Beyond the isoplanatic patch in the VLA low frequency sky survey

Ionospheric phase errors degrade high-resolution radio images below 100 MHz, and they differ significantly from the tropospheric errors which dominate at high frequencies. The ionosphere is so high (similar to 400 km) and the VLA primary beam is so wide (similar to 0.2 rad) that the intersection of the beam with the ionospheric screen is larger than the "isoplanatic patch" size, a phase coherent region on the sky. Antenna-based calibration techniques developed at higher frequencies cannot be used because ionospheric phase errors vary significantly across the field-of-view of each antenna. This paper describes the "field-based calibration" technique adopted for the 74 MHz VLA Low-frequency Sky Survey (VLSS) being made with the 10 km "B" configuration. This technique is useful for a range of array sizes but fails on baselines longer than the linear size of the isoplanatic patch, a few 10s of km at 74 MHz. Implications for designing larger low-frequency arrays are discussed.