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 (2016B).
Right now, for KaVA aray, these are still some limitations of fast antenna nodding mode.
Now, we are keep working to improve KaVA phase-reference and astrometry capabilities: (1) improving accuracy of troposphere and clock model; (2) hybrid phasereference mode: KVN nodding + VERA two beam staring-mode to achieve a faster nodding speed.
KaVA will enable us to conduct VLBI observations in combinations of different types of antennas (antenna beams), receiving bands, recording rates (namely total band widths), and filtered base band channels in one observing session, whose cross correlation is still valid for the whole or some parts of KaVA. In such “hybrid” observing modes with KaVA, there are some modes that are available in the 2016B CfP described as follows.
Although VERA shall use only one of dual beams in a single frequency band (K or Q), the KVN is able to observe in two to three of K/Q/W bands simultaneously in a common observing session. Please check the KVN status report for W-band information (http://radio.kasi.re.kr/kvn/status_report_2016/array.html). Signal correlation for all the KaVA baselines is valid for the band in which both the KVN and VERA observe, while that for all the observed bands is valid for the KVN baselines.
Frequency allocations should be made separately to the KVN and VERA, including base band channels that are common between the two arrays in a specific band (K or Q). Note that the number of base band channels or the total bandwidth available per frequency band is limited, therefore brighter continuum sources should be selected for group-delay calibration.
Moreover, different frequency-band assignment (K or Q) is also available for each of VERA stations to observe in K/Q-bands simultaneously. In this case, it is necessary to confirm that uv-coverages in both bands are suitable for a target source. For example, suppose a simultaneous observation of faint 44 GHz methanol maser and compact 22 GHz water masers. In this case, VERA Iriki station is assigned to a Q-band observation while other VERA stations are assigned to K-band in addition to a simultaneous K/Qband observation with the KVN. The Q-band array becomes a compact array consisted with KVN three + VERA Iriki stations, while the K-band array becomes an extended array consisted with KVN three + VERA Mizusawa/Ogasawara/Ishigakijima stations.