SELENE Main Orbiter (KAGUYA) has separated two small sub-satellites; (1) the Relay Satellite “OKINA (Rstar)”, and (2) the VLBI Radio Satellite “OUNA (Vstar)”. These sub-satellites started to perform 4-way Doppler measurements using Relay Satellite Transponder (RSAT) and multi-frequency phase-delay differential VLBI using VLBI Radio Sources (VRAD) for lunar gravity mapping. We have developed the frequency conversion system, multi frequency S/X-band vertical dipole antenna, and light weighted S-band patch antenna to perform these missions. Simple structured release mechanism has also been developed and confirmed its performance by ground test and orbital demonstration using micro-Lab Sat. Initial check out were executed and properties of satellite bus equipments, onboard mission instruments, and observation systems including ground stations were evaluated. Electric power and thermal control subsystems have shown that they conduct as designed and inspected in the ground tests. The release mechanisms have given the spin which can maintain the stability of the satellite attitudes. Communication functions of mission instruments conform to the link budgets. These results suggest that OKINA and OUNA have enough performances to produce efficient data by RSAT/VRAD gravity observations.
Lunar gravimetry on the far side is one of the most important missions in SELENE. A stable frequency transmitted from a ground station is relayed to Main Orbiter by Rstar, and -Main Orbiter returns back the signal to Rstar. Then the signal is changed to X-band in Rstar, and transmitted to the ground station. This measurement is called 4way Doppler method. When the 4way Doppler measurement is performed successfully, the Rstar receives the signal from the ground station and transmits a different frequency signal synchronized with the received signal to Main Orbiter by using a PLL. In this case, the PLL is in “Locked” status. Main Orbiter also relays the signal by the same way. Usually the status of “Locked” or “Unlocked” is sent to the ground station as telemetry information to confirm the completion of the 4way link. However, the ground station is not able to get the telemetry information from Main Orbiter on real time, because the orbiter is on the far side during 4way Doppler measurements. The new methods have been proposed to know the status of the PLL from variations of the Doppler rate and/or frequency shift of the received signal itself on the ground station. They were applied to actual operations of SELENE and were very useful.
Temperature and power supply of two sub satellites of SELENE (Kaguya), Rstar and Vstar, are controlled either by automatically or by commands from a ground tracking station, and their conditions are judged from related telemetries such as voltage, current and temperature of the batteries. After two sub satellites were separated from the main orbiter of SELENE, first test operations of all the instruments onboard the two sub satellites were carried out in the initial check-out for six days, and the electric power system such as solar arrays and batteries were confirmed to have enough power to supply for all the instruments. It was also confirmed that temperatures of all the instruments were controlled in the range of designed value. Throughout the nominal mission period from the beginning of November in 2007 to the end of October in 2008, there was no serious problem in the temperature control system and the electric power system even in an eclipse of the moon, and it was clarified that Rstar and Vstar were capable of producing more power than expected from detailed analysis of the telemetry data obtained during the eclipse. As the result, we could increase chances of 4-Way Doppler measurements and VLBI measurements by several tens of percents, and could obtain a better gravity anomaly map of the lunar far side.
The Laser altimeter (LALT) is one of the 14 instruments boarded on KAGUYA lunar orbiter. The LALT emits a laser pulses every 1 s or 2 s from about 100 km altitude to the lunar surface and the round trip times of these pulses are converted to range data. The LALT incorporates Cr doped Nd:YAG laser whose pulse energy is 100 mJ and the beam divergence angle is 0.4 milli radian yielding the footprint diameter of 40 m. The aim of the LALT is to conduct precise and accurate measurements of the global topography of the Moon. The first laser ranging test of the LALT was carried out on 25 November, 2007 when the high voltage and laser transmit and receiving unit were turned on for the first time. The LALT has started nominal observations since 30 December, 2007 after the initial tuning of the avalanche photo diode (APD) detector in the receiving system. The LALT acquired more than 11 million range data by the end of November, 2008. Detailed descriptions of the measurement principle, the hardware design, structures, and several functions of the LALT are presented in the first half of this article; then followed by reviews of several tests and their results on the ground and in orbit.
