Radar Velocity Meter Installation
The radar emits electromagnetic waves to the surface of the water through its antenna. The electromagnetic waves reflected by the water flow generate a Doppler frequency shift. The shift is received by the receiver. The magnitude of the Doppler frequency shift is obtained through self-mixing, and then the flow velocity is calculated. Therefore, the following points should be noted for accurate flow velocity measurement:
Selection of the test area
(1) The direction and velocity of the water flow in the test section should not change significantly. And the water surface should have some undulations.
(2) There should be no large eddies, turbulence, or other phenomena, and the river section should be straight.
(3) Floating objects: Floating objects can interfere with the radar signal. It causes a certain jump in the flow velocity measurement result. The measurement can return to normal after the floating object leaves the radar’s field of view.
(4) Rainfall: Rainwater itself is a moving object and will also be measured by the radar. The radar has a built-in tracking system, which can avoid jumps in the measurement result. At the same time, for better measurement, it is recommended to adjust the installation angle to about 30°.
Note: If the water surface being measured is calm and still, the electromagnetic waves emitted by the radar will not be reflected to the receiver due to specular reflection. This results in no echo signal being received by the radar and thus no accurate flow velocity result being provided.
Influence of Installation Angle
The relationship between water flow velocity and Doppler frequency shift is as follows:
V = F Doppler × λ0²cos(α).
Radar current meter installation generally requires tilting the current meter at an angle to the water surface. The installation angle can be adjusted appropriately within the range of 45–60 degrees. As the installation angle increases, the echo signal received by the radar becomes stronger. But the corresponding Doppler frequency shift decreases. Conversely, as the installation angle decreases, the echo signal received by the radar becomes weaker. But the Doppler frequency shift increases. Therefore, an installation angle of approximately 45° is recommended.
The radar has a built-in gyroscope that automatically compensates for the flow velocity based on the time deflection angle, thus eliminating the need for precise adjustment.
Note that the installation angle refers to the angle between the current meter antenna’s transmitted beam and the direction of the water flow within the radar beam’s illumination area.
Influence of Installation Height
(1) The farther the radar is from the water surface, the weaker the radar echo received. And the worse the radar’s ability to accurately detect water flow velocity.
(2) The radar should not be too close to the water surface, as it is easily submerged by the water flow.
(3) If you want to measure extremely low flow velocities (<0.1m/s) or relatively calm water surfaces, you need to test at close range to improve the energy of the radar echo received. It is recommended to keep the distance from the water surface between 10-30cm.
Influence of Radar FOV
The radar’s field of view is called the FOV. The radar’s field of view presents a cone-shaped beam that spreads outward. Within the field of view, there should be no other moving objects besides the water flow being measured, such as turbines, pedestrians, cars, etc., that could interfere.
The product’s field of view is approximately 12°*25°. You can roughly estimate the radar’s illumination range at the corresponding distance based on the following diagram. If the distance is 10m, the radar illumination range is (2*0.105*10)*(2*0.222*10) = 2.1m*4.44m.
The radar illumination area will rotate depending on the installation direction. When the radar power supply line is horizontally led out, the illumination area is as shown in Figure (a). If the radar power supply line is vertically led out, the illumination area is as shown in Figure (b).
How does radar measure velocity?
Radar velocity meters operate based on the Doppler frequency shift principle. When a 24GHz/60GHz millimeter-wave beam is directed at the water surface at an angle θ (typically 45-60°), the frequency change of the reflected wave generated by the moving water conforms to the formula:
Δf = (2v·cosθ)/λ
Where:
Δf: Frequency shift (Hz)
v: Water flow velocity (m/s)
λ: Radar wavelength (mm)
Through accurate calculation of the frequency shift using a DSP digital signal processor, a measurement accuracy of ±0.03m/s can be achieved. This high-precision measurement capability makes it more stable in complex hydrological environments.
The choice of millimeter-wave technology not only improves measurement sensitivity but also significantly enhances the equipment’s anti-interference performance. So, it can keep stable operation under complex weather conditions.
What are the three types of radar?
Microwave radar is a radar system that uses microwaves as the carrier wave, with wavelengths typically between 1mm and 1m. Microwave radar has a lower resolution due to its longer wavelength. But it has a strong penetrating power. It can operate normally in adverse weather conditions such as rain and snow.
Millimeter-wave radar, on the other hand, operates using electromagnetic waves with wavelengths between 1 and 10 mm. Its wavelength falls between microwaves and light waves, thus combining the advantages of both microwave and photoelectric radar. Millimeter-wave radar typically has a detection range of 0-200 meters, making it suitable for applications. such as automotive collision avoidance, drone obstacle avoidance, and intelligent transportation.
Lidar, or laser radar, uses laser light as its carrier wave. Its wavelength is generally between infrared and ultraviolet. Lidar has a very short wavelength, resulting in extremely high angular resolution, velocity resolution, and range resolution. This makes lidar widely used in autonomous driving, robot navigation, and other fields.
How accurate is the radar velocity?
Sino-Inst’s radar velocity meter has an accuracy of ±0.5%.
