Simulation and Validation of UV–Visible Limb-Scattered Imaging Spectroscopy Based on SASKTRAN-HR

UV–visible limb-scattered observations provide key information on the vertical distributions of trace gases in the middle atmosphere. As Earth-observing satellites progressively acquire large off-nadir limb-imaging capability, imaging spectroscopy is expected to further enhance the spatiotemporal information content of limb observations. However, existing models typically focus on single line-of-sight spectral radiance calculations and do not explicitly incorporate the instrument imaging chain, which hinders end-to-end assessment and broader application of limb-imaging spectrometers. In this study, starting from atmospheric composition and including selected instrument parameters, we implement UV–visible Earth limb-imaging spectral simulations and perform validation with quantitative error analysis.

The proposed approach builds a forward-simulation framework for UV–visible limb-imaging spectroscopy based on the SASKTRAN-HR radiative transfer engine, generating physics-based limb images and hyperspectral three-dimensional data cubes over 300–800 nm and 6–97 km. A three-dimensional atmospheric scene is constructed using CAMS reanalysis data (deriving air number density from pressure and temperature and incorporating ozone and sulfate aerosols, among others), while the upper-atmospheric background state is extended using the CIRA-86 model. The instrument imaging chain is further coupled, including field of view (FOV), spectral response function (SRF), and point spread function (PSF), to represent pixel-level spectral–spatial coupling effects.

Validation is conducted in the spectral domain. Simulated radiances are convolved to the effective spectral resolution of OSIRIS, and a height-by-wavelength evaluation is performed against a single OSIRIS limb scan (scan No. 54300035). Over 300–800 nm and 6–97 km, the simulations exhibit an overall systematic underestimation, with a mean absolute relative error (MARE) of 31.5% (median 25.1%). The dominant error source is attributed to discrepancies between the constructed atmospheric scene and the actual atmospheric state. Within the 20–55 km altitude range commonly used for trace-gas profile retrievals, the MARE is 10.9% (median 8.5%). Errors increase substantially above 80 km (MARE = 63.1%), likely related to stray light and reduced signal-to-noise ratio due to weak scattering signals. In addition, the simulated results are converted into pseudo-color imagery using the CIE 1931 color matching functions to enable a qualitative consistency check of limb radiance gradients and chromaticity variations.

This imaging-spectroscopy simulation framework provides a testbed for limb-imaging instrument design, development and evaluation of trace-gas retrieval algorithms, and satellite validation activities for current and future limb-imaging missions (e.g., ALTIUS).