Comparisons Of Space-Based Gps Occultation Ionospheric Scintillation Measurements With Ground-Based Vhf Measurements

Satellites in low earth orbit (LEO) equipped with dual-frequency GPS receivers, such as the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC), have enabled a new means of retrieving ionospheric electron density profiles through the so-called radio occultation technique. Essentially the on-board GPS receiver tracks signals from GPS satellites, measuring total electron content (TEC) between the LEO satellite and the distant GPS platform. In cases where a rising or setting satellite geometry results in the raypath scanning through ionospheric altitudes (60-1000 km), it is possible to invert the observed TEC variations to determine the electron density profile along the line-of-sight. Because the technique is completely space-based, it is possible to achieve global real-time coverage with an appropriate constellation of LEO satellites. Furthermore, using high sample rate data, it seems promising to extract ionospheric scintillation parameters during occultation events as the GPS radio signal encounters turbulence or structure along the radio ray path. We have conducted an analysis of more than 200 COSMIC GPS occultation events in the vicinity of ground-based scintillation observations to determine if signal fluctuations observed with the satellite GPS receiver are correlated with scintillation observations on the ground. The data show a reasonable correlation with ground-based observations when a variety of factors such as spatial and temporal differences are accounted for. The COSMIC data are not ideal for such investigations because the high data rate (50 Hz) sampling is not enabled until the occulting raypath altitude is below 120 km. Thus, a number of occultations from the AFRL Communications/Navigation Outage Forecast System (C/NOFS) have also been examined. C/NOFS flies in a low inclination orbit near the equator and frequently encounters electron density irregularities associated with Spread F activity at low latitudes. Results of observations from both satellites will be presented.