  Foreground mitigation is critical to all next-generation radio
interferometers that target cosmology using the redshifted neutral hydrogen 21
cm emission line. Attempts to remove this foreground emission have led to new
analysis techniques as well as new developments in hardware specifically
dedicated to instrument beam and gain calibration, including stabilized signal
injection into the interferometric array and drone-based platforms for beam
mapping. The radio calibration sources currently used in the literature are
broad-band incoherent sources that can only be detected as excess power and
with no direct sensitivity to phase information. In this paper, we describe a
digital radio source which uses Global Positioning Satellite (GPS) derived time
stamps to form a deterministic signal that can be broadcast from an aerial
platform. A copy of this source can be deployed locally at the instrument
correlator such that the received signal from the aerial platform can be
correlated with the local copy, and the resulting correlation can be measured
in both amplitude and phase for each interferometric element. We define the
requirements for such a source, describe an initial implementation and
verification of this source using commercial Software Defined Radio boards, and
present beam map slices from antenna range measurements using the commercial
boards. We found that the commercial board did not meet all requirements, so we
also suggest future directions using a more sophisticated chipset.
