This surveillance is used when an aircraft is out of radar range. Not all aircraft currently have this facility, but it seems likely that it will be installed on most international passenger aircraft. Electromagnetic radiation is simply energy which travels through space at about 300,000 kilometres per second - the speed of light. We imagine radiation moving like a wave. The distance between two adjacent wave crests is called a wavelength.
Electromagnetic radiation travels in packets called photons.
The energy in a photon is related to wavelength - higher energy photons have shorter wavelengths. Also, the shorter the wavelength, the greater the frequency of the radiation. Other than wavelength, frequency and energy there is no difference between a radio wave, an X-ray and the colour green. They all possess the same physical nature. Global Positioning System.