The past decades have seen a significant increase in popularity of CubeSats for commercial applications, leading to a strong demand for enhanced reliability in these satellite systems. A critical component of these systems is the Attitude Determination and Control System (ADCS), which plays a pivotal role in managing the satellite’s orientation in space. However, this importance highlights a concerning issue: the ADCS has been identified as a leading contributor to failures in small satellite missions, often due to inadequacies in testing environments. To address this challenge, this paper introduces the FLIght equivalent Testing Environment Simulator (FLITESIM), developed by the Satellite Technology and Research (STAR) Centre of National University of Singapore. FLITESIM is designed to offer a comprehensive verification and validation (V&V) environment tailored for ADCS. This paper elaborates on FLITESIM’s design, its key features and reports experimental results from V&V activities on STAR’s Lumelite satellites, demonstrating its effectiveness in enhancing mission assurance, increasing early orbit readiness and improving ADCS operational stability.