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Q1. What does "VLBI" stand for? Back to Q&A List Go to TOP of This Page
VLBI is the acronym for "Very Long Baseline Interferometry".
 
Although it has a Japanese-translated name,

the acronym "VLBI" is more familiar to people both in Japan and overseas.
 
Q2. When and Where was VLBI born? Back to Q&A List Go to TOP of This Page
The first successful experiment of VLBI was made by Canadian and US radio astronomy groups in 1967. The experiments for geodetic use were started in the 1970s and proved that a distance of thousands of kilometers was possible to be measured with an accuracy of few centimeters (few mm at the present) by VLBI.
 
VLBI is a technique originally from "Radio Astronomy". The field of radio astronomy began with a series of discoveries of radio waves emitted from the Sun and stars in space, which were made individually by engineers, researchers and armies in the USA and Britain from the 1930s through the 40s. Since then it has developed at a rapid pace in Europe, the United States, and Japan.
 
Q3. When did GSI start VLBI measurements? Back to Q&A List Go to TOP of This Page
Launching production of antenna and equipment in 1981, it was in 1986 that GSI officially started VLBI observations. Its main objectives were to correct the geodetic reference frame of Japan, and to detect plate motions and crustal deformations.
 
Q4. Why are VLBI antennas so big? Back to Q&A List Go to TOP of This Page
VLBI antennas are required to receive very faint radio signals from stars (quasars) in deep space as far as several billion light years away.
 
Although "quasars" are celestial objects emitting extraordinary radio energy, their radio waves weaken gradually as they travel the great distance to Earth. In fact, when arrived on earth, they are millions of times weaker than the radio signals by the cell-phones we use today. In VLBI, such faint signals are targeted in the circumstance where many other stronger radio signals flying through the air.
 
Therefore, in VLBI, it is indispensable to use the large diameter antennas with high receiving capability.
 
Q5. What are those big antenna made of? Back to Q&A List Go to TOP of This Page
In the Tsukuba 32-m antenna, which is one of the largest VLBI antennas in Japan and the core of all GSI VLBI, its dish is made of rigid aluminum alloy plates, and its support base is made of steel pipes and H-steels.
 
Q6.
 
How far can VLBI antennas stand against earthquakes and typhoons? Back to Q&A List Go to TOP of This Page
In the Tsukuba 32-m antenna, it is designed to strong enough to withstand an intensity 7 earthquake on the Japanese scale or a typhoon with a wind speed of 60 meters a second.
 
Q7.
 
What is the longest baseline in the past GSI VLBI observations?
What is the distance limit possible for measurements by VLBI?
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The longest baseline we've ever done is "Tsukuba(Ibaraki)-Conception(Chile)"--approx. 12400.5 km. Considering the Earth's diameter (12741.9 km), it should be one of the longest baselines for a VLBI observation with the antennas on Earth.
 
However, in cases like "Tsukuba-Conception" that one antenna of a baseline is located on the other side of the Earth from another one, it is impossible to implement a VLBI measurement with only two antennas. This is because the Earth itself becomes a stumbling block to the antennas in receiving targeted radio signals from space. Therefore, the distance of such a baseline required to be measured with a network of several other antennas located between the two antennas of the baseline, observing targeted signals in parallel. The distance can be figured out only after correlation processing and analysis of all the data obtained from both the two stations of the baseline and all the network stations. In this way a network VLBI observation makes it possible to measure any long distance on Earth.
 
Additionally, there is a way to measure far longer distance. This technique is called "Space VLBI". A Space VLBI observation is performed between an antenna on Earth and an antenna set up in space, thus, the baseline is dramatically longer than the one above mentioned. In Japan, Space VLBI was being performed between a satellite "Halca", which was launched by the Institute of Space and Astronautical Science (ISAS: now JAXA), and the Usuda 64-m antenna, which is also operated by JAXA, with other participating antennas in the United States, Spain and Australia.
 
Q8. Aren't non-targeted radio signals received in observations? Back to Q&A List Go to TOP of This Page
99.9 % of the received and recorded radio signals are other than our target ones. Although Geodetic VLBI antennas are basically set for receiving only particular frequency bands (2 GHZ and 8GHZ), which are not for commercial use such as TV and radio broadcastings, or cellular phone calls, it is impossible to receive only our targets, because there are so much other radio waves emitted from different kinds of objects in nature or space.
 
All observed data are, therefore, processed in correlation processing performed after observation, and only the targeted signals are detected.
 
Q9.
 
What is the difference between "geodetic VLBI" and "astronomical VLBI"? Back to Q&A List Go to TOP of This Page
VLBI is classified into two categories, geodetic VLBI and astronomical VLBI, by purpose, although their principles are basically the same.
 
