Zephyr: A High Altitude Long Endurance Unmanned Air Vehicle ...

1.2.4 Mission EnvelopeAs Zephyr is powered by solar energy it is essential that it is flown when there isenough solar power to power the aircraft. In the northern hemisphere this is during thethree months surrounding the summer solstice (May, June and July). As seen byGraph 2, the most solar energy is not at the equator, but it is in fact on the Tropic ofCancer at a latitude of 23° 26′ 22″. The closer you get to this latitude the more powerwill be available and the longer Zephyr will be able to fly. In London (51° 32’) theperiod of time when the solar power is large enough for Zephyr to fly is lower, andthus the mission length is shorter.Wm 2Graph 2. Graph to show how solar power varies depending on the time of the year andthe latitude [1]As aforementioned, another important condition is the current weather conditions. Itis important to try and begin Zephyr’s flight when there is a cloudless sky with low14