Ultrasonic anemometer

© Fraunhofer IOSB, Erik Sucher

For the determination of local Cn2 values, 3D ultrasonic anemometers of type HS-50 (Gill Instruments, GB) are used. These measure with a high temporal resolution of 50 Hz time series of the sound velocity between 3 opposite pairs of transmitter and receiver units in 3 directions. The horizontal wind speed components u and v as well as the vertical wind speed w can be derived from the time differences of the sound velocity. From the dependence of the sound velocity on the temperature, the time series of temperature variations can be determined.

For wavelengths in the visible and near infrared, temperature variations are mainly responsible for variations in the refractive index. From the spectral analysis of the time series of the temperature variations, the structural function constant of the temperature CT2 can be determined by a Fourier transformation. Cn2 is proportional to CT2 for the wavelength range under consideration and one obtains Cn2 [1] after

© Fraunhofer IOSB, Erik Sucher

with the average air pressure, the average absolute temperature, the average wind speed, the frequency f and the variance spectrum of the temperature over the period under consideration

This measuring method can also be used on mobile measuring instrument platforms to detect spatial variations of optical turbulence. The decisive factor is to take into account or compensate for the inherent motion of the mobile platform.

Thanks to our extensive expertise in this field, the metrological procedures are continuously refined.