A Look Ahead: Expected Ionospheric Effects on GPS in 2000

Abstract

he next maximum in the approximate 11-year cycle of solar ultraviolet (UV) activity is expected to occur near 2000. Two major ionospheric effects on GPS signals are closely related to long-term solar UV activity, and also will maximize at that time. These are ionospheric range delays and amplitude fading and phase scintillation effects. Dual-frequency GPS receivers automatically correct for the ionospheric range delay by measuring the difference in this dispersive effect on both frequencies. Civilian users of singlefrequency LI GPS receivers must either rely on the ionospheric correction algorithm sent as part of the user message, designed to correct for only 50% rms of the range delay, or they must use a nearby, in time and space, actual measurement of the ionospheric range delay to provide a correction for the ionospheric range error. Ionospheric range delays are directly proportional to the total electron content (TEC), encountered along the path from each GPS satellite to the user. The TEC increases with increasing solar cycle activity. As the absolute values of range delay increase with the solar cycle, the need for improved corrections also will increase. Irregularities in the ionosphere that produce amplitude fading and phase scintillation effects on GPS frequencies can become significant as the solar cycle