Nobuyuki Kawano

Lunar global shape and polar topography derived from Kaguya-LALT laser altimetry.

A global lunar topographic map with a spatial resolution of finer than 0.5 degree has been derived using data from the laser altimeter (LALT) on board the Japanese lunar explorer Selenological and Engineering Explorer (SELENE or Kaguya). In comparison with the previous Unified Lunar Control Network (ULCN 2005) model, the new map reveals unbiased lunar topography for scales finer than a few hundred kilometers. Spherical harmonic analysis of global topographic data for the Moon, Earth, Mars, and Venus suggests that isostatic compensation is the prevailing lithospheric support mechanism at large scales. However, simple rigid support is suggested to dominate for the Moon, Venus, and Mars for smaller scales, which may indicate a drier lithosphere than on Earth, especially for the Moon and Venus.

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.

Selenodetic experiments of SELENE: Relay subsatellite, differential VLBI, and laser altimeter

Abstract Since 1960s, the gravitational potential of the Moon has been extensively studied from Doppler tracking data between a ground station and spacecraft orbiting in front of the Moon (e. g., Lorell and Sjogren, 1968; Bills and Ferrari, 1980; Konopliv et al. , 1993; Lemoine et al. , 1997). Because direct radio communication is interrupted while spacecraft is orbiting behind the Moon, however, the coverage of tracking data has been limited mostly to the nearside of the Moon so far. In order to compensate for such lack of tracking data, we propose satellite-to-satellite Doppler measurement by using a relay subsatellite in Japanese mission to the Moon in 2003. A complete coverage of Doppler tracking from an orbiter at sufficiently low altitude will significantly improve lunar gravity model and will contribute for future geophysical study of interior and tectonics on the Moon. Further, we propose differential VLBI experiment between the subsatellite and a propulsion module landed on the surface of the Moon. The differential VLBI is about 10 times more accurate than conventional Doppler measurement for long-wavelength gravity field. Besides, differential VLBI is sensitive to the displacement perpendicular to the line of sight. Thus the VLBI experiment provides precise estimates of the lunar gravity potential at low degree. The last proposal for selenodetic experiments is a laser altimeter. Global topography model has been already developed from the analysis of Clementine LIDAR data (Zuber et al. , 1994), but it is suggested that the model includes appreciable anisotropy between NS and E-W directions due to highly eccentric orbit of Clementine spacecraft (Bills and Lemoine, 1995). The laser altimeter experiment from an orbiter in nearly circular orbit will provide a new reference for the isotropic lunar topography model.

Precise positioning of spacecrafts by multi-frequency VLBI

Multi-Frequency VLBI (MFV) is one of the most powerful methods for precise positioning of spacecrafts. The system transmits three carrier waves at S-band and one wave at X-band. These frequencies are set to resolve the cycle ambiguity of carrier wave at X-band from two group delays between carrier waves and one phase delay of the carrier wave at S-band. The procedure to resolve the cycle ambiguity is proposed in this article. Some conditions about frequency variation and prediction of position and the ionosphere are also clarified for resolving the cycle ambiguity. The dedicated recording system for MFV is developed. A preliminary observation of MFV is carried out with this system by using Lunar Prospector. As a result of the experiment, residual phases from predicted ones are within ±2π, and the RMS of the residual for the period of several seconds is about 4 degrees. This result supports realization of the MFV.

Development of an ultrastable fiber optic frequency distribution system using an optical delay control module [for frequency standard and VLBI]

An ultrastable fiber optic frequency distribution system is described. The ultrastable phase stability is achieved by configuring a closed phase-locked loop with a single-mode optical fiber transmission line in which two optical carrier signals with different wavelengths are transmitted as a forward and a backward signal, and installing an optical delay control module which has no differential dispersion effect between the two optical carrier waves and induces no electrical noise. A phase stabilized optical fiber (PSOF) is used for the signal transmission line. The stabilities of this system are 7.5/spl times/10/sup -17/ and 1.1/spl times/10/sup -17/ in Allan standard deviation at 1000 and 10000 s averaging time, respectively, while the environmental temperature of the PSOF cable varies as much as the range of 10/spl deg/C and the rate of 10/spl deg/C/12 h.

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.

Statistical Characteristics of atmospheric phase fluctuations observed by a VLBI system using a beacon wave from a geostationary Satellite

This paper reports the statistical characteristics of phase fluctuations obtained by a very long baseline interferometry, which received beacon waves from a geostationary satellite. Observations were made in different weather conditions by the 6-m Kagoshima and the 10-m Mizusawa radio telescopes, which were 1284 km apart from one another. Atmospheric phase fluctuations ranging from 0.2 to about 1000 s were detected. To study a variety of statistical characteristics of these phase fluctuations, the Allan standard deviation, /spl sigma//sub y/(/spl tau/), the temporal structure function, D/sub /spl phi//(/spl tau/), and the square root of power spectrum, G/sub /spl phi//(f), were calculated from the observation results. These qualities were found to depend on the time interval /spl tau/ or the frequency f (f=1/2/spl tau/) as follows: /spl sigma//sub y/(/spl tau/)/spl prop//spl tau//sup -0.6/, /spl tau//sup -0.3/, D/sub /spl phi//(/spl tau/)/spl prop//spl tau//sup 0.9/, /spl tau//sup 1.3/ for 0.2 sf>0.33 Hz and G/sub /spl phi//(f)/spl prop/f/sup -1.3/ for 0.33 Hz>f>0.00125 Hz. The curves of /spl sigma//sub y/(/spl tau/) and D/sub /spl phi//(/spl tau/) exhibited shifts whose magnitudes followed the weather order clear, cloudy, and rainy. For /spl tau/>1.5 s (or f<0.33 Hz), the experimental results were consistent with the theoretical values predicted by Kolmogorovs turbulence theory and a frozen-screen model, but for /spl tau/<1.5 s (or f>0.33 Hz), the model has to be modified to explain the experimental results.

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.