Groundwave propagation, tropospheric scatter propagation, ionospheric propagation (skywave), communication satellites
Lower frequency radio waves, below 3 MHz, propagate through the ground and in general through the Earth's surface, including the sea surface, due to its conductivity. As the sea's conductivity is greater, propagation is enhanced. The lower the frequency, the better the waves penetrate sea water. ELF waves (below 3 KHz) have been used to communicate with deeply submerged submarines. The frequencies in the range of VLF (3 - 30 KHz) and LF (30 - 300 KHz) are mostly used for military communications, especially with ships and submarines.
Combined groundwave and skywave/ionospheric propagation
Mediumwave radio transmissions (300 KHz - 3 MHz), including the AM broadcast band, travel both as groundwaves and, for longer distances at night, as skywaves i.e. using ionospheric propagation, to be described further below.
High frequencies or radiofrequencies (3–30 MHz) as well as microwave frequencies (300 MHz - 300 GHz) travel in straight lines and therefore would require "line of sight", that is, the transmitter would have to be able to see the receiver. The visual horizon limits the transmissions to approximately 55 Km. This problem was resolved in the 1950s, when tropospheric scatter or troposcatter transmission was developed for military communications. This consists of the transmission of microwaves at an angle just above the horizon with the purpose of having the microwaves scattered/reflected from the troposphere back to the Earth's surface towards the receiver. Due to the turbulence and high moisture content of the troposphere, microwaves are refracted, that is, they change direction and are therefore dispersed before reaching the receiver. Microwaves of approximately 2 GHz are best suited for trophospheric scatter transmission. Troposcatter was replaced in the 1970s by satellite communications. These create a communication channel between a transmitter and a receiver.
Skywave or Ionosopheric propagation refers to the propagation of radio waves reflected or refracted back to Earth from the ionosphere which constitutes an electrically charged layer of the upper atmosphere (reflection from ionized atom layers). It can be used to communicate at intercontinental distances. It is mostly used in the shortwave frequency bands i.e. radiofrequencies (3–30 MHz). (https://en.wikipedia.org/wiki/Skywave - includes above propagation type categories)