ACRI-ST has developed several Earth Observation System Simulators. These software are used during all phases of space and ground segment design, implementation and exploitation for an Earth observation mission. They allow characterisation and quantification of the detailed specifications, and validation of the different modules of the system. The simulation softwares describe the different modules or components of a space system from the signal emission to the ground receiving and processing facility including the sensor and data acquisition on board the space platforms. The application is Earth Observation from space but also applicable to airbone systems and ground based or underwater systems to monitor environment. The actual simulators are :