Korean VLBI Network (KVN)

Amplitude calibration

System temperatures in Kelvin (Tsys) are measured during observations at KVN stations once every user-specified interval (default 10 sec) in order to calibrate amplitude variation in time due mainly to atmospheric fluctuation. The measured Tsys is a sum of three temperatures: the receiver temperature, the spillover temperature, and the contribution of the atmosphere as described in Petrov et al.(2012). These Tsys values can be converted to SEFD (System Equivalent Flux Density) by dividing by the KVN antenna gains in K/Jy.

Additional amplitude correction for the atmospheric opacity above an antenna is performed by conducting a sky tipping curve analysis according to the method described in Mangum (2000). In practice, the system temperatures (Tsys*) corrected for the atmospheric opacity are estimated based on the sky tipping curve measurements once every user-specified interval (default before and after an experiment).

Further corrections are made to the KVN observations taken with 2-bit (4-level) sampling, for the systematic effects of non-optimal setting of the quantizer voltage thresholds.

The amplitude calibrations with KVN are accurate to 15% or better at 22 and 43GHz. However, it is recommended to observe a few amplitude calibrators during KVN experiment, allowing for (a) the assessment of the relative gains of KVN antennas and gain differences between IF-bands at each station, and (b) the confirmation of the correlation coefficient correction assuming that you have contemporaneous source flux densities obtained with other VLBI networks independent of the KVN observations.

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Korean VLBI Network