Ground Based Communications

Shortwave radio signals at HF frequencies below about 30 megahertz are efficiently reflected by the Earth's ionosphere. This allows radio communications over several thousands of kilometers. Radio signals at higher frequencies however, are able to penetrate the ionosphere, and are, therefore, useful for ground-space communications. The following figure illustrates some of the key frequencies used for radio communications.

The coloureed bands indicate ham radio operator's frequency band allocations from LF/MF to VHF/UHF. MF and HF bands are where most of the worldwide short-wave stations are located, including Ham Radio and citizens band (CB). Gaps represent frequencies used by other services such as commercial or governmental.

Space weather is able to influence the propagation of signals through the influence of solar activity on the ionosphere. In the case of lower frequencies, solar x-ray and ultraviolet bursts caused by solar flares can lead to increased absorption and for higher frequencies, unexpected reflections at frequencies can result, causing radio interference.

Scattering caused by ionospheric irregularities may also produce signal fluctuations (scintillations) and propagation might take unexpected paths. This is particularly true for HF ground-to-air, ship-to-shore and ham radio signals.

Solar flare radiation can lead to a sudden ionospheric disturbance (SID) - large increase in ionisation on the sunward facing side of the Earth. At HF frequencies, a SID may appear as a short-wave fade (SWF). This disturbance might last for days or hours, depending on the strength and duration of the flare.

Solar flares also release a wide spectrum of radio noise which can interfere directly with an expected signal.

In addition, solar particle events may lead to increased ionisation of the lower ionosphere at polar latitudes as high energy particles enter the magnetosphere near the poles. This increased ionisation can lead to severe HF and VHF signal absorption called a polar cap absorption event (PCA) event. PCA events can last from days to weeks, during which polar HF radio propagation becomes impossible.

Geomagnetic storms also effect radio propagation, especially at auroral latitudes.

Ionisation produced by the solar protons also alters the path taken by the waves reflecting from the ionosphere.

Ionospheric changes that occur during disturbed times also increase the incidence of electron density irregularities, leading to sometimes severe variations or scintillations in the phase strength of signals sent from the ground to satellites at VHF and UHF frequencies (30 megahertz to 3 gigahertz).

Communication Systems

Mobile Phone