  The ambitious scientific goals of the SKA require a matching capability for
calibration of atmospheric propagation errors, which contaminate the observed
signals. We demonstrate a scheme for correcting the direction-dependent
ionospheric and instrumental phase effects at the low frequencies and with the
wide fields of view planned for SKA-Low. It leverages bandwidth smearing, to
filter-out signals from off-axis directions, allowing the measurement of the
direction-dependent antenna-based gains in the visibility domain; by doing this
towards multiple directions it is possible to calibrate across wide fields of
view. This strategy removes the need for a global sky model, therefore all
directions are independent. We use MWA results at 88 and 154 MHz under various
weather conditions to characterise the performance and applicability of the
technique. We conclude that this method is suitable to measure and correct for
temporal fluctuations and direction-dependent spatial ionospheric phase
distortions on a wide range of scales: both larger and smaller than the array
size. The latter are the most intractable and pose a major challenge for future
instruments. Moreover this scheme is an embarrassingly parallel process, as
multiple directions can be processed independently and simultaneously. This is
an important consideration for the SKA, where the current planned architecture
is one of compute-islands with limited interconnects. Current implementation of
the algorithm and on-going developments are discussed.
