ALTIUS will be ESA’s next limb-sounding mission for operational ozone monitoring. The space segment is foreseen to be based on the flight-proven PROBA platform, and an innovative instrument that will image the atmospheric limb at tuneable wavelengths simultaneously in its ultraviolet, visible and near-infrared channels, between 250 and 1800 nm. The spectral tuning is achieved thanks to Fabry-Perot interferometer and acousto-optical tuneable filters technologies. The System Performance Simulator (SPS) described in this paper has been developed in a standardised and fully modular framework, inspired by the ESA OpenSF framework but implemented within the MATLAB environment. Its overall architecture and features are described for each module: (1) scene stimuli (simulating the spectral radiances at the aperture of the instrument), (2) geometry (simulating the spacecraft state), (3) instrument (simulating the electro-optical behaviour of the instrument), (4) platform (simulating the ancillary data generation), (5) L0 processor (raw data formatting), (6) L1 processor simulation (radiometric data correction and geo-referencing), and (7) performance assessment modules (performance assessment and verification based on the generated data up to level 1). Their modularity allows both the update of any parameter of the models and the re-use of modules by external parties. Moreover, the framework can be easily adapted to different missions and applications. The simulation of data generation - up to level 1- and the associated temporal, spectral, radiometric and geometric performance assessment can be conducted in a variety of different observation modes such as: limb scattering (limb sounding, tomography), occultation (solar, stellar, lunar or planetary) or even Earth observation (geographic pointing, scanning) geometries. The Simulator represents a powerful tool supporting the overall system (payload, platform & ground processing) specification and design by sensitivity analyses, which are enabled thanks to the extensive amount of parameters and scenarios that can be simulated and to the high efficiency implementation. The System Performance Simulator will be integrated, thanks to its modularity, with a future level 2 processor prototype into an End-to-End Simulator for the ALTIUS System.