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Foreground mitigation is critical to all next-generation radiointerferometers that target cosmology using the redshifted neutral hydrogen 21cm emission line. Attempts to remove this foreground emission have led to newanalysis techniques as well as new developments in hardware specificallydedicated to instrument beam and gain calibration, including stabilized signalinjection into the interferometric array and drone-based platforms for beammapping. The radio calibration sources currently used in the literature arebroad-band incoherent sources that can only be detected as excess power andwith no direct sensitivity to phase information. In this paper, we describe adigital radio source which uses Global Positioning Satellite (GPS) derived timestamps to form a deterministic signal that can be broadcast from an aerialplatform. A copy of this source can be deployed locally at the instrumentcorrelator such that the received signal from the aerial platform can becorrelated with the local copy, and the resulting correlation can be measuredin both amplitude and phase for each interferometric element. We define therequirements for such a source, describe an initial implementation andverification of this source using commercial Software Defined Radio boards, andpresent beam map slices from antenna range measurements using the commercialboards. We found that the commercial board did not meet all requirements, so wealso suggest future directions using a more sophisticated chipset.

A Digital Calibration Source for 21cm Cosmology Telescopes