N. Kawano

Regional ionosphere map over Japanese Islands

Based on the high time and spatial resolution total electron content (TEC) data, which is estimated from the phase and code observables obtained by using GPS (Global Positioning System) Earth Observation Network (GEONET), the TEC distribution and its time variation over Japanese Islands are scaled into 0.5° × 0.5° grid data for each 10 minutes. The TEC daily map time series are arranged in an array to show the TEC evolution. Based on the spherical harmonics expansion of global ionospheric TEC model (GIM), which is estimated from global GPS observation, the TEC maps are expanded firstly through as high as 60 degrees and orders for a spherical harmonic function as a regional ionosphere map (RIM). The evolution history of medium scale traveling ionospheric disturbances (MSTIDs), i.e. polarward intense TEC enhancement and pre-noon rapid irregular fluctuations near the geomagnetic equator, are identified and confirmed in a quiet geomagnetic period.

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.

Application of a PZT telescope to In situ Lunar Orientation Measurement (ILOM)

We are proposing a selenodetic mission, e.g. In situ Lunar Orientation Measurement (ILOM) to study lunar rotational dynamics by direct observations of the lunar physical liberation and the free librations from the lunar surface with an accuracy of 1 millisecond of arc in the post-SELENE project. Year-long trajectories of the stars provide information on various components of the physical librations and they can also be used to possibly detect the lunar free librations in order to investigate the lunar mantle and the liquid core. The PZT on the moon is similar to that used for latitude observations of the Earth except the half mirror above the objective, a CCD with high well capacity, and the heater beneath the mercury pool. Although a star position on the focal plane does not change even if the telescope inclines in principle, the tilt of the telescope affects the star position due to aberrations of the objective in the actual case. We obtained the relation between the deviation of the star position in CCD and the tilt of the telescope by ray tracing method and found that the effect of the tilt less than 100 arc seconds upon the star position does not exceed 1 mas. Thermal test of a tube made of CFRP showed that the tube did not incline by more than 100 seconds of arc even in the conditions of the lunar surface. We have a prospect to attain an accuracy of positioning of better than 1 mas from simulated experiments in laboratory using a CCD.

Observation Site Atmospheric Phase Fluctuations Observed by Three-Element VLBI

The beacon signals from a geostationary satellite were observed using three-element very long baseline interferometry (VLBI), and the phase fluctuations along the baselines between three sites were obtained. The atmospheric phase fluctuations at each observation site were derived from the baseline phase fluctuation data. The fluctuations were classified into three time-interval regions based on the dependence of the Allan standard deviation of the fluctuations on the time interval. In the region where the interval was less than a few seconds and in the one where it was greater than one hundred seconds, the curve of the Allan standard deviation was steep and showed the property of white phase noise. In the region between these two regions, the dependence of the Allan standard deviation on the time interval was weak. The magnitude of the Allan standard deviations for the three observation sites showed time variations in the region where the time interval was longer than a few seconds. Comparison with the weather conditions suggested that the time variations of the Allan standard deviation reflected atmospheric instability above the sites.

A new frequency synthesis approach to precisely track the spacecrafts using Same-Beam Interferometry

This article describes a new frequency synthesis approach to realize extremely accurate relative position determination for two spacecrafts with phase measurements at a variety of frequency separations. Both of the spacecrafts transmit one carrier wave phase modulated by two DOR sine tones at X-band, and one wave at S-band. These frequencies are set to resolve the cycle ambiguity of carrier wave at X-band from two group delays corresponding to the four DOR sub-carriers and one phase delay of the carrier wave at S-band. The procedure to resolve the cycle ambiguity is analysed and discussed in detail, and the corresponding conditions, such as prediction of light time, the transmission media, are also clarified based on mathematical analysis and the same-beam tracking data from SELENE mission. The results show that all the conditions to resolve the cycle ambiguity can be satisfied in Same-Beam Interferometry. Thus, the accuracy of the differential phase delay could be achieved within several picoseconds. This method can be used in the missions with challenging navigation requirements, such as planet-relative targeting, as well as rendezvous and docking around Moon and planets beyond.

Observation System in Radio Interferometry for Selenodesy (RISE)

We are investigating the project RISE (Radio Interferometry for SElenodesy) which could measures angular distances between a radio transmitter on a lunar orbiter or on the Moon and quasars by differential VLBI. RISE is an equipment used for the selenodesy mission, which will be performed together with a laser altimeter and a relay satellite. We can measure amplitudes of the physical librations and gravitational harmonic coefficients of the Moon through this technique with an accuracy one or two orders higher than before. It can put stronger constraint on the bulk composition of the Moon through the moment of inertia. It is necessary to develop radio transmitters which can emit several carrier waves at different frequencies in S and X bands with bandwidths of about 1 kHz and with powers of a few hundreds of milliwatts. VLBI antennas forming networks on the Earth receive the radio waves from the lunar orbiter, the lunar surface and the quasars alternatively. It is also necessary to develop a special receiving system with very narrow frequency bands in order to improve the sensitivity and to simplify the data reduction. The narrow band receiving system is appropriate for RISE since there is very limited power on the Moon.

New method to resolve 2π ambiguity in NBV

The Narrow Bandwidth VLBI (NBV) technique is utilized to track spacecrafts in Japanese lunar exploration. However, the problem of 2π ambiguity affects the phase delay of the carrier waves transmitted by satellites. A probabilistic algorithm called coarse search and fine delay search, in combination with a simple ambiguity judgment procedure, is presented in the current paper to resolve the 2π ambiguity. This method is employed to estimate both the delay and the delay rate from the residual phases obtained using NBV. Compared with previous analytic methods, it does not require strict constraint conditions. The ambiguity can be resolved even when the phase variations in the residual phase are 0.2 rad (approximately 11.6 degrees) compared with the less than 4.3 degree phase variations in the analytic method. The method also has the advantage of giving short time variations in the orbital motion of a satellite without ambiguity using NBV.

Laser Altimetry in the Selene Project

Laser altimetry of the Moon is now under investigation as a part of the RISE project in the Moon-Orbiting exploration (SELENE; SELenological and ENgineering Explorer) by Japan in 2002–2003. Main objective of this measurement is to construct a precise and global topography of the moon with the 5m vertical resolution and with a center-to-center along-track footprint spacing of 330m. The topography will be a great contribution for addressing several problems of lunar geology and geophysics such as morphological dichotomy, internal structure, isostatic state, and various surface modification processes of the moon.