VLBI System Research Articles

근접사진측량과 Total Least Squares를 활용한 VLBI 안테나 형상 변형 모니터링 방안 연구

In order to maintain the precise positioning accuracy of the VLBI system, the shape deformation found in antenna structure should be monitored. In fact, reduced the antenna gaining of an electromagnetic wave reception from the Quasar has been particularly expected due to the shape deformation of main reflector in VLBI antenna. Therefore, the importance of shape deformation monitoring for the main reflector has been significantly increased. The main reflector has come out as the high potential for deformation in the VLBI structure. The fact has led us to investigate the monitoring system for the main reflector based on the efficient algorithm in accordance with the close-range photogrammetry, which of expecting to be utilized as the continuous and automated monitoring system for the structure deformation in the near future. Ten fitting lines were estimated with the TLS for feature points of distributed in all directions from the main reflector. The resultant intersection point of estimated fitting lines was calculated by using the nearest point calculation algorithm, based on those non-intersection lines. Following to the intuitive basis for the time series analysis, the results was able to provide the calculation of numerical variation in the intersection point, which is represented in 3-axis,; that we are expecting to open the way for predicting a deformation rate as well as deformation direction

Earth orientation parameters estimated from VLBI during the CONT11 campaign

In this paper we investigate the accuracy of the earth orientation parameters (EOP) estimated from the continuous VLBI campaign CONT11. We first estimated EOP with daily resolution and compared these to EOP estimated from GNSS data. We find that the WRMS differences are about 31 $$\upmu $$ as for polar motion and 7 $$\upmu $$ s for length of day. This is about the precision we could expect, based on Monte Carlo simulations and the results of the previous CONT campaigns. We also estimated EOP with hourly resolution to study the sub-diurnal variations. The results confirm the results of previous studies, showing that the current IERS model for high-frequency EOP variations does not explain all the sub-diurnal variations seen in the estimated time series. We then compared our results to various empirical high-frequency EOP models. However, we did not find that any of these gave any unambiguous improvement. Several simulations testing the impact of various aspects of, e.g. the observing network were also made. For example, we made simulations assuming that all CONT11 stations were equipped with fast VLBI2010 antennas. We found that the WRMS error decreased by about a factor five compared to the current VLBI system. Furthermore, the simulations showed that it is very important to have a homogenous global distribution of the stations for achieving the highest precision for the EOP.

Flux monitoring observations of Sgr A* at 8 GHz and 2 GHz with the NICT Kashima–Koganei VLBI System

AbstractWe have been conducting flux monitoring observations of Sgr A* at 8 GHz and 2 GHz using the NICT Kashima-Koganei VLBI system (109 km baseline) since mid-February 2013. The primary objective of the monitoring is a search for flux variation which is expected to be caused by the interaction between the G2 cloud and the accretion disk. Until 2013 September 22, we observed Sgr A* for 39 days, five hours on each day. Four quasars (NRAO 530, PKS 1622–253, PKS 1622–297, PKS 1921–293) were also observed as flux calibrators every 6 minutes. No significant change nor variation has been detected in the 8 GHz flux density of Sgr A*. The 8 GHz flux density was 0.81 ± 0.07 Jy (preliminary), while no significant 2 GHz emission was detected by our system. We will continue monitoring as often as possible until at least 2014 May.

Signal delay in the RadioAstron ground-space interferometer

The commissioning of the RadioAstron space radio telescope observing in a radio-interferometer mode with baselines exceeding the baselines of ground-based VLBI systems by more than an order of magnitude requires revision of the formula used to calculate the delay of the arrival of the signal from a distant source to the telescopes in ground-based observations. This formula is derived taking into account all corrections necessary to achieve the required accuracy; numerical estimates of the contributions of all terms are presented.

Parameters of giant pulses from the Crab pulsar measured with the Mark5A VLBI system

Radio observations of the Crab pulsar were performed on the 100-m radio telescope of the Green Bank Observatory at a frequency of 2100 MHz in a 64-MHz band in two channels with right-and left-circular polarization. The Mark5A recording system was used. During 15 min of observing time, 609 giant pulses were recorded; the brightest had a peak flux density of 670 kJy. The energy distribution has been constructed, polarization properties have been analyzed, and the characteristic temporal and frequency scales in the radio emission of the detected giant pulses have been found. Comparison of these parameters indicates that the properties of giant pulses detected at the main-pulse and interpulse longitudes do not differ, as is clearly observed at frequencies above 4 GHz. Probable origins of the frequency evolution of the properties of giant pulses are discussed.