Astronomical VLBI uses observed radio wave data to obtain information on space such as a star's position and distance from the Earth, or motion of a star. On the other hand, geodetic VLBI (which is applied at GSI) uses the data to obtain information on the Earth such as a position and a distance located on Earth, crustal movements, and Earth's rotation speed.
 
Q10.
 
 
I hear that parabolic antennas could be used for a variety of purposes. Are GSI's VLBI antennas used for any other purposes? or used not only for observations in the field of Geodesy but Astronomy? Back to Q&A List Go to TOP of This Page
All of the GSI's VLBI antennas are used only for geodetic VLBI.
 
It is sure that parabolic antennas may generally be used for multi purpose. However, in GSI, the systems of VLBI including its parabolic antennas were built and are operated for VLBI only, and there is no plan to use them for other purposes in the future.
 
Additionally, it is possible to perform a VLBI observation in the field of Astronomy with the GSI's antennas, but the targets to be observed are only 2 GHZ and 8 GHZ radio waves. Because the receivers installed in them are only for these two frequency bands, other radio waves can not be received with the GSI's antennas.
 
Q11.
 
How high has the VLBI observation accuracy been improved to since the 1990's? Back to Q&A List Go to TOP of This Page
It is difficult to make a simple comparison between the past and the present, because the accuracy depends on several interrelated factors. Considering all the factors together, it can be said that it has improved several folds. Especially the accuracy of the current hydrogen maser atomic clocks is considerably higher than that in the 1990's. Additionally, one of the most significant improvements is the certainty of observation operations. Previously sometimes there was a failure to obtain observed data, but presently observations are almost certain to result.
 
Q12.
 
 
The parabolic (dish) antenna in the Tsukuba VLBI station is tremendously large compared to other stations. What are the advantages and disadvantages of supersizing antennas? Back to Q&A List Go to TOP of This Page
The parabolic (dish) antenna in the Tsukuba VLBI station is tremendously large compared to other stations. What are the advantages and disadvantages of supersizing antennas?
 
The advantages and disadvantages are as follows.
 
( Advantages )
・Weaker radio signals can be received with smaller signal-to-noise ratios.
・Higher positioning accuracy is achieved
 
(Disadvantages)
・It is difficult to maintain the accuracy of reflectors (antenna dish surfaces) with deformation due to gravity.
・Enormous power is required to operate.
・It costs a great deal to build and maintain.
 
Basically, it's not like the larger, the better. In VLBI observations, more than two stations receive a particular radio wave simultaneously, and the required antenna sizes are considered to be that one of the two antenna diameters times the other is more than 100 meters. The presently smallest antenna (3.8-m diameter) of all the GSI VLBI stations is, therefore, enough size to carry out VLBI observations if the other of the pair is the Tsukuba station's antenna (32-m diameter)--3.8 m * 32 m = 121.6 m.
 
Q13.
 
What are the reflector surface / the pointing accuracy of the Tsukuba 32-m antenna? Back to Q&A List Go to TOP of This Page
The accuracy of the Tsukuba 32-m antenna is as follows:
 
[reflector surface] -- 0.5 mm (rms) and below
[pointing] -- 0.008 degrees (rms) and below (at a wind speed of 5m/s at night)
[axis] ・intersection point -- 1 mm and below (the intersection of Az axis with El axis)
・intersection angle -- 0.01 degrees and below (the angle of Az axis with El axis)
・vertical degree -- 0.01 degrees and below
 
The Tsukuba VLBI station has the high antenna accuracy described above, which meets the requirements for precise geodetic activities, and keeps such a high accuracy taking measures to prevent the effect of temperature variations on accuracy, such as covering its antenna main reflector with back panels, and making air circulation with fans.
 
Q14.
 
 
I presume such a big antenna should become severely deformed due to a temperature variation of its surface, and the accuracy in receiving radio waves should be affected. What measures are taken to prevent this? Back to Q&A List Go to TOP of This Page
The Tsukuba 32-m antenna main reflector is made of curved panels which are made of rigid aluminum alloy molded by a method called androforming. The reflectors produced by this method have a high accuracy. In the Tsukuba antenna, the reflector surface accuracy is as high as 0.14 mm (rms) by actual measurement.
 
To prevent the worsening of the accuracy due to its deformation or uneven temperature distribution, the underlying surface of its reflector is covered with white panels, and the air is circulated with 8 temperature sensors and 16 fans. However it is practically impossible to keep it with no deformation, so VLBI observations are performed for 24 consecutive hours in order to clear the factors causing errors such as a temperature variation, a solar irradiation and a wind load.
 

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