1. Abbasi, S. (2018). Development of a UAV traffic management system using mixed integer programming. Masters' thesis, Indian Institute of Technology, Kanpur.
2. AltitudeAngel. (2018). “Altitude Angel and NATS enter strategic partnership to deliver UTM in the UK Altitude Angel", Altitudeangel.com. Retrieved January 04, 2019, from https://www.alti-tudeangel.com/blog/nats-enters-strategic-partnership-with-altitude-angel/.
3. AltitudeAngel (2018). "GuardianUTM O/S successfully powered operation zenith: The most comprehensive global drone trial Altitude Angel", Altitudeangel.com. Retrieved January 03, 2019, from https://www.al-titudeangel.com/blog/guardianutm-os-successfully-powered-operation-zenith-the-uk-s-most-comprehensive-drone-trial/.
4. CAA. (2018). Civil aviation authority, "Drone safety risk: An assessment", West Sussex, 2018.
5. Dedrone. (2018). "Worldwide drone incidents", Dedrone.com. Retrieved December 25, 2018, from https://www.dedrone.com/resources/incidents/all.