Scientific data obtained by 14 mission instruments of SELENE (Kaguya) are managed by JAXA in a unified way at SELENE Operation and Analyses Center (SOAC). Among them, selenodetic data obtained with the laser altimeter and two sub-satellites (Rstar and Vstar) are archived and processed at National Astronomical Observatory of Japan (NAOJ) in Mizusawa. The stream of the data including the altimeter data, Doppler- and range-type satellite tracking data is described in association with computer systems at SOAC and NAOJ. Also described is a system for VLBI data acquisition at four domestic and four foreign stations which are remotely controlled from Mizusawa as well as a system for correlation of VLBI data. These systems have been stably operated to produce scientific data products related to lunar topography and gravity field.
The Japanese lunar explorer SELENE (KAGUYA), which has been launched on Sep. 14th, 2007, utilizes VLBI observations in lunar gravimetry investigations. This can particularly improve the accuracy of the low degree gravitational harmonics. Combination of ground based VLBI observations and Doppler measurements of the spacecrafts enable three dimensional orbit determinations and it can improve the knowledge of the gravity field near the limb. Differential VLBI Radio sources called VRAD experiment involves two on-board sub-satellites, Rstar (Okina) and Vstar (Ouna). These will be observed using differential VLBI to measure the trajectories of the satellites with the Japanese network named VERA (VLBI Exploration of Radio Astrometry) and an international VLBI network. Two new techniques, a multi-frequency VLBI method and the same-beam VLBI method, are used to precisely measure the angular distance between the two sub-satellite radio sources Okina and Ouna. The observations are at three frequencies in S-band, 2212, 2218 and 2287 MHz, and one in X-band, 8456 MHz. We have succeeded in making VLBI observations of Okina/Ouna with VERA and the international network, and have also succeeded in correlating of signals from Okina/Ouna, and obtained phase delays with an accuracy of several pico-seconds in S-band.
Ground systems of “KAGUYA (SELENE)” mainly consist of “Tracking and control system” and “Mission operation and analyses system”. This paper presents the overview of KAGUYA’s ground systems for flight dynamics operation describing operation flow related to orbital information.
VLBI data analysis system has been developed for the VRAD mission of Japanese lunar explorer program KAGUYA. The system is composed of correlation and delay estimation parts. Carrier search program has been newly developed in the correlation part. This program automatically distinguishes the carrier wave signal of the lunar orbiter from spurious signal. The delay estimation part has been newly developed. This program corresponds to the multi frequency VLBI method and estimate the cycle ambiguity of S-band and X-band signal automatically. Graphical user interfaces has been also developed both for the correlation and delay estimation parts. VLBI data analysis system contributes a great deal to reduce processing time. This makes it possible to accumulate the delay data which is enough to estimate the orbit of lunar orbiters and lunar gravity field precisely. Moreover, our system is applicable to future deep space mission in terms of orbit determination by using VLBI technique.
The Japanese lunar mission, SELENE (Kaguya) consists of a Main satellite and two small satellites, Rstar and Vstar. In S-band same-beam VLBI observations of Rstar and Vstar, phase fluctuations caused by the atmosphere, ionosphere and instruments were reduced to a low level of 1deg～2 deg, and the S-band differential phase delay between Rstar and Vstar was obtained with an error of 1 pico-second. We corrected the long-time atmospheric and ionospheric delays by using GPS techniques. According to the VLBI observation results up to February 2008, we changed the VLBI observation method that the telescope tracked the midpoint of Rstar and Vstar even if when the separation angle is as large as 0.56 deg. From March 2008, the chances for S-band same-beam VLBI observations are thus much increased and more data were obtained for orbit determination and estimating the global lunar gravity field model. We performed orbit determination for Rstar and Vstar, the accuracy was improved from a few tens meters when only using Doppler and range data, to a level of about 10 m when S-band same-beam VLBI data are also used.
