Propagation through Random Ionospheric Irregularities
To predict the impact of ionospheric scintillation on communications, navigation, and space radar systems it is often necessary to simulate the RF conditions under which these systems must operate. This involves physical modeling of the disturbed ionosphere followed by phase screen or full-wave propagation techniques. While the simplest ionospheric irregularity models assume statistically homogenous turbulence, in reality the electron density fluctuations are most commonly distributed within discrete plumes that are best described by an inhomogeneous phenomenological model. A realistic simulation must account for the motion of the transmitter and receiver platforms, the drift and anisotropy of the irregularities, and the oblique angle of propagation, all of which determine the scale sizes of the turbulence sampled by the radio wave. ISR staff have developed algorithms to simulate RF conditions in highly realistic scenarios which take all of these aspects into account for space-to-ground, ground-to-space, and space-to-space propagation scenarios.
For example, the top panel in the figure below shows the total phase change imparted to a 250 MHz radio wave after propagation through the ionosphere. In this example, the ionosphere is modeled as a homogenous layer of irregularities statistically described by a power law with a phase spectral index of 3. The bottom panel in the figure shows the intensity of the radio wave as a function of distance below the layer and also distance along the ground. The dark and light regions represent fades and enhancements of the wave, respectively, caused by defocusing and focusing of the wave as it propagates. A receiver samples the radio signal along the ground, and when the signal fades below the margin of the receiver, communications or navigation systems that depend on it are disrupted. Note the qualitative resemblance to the dark and light regions at the bottom of a pool of water, which are caused by refraction of the light from above. The physical mechanisms at work in both cases are closely related.