  A radio interferometer uses time delays to maximize its response to radiation
coming from a particular direction. These time delays compensate for
differences in the time of arrival of the wavefront at the different elements
of the interferometer, and for delays in the instrument's signal chain. If the
radio interferometer is operated as a phased array (tied array), the time
delays cannot be accounted for after an observation, so they must be determined
in advance. Our aim is to characterize the time delays between the stations in
the core of the LOw Frequency ARray (LOFAR). We used radio holography to
determine the time delays for the core stations of LOFAR (innermost 3.5 km).
Using the multibeaming capability of LOFAR we map the voltage beam faster than
with a raster scan, while simultaneously calibrating the observed beam
continuously. For short radio holographic observations (60 s and 600 s) of
3C196, 3C147, and 3C48 we are able to derive time delays with errors of less
than one nanosecond. After applying the derived time delays to the beamformer,
the beam shows residuals of less than $20\%$ with respect to the theoretical
beam shape. Tied-array holography could be a way towards semi-real-time beam
calibration for the Square Kilometer Array.
