Tetsu Iwata

An improved lunar gravity field model from SELENE and historical tracking data: Revealing the farside gravity features

[1]xa0A new spherical harmonic solution of the lunar gravity field to degree and order 100, called SGM100h, has been developed using historical tracking data and 14.2 months of SELENE tracking data (from 20 October 2007 to 26 December 2008 plus 30 January 2009). The latter includes all usable 4-way Doppler data collected which allowed direct observations of the farside gravity field for the first time. The new model successfully reveals farside features in free-air gravity anomalies which are characterized by ring-shaped structures for large impact basins and negative spots for large craters. SGM100h produces a correlation with SELENE-derived topography as high as about 0.9, through degree 70. Comparison between SGM100h and LP100K (one of the pre-SELENE models) shows that the large gravity errors which existed in LP100K are drastically reduced and the asymmetric error distribution between the nearside and the farside almost disappears. The gravity anomaly errors predicted from the error covariance, through degree and order 100, are 26 mGal and 35 mGal for the nearside and the farside, respectively. Owing to the 4-way Doppler measurements the gravity coefficients below degree and order 70 are now determined by real observations with contribution factors larger than 80 percent. With the SELENE farside data coverage, it is possible to estimate the gravity field to degree and order 70 without applying any a priori constraint or regularization. SGM100h can be used for global geophysical interpretation through degree and order 70.

Picosecond accuracy VLBI of the two subsatellites of SELENE (KAGUYA) using multifrequency and same beam methods

[1]xa0Same beam very long baseline interferometry (VLBI) observations of the two subsatellites of SELENE (KAGUYA) are demonstrated for purpose of the precise gravimetry of the Moon. Same beam VLBI contributes a great deal to cancel out the tropospheric and ionospheric delays and to determine the absolute value of the cycle ambiguity by using the multifrequency VLBI method. As a result, the differential phase delay of the X-band signal is estimated within an error of below 1 ps. This accuracy is more than 1 order of magnitude smaller than former VLBI results. The preliminary results for the orbit determination of the subsatellites show a decrease of the orbit error from a few hundreds of meters to around 10 m when the differential phase delay data are added to the conventional range and Doppler data. These results reveal the possibility of precise gravimetry.

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.

Dual‐spacecraft radio occultation measurement of the electron density near the lunar surface by the SELENE mission

[1]xa0During the SELENE (Kaguya) mission the dual-spacecraft radio occultation technique was used to investigate the electron population in the vicinity of the lunar surface. One pair of coherent S-band radio signals from one spacecraft was used to probe the possible electron density enhancement near the Moon, and another signal pair from the other spacecraft measured the solar wind and the terrestrial ionosphere plasma fluctuations, which also exist in the measurement by the former signal pair. The results suggest that any stable ionosphere with densities comparable to the ones observed by the Soviet Luna 19 and 22 missions does not exist near the terminator at high latitudes, although the occurrence of temporal or localized density enhancements cannot be ruled out.

Results of the Critical Design of RSAT/VRAD Mission Instruments on SELENE Sub-satellites Rstar/Vstar for Selenodesy

Four-way Doppler measurements and differential VLBI will be executed by SELENE to obtain highly accurate global mapping and improve the model of lunar gravity. Two small sub-satellites: Relay Satellite (Rstar) and VLBI Radio Satellite (Vstar), which are separated from SELENE Main Orbiter, perform these selenodetic observation using Relay Satellite Transponder (RAST) and VLBI Radio Sources (VRAD). These sub-satellites have no thrusters to control orbits and attitude to yield precise measurements of orbits perturbed by gravity anomaly. Lack of thrusters, however, causes the instability of attitude that interfere the selenodesy observation. The tip off at the separation and the solar radiation pressure torque dominantly affect the attitude, hence, we evaluated them in the critical design phase of the sub-satellite development. Properties of the newly developed release mechanism were obtained by ground tests, and the satellite attitude inclination by disturbances was analyzed. These results show that the design of the sub-satellites has enough properties to produce selenodesy data.