This paper focuses on SSA's Sensor Simulator (SSIM), and how it is defined to support the testing and evaluation of Sensor Planning System and Data Processing Chain prior to the deployment of real sensors, in the frame of SSA programme. The Sensor Simulator for the Pilot Data Centres reproduces physical models for all system elements involved in the data generation process: observation constraints and strategies (tracking and survey), debris orbit propagation, Near Earth Objects (NEO) orbit propagation, generation of radar, ground based optical and space based optical measurements. A review of the capabilities, main models and associated algorithms is presented in this paper. Examples of the use of SSIM for the simulation of observations of both Space Surveillance and Tracking (SST) and NEO objects are provided, highlighting the differences between these two operational cases. SSIM is designed and implemented to make use of the ESA SIMULUS infrastructure and it will be deployed on top of the Common SSA Integration Framework. A brief description of the architecture of the system is provided.