The 2nd Commission 4 Symposium brings together scientists, researchers, and professionals dealing with the broad area of positioning and applications. For this purpose, it will facilitate the discussion of research that leverages current and emerging positioning techniques and technologies aiming at delivering both practical and theoretical solutions for engineering and mapping applications, GNSS-based technologies, sensor fusion, and atmospheric remote sensing, modelling, and applications, based on geodetic techniques. Recognizing the central role of Global Navigation Satellite Systems (GNSS) in providing high-accuracy positioning information today and in the future, the 2nd IAG Commission 4 Symposium will focus on ongoing research for improving models and methods that enhance and ensure the performance of GNSS-based positioning solutions for a wide range of geodetic applications. The 2nd IAG Commission 4 Symposium will be devoted to the discussion of theory, strategies, and tools for modelling and/or mitigating the effects of interference, signal loss, and atmospheric effects as they apply to precise GNSS and other space geodetic techniques. They will address the technical and institutional issues necessary for developing backups for GNSS, integrated positioning solutions, automated processing capabilities, and quality control measures. Besides positioning, another focus of the 2nd IAG Commission 4 Symposium will be on remote sensing and modelling of the lower and upper atmosphere.