Maximum usable frequency Maximum usable frequency In radio transmission maximum usable frequency (MUF) is the highest radio frequency that can be used for transmission between two points via reflection from the ionosphere (skywave or "skip" propagation) at a specified time, independent of transmitter power. This index is especially useful in regard to shortwave transmissions. In shortwave radio communication, a major mode of long distance propagation is for the radio waves to reflect off the ionized layers of the atmosphere and return diagonally back to Earth. In this way radio waves can travel beyond the horizon, around the curve of the Earth. However the
Atmospheric entry not far above it) cannot create enough velocity to cause significant atmospheric heating. For Earth, atmospheric entry occurs at the Kármán line at an altitude of 100 km (62.14 mi / ~ 54 nautical mi) above the surface, while at Venus atmospheric entry occurs at 250 km (155.3 mi / ~ 135 nautical mi) and at Mars atmospheric entry at about 80 km (50 mi / ~ 43.2 nautical mi). Uncontrolled, objects reach high velocities while accelerating through space toward the Earth under the influence of Earth's gravity, and are slowed by friction upon encountering Earth's atmosphere. Meteors are also