Signal variation due to naturally-ocurring (or sometimes man-made) irregularities in the ionopshere. The effect is the same as the visible twinkling of stars due to variations in the atmosphere. Scintillation is readily identified in the data. Scintillation is most easily explained by analogy to a diffraction grating. Ionospheric irregularities are the equivalent of the diffraction grating. The received signal is then the sum of signal from multiple paths. As the path lengths change the signal varies due to constructive and destructive interference, hence the star twinkles. In other words, scintillation is variations in amplitude, phase, polarization and angle of arrival of a radio wave upon passing through the ionosphere, such as would occur with a satellite signal. Scintillations can be severe (especially near the equator) and present problems to communication services.