@misc{oai:ir.soken.ac.jp:00000399,
 author = {寺家, 孝明 and ジケ, タカアキ and JIKE, Takaaki},
 month = {2016-02-17, 2016-02-17},
 note = {Antarctica, most of which is covered with thick ice sheet, is a geologically distinct continent from the others and its characteristics have not been well revealed yet. In order to uncover interesting Antarctic tectonics, which are motion of the Antarctic plate, intra-plate deformation and uplifting by the Post Glacial Rebound, space geodetic methods play more important role than those for the other continents.
　　The Antarctic plate is unique because it surrounded by the expansion area. There have been established several GPS and DORIS points in Antarctica. It has been reported that the crustal velocities in Antarctica acquired by these space geodetic points does not show the systematic residuals after subtracting fitted rigid plate motion. Also, shortness of the observation period prevents interpretations of more complicated movements.
    Current uplifting velocity of the Ongul Islands periphery was not clarified. Tide gauge's data that is recorded at Syowa Station shows subsiding of the sea level at a speed of 9.5mm/y. However, the observation period is not long enough to obtain a quantitative estimate. This result is larger than the averaged uplifting speed of 4mm/y, which is estimated as the Postglacial Rebound by geological survey in the Ongul Islands periphery.
   Syowa Station is built on a continental shield formed by old rocks. Generally, such a place is geologically very stable. Four space-geodetic techniques, which are GPS, DORIS, PRARE and VLBI, establish the observation points on the base-rock in Syowa Station. Accordingly, this Antarctic station is one of the largest space-geodetic observatories in Antarctica and plays the central role to the clarification of the Antarctic tectonics.
   Among the space geodetic techniques, VLBI occupies a distinct position that it can directly determine positions on the Earth's surface with respect to a quasi-inertial reference frame realized by positions of extra-galactic radio sources. Then VLBI is free from the influence by the fluctuation of the Earth's gravity center. This is the important characteristic that differs largely from other space geodetic techniques that the satellite's orbits used as the reference of measurement method. Accordingly, it is expected that the VLBI's results become a reference to the other space geodetic techniques' in Syowa Station.
   Also, because there are only a few VLBI stations in the southern hemisphere, the accuracy of the radio reference frame is worse compared with that in the northern hemisphere. As the south extremity of VLBI stations, Syowa Station is expected to play very important role in the construction of an accurate reference frame in the southern hemisphere. 
The multipurpose 11m parabolic antenna at Syowa Station is one of the most important geodetic instruments in Antarctica since it is the only antenna for VLBI in East Antarctica..The first experimental VLBI observation was carried out in Januarv, 1990 by JARE30 and the position of the VLBI reference point was estimated. However, after this experimental observation, logistic difficulties and severe observational environment made subsequent VLBI observations abandoned until 1998. The regular VLBI observation project using this antenna started by JARE39, as a core of the research project for monitoring motion of the Antarctic plate and crustal movement around Lützow-Holm Bay This project is called the Syowa experiment. During the summer operation, the VLBI observation system including hydrogen maser frequency standards  was built up and its performance was checked. JARE39 brought two hydrogen maser frequency standards to Syowa Station and installed them in the seismograph room. Other equipments such as the front-end, the back-end and antenna control system were also installed in the 11m  antenna and the satellites data reception room. It is the first case that a complete system for geodetic VLBI observation including two hydrogen maser frequency standards was installed at Syowa Station.
   An important characteristic of the Antarctic VLBI network is that two different types of data recording system, namely, K4 at Syowa and S2 at Hobart and HartRAO are used. It is the first case in the world to carry out geodetic VLBI experiment in the network that S2 and K4 are simultaneously used. The Mitaka FX correlator correlated the VLBI raw data obtained with this network because it is the only facility that can process both the types of recorders. This correlator has been primarily used for processing astrophysical data and no geodetic data have been processed with this. So, this is the first occasion to process the geodetic VLBI observation data with the Mitaka FX correlator.
