This chapter establishes a systematic approach to addressing unmodeled errors in Global Navigation Satellite System (GNSS) observations, recognizing their inevitable presence due to their complex spatiotemporal characteristics. Building on prior findings that suggest the existence of such errors, the chapter focuses on both their detection and mitigation. Commencing with an in-depth analysis of error detection, advanced methodologies to identify unmodeled disturbances within GNSS measurements is introduced firstly. Following this, compensation strategies are explored, presenting innovative techniques to minimize their impact on positioning accuracy. Through a combination of theoretical insights, experimental validation, and practical applications, the chapter provides a comprehensive framework for understanding, quantifying, and addressing unmodeled errors, ultimately enhancing the positioning performance.