There are 3 signals for centering from Themis's lights:
1 light on or
1 light flashing or
both lights on.
Their meaning will be explained in deteil in the chapter "Themi's Signals".
The Themi centering device offers a new tool for glider pilots. Themi measures and computes constantly the values of vertical velocity, position and heading, direction and velocity of the wind. Using these data Themi computes the position of the virtual center of the thermal. During flight in the thermal this virtual center becomes more and more identical with the real center of the thermal.
The position of this virtual center can be indicated with signal lights. Themi uses red LEDs (light emitting diodes) for signal lights. The two signal lights are installed in the normal view of the pilot. He can recognize a signal of the lights without directly looking at them.
In response to a light signal the pilot can steer the glider in ways that will improve his flight around the center.
When a signal light comes on, it can be thought of the illuminated center itself representing the center relatively to the glider. For example the right signal light means that the center is on the right side and in front of the glider.
The signal lights only come on if the present flight situation concerning the center can be improved. If the pilot initiates the right corrections on his own , Themi will not give light signals. Themi is updating the virtual center every second. Therefore Themi always has correct informations of the flight. Themi will always give signals if the flight could be improved.
The following pages show some general facts about thermalling without Themi. Later it will be shown how much easier it is to center a flight with the help of Themi.