Nearfield to Farfield Methods for Drone Beam Mapping

Extracting cosmological 21 cm emission from the radio foregrounds which dominate requires precision calibration, including sub-percent measurements of the complex instrument beam. 21 cm cosmology experiments are typically driven to be compact transit interferometers with poor point-source sensitivity, and have found it difficult to constrain the beam shape to this precision with sky data alone. A technique that has been developed and demonstrated by multiple groups to address this is to transmit a calibrated RF signal from a drone into the telescope to measure the beam pattern.