Ionospheric and plasmaspheric effects in satellite navigation systems

A satellite navigation concept requires determination of time (or phase) delays that satellite-emitted signals experience when traversing the distance between satellite and observer. A pulse propagating this distance is slowed down somewhat from its free space velocity-by an amount directly proportional to the total number of free electrons (TEC) along its path. Ranging accuracy requirements mandate compensation for this additional signal delay. The total electron content for high-orbit satellites includes the ionospheric as well as the plasmaspheric electron content. The Radio Beacon Experiment (RBE) aboard the geostationary ATS-6 Satellite provides the opportunity to determine the ionospheric content (up to heights of
\sim 1500
km), the total electron content (from observer to satellite), and the plasmaspheric content (the difference of the two). Observations of ionospheric contents were carried out at Fort Monmouth from the launch of the ATS-6 in May 1974 to its planned drift eastward in June 1975. Diurnal, day-to-day, and seasonal variations of the contents were observed. Diurnal plasmaspheric changes were smaller in percent and in absolute terms than ionospheric changes. While, in general, ionospheric contents were larger during the fall than during the spring, the plasmaspheric contents exhibited the opposite behavior. The diurnal behavior of the ratio of plasmaspheric-to-total contents exhibited variations from
\sim 45
percent at night to
\gsim 10
percent during the day, with rapid changes after local sunrise and after local sunset.