  This paper presents the feasibility study of deploying Quantum Key
Distribution (QKD) from High Altitude Platforms (HAPs), as a way of securing
future communications applications and services. The paper provides a thorough
review of the state of the art HAP technologies and summarises the benefits
that HAPs can bring to the QKD services. A detailed link budget analysis is
presented in the paper to evaluate the feasibility of delivering QKD from
stratospheric HAPs flying at 20 km altitude. The results show a generous link
budget under most operating conditions which brings the possibility of using
diverged beams, thereby simplifying the Pointing, Acquisition and Tracking
(PAT) of the optical system on the HAPs and ground, potentially widening the
range of future use cases where QKD could be a viable solution.
