1. Introduction

Korean VLBI Network (KVN) is the only VLBI facility in Korea. It consists of three 21 m radio telescopes, which are located in Seoul (Yonsei Univ.), Ulsan (Univ. of Ulsan), and Jeju island (Tamna Univ.), and produce an effective spatial resolution equivalent to that of an 500 km radio telescope. Compared to American and European VLBI networks such as VLBA and EVN, KVN is still small. In order to overcome this shortcoming, KASI is developing innovative multi-frequency band receiver systems with which we will be able to make simultaneous VLBI observations at four different frequencies: 22, 43, 86, 129 GHz. Since millimeter-wave VLBI (mm-VLBI) is still in the developing stages around the world, we expect that KVN will play an important role in promoting mm-VLBI research activities. In addition, KVN will be often operated in combination with Japanese and Chinese VLBI networks to form so-called East Asian VLBI network (EAVN), which is expected to be comparable to VLBA and EVN in spatial resolution, sensitivity, and imaging fidelity. KVN will provide opportunities to study the formation and death processes of stars, the structure and dynamics of our own Galaxy, the nature of Active Galactic Nuclei and so on at milli-arcsecond resolutions. KVN will be also utilized to precisely detect tectonic movements in the Korean peninsula. KASI is building the Korea-Japan Joint VLBI Correlator, as well, in collaboration with the National Astronomical Observatory of Japan.
The first set of 22/43 GHz receivers were installed at the KVN Yosei telescope in 2008 August. The first fringes were detected in the VLBI test observations with the Japanese VLBI network VERA at both frequencies. KVN Yonsei telescope is operational as a single dish. The VLBI test observations are being carried out to evaluate and improve the performance. The second and third sets of 22/43 GHz receivers will be installed on the other telescopes by the end of 2009. All KVN telescopes will be equipped with 22/43/86/129 GHz receivers in 2011. As the world-first dedicated mm-VLBI network, KVN will make KASI take another big step up to the next level in radio astronomy. More detailed information of KVN is as follows.

2. KVN sites

The three KVN observatory sites have been selected from among many competitors. Table 1 presents the locations, while Table 2 lists the baseline lengths, ranging between 300 km and 500 km. The antenna constructions were finished at all three sites in 2008. Figure 1 shows an example of the u-v coverage and the synthesized beam for the combination of three KVN antennas and TRAO 14m radio telescope located in Daejeon (KASI headquarter). In the future, these four antennas will be connected using the optical fibre networks both for the real-time unmanned operations and for ‘online’ operations (e-VLBI).

*Site locations of KVN and TRAO

*Baselines of KVN sites and TRAO 14m (km)

*Example u-v coverage (Left) and synthesized beam (Right) of KVN plus TRAO 14m for a source with a declination of 40o

3. KVN 21m Antennae

The KVN antennae are shaped Cassegrain antennae with a diameter of 21m. They were built between 2006 September and 2008 September. Figure 2 exhibits three KVN sites with the 21m antennae. The surface accuracies of the primary reflectors are ≤150 μm rms. The aperture efficiencies were measured to be about 50, 70, 70 % at 22, 43, 100 GHz, respectively. The pointing accuracy of each antenna was estimated to be better than 4 arcsecond rms. The maximum slewing speed is 3°/sec in both azimuthal angle and elevation with an acceleration of 3°/sec2. This high slewing speed ensures weak source detection via 'fast position switching phase referencing'.

*KVN Radio Observatories

4. KVN Receiver Systems

For the KVN front-ends, several cryogenic HEMT receivers will be installed at the Cassegrain focus for 2/8, 22, 43, 86 GHz and SIS receiver for 129 GHz operations. The 2/8 GHz receivers will be mainly for geodetic observations, whereas the others will be for astronomical observations. The 22/43GHz receivers are installed first, to set up the antennas and for the initial VLBI observations. The 86/129GHz receivers will be installed in 2011. Full polarization observation mode is supported at 22, 43, 86, 129 GHz.
For KVN, which is designed for mm-VLBI, we adopted innovative multi-channel quasi-optical beam transportation system, which can be used for simultaneous on-source phase referencing of mm-VLBI, without losing observing time, and without the necessity to look for reference sources. In addition, this method enables us to observe at four frequencies simultaneously. Figure 3 (Left) shows a schematic diagram of the KVN beam transporting system employing low-pass filters (LPFs), while Figure 3 (Right) displays the lower frequency part of the beam transporting system currently installed at KVN Yonsei telescope.
The 22GHz band is used as a phase calibration reference for the higher frequency band observations, made toward the same source at the same time. This multi-channel idea is expected to give very reliable phase corrections in mm-VLBI.

*Schematic diagram of KVN quasi-optics (Left) and Receiver plate of KVN Yonsei telescope (Right). Only 22/43 GHz receivers are currently installed.

5. KVN DAS and MK5 recorder

The KVN data acquisition system (DAS) has been developed and installed at the all stations in 2008. For our multi-channel receiver system, we employ four high-speed samplers operated at 1 Gbps. These four data streams of 2 Gbps are transported via optical fibres to the operation building, and then distributed among sixteen FIR digital filters. With these filters, we can choose a passband whose centre frequency is arbitrarily programmable in the input bandwidth, and then resample the filtered data at 2 bits per sample. These re-sampled data streams are then formatted and sent to the recorder. The digital spectrometers are deployed for single-dish observation of total-power and polarization.
KASI participated in the Mark5 VLBI recorder development consortium. The KVN-Mark5B is designed to support VLBI Standard Interface specification fully. With this new recording system, not only the highly sensitive offline observation but also e-VLBI is possible. e-VLBI will ensure more reliable VLBI operation for astronomy, astrometry, geodesy, and space mission.

6. East-Asian VLBI Network (EAVN)

As mentioned earlier, KVN will be often operated together with Japanese and Chinese VLBI networks to form EAVN. EAVN for 22 GHz will consist of 19 antennae scattered over 5000 km, while EAVN for 43 GHz will comprise 9 antennae. In 2009 May the first fringes of EAVN for 22 GHz were successfully detected. Figure 4 shows the results. Full-fledged observations using EAVN are expected to get under way in 2010.

*First fringes of K-band EAVN (KVN Yonsei-VERA-CVN Sheshan)