Zen-ichi Yamamoto

Phase scintillation observation during coronal sounding experiments with NOZOMI spacecraft

Radio occultation observations of the solar corona at solar offset distances of 12.8-36.9 RS were performed from December of 2000 to January of 2001, using the Nozomi spacecraft (Planet-B) of Japan. Phase scintillation spectra up to the frequency of ∼10 Hz were obtained in two-way mode with S -band uplink and X-band downlink. The spectra cannot be represented by a single power law especially for small offset distances. That is, at low frequencies (large scales) the spectra show slopes indicative of Kolmogorov, a local flattening occurs from ∼0.5 Hz (scale ∼600 km) to ∼ 3H z (∼100 km), and a steepening occurs again at higher frequencies. This three-component nature of the spectrum was observed with a single method for the first time, although the spectral shape is variable and the three-component feature is not always evident.

Space Vlbi Satellite Halca and its Engineering Accomplishments

Abstract The Institute of Space and Astronautical Science (ISAS), Japan, launched a satellite named HALCA in February 1997 by the ISASs new rocket M-V. It has become the first space Very Long Baseline Interferometry (VLBI) satellite of the world by accomplishing a series of engineering experiments, representative ones of which are deployment of 8 m diameter parabolic antenna, precise attitude control of spacecraft, transfer of phase reference signal, high data-rate telemetry, single dish telescope operation, and interferometry with ground radio telescopes. Following the successful engineering experiments, HALCA has been operated for science observations under the science program named “VSOP” (VLBI Space Observatory Programme) in cooperation with many organizations and radio telescopes around the world.

Radio occultation measurement of the electron density near the lunar surface using a subsatellite on the SELENE mission

The electron density distribution in the vicinity of the lunar surface was explored with the radio occultation technique using a subsatellite on the SELENE mission. Although the measurements suffer from contamination by the terrestrial ionosphere and interplanetary plasma, an analysis of more than 300 measurements provides adequate statistics and reveals a general trend. The result suggests that a dense ionosphere covering the whole sunlit side, as suggested by the radio occultation measurements on the Soviet Luna 19 and 22 missions, does not exist. However, weak signatures of electron density enhancement with densities on the order of 100 cm(-3) are observed below 30 km altitude at solar zenith angles less than 60 degrees. The statistically averaged density reaches a peak at around 15 km altitude and decreases gradually at higher altitudes and toward the surface. Although the suggested electron layer is thinner and less extended horizontally than that reported by Luna 19 and 22, the existence of such an ionized layer is still difficult to explain by conventional ionosphere generation mechanisms. An alternative source of electrons may be required.

CISLUNAR PLASMA EXPLORATION WITH THE SELENE RADIO SCIENCE SYSTEM

Abstract The SELENE Project of Japan includes three spacecraft: a main orbiting satellite and two subsatellites. Among many scientific objectives of the Project is the exploration of plasma formations above the lunar surface. The thin plasma layer above the area of the Moon, presented to the Sun, was first detected in radio occultation experiments conducted during 1972 with the Soviet lunar satellite “Luna-19”. The SELENE Project radio science experiment will be realized by means of a dual S- and X- band transmitter installed onboard the orbiter and Earth based antennas. The effect of the thin plasma layer near the Luna can be hidden by the fluctuations of the total electron content of the Earths ionosphere. Dual spacecraft occultation configurations can remove partially the influence of the Earths ionosphere. Several dual spacecraft configurations are discussed and upper error limits of total electron content measurements are estimated.

First test of the NOZOMI radio science system in actual space flight

Abstract The Japanese NOZOMI spacecraft (S/C) launched on the 4th of July, 1998 was equipped with a coherent dual frequency Radio Science System (RSS) to explore the Martian atmosphere and the solar wind plasma by the radio sounding method. Several communication operations were performed in order to test and calibrate the RSS during the cruise phase of the flight to Mars. S- and X- band signals emitted by the S/C were received and recorded at the Usuda tracking station (Japan). Analysis of the data processing results revealed the influence of the S/C spin on all signal parameters. In particular the S/C spin shifted the frequencies of both the S- and X- band signals, destroying their coherency. It was shown that the effect of the spin stabilizing system can be removed from the final occultation results and the Nozomi S/C can be successfully used in radio occultation experiments.

GaN HEMT based rectifier for spacecraft health monitoring system using microwave wireless power transfer

High power operable and miniaturized rectifier is one of the most important issues for spacecraft health monitoring system. This is because the flexibility of location, number, and power consumption regarding sensor tag is indispensable for reliable health monitoring. Previous single rectifiers focusing on schottky diode are unsuitable since operating at high power and considering integrated MMIC sensor tags are quite difficult. This research proposes GaN HEMT based rectifier for these issues due to its excellent properties such as structurally-preferable for MMIC, and suitable for high power operation. Design and evaluation of GaN HEMT based rectifier and an experiment using the rectifier and a thermal sensor with microwave wireless power transfer has been conducted. The proposed rectifier can be operated with 47 dBm input RF signal.

Four-way Doppler tracking for lunar gravity measurements executed by Kaguya and its relay satellite: Okina

SELENE (Selenological and Engineering Explorer) is a Japans lunar probe which was launched and injected into the lunar polar orbit in 2007. The Main Orbiter of SELENE named Kaguya has separated the Relay Satellite: Rstar (Okina). We have executed four-way Doppler measurements which determined the orbit of Kaguya aviating above the lunar far side. The ground station up-links ranging signals, and the relay satellite transponder on Rstar (RSAT-1) relays the carrier waves to Kaguya. Then the transponder on Kaguya (RSAT-2) receives the signals and returns to Rstar, and down-linked to UDSC. Three of four receivers through four-way links acquires signals and tracks Doppler frequency shift with phased lock loops. For that purpose, the receivers should be locked sequentially under the condition of large Doppler shift due to mutual velocities of three moving bodies. The receivers were designed in consideration of the above condition and system operability. Eventually, our system has realized the first case to track two fully moving links between the lunar orbiters and carried out Doppler measurements. Results of the orbit determinations for Kaguya above the lunar far side have shown the anomaly distributions of the gravity fields which could be scarcely found by the conventional two-way RARR methods.

HALCA phase transfer and statistical property of atmospheric phase fluctuations

Abstract The space orbiting radio telescope, HALCA, was placed into highly elliptical earth orbit on February 12, 1997. A master oscillator on board is phase locked to a reference signal generated by a hydrogen maser frequency standard in a ground tracking station. The frequency and the phase of the satellite oscillator are monitored in the ground tracking station. This paper provides a brief summary of the phase transfer link and gives the stability of the phase transfer link before and after the launch. Also this paper gives the discussions on the atmospheric phase fluctuations and the statistical property.