Results are presented for the orbit determination of the three satellites of Kaguya. Doppler data fit for the main satellite is typically at a level of 0.5 mm/s, and that for the subsatellites is typically less than 0.5 mm/s. Orbit accuracy for the main satellite from altimeter crossovers gives a total orbit accuracy of 50 m. Orbit overlaps confirm this total accuracy, and show a radial accuracy of generally less than 1 m. For the subsatellites Rstar and Vstar, accuracy is restricted due to sparse data coverage, but the inclusion of high-precision differential VLBI data improves the consistency of the orbits greatly, to a level of ten meter in a three-dimensional sense.
A new global lunar gravity field model SGM90f is developed from historical tracking data and about 5 months of SELENE (Kaguya) tracking data. The latter includes 4-way Doppler data which allowed direct observations of the far-side gravity field for the first time. The new model successfully reveals the ring-shaped free-air gravity anomalies on the far-side which correlate well with the topographic features of impact basins. Comparison between the SGM90f model with the LP100K model shows that the large gravity errors which existed in the pre-SELENE model are drastically reduced and asymmetric error distribution between the near-side and the far-side is much improved. Owing to the 4-way Doppler measurements the gravity coefficients below degree and order 55 are now determined by real observations with contribution factor larger than 90 percent.
New global topographic model of the Moon with a spatial resolution less than 0.5 degrees (its angular distance is about 15 km) has been derived from the laser altimeter (LALT: Laser ALTimeter) on the Japanese lunar explorer KAGUYA (SELENE). The model reveals more detailed lunar topography than the previous global one (ULCN2005) for scales less than a few hundred kilometers. Spherical harmonic analysis of the new lunar topography shows the Moon has rougher topography than was in the previous models. Global correlation between this topographic model (STM359_grid-02) and the latest gravity model (SGM90d) and their admittance spectrum give fundamental clues for compensation of lunar topography and its evolutional restrictions. Topographic features for scales more than about 180 km (6 degrees in angular distance) are supported partly by the isostatic compensation, while other features are done by the crustal rigidity.
On the basis of the gravity model of the Moon developed by SELENE (Kaguya), we propose new classification and compensation mechanism of lunar impact basins. Impact basins on lunar far side and limb are classified into Type I and II basins depending on the magnitude of central gravity high in free-air and Bouguer gravity anomalies. Among previously known gravity anomalies, most typical mascons are referred as primary mascon basins. Topographic depression and rim of both Type I and II basins show good correlation between topography and free-air gravity anomaly suggesting elastic support of lunar lithosphere. Central gravity high of Type I basin is inferred to be a result of mantle uplift at the time of basin formation, and is elastically supported, too. On the other hand, free-air anomalies at the center of Type II basins are lower than Bouguer anomalies indicating brittle deformation of the basins. Topographic depression and rim of primary mascon basins on near side of the Moon show little to no free-air gravity anomalies. This indicates a result of elastic relaxation that occurred probably after eruption of mare basalts. Plateau-like signature of gravity anomalies of primary mascon basins implies viscous relaxation at crust-mantle boundary beneath the basins and significant heat (or volatile) transport by basaltic magma.
The electron density profiles above the lunar surface are being observed by the radio occultation technique during the mission using the Vstar and Rstar sub-satellites. In addition to a traditional technique which uses one orbiter, we are conducting another method which uses two orbiters with the second one being used to measure the terrestrial ionosphere contribution. Previous radio occultation measurements have indicated the existence of an ionosphere with densities of up to 1000 cm-3 above the dayside lunar surface. These densities are difficult to explain theoretically when the removal of plasma by the solar wind is considered, and thus the generation mechanism of the lunar ionosphere is a major issue, with even the validity of previous observations still under debate. The SELENE radio science experiment will establish the morphology of the lunar ionosphere and will reveal its relationship with various conditions to provide possible clues to the mechanism.