The development of a monitoring system based on global satellite navigation systems (GNSS) of ballast compaction quality during the construction and overhaul of railways is covered in the article. Traditional geodetic methods for determining the quality of ballast compaction are tedious. Non-geodetic methods (dynamic control systems, empirical models and geophysical methods) are not widely used on railways due to the low reliability of the ballast compaction quality, as well as the high complexity of the work. The proposed method and device of a quality control system for ballast compaction are based on the measurement of draft and residual deformations during compaction in dynamic mode. The current coordinates are determined using GNSS with dual-antenna positioning receivers performing advanced functions, including determining the relative position of the antennas in plan and height. The monitoring system developed at the Siberian State University of Railway Engineering enables real-time determining parameters which characterize the quality of compaction with high accuracy and the ability of controlling the compaction process according to the current parameters.