Monte Carlo simulations of the impact of troposphere, clock and measurement errors on the repeatability of VLBI positions

Within the International VLBI Service for Geodesy and Astrometry (IVS) Monte Carlo simulations have been carried out to design the next generation VLBI system (“VLBI2010”). Simulated VLBI observables were generated taking into account the three most important stochastic error sources in VLBI, i.e. wet troposphere delay, station clock, and measurement error. Based on realistic physical properties of the troposphere and clocks we ran simulations to investigate the influence of the troposphere on VLBI analyses, and to gain information about the role of clock performance and measurement errors of the receiving system in the process of reaching VLBI2010’s goal of mm position accuracy on a global scale. Our simulations confirm that the wet troposphere delay is the most important of these three error sources. We did not observe significant improvement of geodetic parameters if the clocks were simulated with an Allan standard deviation better than 1 × 10−14 at 50 min and found the impact of measurement errors to be relatively small compared with the impact of the troposphere. Along with simulations to test different network sizes, scheduling strategies, and antenna slew rates these studies were used as a basis for the definition and specification of VLBI2010 antennas and recording system and might also be an example for other space geodetic techniques.

A Study on the Development of Playback Control Software for Mark5B VSI System

We developed the playback control software for a high-speed playback system which is a component of the Korea-Japan Joint VLBI Correlator (KJJVC). The Mark5B system, which is a recorder and playback system used in the Korean VLBI Network (KVN), has two kinds of operation mode. That is to say, the station unit (SU) mode, which is for the present Mark4 system, and the VSI mode, which is for the new VLBI standard interface (VSI) system. The software for SU is already developed and widely used in the Mark4 type VLBI system, but the software for VSI has only been developed for recording. The new VLBI system is designed with a VSI interface for compatibility between different systems. Therefore, the playback control software development of the VSI mode is needed for KVN. In this work, we developed the playback control software of the Mark5B VSI mode. The developed playback control software consists of an application part for data playing back, a data input/output part for the VSI board, a module for the StreamStor RAID board, and a user interface part, including an observation time control part. To verify the performance of developed playback control software, the playback and correlation experiments were performed using the real observation data in Mark5B system and KJJVC. To check the observation time control, the data playback experiment was performed between the Mark5B and Raw VLBI Data Buffer (RVDB) systems. Through the experimental results, we confirmed the performance of developed playback control software in the Mark5B VSI mode.

Ultra-Rapid DUT1-Observations with E-VLBI

We give a short overview about the achievements of the Fennoscandian- Japanese ultra-rapid dUT1-project that was initiated in early 2007. The combination of real-time data transfer, near real-time data conversion and correlation, together with near-real time data analysis allows to determine dUT1 with a very low latency of less than 5 minutes after the end of a VLBI-session. The accuracy of these ultra-rapid dUT1-results is on the same order than the results of the standard rapid-service of the International Earth Rotation and Reference Frame Service (IERS). The ultra-rapid approach is currently extended to 24 hour sessions and is expected to become an important contribution for the future next generation VLBI system called VLBI2010.

Constructing space-based continuous navigation and communication system utilizing formation flying satellites on GEO orbit in deep space exploration

Two clusters of formation flying satellites with a relative angle on geosynchronous Earth orbit were used to construct a space-based continuous navigation system for deep space exploration. Adopting technique of passive navigation by multiple base-stations, formation flying satellites obtained high precision of navigation according to time and frequency difference of arrival of downlink signal from deep space explorer. Enhancing system timing and carrier frequency, and increasing the length of baselines help to improve precision of navigation. The precision of this navigation system is close to that of NASA's VLBI system.