   The VLBI experiments were carried out (February, May, August, and November) 4 times in 1998. Participating stations were Syowa, HartRAO, Hobart, and other temporal stations. Two problems specific to Syowa Station arose during the wintering.. They were decline of sensitivity in the X band and sudden increase of recording error rate. Due to these problems we could not detect fringes in the X band in the February observation and quality of the data of the May and August experiments was not enough to perform subsequent analyses. The increase of the error rate was considered to be due to insufficient control of humidity in the observing room. In the November experiment the recording head of the recorder was cleaned carefully and this kind of trouble did not occur during the observation. Good correlation results were obtained for all the baselines in the November experiment. 
  The Mitaka FX correlatior provides the correlated data in the form of FITS. However, the structure of FITS is not suited to geodetic analysis. So, the various steps are necessary before starting geodetic analyses. The new geodetic analysis software was developed. The bandwidth synthesis and time system transformation are mainly developed in this software.
   A standard algorithm is used in the bandwidth synthesis except for input and output interfaces. However, the observed delay that the software outputs is expressed in a time system that differs from the one used in the standard geodetic analysis. In processing of the Mitaka FX correlator, a delay is defined as a difference of epochs when the same wave-front from a celestial body passes the geocenter and a ground station, respectively, while observed delay is the difference in the time when this wave front passes two ground stations. The geometrical delay between two ground stations is computed as a difference of the delays between the geocenter and individual ground stations. On the other hand the reference time of a prior delay used in geodetic analyses is the time when the wave-front from a celestial body reaches a reference station on the ground. Therefore, time system transformation is necessary to compensate the influence of the difference of time systems between the observed and a priori delays. The magnitude of the delay due to the difference of the time systems is 1～10 nanoseconds.
   The most important process in the conversion is addition of twice the atmosphere propagation delay 
  The geodetic estimation was performed for the data in November, 1998, by using a program called MSLV3. The position of the Syowa VLBI point is estimated to be 1766194.143m, 1460410.932m and -5932273.353m with standard deviations of 2.9cm, 2.5cm and 6.2cm in the x-, y- and z-components in the geocentric Cartesian system. These errors are larger than typical values of recent inter-continental VLBI experiments. Nevertheless, the movement of Syowa VLBI reference point is shown clearly, when this estimated position is combined with other experiments in January, 1990, and November, 1999. The velocity vector of Syowa Station in the local topocentric frame is 0.53cm/y, 0.01cm/y and 1.17cm/y in the north, east- and upward components and their errors are 0.22cm/y, 0.17cm/y and 1.00cm/y, respectively.
   This movement of VLBI reference point is compared with velocities observed with GPS and DORIS, predicted by NNR-NUVEL1A plate motion model, and taken from ITRF2000. The horizontal movements of Syowa Station can evidently be classified into two groups. One is the northward motion composed of VLBI and NUVEL1A. The other is the north-east motion composed of DORIS, IGS and ITRF2000. Close agreement of the horizontal movements between the VLBI observations and the NNR-NUVEL1A prediction may support the rigid plate motion model. The vertical motion obtained from VLBI shows that Syowa Station is uplifting. Its speed is close to that of the sea level change estimated from Syowa tide gauge data. Movements determined from GPS, Doris and the geological survey support this direction. In summary, the result of the present thesis is in good accordance with the global plate motion model and the Post Glacial Rebound. 
  Standard deviation of the post-fit residual is about 300 pico seconds, which is fairly larger than typical values. The causes of the large standard deviation must be clarified to confirm the reliability of the newly developed analysis system and the analysis result. However, the reason of this large standard deviation is not yet revealed and left to the future investigation. In order to distinguish possible error sources, namely, those specific to this experiment and those of the analysis system, a comparison is necessary between the Syowa experiment and referential geodetic VLBI experiments by using baselines and radio sources with well determined positions, and a correlator and a analysis system with established performance.
   The Syowa experiment is continuing after JARE39 and improvement of reliability is expected to the position and velocity of Syowa VLBI reference point. The main part of the geodetic analysis system for the Mitaka FX correlator is considered to be usable as one of the core parts of a VERA analysis system and a large contribution is expected in the VERA project., application/pdf, 総研大甲第551号},
 title = {Result of the Antarctic VLBI experiments duringJARE39 and their geodetic interpretations},
 year = {}
}