Using a Sodar to Measure Optical Turbulence and Wind Speed for the Thirty Meter Telescope Site Testing. Part I: Reproducibility

In this two-part study, we investigate the usefulness of Sodars as part of a large instrument suite for the study of high mountains in the site selection process of the Thirty Meter Telescope (TMT). In this first part, we describe the reproducibility of the measurements and the comparability of results from different sites for data taken with two complementary Sodar models: the XFAS and SFAS models manufactured by Scintec Inc. To this end, a cross-calibration campaign was conducted on two of the sites comparing both the wind speeds and the optical turbulence measurements of the different units. The specific set-up conditions and the low atmospheric pressure require us to make a compromise between the amount of data available for statistics and the quality of the data. For the comparison of the wind speed, results from the same models show a systematic difference of 12 and 9% for the XFAS and SFAS, respectively. The scatter between individual measurements, which includes instrumental, set-up and statistical fluctuation contributions, was found to be 21 and 23%. For optical turbulence, the respective values are 6 and 3% for the systematic difference and 46 and 67% for the scatter. These results show that Sodars can be useful tools for astronomical site testing for projects such as the TMT.