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• Investigating the performance of a GNSS + ROBS deformation monitoring method in harsh environment with severe blockage of GNSS signals. • A new Multi-sensor Coaxial Instrument (MSCI) was designed and developed firstly for the installation of multiple sensors on each single station. • The GNSS + ROBS model was then advocated, and its performance was assessed by an experiment at a concrete faced rockfill dam (CFRD) located in the center of China. Global Navigation Satellite System (GNSS) has become one of the most essential deformation monitoring techniques for large structs due to its high precision, automatability, and continuity. Normally, excellent observation circumstances are required to achieve an accuracy of millimeter level. However, the monitoring stations are sometimes located in harsh environments, which results in accuracy degradation of GNSS positioning caused by signal blockage and corruption. In this study, GPS, BDS and ground ranging observations (GNSS + ROBS) are combined to achieve an accuracy of mm level in harsh environment, where the ROBS could be obtained from a ranging technology. Experiments were carried out at a concrete faced rockfill dam in central China. The results show that GNSS + ROBS can achieve positioning precision of 0.5 mm, 0.6 mm, and 1.2 mm in the east (E), north (N), and up (U) directions, respectively. In comparison with GNSS-only positioning, the positioning accuracy of GNSS + ROBS is improved by an average of 25.4 %, 37.7 %, and 21.4 % in the E, N, and U directions, respectively. Additionally, it is found that the ROBS can bring up to 64 % improvement in positioning accuracy in the N direction, when the ROBS are located in the direction of GNSS signal shield. The results indicate that GNSS + ROBS have the potential to enhance precision and reliability of positioning in challenging environments. [ABSTRACT FROM AUTHOR]