From AMS Glossary
(Also called scintillation meter.) A type of photoelectric photometer used in a method of determining high-altitude winds on the assumption that stellar scintillation is caused by atmospheric inhomogeneities (schlieren) being carried along by the wind near tropopause level.
In one design a star is observed with a single telescope that looks through a four-inch long rotatable slit. Two scintillation frequencies are monitored, one high and one low. The slit is rotated until the ratio of high-frequency to low-frequency scintillation is a minimum. At this point, the slit is oriented parallel to the wind. The magnitude of the frequency ratio is a function of the wind speed. In another design, two telescopes mounted four inches apart are focused on a star. Only one frequency need be monitored. By direct observation of phase relationships as the telescopes are rotated about a mutual axis, both direction and speed can be determined with less ambiguity and uncertainty than with the single telescope type.