From AMS Glossary
A lightweight version of a propeller anemometer sometimes used instead of the more expensive sonic anemometer for the measurement of turbulent velocity fluctuations.
The "traditional" Gill propeller anemometer is a three-axis arrangement, which is particularly suited for the measurement of the vertical wind. The so-called K-Gill propeller vane has two propellers, mounted at an angle of 90°, that are aligned with the mean wind by a vane. One particular type of sonic anemometer is sometimes referred to as "Gill sonic anemometer" in the literature of the early 1990s.