This mode is the conventional mode for VLBI phase reference observations. In this mode, the antenna is nodding between the bright phase calibrator and target source. By fringe fitting of the phase calibrator, the residual phase from the unmodeled clock and atmosphere can be dertermined and then interpolated to the target source to achieve a stable phase of the target source.
With this mode, we can detected and image weak sources, which can not be imaged directly by fringe fitting. This mode will also be used for high accuracy astrometry to derive accurate position, proper motion and parallax of VLBI targets, although we will not guarantee the calibration accuracy for astrometry in the current CfP (2018A).
Right now, for KaVA aray, these are still some limitations of fast antenna nodding mode.
1. This mode can only working at K band. Since Q band, with higher frequency then need shorter switch time, which is beyond the ability of VERA antennas. The fastest switching time is 1 min.
2. Currently, the KJCC troposphere model (accuracy 10 cm) are not accurate enough for -arcsecond astrometry. For K band fast antenna nodding mode, the accuracy is 3 mili-arcseconds.
Now, we are keep working to improve KaVA phase-reference and astrometry capabilities: (1) improving accuracy of troposphere and clock model; (2) hybrid phase-reference mode: KVN nodding + VERA two beam staring-mode to achieve a faster nodding speed.