26 IVS CRL-TDC News No.19 NTT’s Ultra High-speed Net- observation system will become better. work Experiment and Realtime In the second phase of the project started in 1998, therefore, the focus is placed on the advance- VLBI Trials ment of this real-time VLBI technology. Exten- sive research items regarding the realtime VLBI 1 Hisao Uose ([email protected]), Sotesu technology are being conducted, together with the 1 2 Iwamura , and Takashi Hoshino scientific observations using the testbed. So far, through the experiments using the developed re- 1 NTT Information Sharing Platform Laboratories altime VLBI system, great improvement in obser- 3-9-11 Midori-cho, Musasino-shi, vation performance has been achieved. This pa- Tokyo 180-8585, Japan per explains the outline of this research project 2 NTT Advanced Technology from the viewpoint of communications technologies 549-2 Shinano-cho, Totsuka-ku, Yokohama-shi which play a major role in the realtime VLBI sys- Kanagawa 244-0801, Japan tem. 1. Introduction 2. Realtime VLBI trial in the framework of NTT s “Very high speed network exper- In 1995, NTT Laboratories have started a joint ’ iment research project on ultra high-speed communica- ” tions with public research organizations using a NTT Laboratories have started a joint research large scale network testbed having the maximum project on very high speed networking with pub- speed of 2.4 Gb/s. Among many applications lic research institutes including Communications tried in the first phase of the project, the real- Research Laboratory (CRL), National Astronomi- time VLBI (very long baseline interferometry) has cal Observatory (NAO) and Institute of Space and proved to be benefited greatly from the high per- Astronautical Science (ISAS) in June 1995. The formance communications technologies. With the purposes of the projects at NTT side were two- realtime data transmission using high-speed com- fold. One is to establish technologies for very high- munications network, the bottleneck regarding the speed communications (Gb/s class for each appli- data transfer in the conventional VLBI system can cation) preparing for the very high speed com- be removed. Furthermore, as the bandwidths of munications services in the future. The second the network become larger, the performance of the aim is to explore and prove the effectiveness of Figure 1. Realtime VLBI testbed. November 2001 27 Figure 2. Antennas used for experiments. those technologies in advanced scientific areas which seemed to be the first applications of this kind utilizing the vast capabilities of high-speed communications technologies. With the belief that the joint effort with the ap- plications users, in this case, VLBI researchers is indispensable for a fruitful achievement, we have constructed a dedicated ATM network spanning Kanto/Shinetsu area and provided the facilities to our research partners together with the necessary technologies to adapt their equipment to connect to the experimental network. The network con- nected the NTT R&D centers and participating Figure 3. Example of Correlated Data. research organizations with 2.4 Gb/s circuits, re- peaters, ATM Switches and high-performance IP routers. We have also connected six antennas as (antennas, receivers and samplers, etc.) and pro- shown in the Figure 1 for our trials. Figure 2 shows cessing units (cross correlators). In addition, re- the pictures of those antennas and Figure 3 shows altime cross correlation during the observation is an example of“ fringes ”obtained with this exper- made possible allowing researchers to check the sta- imental system. tus of their observation at the spot. Consequently, realtime VLBI brings great advantages over the 3. Realtime VLBI Applications conventional tape-based system in both aspects of With the conventional tape-based VLBI system, system performance and observation flexibility and the amount of observation data was restricted by efficiency. We have two applications using the re- the data rate and storage capacity of the data altime VLBI in our project, geodesy and radio as- recorder used to store observed radio signals at tronomy. each antenna site, limiting the performance of the 3.1 Geodesy (KSP Project) total observation system. With the realtime VLBI system directly connecting antennas and data pro- This is a joint trial with CRL to implement cessing units with communications network, how- a high precision crustal deformation measurement ever, this restriction can be removed and the larger system using realtime VLBI. CRL has constructed amount of data can be utilized for analyses. Thus, four dedicated antennas shown in Figure 4 for this the total performance of the VLBI system is only project called KSP (Key Stone Project). By con- limited by the performances of observation system necting those antennas with the central cross cor- 28 IVS CRL-TDC News No.19 Figure 4. Location of KSP measurement sites. Figure 6. Observation of Bursty Radio Source. The synthesized virtual telescope has the same an- gular resolution