Ionospheric Corrections for Timing Applications

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.