Radio Wave Propagation Modeling
isr staff simulate the impacts of ionospheric scintillation on communications, navigation, and radar systems
The distribution of free electrons in the ionosphere is dictated by production from solar radiation, transport, and loss through chemical recombination. It is also subject to instability mechanisms that generate large-scale depletions and irregularities in the ambient electron density over a wide range of spatial scales (plasma turbulence). Radio waves that propagate through these irregularities experience scattering and diffraction, causing random fluctuations in amplitude and phase referred to as scintillations. The scintillation of satellite signals can severely degrade the performance of satellite communications systems, global navigation systems such as the Global Positioning System (GPS), and space radars used to conduct cloud-free, day-and-night observations of the Earth’s surface. Ionospheric irregularities and scintillations constitute one of the most important space weather threats to technological systems of the modern world which increasingly rely on trans-ionospheric radio propagation.
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.
Severe scintillation of the GPS satellite signals can result in loss of satellite tracking, which degrades GPS positioning accuracy. Even when satellite tracking is maintained, scintillation can cause errors decoding the GPS data messages, cycle slips, and ranging errors. To predict the performance of GPS under different ionospheric conditions, phase screen techniques can be used to simulate the received signal on the ground. The figure below compares the measured (black) carrier-to-noise ratio of a GPS satellite signal and the results of a phase screen simulation (red) using the measured phase as a proxy for the phase screen. The phase screen model accurately reproduces the statistics of signal fluctuations, and in this case reproduces the temporal structure of the signal on a fade-by-fade basis.
Radio Occultation Scintillation
Staff at the ISR have developed the Radio Occultation Scintillation Simulator (ROSS), which uses the multiple phase screen technique to simulate the forward-scatter of radio waves by irregularities in the equatorial ionosphere during radio occultation experiments. ROSS simulates propagation through equatorial plasma bubbles which are modeled as homogeneous turbulence modulated by spatial functions with local support. The figure below shows a comparison of the simulated signal intensity with the actual signal intensity measured by the CORISS instrument onboard the C/NOFS satellite.
Scintillation Impacts on Space Radar
Synthetic aperture radar (SAR) is a widely used remote sensing technique for continuously monitoring changes on the Earth’s surface from space. The superior resolution of the SAR technique is achieved by the coherent processing of multiple pulses transmitted by the radar as it moves in its orbit. This coherent processing creates a synthetic aperture, much larger than the aperture of the physical antenna, along the satellite track. Amplitude and phase fluctuations which decorrelate across the synthetic aperture, however, reduce the effective resolution of the SAR image. Staff at the ISR have developed the SAR Scintillation Simulator (SAR-SS), a phase screen technique for simulating the impact of small-scale ionospheric structure on SAR image formation and interferometry. The figure below compares simulated and actual imagery of the Amazon River Basin in Brazil measured by the PALSAR instrument onboard the ALOS satellite. The figure demonstrates that SAR-SS can reproduce the essential features of streaking and contrast degradation caused by small-scale structure in the ionosphere. The arrow points in the local magnetic field direction, which is closely related to the direction of the ionospheric streaks.
Point of Contact for this project is Charles Carrano.