Impact of GNSS Monitoring Point Spacing on Dam Displacement Measurement Accuracy

In dam displacement monitoring, the distance between GNSS points—defined by the baseline length—is a critical control parameter. It is recommended that the spacing between the reference station and the monitoring station be kept within 500 meters, with a maximum baseline not exceeding 2,000 meters. The system utilizes differential RTK technology to achieve millimeter-level displacement data acquisition, making it suitable for monitoring applications such as dams, landslides, and bridges.

GNSS displacement monitoring stations rely on signals from the BeiDou and GPS satellite constellations. The baseline length is a primary factor affecting measurement accuracy; excessively long baselines are more susceptible to interference from terrain and electromagnetic environments, which can degrade data accuracy and hinder the achievement of millimeter-level precision. The equipment features a choke-ring antenna combined with magnetic wave-absorbing materials to mitigate signal deviations caused by multipath effects. In static differential mode, the system achieves a horizontal accuracy of ±(2.5 mm + 1 ppm) and a vertical accuracy of ±(5 mm + 1 ppm). Powered by a 10V–15V DC supply—supported by a 30W solar panel and a 20Ah battery—the unit can operate independently and continuously in areas lacking external power, with stable power consumption, making it ideal for long-term field deployment.

When deploying monitoring points, reference and monitoring stations should be situated away from tall structures, with a minimum clearance of 20 meters from any obstructions. Ideally, the antenna should be positioned at the highest point within a 20-meter radius, and the mounting base must be securely anchored to minimize data fluctuations caused by wind. A clear line of sight between stations is preferred, and sites with similar elevations should be selected to reduce the impact of height differences on differential calculations. The equipment supports the Modbus-RTU protocol, enabling local displacement computation; it can independently output displacement values and angles, as well as trigger alarms, allowing for basic monitoring operations without reliance on a cloud platform.

Data transmission options include 4G, wireless bridges, Wi-Fi, and Ethernet. The data acquisition unit includes an RS485 interface for connecting external environmental sensors. It can output data in ASCII format locally or transmit it directly to a user-defined platform via TCP, HTTP, or UDP protocols. The supporting cloud platform enables data forwarding and 3D visualization, allowing personnel to intuitively assess the displacement direction and magnitude at specific monitoring points—facilitating long-term trend analysis. The system is simple to install and supports remote access to monitoring data. With high interface compatibility, it can be integrated into existing engineering monitoring systems to achieve data sharing and fusion, providing stable and reliable deformation monitoring data to support the safe operation and maintenance of dams.