Abstract:         Radar satellites typically do not provide the solar vector information required to analyze data from their co-flying optical payloads. To process and interpret such data effectively, it is first necessary to determine the time intervals and spatial extent of nighttime illumination within the payload’s field of view. Conventional calculations of illumination conditions usually rely on Earth shadow models to identify illuminated periods and regions, but these models are not well suited to deriving illumination for an arbitrary payload viewing geometry. In this study, we propose two methods for determining illumination conditions in the payload’s field of view without explicitly using shadow models: an illumination-simulation method and a solar zenith angle filtering method. Together, these methods resolve the problem of retrieving illumination conditions for the optical payload; their results are mutually consistent and provide a solid basis for subsequent processing and analysis of the payload observations. Complete nighttime illumination masks obtained from the two methods are matched with actual observations from an optical payload onboard a radar satellite, and the matched results are further validated using radiometric measurements from the optical payload onboard the FengYun-3E (FY-3E) satellite. The comparisons show that the proposed methods can reliably determine both the timing and spatial extent of nighttime illumination within the payload’s field of view: for observations that satisfy the derived illumination criteria, the correlation with FY-3E radiances exceeds 90%. This high level of agreement, combined with the relatively low computational cost, demonstrates that the methods are both accurate and efficient. Moreover, they are applicable not only at the sub-satellite point but also to any location associated with the satellite, providing a practical new approach for future satellite illumination calculations.