Open loop doppler tracking in Chinese forthcoming Mars mission

AbstractUsing the radio telescopes in Chinese VLBI Network and the K5/VSSP32 VLBI system of NICT in Japan, we have developed algorithms that can extract open loop Doppler information from the Chang'E-1 and Mars Express radio tracking data. Our latest results indicate the Doppler accuracy of open loop three-way Doppler is about 1mm/s in 1 second integration time, relative to a 8.4GHz carrier. In the forthcoming joint Russian-Chinese Martian mission in 2009, the current software algorithms and hardware performance will be improved, and we will attempt to use the high precision Doppler shift and phase information to test gravitation theories.

Very long baseline interferometry: accuracy limits and relativistic tests

AbstractWe present a review on relativistic effects and best estimates of the relativistic PPN parameter γ obtained by analysis of data from the International VLBI Service for Geodesy and Astrometry (IVS). Relativistic implications are also considered in view of the upcoming new generation VLBI System: VLBI2010.

Direct determination of astronomical distances and proper motions by interferometric parallax

Aims. We discuss a new method for measuring the distances to astronomical objects and their transverse proper velocities. Methods. The phenomenon of interferometric parallax identifies a component of 2- and 4-point amplitude and the intensity correlations that can be observed at frequencies ranging from radio to optical. The calculation hinges on terms depending on the source to receiver separation that are conventionally neglected in the Van Cittert-Zernicke theorem. Results. Order of magnitude estimates find that the baselines of Earth-bound VLBI systems might be capable of measuring 10−100 kpc distance scales. In either the optical or radio regime, space-borne detectors with fine baseline control might resolve source distances of Gigaparsec order. We discuss the possibility that an independent distance ladder based directly on experimental measurement might be constructed on multiple scales.

KVN을 위한 디지털 데이터 처리 시스템의 성능평가

In this paper, we introduce the performance test results of digital data processing system for KVN (Korean VLBI Network). The digital data processing system for KVN consists of DAS (Data Acquisition System) and high-speed recorder which called Mark5B system. DAS system performs the digitalization of analog radio signal through ADS-1000 gigabit sampler with 1 Gsps/2-bit and process the digital filtering of digital signal. Mark5B system records the output data of DFB (Digital Filter Bank) with about 1 Gbps. In this paper, we carried out the preliminary evaluation experiments of the KVN digital data processing system connected between DAS system and Mark5B with VSI (VLBI Standard Interface) interface which is designed for compatible in each VLBI system. We first performed all of the KVN digital data processing system connected by VSI interface in the world. In factory inspection phase, we found that the DAS system has a memory read/write error in DSM (Digital Spectrometer) by analyzing the recorded data in Mark5B system. We confirmed that the DSM memory error has been correctly solved by comparing DSM results with Mark5B results. The effectiveness of KVN digital data processing system has been verified through the preliminary experiments such as data transmission, recording with VSI interface connection and data analysis between DSM and Mark5B system. In future work, we will perform the real astronomical observation by using the KVN 21m radio telescopes so as to verify its stability and performance.

