1. INTRODUCTION

1.1 Radio Occultation Measurements by Nozomi and Selene

The Japanese Mars spacecraft Nozomi was launched on July 4th, 1998, and will be put into the Mars orbit in 2004. Radio occultation measurement is one of the scientific objec-tives of the Nozomi mission. We also have a plan to carry out a radio occultation mea-surement using the lunar spacecraft SELenological and ENgineering Explorer (selene), which will be launched in 2004.

Radio occultation measurements give us the information on the aeronomy of the plan-ets. The measurement is carried out by using a spacecraft which emits unmodulated radio waves to the Earth [Fjeldbo et al., 1965]. The phases of radio waves are perturbed when the rays pass through the planetary atmosphere. An antenna on the earth receives the radio wave. The density profiles of the neutral atmosphere and electrons are derived from the phase perturbation assuming the spherical symmetry of the atmosphere [Phinney et al., 1968; Fjeldbo et al., 1971].

Radio occultation measurements can be carried out in both "1-way mode" and "2-way mode". In 1-way mode, the spacecraft emits unmodulated radio waves which are generated by an onboard oscillator. The onboard oscillator must be very stable in this case. In 2-way mode, on the other hand, the receiver on the spacecraft is phase-locked to the uplink signal which is generated by a stable oscillator on the earth. The spacecraft transponder generates the downlink signal from the uplink signal coherently.

We often use more than one frequency. The effect of the neutral atmosphere on the frequency is proportional to the signal frequency. The errors due to the fluctuation of the onboard oscillator and the incorrect orbital information are also proportional to the signal frequency. The effect of plasma is, on the other hand, inversely proportional to the signal frequency. By using two frequencies, we can distinguish the effect of plasma from other effects. In the Nozomi mission, we were to carry out measurements with S-band (2.3 GHz) and X-band (8.4 GHz) signals in 1-way mode using a stable onboard oscillator, so-called Ultra Stable Oscillator (USO). However, as the onboard S-band transmitter and the USO were dead possibly due to a trouble of the field effect transistor used for the last stage of the transmitter, we can not use the downlink S-band signal and the USO. We are forced to carry out measurements in 2-way mode using the uplink S-band signal and the downlink X-band signal.

In the Selene mission, we will carry out measurements using an ordinary oscillator, whose frequency stability is 1.0 X 10^-9 rms s^-1 in the short term, since the spacecraft dose not have any USO. It will be carried out in 1-way mode using S-band (2.3 GHz) and X-band (8.4 GHz) signals for the downlink.

The data recording system at Usuda Deep Space Center (UDSC) is illustrated in Figure 1. The radio waves from the spacecraft are mixed with local signals to cancel out the predicted Doppler shift due to the relative motion between the spacecraft and the tracking station. By this mixing operation, the frequency of the RF signal from the space-craft is converted down to the vicinity of 20 kHz. This analog signal is digitally recorded on CD-ROM media with 16 bit quantization at a sampling rate of 80 kHz.

Fig.1.Data recording system at Usuda station and the data processing procedure.