Aim of the project TDP-1 – Technology Demonstration Payload No.1 is the demonstration of a data relay service, using an optical High Data Rate Inter-Satellite Link (ISL) between a Laser Communication Terminal (LCT) flown on a low earth orbiting satellite (LEO-LCT) and a second LCT (GEO-LCT) placed on the geostationary communication satellite AlphaSat (of INMARSAT) . The LCT planning system consists of one geostationary satellite (GEO) and up to five low orbiting satellites (LEO) which are also referred to as customers. The main task of GEO within this system is to serve as service provider for the LEOs and one optional optical ground station (OGS). The service consists of an optical data link between the Laser Communication Terminals (LCT) of the satellites (inter-satellite-link,ISL) and a link from a satellite to a ground station (space-to-ground-link, SGL).