Tropospheric scatter is a method of transmitting and receiving microwave radio signals over considerable distances – often up to 300 km. This method of propagation uses the tropospheric scatter phenomenon, where radio waves at particular frequencies are randomly scattered as they pass through the upper layers of the troposphere

Troposphere

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

 (hence troposcatter). Radio signals are transmitted in a tight beam aimed at the tropopause

Tropopause

The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...

, midway between the transmitter and receiver sites; as the signals pass through the troposphere they are scattered, allowing the receiver station to pick up the signal.

Normally, microwave signals, transmitted at various frequencies, usually around 12 Gigahertz (GHz) or 19 GHz, are only used for ‘line of sight

Line-of-sight propagation

Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...

’ applications, where the receiver can be ‘seen’ from the transmitter.