KVN (Korean VLBI Network)의 우주측지학적 기여도 분석

전파천문학과 우주측지학적 활용을 목적으로 3기의 전파망원경으로 국내에 구축되고 있는 초장 거리간섭계(VLBI; Very Long Baseline Interferometry)인 KVN의 우주측지학적 기여도를 분석하기 위한 모의실험을 수행하였다. KVN의 우주측지학적 기여도를 두 가지 서로 다른 규모의 관측망에 대해 분석하기 위해 동아시아 지역의 local관측망(KVN-Asia)과 태평양 주변 지역의 global 관측망(KVN-Pacific)을 구성하였다. KVN이 이들 두 관측망에 포함되기 전 후에 대한 정량적 기여도 분석이 이 연구를 통해 수행된 모의실험의 주목적이다. 이를 위해 관측소 좌표와 지구회전계수(EOPs; Earth Orientation Parameters)를 정량적 기여도 분석의 기준 파라미터로 선택하였다. 이들 파라미터는 모의실험을 통해 추정되는 대상인 동시에 KVN 포함 전 후의 추정 정밀도 향상에 대한 기준 비교인자이기도 하다. 관측소 좌표는 KVN-Asia에 대한 기준 비교인자로 사용되었고, EOP는 KVN-Pacific에 대한 기준 비교인자로 사용되었다. 모의 실험 결과 KVN-Asia와 KVN-Pacific의 기준 비교인자 추정정밀도 개선에 대한 KVN의 기여도는 최대 50%, 20%수준이 될 것으로 예상되었다. 또한 연구결과를 토대로 제안된 KVN의 우주측지학적 유망연구 분야들은 향후 KVN우주측지학적 활용의 핵심분야가 될 것으로 기대한다. KVN simulation, which is focused on evaluating the impacts of KVN to geodetic VLBI network, was performed. The KVN is under construction with three radio telescopes VLBI system for radio astronomy and space geodesy. To distinguish the impacts of KVN on global and local scale networks, we designed two different sizes of VLBI networks, namely, KVN-Asia and KVN-Pacific. While the former consisted of Far East Asia region VLBI stations, the latter consisted of pacific region VLBT stations. The primary purpose of our simulation is quantitative evaluation of KVN impacts before and after the participation of KVN in the previous two virtual networks. We selected two different sets of parameters to be estimated in the simulation as indices of evaluating estimation precision: station coordinates and EOPs. The station coordinates are evaluating index for KVN-Asia and the EOPs are another evaluating index for KVN-Pacific. From the simulation results of comparisons between evaluating indexes, 50% and 20% of maximum improvements for KVN-Asia and KVN-Pacific were anticipated respectively. We expect that the space geodetic use of KVN will be focused on the several promising research fields which are proposed through the simulation results.

Astronomy in the former Soviet Union 15 years after the breakup of the USSR

AbstractDuring the immediate post-Soviet period, the main infrastructure of astronomy over the territory of the former Soviet Union (FSU) was saved, in spite of dramatic decreases in financial support. Overall the situation for FSU astronomy is now stable. In Latvia, the 32-m radio-dish is in working order. This allows it to participate in VLBI programmes. In Russia, all three 32-metre radio dishes of the QUASAR VLBI system are operational, as well as the 2-m telescope with a high-resolution spectrograph (up to resolution R≃ 500 000) and the horizontal solar telescope (R= 320 000) of the Russian-Ukrainian Observatory on Peak Terskol (Caucasus, altitude 3100 m). However the situation with the observatory itself is worrying, because of the regional authorities' attempt to privatize its infrastructure.The process of equipping a number of Commonwealth of Independent States (CIS) (including Russian) observatories with CCD-cameras is in progress. To solve staff problems, Kazakhstan, Tajikistan and Uzbekistan have begun to prepare national specialists in astronomy, and the Baltic States, Armenia, Azerbaijan, Georgia, Russia, and Ukraine continue to prepare astronomers.Teaching of astronomy at schools is obligatory only in the Ukraine and partially in the Baltic states. To maintain a “common astronomical area”, the Eurasian Astronomical Society (EAAS) continues its programme of reduced-price subscription to Russian-language astronomical journals and magazines in the territory of FSU, the organization of international conferences and Olympiads for school students, and lectures for school teachers and planetarium lecturers, etc.Telescopes in Russia and other CIS territories permit to monitor an object more then 12 hours and can be used in global monitoring programmes. The Central Asian sites have some of the very best astro-climates in the world. They are similar to (or a little better than) the well known Chilean sites (median seeing 0.7′′, very high fraction of clear nights, no light pollution and no high wind). It is imperative that these sites be protected and intensively used by the international astronomical community.

Collaboration and development of radio-astronomy in Australasia and the South-Pacific region: New Zealand perspectives

AbstractAs a result of collective efforts of an Australian–New Zealand VLBI team, the first New Zealand VLBI system was developed, and a series of test observations between New Zealand and Australia conducted. The equipment and techniques used to conduct New Zealand's first VLBI observations are discussed and results of work in Australia and New Zealand to obtain fringes and the image of the source (PKS1921-231) are presented. The road map for New Zealand radio-astronomy as well as New Zealand involvement in the SKA is discussed.

