We report results of investigation of amplitude calibration for very long baseline interferome- try (VLBI) observations with Korean VLBI Network (KVN). Amplitude correction factors are estimated based on comparison of KVN observations at 22 GHz correlated by Daejeon hardware correlator and DiFX software correlator in Korea Astronomy and Space Science Institute (KASI) with Very Long Base- line Array (VLBA) observations at 22 GHz by DiFX software correlator in National Radio Astronomy Observatory (NRAO). We used the observations for compact radio sources, 3C 454.3, NRAO 512, OJ 287, BL Lac, 3C 279, 1633+382, and 1510−089, which are almost unresolved for baselines in a range of 350-477 km. Visibility data of the sources obtained with similar baselines at KVN and VLBA are se- lected, fringe-fitted, calibrated, and compared for their amplitudes. We find that visibility amplitudes of KVN observations should be corrected by factors of 1.10 and 1.35 when correlated by DiFX and Daejeon correlators, respectively. These correction factors are attributed to the combination of two steps of 2-bit quantization in KVN observing systems and characteristics of Daejeon correlator.

Abstract
1. INTRODUCTION
2. OVERVIEW OF THE KVN OBSERVING SYSTEMS
    2.1. Digital sampler and Digital filter bank
    2.2. DiFX and Daejeon correlator
3. OBSERVATIONAL DATA AND DATA REDUCTION
    3.1. Observational data
    3.2. Data reduction
    3.3. Amplitude correction
4. RESULTS
5. DISCUSSION
    5.1. Quantization loss
    5.2. Characteristics of Daejeon correlator
    5.3. Calibration uncertainty and source variability
    5.4. Amplitude correction factors
6. CONCLUSION
REFERENCES

• Sang-Sung Lee(Korea Astronomy and Space Science Institute/Korea University of Science and Technology) Corresponding author
• Do-Young Byun(Korea Astronomy and Space Science Institute)
• Chung Sik Oh(Korea Astronomy and Space Science Institute)
• Hyo Ryoung Kim(Korea Astronomy and Space Science Institute)
• Jongsoo Kim(Korea Astronomy and Space Science Institute/Korea University of Science and Technology)
• Taehyun Jung(Korea Astronomy and Space Science Institute/Korea University of Science and Technology)
• Se-Jin Oh(Korea Astronomy and Space Science Institute)
• Duk-Gyoo Roh(Korea Astronomy and Space Science Institute)
• Dong-Kyu Jung(Korea Astronomy and Space Science Institute)
• Jae-Hwan Yeom(Korea Astronomy and Space Science Institute)