The wave propagation of radio waves depends on the frequency, as certain waves propagate as ground waves and others as space waves
. The propagation of space waves, which reflect off
, depends onthe activity of
the ionosphere, which in turn depends on the position of the sun and the time of day. The Frequency of Optimum Transmission (FOT) is the optimum frequency, over a period of days or weeks, which is most suitable as a transmission frequency and which can be used to establish a radio link between two locations on the surface of the earth. The optimum frequency is usually slightly below the frequency that provides the most effective transmission, the Maximum Usable Frequency (MUF).