竹内 央

VSOP-2 is a next generation space-VLBI (SVLBI) mission for millimeter-wave VLBI astronomy. The VSOP-2 mission has a radio telescope satellite, ASTRO-G, as a space segment, which will be launched in JAXA's fiscal year 2012. The ASTRO-G satellite will be orbiting around the Earth with the apogee and perigee altitudes of 25000 and 1000 km, respectively. In VSOP-2, Precise Orbit Determination (POD) with an accuracy of 10 cm for ASTRO-G is required to conduct an advanced observation mode of phase referencing. To achieve the POD for such a highly elliptical orbit, two on-board instruments are proposed to be installed on ASTRO-G: one is a Global Navigation Satellite System (GNSS) receiver mainly used at lower altitudes, and another is a retroreflector array for Satellite Laser Ranging (SLR) at higher altitudes. In this paper, ASTRO-G's POD plan with the above two instruments are reported. POD simulations indicate that the proposed method is promising, while development of the acceleration model to treat non-gravitational perturbation is essential for the ASTRO-G's POD.

This report describes the evaluation of Δ VLBI delay measurement accuracy for Spacecraft Navigation. At the epochs of Hayabusa's touchdown to asteroid Itokawa, we made Δ VLBI observation for Hayabusa with Japanese domestic VLBI stations. Since the orbit of Itokawa is accurately known, we could use this occasion for evaluation of the delay calibration accuracy by Δ VLBI. By using a calibration technique of modeling the excess delay with group delays of multiple reference sources, calibration accuracy in the order of several hundreds pico seconds are obtained. Further improvements are expected by using dual-band observation for reference sources. As another approach, phase delay data of reference radio sources were applied for calibration with expecting improved precision and temporal resolution, though it did not show significant improvement for calibration in this case. © 2007 SICE.

This report describes the evaluation of Delta VLBI delay measurement accuracy for Spacecraft Navigation. At the epochs of Hayabusa's touchdown to asteroid Itokawa, we made Delta VLBI observation for Hayabusa with Japanese domestic VLBI stations. Since the orbit of Itokawa is accurately known, we could use this occasion for evaluation of the delay calibration accuracy by A VLBI. By using a calibration technique of modeling the excess delay with group delays of multiple reference sources, calibration accuracy in the order of several hundreds pico seconds are obtained. Further improvements are expected by using dual-band observation for reference sources. As another approach, phase delay data of reference radio sources were applied for calibration with expecting improved precision and temporal resolution, though it did not show significant improvement for calibration in this case.

深宇宙探査機との通信にはX-bandを用いることが定められている.しかし,帯域は既に混み合っておりX-bandの使用は徐々に困難になりつつある.そのような状況でKa-bandを高速伝送のために使用することは魅力で,今後の深宇宙探査には必須と言われている.宇宙航空研究開発機構の深宇宙局は64mを擁するが,X-bandまでの対応で鏡面精度などKa-bandには使用できない.そこで,より小型のアンテナをアレイ化し64mと等価な機能を持たせ,更にKa-bandへの対応を図る考えが生まれてきた.システムの実現性を検討するにあたり,受信信号の位相差推定の方法が鍵となる.本稿では,これまでに調査・研究してきた深宇宙通信用アレイ地上局のダウンリンク合成の手法について報告する.

© World Scientific Publishing Company. In the conventional very long baseline interferometry (VLBI) measurements, radio signals received at radio telescopes are digitized and recorded with highspeed data recorders and the recorded media are transported to a correlator site for data processing. Because of the rapid developments in the high-speed network and communication technologies, it is becoming possible to perform real-time high-volume data transfer for VLBI observation data from the radio telescopes to the correlator and this mode of operation is called as e-VLBI. To ensure the inter-operability among heterogeneous observing systems, it is very important to define the standard specifications. For this purpose, a series of standard specifications were defined and are being defined as the VLBI standard interface (VSI) specifications including the data transfer protocol VSI electric (VSI-E). Efforts to support such standardizations and the current status of the e-VLBI developments will be introduced.

A 2048 Msps (megasamples per second) very long baseline interferometry (VLBI) data acquisition system (DAS) that has the ability to directly acquire intermediate frequency signals up to 1024 MHz bandwidth has been developed. By using these systems, 4 Gbps (4 gigabits per second = 2048 Msps, 2 bits) VLBI experimental observations were performed on 2005 November 25, and fringes were successfully detected with a newly developed software correlator. Both the data sampling rate (2 Gsps) and the total data rate (4 Gbps) were the highest rate ever used for VLBI observations. One of the applications realized with the DAS is differential VLBI measurements of deep space spacecraft, for which a wide-band recording is used for phase-reference quasars and a narrow bandwidth digital filtering is used for spacecraft. This paper describes the features of the developed DAS and cross-correlation system, results of the experimental observations, expected applications realized with the newly developed systems, and effectiveness of wide-band data acquisition for differential VLBI experiments for spacecraft navigation. © 2006. The Astronomical Society of the Pacific. All rights reserved.

An 8×8 = 64 element digital interferometer was developed at Waseda University (1989-1995). It was an equally spaced two dimentional array of 2.4 m dishes at 10.6 GHz, and a 2D FFT processor was used for Nyquist rate imaging. Recently a spatial-temporal (2+1)D FFT processor has been developed for pulsar survey (1996-2000), in which a first 2D spatial FFT transforms the electric fields of cordinate represented to those of momentum represented. We obtain the electric fields in each 64 direction at Nyquist rate by the 2D spatial Fourier transform. The following temporal 1D FFT gives the spectrum of 64ch or 256ch to 64 directions respectively also at Nyquist rate. The total bandwidths of each direction are 20 MHz and the resulting frequency resolutions are 20/64 MHz or 20/256 MHz. For sensitive pulsar survey, we also have developed an interferometric array of 20 m spherical dishes in Nasu flat, 160 km north of Tokyo. Five dishes are completed at present. Normal temperature receivers of high electron mobility transistors (HEMT) are used at 1.4GHz observation and the receiver noise temperatures are below 40K. Expected rms detection limit ΔT is about 10-2 K under the condition of 20 MHz band widths and 1s integration time. Preliminary results of zenith survey at δ = 40 deg using a single dish show ΔT = 0.03 K without switching. Sub-reflecters and feed horns were designed so as to observe 5 deg off directions from zenith and they rotate around Az axis. It enable us to survey the declination zone of 32≤δ≤42 deg.