AFRL scintillation images produced by the model PBMOD.

• S4 is an index for the strength of scintillation in the strength of a radio signal; it ranges up to values near 1, implying fluctuations comparable to the average signal intensity. A value of 0.6 implies fades of 10 dB are likely.

• S4 is calculated here for the low-latitude region (+/- 20 degrees about the geomagnetic equator), where ionospheric instabilities in the evening can create plasma irregularities that cause scintillation.

• Ionospheric scintillation affects radio transmissions most strongly at lower frequencies (UHF band), but can affect the L band, leading to loss of lock on GPS satellites and degradation of GPS accuracy.