The overall system configuration of KVN DAS system (Oh et al. 2011), which was developed by Korea Astronomy and Space Science Institute (KASI), is shown in figure 1. The frequency bandwidth adopted in the KVN receiver system is 512-MHz bandwidth in 512~1024-MHz region. To obtain the digital signal from radio, ADS-1000 gigabit sampler is adopted in the KVN, which is developed by NiCT in Japan (Nakajima et al. 2000). The radio signal is sampled by 1024-MHz sampling rate with 2-bit quantization level. The KVN can support multi-frequency simultaneous 4-IF receiver system (Han et al. 2008), so the total data rate is 8-Gbps and it can support the VSI interface.
Four data streams through the Gigabit sampler can be simultaneously processed or configured to be selected several data stream. To connect the data path from the antenna to the observation building, optical transmission units (OTU) are used to transmit the digital data streams, which are able to convert the VSI data format to optical data with ATM cell and WDM (Wavelength Division Multiplexing) module. There are two OTU systems, one (OTX) is for data converting from VSI format to optical for transmission to observation building, the other (OTR) is for data converting from optical to VSI format for receiving from the antenna.
Transmitted data streams are selected in digital filter bank (DFB) (see table 1) with KVN observation mode (see table 2), which consists of 256-MHz bandwidth one filter unit to 8-MHz bandwidth sixteen filter units. VERA (VLBI Exploration of Radio Astrometry) GDFB (Gigabit Digital Filter Bank) was modified and adopted in KVN DFB (Iguchi et al. 2005). It can be performed the digital filtering by various observation modes.
To measure the power spectrum of data streams through single-dish observation, the digital spectrometer (DSM) is applied, which consists of auto- and cross-correlator with FX-type. Filtered data streams could be processed with the DSM and the transmitted data steam through the OTR could be processed by DSM with 512-MHz bandwidth directly.
Figure 1. KVN DAS system configuration
Table 1. KVN observation modes
2. Recorder
The recording system of the KVN is basically Mark5B system. However, to check firstly the VLBI experiment results of the KVN, the KVN DAS was designed and adopted ID-1 output port for DIR1000 system. To do this, the correlation processing of observed data is needed by Mitaka FX correlator of NAOJ (National Astronomical Observatory of Japan. Therefore the DIR1000 system was introduced by NAOJ for VLBI experiments between KVN, VERA, and EAVN (East Asian VLBI Network). The DIR1000 and Mark5B system is briefly introduced as follows.
2.1. DIR1000
The VERA has two types of high speed magnetic tape recorders. These data recording systems are the DIR1000 (Kobayashi et al. 2003) developed for the VSOP (VLBI Space Observatory Programe), and the DIR2000 which is newly developed and widely used for the VERA. In case of DIR1000, the maximum recording speed is 256-Mbps. And the total bandwidths are 64-MHz in the DIR1000 system, because of 2-bit sampling. The recording time per roll of tape is 2 hours in the DIR1000 system. It adopts ID-1 specification, which records and playback at 1.3, 2, 4, 8, 16, and 32-MB/sec. The ID-1 format utilizes azimuth recording techniques, which minimizes the effects of mis-tracking and insures greater cross-play among all ID-1 type units.
2.2. Mark5B
The Mark5B (Whitney 2003) system was developed at Haystack Observatory as the first high-data-rate VLBI data system based on magnetic-disc technology. Incorporating primarily low-cost PC-based components, the Mark5B system supports data rates up to 1024-Mbps, recording to an array of 8 inexpensive removable IDE or SATA disks. Support for Mark5B development was provided by BKG, EVN, KVN, MPI, NASA, NRAO and USNO as like international consortium.
References
http://kvn-web.kasi.re.kr (written in Korean)
Han, S.-T., Lee, J.-W., Kang, J.M., Je, D.-H., Chung, M.-H., Wi, S.-O., Sasao, T, & Wylde, R. 2008, Int. J. Infrared Milli. Waves, 29, 1
Iguchi, S. et al. 2005, PASJ, 57, 259
Kobayashi, H., et al. 2003, in ASP Conf. Ser., 306, in New Technologies in VLBI, ed. Y.C. Minh (San Francisco: ASP), 367
Nakajima, J. et al. 2000, IVS CRL-TDC News, 17, 18
Oh, S.-J. et al. 2011, PASJ, 63(6) will be published.
Oh, S.-J. et al. 2010, GM proceedings
Oyama, T., Y. Kono, T. Hara, N. Kawaguchi, 2008, General Meeting Abstract Book, 68.
Whitney, A. R. 2003, in ASP Conf. Ser., 306, New Technologies in VLBI, ed. Y.C. Minh (San Francisco: ASP), 123
Copyright by Korea Astronomy and Space Science Institute KVN Korean VLBI Network (KVN)
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