The Changing-Atmosphere Infra-Red Tomography Explorer (CAIRT) was proposed as a candidate for the European Space Agency’s eleventh Earth Explorer mission to address critical knowledge gaps in middle atmosphere dynamics and composition. Utilizing an innovative Imaging Fourier Transform Spectrometer (IFTS) in a limb-sounding geometry, CAIRT is designed to provide high-resolution, three-dimensional (3D) tomographic observations of the atmosphere from the free troposphere (5 km) to the lower thermosphere (115 km). To support the scientific evaluation and requirement consolidation of the mission, this paper introduces two computationally efficient performance modeling tools: the Linear Performance Estimator (LPE) and the Fast L2 Simulator (FL2S).</p> <p>The Linear Performance Estimator (LPE) serves as a diagnostic tool that applies linear retrieval theory to quantify spatial resolution and retrieval uncertainties for atmospheric targets like temperature and trace gas concentrations. It features a novel extension to 2D tomographic retrievals, enabling the assessment of how instrument characteristics and uncertainties propagate into final Level-2 (L2) geophysical data products. The Fast Level-2 Simulator (FL2S) complements this by utilizing LPE-derived metrics to project high-resolution atmospheric model data fields onto a simulated CAIRT measurement grid, generating synthetic L2 data products that mimic the full retrieval chain. This allows for a streamlined assessment of how CAIRT’s spatial resolution and uncertainties affect specific scientific use cases.</p> <p>Applying these tools to the CAIRT instrument characteristics, we demonstrate the mission’s capability to meet specified L2 and science performance requirements. The results confirm that the modular end-to-end simulation framework effectively translates instrument specifications into robust scientific performance metrics, providing a mature basis for possible future implementations of limb-imaging space missions.