Space operation system for Chang’E program and its capability evaluation

Space operation for China’s first lunar exploration program, Chang’E will be provided by the S-band aerospace Telemetry, Tracking and Command (TT&C) network designed for China’s manned space program. This is undoubtedly a great challenge to the ground TT&C system. The largest antennas of China’s S-band aerospace TT&C network has an aperture of only 12 m. A series of technical measures have been taken into the designing of the spacecraft-ground TT&C system to ensure that such antennas can communicate with Chang’E-1 lunar probe 400,000 km away. These include installation of high-gain directional antennae and medium-gain omni-directional antennae for the probe, adding channel encoding to the downlink channel, using both high and low data rates for information transmission and upgrade and design of ground equipment terminals. Among them, the omni-directional antenna will operate in the earth-ground transfer orbit phase and the directional antenna will operate in the lunar orbit phase. These measures satisfy the spacecraft-ground link and program design requirements.To provide accurate navigation for the probe during its Earth-Moon flight and initial lunar orbiting flight, China’s VLBI system designed for astronomical observations, will also be used besides the ranging and range rate measurement capabilities of the S-band TT&C network. The purpose is to provide 100 m accuracy in position determination during lunar orbit. This paper describes the system design, technical challenges, solutions and capability evaluation of space operation for Chang’E-1.

Evaluation of co-location ties relating the VLBI and GPS reference frames

We have compared the VLBI and GPS terrestrial reference frames, realized using 5 years of time-series observations of station positions and polar motion, with surveyed co-location tie vectors for 25 sites. The goal was to assess the overall quality of the ties and to determine whether a subset of co-location sites might be found with VLBI–GPS ties that are self-consistent within a few millimeters. Our procedure was designed to guard against internal distortion of the two space-geodetic networks and takes advantage of the reduction in tie information needed with the time-series combination method by using the very strong contribution due to co-location of the daily pole of rotation. The general quality of the available ties is somewhat discouraging in that most have residuals, compared to the space-geodetic frames, at the level of 1–2 cm. However, by a careful selection process, we have identified a subset of nine local VLBI–GPS ties that are consistent with each other and with space geodesy to better than 4 mm (RMS) in each component. While certainly promising, it is not possible to confidently assess the reliability of this particular subset without new information to verify the absolute accuracy of at least a few of the highest-quality ties. Particular care must be taken to demonstrate that possible systematic errors within the VLBI and GPS systems have been properly accounted for. A minimum of two (preferably three or four) ties must be measured with accuracies of 1 mm or better in each component, including any potential systematic effects. If this can be done, then the VLBI and GPS frames can be globally aligned to less than 1 mm in each Helmert component using our subset of nine ties. In any case, the X and Y rotations are better determined, to about 0.5 mm, due to the contribution of co-located polar motion.

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 s f>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 model has to be modified to explain the experimental results.

Giant Pulses from PSR B1937+21 with Widths ≤15 Nanoseconds and \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $T_{b}\geq 5\times 10^{39}$ \end{document} K, the Highest Brightness Temperature Observed in the Universe

Giant radio pulses of the millisecond pulsar B1937+21 were recorded with the S2 VLBI system at 1.65 GHz with NASA/JPL's 70 m radio telescope at Tidbinbilla, Australia. These pulses have been observed as strong as 65,000 Jy with widths ≤15 ns, corresponding to a brightness temperature of Tb ≥ 5 × 1039 K, the highest observed in the universe. The vast majority of these pulses occur in 5.8 and 8.2 μs windows at the very trailing edges of the regular main pulse and interpulse profiles, respectively. Giant pulses occur, in general, with a single spike. Only in one case of 309 was the structure clearly more complex. The cumulative distribution is fitted by a power law with index -1.40 ± 0.01 with a low-energy but no high-energy cutoff. We estimate that giant pulses occur frequently but are only rarely detected. When corrected for the directivity factor, 25 giant pulses are estimated to be generated in one neutron star revolution alone. The intensities of the giant pulses of the main pulses and interpulses are not correlated with each other nor with the intensities or energies of the main pulses and interpulses themselves. Their radiation energy density can exceed 300 times the plasma energy density at the surface of the neutron star and can even exceed the magnetic field energy density at that surface. We therefore do not think that the generation of giant pulses is linked to the plasma mechanisms in the magnetosphere. Instead we suggest that it is directly related to discharges in the polar cap region of the pulsar.