the equipment was installed in a horizontal plane and related to north.
The listening operator aimed as well as possible at the incoming sound of the airplane and by pressing a button he marked at least 3 successive positions in chart angle and elevation. These markings appeared as dots on the outside of the wall of the glass so-called correction cylinder shown in the photographs.
The reading operator had to find a best possible fit of the three markings to one particular target path, to be chosen from a collection. This collection of calculated chart angle/ elevation curves were drawn on a template which could slide along the inside of the cylinder wall.
Due to the low speed of sound in air the target's real chart angle and elevation had already changed at the moment the operator pressed his button. The reading operator could correct for this delay by using a mechanized table (on the photograph below and at the right of the cylinder) and a second, this time transparent template. With the latter he extrapolated along the target path chosen from the last marking to find the corrected values for chart angle and elevation.