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

Abstract

[1] 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.

Results obtained by geodetic instruments of SELENE (KAGUYA)

“…Thus, the 4-way Doppler measurement gave us the first fine global gravity field model. Table 1 shows the results of orbit determinations by Ki- [4], Liu [5] and Goosens [6]. When the orbit is determined only by Doppler and range data, the error is several tens of meters, however, the error drastically decreases and reaches ten meters or so by use of domestic VLBI data in addition to Doppler and range data.…”

Relative position determination of a lunar rover using high-accuracy multi-frequency same-beam VLBI

“…Rstar and Vstar transmitted three carriers in S-band. The differential phase delay between Rstar and Vstar was obtained from the relationship between the correlation phase and the frequency of the three carriers with an error of several picoseconds (1 ps=0.3 mm) [4][5][6][7], which was reduced by 1-2 order compared with the former VLBI results. In this case, the difference in correlation phase at each frequency has to be estimated without the cycle ambiguity.…”

“…In this situation, the influence of the atmosphere, the ionosphere and receivers were almost canceled from the difference in the correlation phases, and differential phase delay was successfully obtained with high accuracy. The orbits of Rstar and Vstar were determined with an accuracy of several meters by using the differential phase delay and a little amount of range and Doppler data [4,6,11].…”

Applications of same-beam VLBI in the orbit determination of multi-spacecrafts in a lunar sample-return mission

“…Rstar and Vstar transmitted three carriers in S-band. The differential phase delay between Rstar and Vstar was obtained from the relationship between correlation phase and frequency of the three carriers with an error of several picoseconds [4][5][6]. This is a reduction of 1 to 2 orders of magnitude compared with the former VLBI results.…”

“…Then the differential correlation phase is obtained from the correlation phases of two or more spacecrafts, which can remove most of the effects from the ionosphere, the atmosphere and the instruments. The differential phase delay with a high accuracy can be obtained from the differential correlation phases [4][5][6]. Here the S-band signals are used instead of X-band signals, because the width of the main beam of the receiving telescope is wider in S-band than in X-band (see Figure 2, left).…”