Ionospheric delay contribution to the uncertainty of time and frequency measurements by one-way satellite time transfer method

The results of international time and frequency comparisons between INPL reference and BIPM using GPS satellite system are presented. The uncertainty of time and frequency comparison by the one-way satellite time transfer method is discussed. Ionospheric delay contribution to this uncertainty is evaluated. The diurnal, seasonal and long-term variations of ionospheric delay are presented and discussed. Ionospheric delay has daily and seasonal oscillations with amplitudes of about 20–60 and 20 ns, respectively. These oscillations are due to variations of solar activity and the variations of total electron content in ionosphere. Long-term variations of ionospheric delay correlate with 11-year sunspot cycle. The difference between ionospheric delay values at minimum (in 1996) and maximum (in 2002) solar activity is 3 times. The increased values of ionospheric delay have caused the increase of the standard deviation of the time comparison results in the same proportion.