Hideo Hanada

Same-beam VLBI observations of SELENE for improving lunar gravity field model

The Japanese lunar mission, Selenological and Engineering Explorer (Kaguya), which was successfully launched on 14 September 2007, consists of a main satellite and two small satellites, Rstar and Vstar. Same-beam very long baseline interferometry (VLBI) observations of Rstar and Vstar were performed for 15.4 months from November 2007 to February 2009 using eight VLBI stations. In 2008, S band same-beam VLBI observations totaling 476 h on 179 days were undertaken. The differential phase delays were successfully estimated for most ( about 85%) of the same-beam VLBI observation periods. The high success rate was mainly due to the continuous data series measuring the differential correlation phase between Rstar and Vstar. The intrinsic measurement error in the differential phase delay was less than 1 mm RMS for small separation angles and increased to approximately 2.5 mm RMS for the largest separation angles ( up to 0.56 deg). The long-term atmospheric and ionospheric delays along the line of sight were reduced to a low level ( several tens of milimeters) using the same-beam VLBI observations, and further improved through application of GPS techniques. Combining the eight-station ( four Japanese telescopes of VLBI Exploration of Radio Astrometry and four international telescopes) S band same-beam VLBI data with Doppler and range data, the accuracy of the orbit determination was improved from a level of several tens of meters when only using Doppler and range data to a level of 10 m. As a preliminary test of the technique, the coefficient sigma degree variance of the lunar gravity field was compared with and without 4 months of VLBI data included. A significant reduction below around 10 deg ( especially for the second degree) was observed when the VLBI data were included. These observations confirm that the VLBI data contribute to improvements in the accuracy of the orbit determination and through this to the lunar gravity field model.

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.

Effects of a physical librations of the moon caused by a liquid core, and determination of the fourth mode of a free libration

An analytical theory of lunar physical librations based on its two-layer model consisting of a non-spherical solid mantle and ellipsoidal liquid core is developed. The Moon moves on a high-precision orbit in the gravitational field of the Earth and other celestial bodies. The defined fourth mode of a free libration is caused by the influence of the liquid core, with a long period of 205.7 yr, with amplitude S = 0″0395 and with an initial phase Π0 = −134° (for the initial epoch 2000.0). Estimates of dynamic (meridional) oblatenesses of a liquid core of the Moon have been estimated: ɛD = 4.42 × 10−4, μD = 2.83 × 10−4 (ɛD + μD = 7.24 × 10−4). These results have been obtained as a result of comparison of the developed analytical theory of physical librations of the Moon with the empirical theory of librations of the Moon constructed on the basis of laser observations.

Development of a small telescope like PZT and effects of vibrations of mercury surface and ground noise

A PZT type telescope for observations of gravity gradient and lunar rotation was developed, and a Bread Board Model (BBM) for ground experiments was completed. Some developments were made for the BBM such as a tripod with attitude control system, a stable mercury pool and a method for collecting the effects of vibrations. Laboratory experiments and field observations were performed from August to September of 2014, in order to check the entire system of the telescope and the software, and the results were compared to the centroid experiments which pursue the best accuracy of determination of the center of star images with a simple optical system. It was also investigated how the vibrations of mercury surface affect the centroid position on Charge Coupled Device (CCD). The results of the experiments showed that the effects of vibrations are almost common to stars in the same view, and they can be corrected by removing mean variation of the stars; and that the vibration of mercury surface can cause errors in centroid as large as 0.2 arcsec; and that there is a strong correlation between the Standard Deviation (SD) of variation of the centroid position and signal to noise ratio (SNR) of star images. It is likely that the accuracy of one (1) milli arcsecond is possible if SNR is high enough and the effects of vibrations are corrected.