For voice communication purposes civilian air traffic uses two ranges of frequencies: Very High Frequency (VHF) and High Frequency (HF). VHF radio is used for relatively short distances (up to 200 miles) and is therefore suitable for populated areas where transmitters can be provided comparatively close to each other to ensure complete radio cover. There are 760 VHF voice radio channels available for air traffic use.
HF (or short wave) radio is used for long distance communication covering hundreds or even thousands of miles. Civil and military air traffic over the oceans of the world and over sparsely populated continents, such as Africa, communicate with radio base stations on HF using a wide range of frequencies which are varied according to the different qualities of reception which occur throughout the year and from day to day.
Whenever a civil or military aircraft carries a member of the Royal Family in the UK, an area of purple airspace is created which is intended to protect the Royal Flight throughout its journey. Usually the Royal Flight will be routed along the normal UK airways system and through established control zones, and the airspace will then be declared Purple Airspace.