The place of the Test and Simulation Assembly for a Modular Attitude and Orbit Control Subsystem (MACS-TSA) in the development process of the ISO and SAX spacecraft is given, with emphasis on the simulation aspects. The MACS-TSA is used as checkout equipment during the integration of AOCS, hardware and software at subsystem level. The other spacecraft subsystems are simulated, as well the dynamic behaviour in the orbital environment. A description of the components of the TSA is given.