1. Introduction#
The Geolux FX200 flow meter uses advanced radar technology to accurately measure surface flow velocity and water level (distance) without requiring contact with the water. Because the sensor operates entirely above the water surface, installation is quick and simple, and long-term operation requires only minimal maintenance. The instrument is mounted above the channel or river, eliminating the need for submerged components and reducing the risk of damage from debris, sediment, or flooding.
The FX200 incorporates two independent radar sensors that simultaneously measure surface velocity and water level, and a third lidar sensor for automatic measurement of the riverbed profile.
Surface velocity is measured using a radar operating in the 60 GHz frequency range (V-band). The radar transmits a modulated electromagnetic wave toward the water surface at 60° angle. The signal reflects from the moving water surface and returns to the sensor. By analyzing the frequency shift of the reflected signal, which is caused by both the movement of the water and the radar signal modulation, the instrument determines the velocity of the surface flow.
This measurement method is known as Frequency-Modulated Continuous Wave (FMCW) radar. Using advanced signal processing techniques, the FX200 analyzes the returned radar signal to determine both the distance to the reflecting surface and the velocity of the moving surface. The processed data are represented internally as a range-Doppler map, which shows the velocity of objects at different distances from the sensor.
Water level is measured independently using a dedicated level radar sensor. The radar periodically transmits a linear frequency chirp ranging from 77 GHz to 81 GHz (W-band). The signal reflects from the water surface and returns to the sensor. The difference between the transmitted and received frequencies is proportional to the distance between the sensor and the reflecting surface. As this distance changes, the frequency difference changes accordingly, allowing the FX200 to determine the water level with high precision.
The radar modulation and signal processing algorithms used in the FX200 enable highly accurate measurements that are not affected by air temperature, humidity, or other environmental conditions.
Unlike conventional surface velocity meters that use simple Doppler radar, FMCW radar technology can separate reflections originating from objects at different distances. This capability is particularly important in hydrological applications. For example, during rainfall events the FX200 can clearly distinguish reflections from raindrops and those from the river surface. Conventional Doppler radars cannot fundamentally separate these signals and must rely on heuristic filtering techniques to reduce rain-induced measurement errors.
The FX200 measures surface velocity in the range 0.020 m/s to 16.000 m/s, with a resolution of 0.001 m/s.
The instrument also includes an integrated tilt sensor that continuously monitors the mounting angle of the device. Measured surface velocity values are automatically corrected using cosine compensation based on the detected tilt angle, ensuring accurate measurements even if the mounting structure shifts over time.
Flow (discharge) is calculated internally by combining the measured surface velocity and water level with the measured or configured cross-section profile of the river or channel. Built-in lidar can automatically measure the cross-section profile of the channel.
Once properly configured, the FX200 can calculate discharge with an accuracy of approximately ±1% compared to ADCP measurements at the same location. In addition to discharge, the sensor simultaneously provides surface velocity and water level measurements via its digital communication interfaces, as well as the channel profile geometry.