5 SUMMARY AND CONCLUSIONS

We focus on the fluctuation of the terrestrial ionosphere, which is a serious error source when we try to obtain the information on planetary ionospheres by the radio occultation technique. In Section 2, the typical phase shifts caused by the radio occulta-tions of the Martian nightside ionosphere and the lunar ionosphere are evaluated. It was shown that the phase shift by the fluctuation of the terrestrial ionosphere is comparable to those by the Martian nightside ionosphere and the lunar ionosphere. In Section 3, the magnitude and the local-time dependence of the TEC fluctuation of the terrestrial ionos-phere over the vicinity of UDSC are clarified using the TEC data obtained by the GPS network. The periods of suitable conditions for the measurement are revealed.In Section 4, the method for the estimation of the fluctuation of the terrestrial ionosphere along the ray path by using the GPS network is presented. The validity of this method is tested by regarding one GPS satellite as the Nozomi or Selene spacecrafts.

Future works to improve the lower limit of the detectability of planetary ionospheres will be:

1. To analyze the TEC data covering several years to clarify the seasonal varia-tion of the fluctuation characteristics of the terrestrial ionosphere.

2. To increase the sampling rate of the observation of the terrestrial ionosphere by the GPS network. The fluctuations with the time scale of less than 1 minutes should be monitored if we want to measure the small ionosphere such as lunar and Martian ionospheres.