Quantitative Scintillation Diagnostics Using Total Electron Content from Commercial Off-The-Shelf GNSS Receivers

Signals from Global Navigation Satellite Systems (GNSS) are routinely monitored using specialized scintillation receivers to detect and characterize irregularities in the ionosphere that cause scintillation. While these specialized receivers provide high fidelity scintillation diagnostics such as the scintillation intensity index (S4), more numerous are inexpensive commercial off-the-shelf receivers capable of measuring the rate of change of total electron content index (ROTI). Several realtime GNSS networks currently operate receivers of this type including the International GNSS Service (IGS), the Continuously Operating Reference Station (CORS), the Crustal Dynamics Data Information System (CDDIS), and the network operated by UNAVCO. Radio occultation (RO) satellite systems, such as the upcoming COSMIC-2 mission, may also be used for this purpose. ROTI is widely used to indicate the presence of ionospheric irregularities, and as a proxy for S4 because of its well-known correlation with that parameter. Due to the Fresnel filtering imposed by diffraction, however, phase and intensity metrics are sensitive to different fluctuation scale-sizes in the turbulence. As a consequence, the search for a quantitative relationship between ROTI and S4 has remained elusive until now. In this paper, we introduce a new theory explaining the dependence of ROTI on the sampling interval, satellite motion, propagation geometry, and the spectral shape, strength, anisotropy, and drift of ionospheric irregularities. The principal dependencies are identified and interpreted. We show, for example, that ROTI varies with the effective scan velocity (Veff) to the power ?-1/2, where 2?+1 is the irregularity spectral index. A consequence of this dependence is that the ratio ROTI/S4 differs from one satellite to another as the propagation geometry changes, and it differs from one geophysical location to another (and also over time) through its dependence on the irregularity drift. While irregularity strength cancels when forming the ratio ROTI/S4, the dependence on Fresnel scale remains and one must also consider this. We validate the theory by comparing predictions of S4 from ROTI with direct measurements of S4 provided by a specialized scintillation monitor. We believe that any multi-frequency GNSS receiver that can monitor total electron content (TEC) should be able to provide quantitative scintillation diagnostics using this technique.