  Recent advances in unmanned aerial vehicle (UAV) technology have made UAVs an
attractive possibility as an airborne calibration platform for astronomical
facilities. This is especially true for arrays of telescopes spread over a
large area such as the Cherenkov Telescope Array (CTA). In this paper, the
feasibility of using UAVs to calibrate CTA is investigated. Assuming a UAV at 1
km altitude above CTA, operating on astronomically clear nights with
stratified, low atmospheric dust content, appropriate thermal protection for
the calibration light source and an onboard photodiode to monitor its absolute
light intensity, inter-calibration of CTA's telescopes of the same size class
is found to be achievable with a 6-8 % uncertainty. For cross-calibration of
different telescope size classes, a systematic uncertainty of 8-10 % is found
to be achievable. Importantly, equipping the UAV with a multi-wavelength
calibration light source affords us the ability to monitor the
wavelength-dependent degradation of CTA telescopes' optical system, allowing us
to not only maintain this 6-10 % uncertainty after the first few years of
telescope deployment, but also to accurately account for the effect of
multi-wavelength degradation on the cross-calibration of CTA by other
techniques, namely with images of air showers and local muons. A UAV-based
system thus provides CTA with several independent and complementary methods of
cross-calibrating the optical throughput of individual telescopes. Furthermore,
housing environmental sensors on the UAV system allows us to not only minimise
the systematic uncertainty associated with the atmospheric transmission of the
calibration signal, it also allows us to map the dust content above CTA as well
as monitor the temperature, humidity and pressure profiles of the first
kilometre of atmosphere above CTA with each UAV flight.
