John A. Klobuchar

Abstract: The group retardation effect of the electrons in the earth's ionosphere can seriously limit the accuracy of time transfer by radio waves relayed via satellite. The dual frequency approach used in the GPS satellite system eliminates this potential problem, but other techniques, such as the use of models of the ionosphere, can only partially remove the ionospheric time delay error. Since the ionosphere is a dispersive medium the best approach to removing this error source is to directly measure the time delay at two, sufficiently widely spaced, frequencies as the dual frequency GPS system does automatically. If one does not have access to dual frequency GPS capability, various other techniques can be used with varying degrees of success in correcting for ionospheric time delay. Work currently being done in the ionospheric research community should help increase the capability of the time transfer community to make corrections for Ionospheric time delay effects on time transfer by satellite. 1) Research is continuing on Improving theoretical models of ionospheric behavior, especially during disturbed conditions when the largest deviations from median model time delay values normally occur. 2) A network of stations making real time measurements of the time delay of the earth's ionosphere is in its preliminary phase and, when completed, will provide qualified DoD users with real time updates of the time delay corrections over large regions of the globe. 3) Work is in progress on development of what is expected to be inexpensive code-free receiving systems using the dual frequency signals from the GPS satellites to directly measure ionospheric time delay in multiple directions simultaneously to allow greatly improved corrections for ionospheric time delay. These code-free GPS receivers should provide an excellent correction for the effects of the ionosphere on time transfer by satellite. Predictions of the magnitude of the current solar cycle are for a nearly record high cycle, with the maximum to be reached in late 1989, or early 1990, and staying high for several years thereafter. The implications for ionospheric time delay errors are for very high values for the next several years, with daytime ionospheric time delay values at 1,6 GHz from several tens of nanoseconds, to well over one hundred nanoseconds along the slant paths normally used with simultaneous satellite viewing for pairs of stations.
Published in: Proceedings of the 20th Annual Precise Time and Time Interval Systems and Applications Meeting
November 29 - 1, 1988
Sheraton Premiere Hotel
Vienna, Virginia
Pages: 193 - 204
Cite this article: Klobuchar, John A., "IONOSPHERIC CORRECTIONS FOR TIMING APPLICATIONS," Proceedings of the 20th Annual Precise Time and Time Interval Systems and Applications Meeting, Vienna, Virginia, November 1988, pp. 193-204.
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