Ultimate turbulence experiment: simultaneous measurements of C_n~2 near the ground using six devices and eight methods

摘要

We have performed a series of experiments in order to simultaneously validate several devices and methods for measurement of the path-averaged refractive index structure constant ( C_n~2). The experiments were carried out along a horizontal urban path near the ground. Measuring turbulence in this layer is particularly important because of the prospect of using adaptive optics for free-space optical communications in an urban environment. On one hand, several commercial sensors were used: laser scintillometer SLS20 (Scintec AG, Rottenburg Germany), large-aperture scintillometer (BLS900, Scintec AG), and 3D sonic anemometer (Thies GmbH, Gottingen Germany). On the other hand, we measured turbulence strength with new approaches and devices developed in-house. Firstly, an LED array combined with a high-speed camera allowed for measurements of C_n~2 from raw- and differential image motion, and secondly a two-part system comprising a Shack-Hartmann sensor and a PSF camera yielded turbulent modulation transfer functions, Zernike variances and angle-of-arrival structure functions, giving three independent estimates of C_n~2. We compare the measured values yielded simultaneously by commercial and in-house developed devices and show very good agreement between C_n~2 values for all the methods. Limitations of each experimental method are also discussed.