of one very large telescope with the diameter which is equal to the distance between the remote antennas, in the case of Kashima-Usuda baseline, 208km. The on-site processing/observation brought by the realtime VLBI has allowed us to conduct very flexible and efficient VLBI observations, by con- stantly checking the obtained data while we are tracking the target radio source. This is only pos- sible with the realtime VLBI technology. We can Figure 5. Distance fluctuations observed in KSP. use antennas shown in Figure 2 together with a space radio telescope HALCA using our network. relator located in CRL Koganei, Tokyo, measure- The first fringe (cross correlation) was detected in ments with very high resolutions have been made 1997 between the signals received by a terrestrial possible. The net data rate from each antenna is antenna (Usuda) and a satellite antenna (HALCA). 256Mb/s and the signals are routed to Koganei Realtime correlation has been successfully estab- through NTT Musashino Research and Develop- lished among large terrestrial antennas (Usuda, ment Center. With this measurement system, the Nobeyama and Kashima) in 1998. Figure 6 shows deformation in Kanto (larger metropolitan Tokyo) an example of observation which proved the effec- area can be measured with the precision of millime- tiveness of the realtime VLBI. This observation of ters. this bursty radio source was possible by catching a Since 1996, regular measurements are being con- sudden variation of signal strength during the re- ducted using the KSP network and the large altime observation. amount of observed data is publicly offered to earth science community through the CRL web 4. Networking issues site. Figure 5 shows the measured distances be- 4.1 Internet VLBI and distributed com- tween Kashima and Tateyama using this system. puting A major deformation due to a sudden volcanic ac- tivity around Izu Island can be observed at the In the first phase of the project, we used ATM right hand side of the figure. (Asynchronous Transfer Mode) technology to carry radio signals. The ATM technology has the pre- 3.2 Radio astronomy (GALAXY Project) cise bandwidth management capabilities enabling a very stable transmission required for the criti- This is a joint effort with NAO, CRL and ISAS cal applications like realtime VLBI. In the second to implement the world’s largest virtual radio tele- phase, however, we started to study to utilize IP scope having ultra-high resolution/sensitivity by (Internet Protocol) technologies extensively within combining conventional VLBI technology and the our experimental network together with ATM. newest Gb/s class communications network. By To make the realtime VLBI experiment more af- adopting the VLBI technology, a large virtual tele- fordable and widespread, the use of IP technolo- scope can be constructed with multiple antennas. gies will be effective, because research and edu- November 2001 29 cation networks around the world are rapidly in- This achievement has a great significance opening creasing their capacities and those networks are ac- up a new vista in the field of VLBI radio astron- cessible from public research organizations around omy by improving the detection sensitivity of the the world. Standing on this viewpoint, we are de- observation system. The detection of the very weak veloping very high speed IP technology applicable radio sources will also accelerate the study to con- to realtime VLBI system. This involves challeng- struct space-time standard infrastructure in space, ing research issues including high speed IP stream which is being conducted at CRL including the transmission and advanced network resource man- real-time and high time-resolution determination agement. In addition, we’ve started the research of the earth orientation parameters. on distributed processing using a number of PCs We plan to further improve the detection sen- connected by high performance network for calcu- sitivity of the real-time VLBI observation system lating cross correlation of the radio signals. This by raising the transmission/processing speeds up approach is in line with other similar effort best to 2Gb/s next year in parallel with the upgrading represented by the GRID project. of the experimental network. The development of “Internet VLBI”systems and the distributed cross- 4.2 Adoption of photonic technologies correlation system using the networked computers are other targets. The cooperation with research NTT Laboratories have a large R&D force institutes abroad is also being sought to realize an in the field of photonic networking technologies. international realtime VLBI observation system. We’ve just started to cooperate with them to fur- ther strengthen the infrastructure of our network References testbed. Now the new pieces of WDM (Wave- length Division Multiplexing) equipment are being [1] GALAXY team: “Ultra High-speed Network deployed in our network with an innovative AWG Experiment,”NTT R&D Special Issue, Vol.50, device developed at our laboratories.