Ionospheric Instability Observed in Low Earth Orbit Using Global Positioning System

Abstract

The global positioning system (GPS) receiver used for navigation on the space shuttle exhibits range rate noise that appears to result from scintillation of the satellite signals by irregularities in ionospheric plasma. The noise events cluster in geographic regions previously identified as being susceptible to instability and disturbed ionospheric conditions. These mechanisms are reviewed in the context of the GPS observations. Range-rate data continuously monitored during the free-orbit phase of several space shuttle missions reveals global-scale distribution of ionospheric irregularities. Equatorial events cluster ±20 ◦ about the magnetic equator and polar events exhibit hemispheric asymmetry suggesting influence of an off-axis geomagnetic polar oval system. The diurnal, seasonal, and geographic distribution is compared to previous work concerning equatorial spread F, Appleton anomaly, and polar oval. The observations provide a succinct demonstration of the utility of space-based ionospheric monitoring using GPS. The susceptibility of GPS receivers to scintillation represents an unanticipated technical risk not factored into the selection of receivers for the U.S. space program.