’s LCTs use a highly sensitive and high-performance coherent transmission technology and a beaconless spatial acquisition system.