The MUF frequency is the highest frequency that is optimally reflected by the ionosphere at a given elevation angle. The MUF frequency refers to two locations or geographic areas on the Earth's surface that are connected by a space wave radio link. In addition, the MUF frequency takes into account different times of day because they affect the reflectivity of the ionosphere. The 85% value of the Maximum Usable Frequency is considered the Frequency of Optimum Transmission ( FOT). The just usable lowest frequency is the Lowest Usable Frequency( LUF).