Scintillation
As electromagnetic waves travel through Earth’s atmosphere, they pass through areas of varying pressure, temperature, and water content. This dynamic medium has rapidly varying indices of refraction, causing the waves to take different paths through the atmosphere. The consequence is that at the point of observation, the waves will be out of phase and appear to be varying in intensity. The effect in the visual range is that stars appear to twinkle and distant scenes on the horizon appear to shimmer (for example, when we see distant “water” mirages in the hot desert). In the radio range, the same phenomenon is called scintillation. The interplanetary and interstellar media can have a similar effect on the electromagnetic waves passing through them.
A star will scintillate or twinkle most violently when it is low over the horizon, as its radiation passes through a thick layer of atmosphere. A planet, which appears as a small disk, rather than a point, will usually scintillate much less than a star, because light waves from one side of the disk are “averaged” with light waves coming from other parts of the disk to smooth out the overall image.
Technology has been developed for both radio and optical telescopes to significantly cancel out the phase changes observed for a given source, thus correcting the resulting distortion. This technology is not implemented on the GAVRT.