Fuyuhiko Kikuchi

Farside Gravity Field of the Moon from Four-Way Doppler Measurements of SELENE (Kaguya)

The farside gravity field of the Moon is improved from the tracking data of the Selenological and Engineering Explorer (SELENE) via a relay subsatellite. The new gravity field model reveals that the farside has negative anomaly rings unlike positive anomalies on the nearside. Several basins have large central gravity highs, likely due to super-isostatic, dynamic uplift of the mantle. Other basins with highs are associated with mare fill, implying basalt eruption facilitated by developed faults. Basin topography and mantle uplift on the farside are supported by a rigid lithosphere, whereas basins on the nearside deformed substantially with eruption. Variable styles of compensation on the near- and farsides suggest that reheating and weakening of the lithosphere on the nearside was more extensive than previously considered.

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.

Effects of Phase Characteristics of Telescopes on Same-Beam Differential VLBI

Phase characteristics, which are systematic phase offsets across the main beam of an actual telescope, may be a problem for achieving the same-beam differential very long baseline interferometer (VLBI) technique. This technique is essential for differential phase delay measurements such as those used in the Japanese Selenological and Engineering Explorer (SELENE) project, where the phase has to be determined to an accuracy of 0.075 radians rms. Accurate measurement and correction of phase characteristics are very important. The phase characteristics at 2.2375 and 2.2807 GHz of the 20-m and 10-m telescopes at Mizusawa were measured to an error of approximately 0.04 radians rms. The phase characteristics were 0.06 radians rms for the 20-m and 0.055 radians rms for the 10-m telescopes in the main beams, and the post-fit residuals decreased to 0.03 and 0.04 radians rms, respectively, after correcting by using quadratic formulas. These results confirmed the effectiveness of the same-beam differential VLBI technique for VLBI observations of SELENE

VLBI observation of narrow bandwidth signals from the spacecraft

We carried out a series of VLBI observations of Nozomi by using a dedicated narrow bandwidth VLBI system. The three carrier waves with frequency interval of 515 kHz were recorded in 3 channels of the system and correlated by a software method. As a result of the correlation, the residual fringe phases of the main carrier wave are obtained for every 1.3 seconds. We can also continuously track them for 100 minutes. The variation of the residual fringe phase is +/− 150 degrees. Moreover, we can derive succesively the group delay for every 100 seconds by using these three carrier waves. The RMS of the group delays is 13 nsec and its average is well accorded with the delay determined by the range and Doppler measurements within an error of 2 nsec. Consequently, we confirmed the validity of the narrow bandwidth VLBI system, and it could be expected that this system, in addition to range and Doppler measurements, can be applied to three-dimensional tracking of a spacecraft and the precise gravity measurement of the Moon and the planets.

New method of measuring phase characteristics of antenna using Doppler frequency measurement technique

This paper reports on a new method of measuring the phase characteristics of an antenna using the Doppler frequency measurement technique. With this method, the antenna being tested is rotated at a rate of f/sub sp/ around an axis through its geometrical center, and the phase characteristics of the antenna are calculated from the harmonic components of f/sub sp/ during time variations in the Doppler frequency of radio waves emitted from the antenna. Using this, we obtained three-dimensional phase characteristics of a patch antenna with a root-mean-square error of about 0.5/spl deg/, and confirmed its efficacy through experimental results.

Simulation analysis of differential phase delay estimation by same beam VLBI method

The same beam VLBI method (SBV) is newly applied to the multi-frequency VLBI method in the VRAD mission of SELENE (KAGUYA). By simultaneously observing two nearby spacecraft with one antenna, the error sources of VLBI measurement common in two propagation paths can be almost canceled out. In this paper, error estimation and simulation analysis are carried out for a feasibility study to apply the SBV method to the VRAD mission. Differential phase delay can be estimated without cycle ambiguity even if tropospheric fluctuation is large and/or traveling ionospheric disturbance occurs. The sensitivity of the differential phase delay with respect to the average elevation angle and the elongation of two spacecraft is also investigated. Moreover, a method is developed for estimating differential phase delay in switching VLBI observations using the cycle ambiguity derived from SBV observations. This method can be performed in more than 90% of the VRAD mission’s total paths. Precise positioning with SBV contributes to accurate estimation of the low degree coefficients of lunar gravity fields by more than one order of magnitude than previous results.

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.

Lunar mare volcanism: lateral heterogeneities in volcanic activity and relationship with crustal structure

Abstract Lunar mare basalts are spatially unevenly distributed, and their abundances differ between the nearside and farside of the Moon. Although mare asymmetry has been attributed to thickness variations in the low-density anorthositic crust, the eruptive mechanism of lunar magma remains unknown. In this study, we investigate the relationship between mare distribution and crustal thickness using geological and geophysical data obtained by the SELENE (Kaguya) and the Gravity Recovery and Interior Laboratory spacecraft, and quantitatively re-evaluate the influence of the anorthositic crust on magma eruption. We identify a lateral heterogeneity in the upper limit of crustal thickness that allows magma extrusion to the surface. In the Procellarum KREEP Terrane, where the surface abundances of heat-producing elements are extremely high, magmas can erupt in regions of crustal thickness below about 30 km. In contrast, magma eruptions are limited to regions of crustal thickness below about 20 km in other nearside regions, around 10 km in the South Pole–Aitken Basin and approximately 5 km in the farside Felspathic Highland Terrane. Such heterogeneity may result from lateral variations in magma production in the lunar mantle and/or crustal density.

Effect of Phase Pattern of Antennas Onboard Flying Spin Satellites on Doppler Measurements

The effect of the phase patterns of antennas onboard flying spin satellites on the Doppler measurements is reported. Phase patterns mean that there are deviations in wave fronts from perfect sphericity, expressed as a function of the angular position around the antenna. We analyzed what effect the phase patterns of dipole and patch antennas onboard two flying spin satellites, Rstar and Vstar, used in the Japanese lunar mission, SELENE (KAGUYA), had on 2-way and 4-way Doppler measurements, and detected higher harmonics in the spin frequency up to an order of 26 in the Doppler frequency. We developed a low-pass filter (LPF) using a Kaiser window, with the optimal parameters empirically determined, to remove the influence of phase patterns and to precisely conserve information on the lunar gravity field. We processed the 2-way and 4-way Doppler data of SELENE by using LPF. After using LPF, a high degree of accuracy of about 0.001 Hz was achieved for the 2-way Doppler measurements, and signals that reflected the gravity field on the far side of the Moon were first detected from the 4-way Doppler data. We also suggested a method for estimating the phase response of satellite antennas using the Doppler frequency variations. In order to estimate the Doppler frequency variation, a filtering technique was adopted to extract the harmonics of interest in the residual signal, from which the antenna phase pattern was derived.

Same-beam VLBI observation of SELENE

The Japanese lunar mission, SELENE (Kaguya) consists of a main satellite and two small satellites, Rstar and Vstar. In same-beam VLBI observations of Rstar and Vstar, phase fluctuations caused by atmosphere, ionosphere and instruments were reduced to a low level of 1–2 deg, and the differential phase delay between Rstar and Vstar was obtained with a very low error of 2 pico-seconds. We corrected the long-term atmospheric and ionospheric delays by using GPS techniques and analyzed other possible influence such as phase-frequency characteristic of the receivers and phase variation in the main beam of telescopes. We performed orbit determination for Rstar and Vstar, the accuracy was much improved from a few tens meters when using only Doppler and range data, to a level of about 10 m when same-beam VLBI data are also used. In addition, the lunar gravity field model was also improved by combing VLBI data.