We present our efforts for extending the simultaneous multi-frequency receiver system of the Korean Very Long Baseline Interferometry (VLBI) Network (KVN) to global baselines in order to measure the frequency-dependent position shifts in Active Galactic Nuclei (AGN) jets, the so called core shift effect, with an unprecedented accuracy (a few micro-arcseconds). Millimeter VLBI observations with simultaneous multi-frequency receiver systems, like those of the KVN, enable us to explore the innermost regions of AGN and high precision astrometry. Such a system is capable of locating the frequency dependent opacity changes accurately. We have conducted the feasibility test-observations with the interested partners by implementing the KVN-compatible systems. Here we describe the science case for measuring the core shift effect in the AGN jet and report progress and future plans on extending the simultaneous multi-frequency system to global baselines.

Abstract
1. INTRODUCTION
2. MEASURING THE CORE SHIFT EFFECTWITH GLOBAL BASELINES
3. GLOBAL EXPANSION OF THE SIMULTANEOUSMULTI-FREQUENCY RECEIVER SYSTEM
    3.1. Simultaneous Multi-Frequency Receiver Systemin mm-VLBI Observation
    3.2. Interested Radio Astronomy Partners
4. TECHNICAL OPTIONS TO PROVIDEKVN-COMPATIBLE VLBI BACKENDS
    4.1. Fast Switching between Receivers
    4.2. Sub-Array Solution
    4.3. Quasi-Optical Receiver System
    4.4. Tri-Band Feed System
    4.5. Triple Feed–Single Dewar System
5. DEMONSTRATIONS
    5.1. Demonstrations with the VERA Antennas
    5.2. Demonstrations with the ATCA Antennas
    5.3. Demonstrations with the Yebes Antenna
6. SUMMARY AND FUTURE PLANS
REFERENCES

• Taehyun Jung(Korea Astronomy and Space Science Institute/University of Science and Technology) Corresponding author
• Richard Dodson(International Centre for Radio Astronomy Research)
• Seog-Tae Han(Korea Astronomy and Space Science Institute)
• Mar´ıa J. Rioja(International Centre for Radio Astronomy Research/Observatorio Astron´omico Nacional (IGN))
• Do-Young Byun(Korea Astronomy and Space Science Institute)
• Mareki Honma(Mizsawa VLBI Observatory, National Astronomical Observatory of Japan)
• Jamie Stevens(Australia Telescope National Facility, CSIRO)
• Pablo de Vicente(Observatorio Astron´omico Nacional (IGN))
• Bong Won Sohn(Korea Astronomy and Space Science Institute/University of Science and Technology)