Phased Array

For phased-array observations the analog sums for the appropriate VLA IFs are recorded. The analog sum for each arm is generated by summing the sampled and delayed IF signals from the working VLA antennas. The analog sums are amplified and filtered, and the signals from the three arms are combined and sent, under Modcomp control, to the VLBI backend. The propagation delay through the VLA waveguide transmission system, the VLA IF processing equipment, and the VLA correlator is usually about 235 $\mu$s.

The algorithm used to phase the array is very simple, and is implemented using the results of a program called ANTSOL that runs in real time in the VLA's Modcomp computers. ANTSOL determines antenna phase errors for each antenna relative to the reference antenna. These phase errors are determined for each integration of the VLA's Modcomp computers, nominally 10 s, in the autophasing observing mode (VLA mode ``VA''). 25% of the observed phase errors are applied to the VLA A and D lobe rotators, so that the time constant of the feedback loop is four integrations of the Modcomp computers, usually totalling 40 s. With these parameters, the VLA will phase up on unresolved sources of about 0.2 Jy at 1.7-8.4 GHz, and of about 0.7 Jy at 15-43 GHz. Even stronger sources are required at 22 and 43 GHz when attempting to phase up at low antenna elevations, due to the increased system temperatures and the reduced elevation-dependent gains. At 0.33 GHz, phased-array observations can encounter problems due to ionospheric activity and the large number of strong confusing sources expected in the primary beam of the VLA antennas; consult the AOC contact assigned to your program for scheduling advice.

In extended-phasing observing mode (VLA mode ``VX''), the phase corrections measured by the last scan in mode VA are applied to the current observation. That is, the previously determined phase corrections are remembered and applied to the current observation. These phase corrections will be destroyed by an intervening observation at a different frequency or in normal interferometer mode (VLA mode `` ''), or by an intervening crash of a VLA Modcomp computer.