Method for supplying a high purity gas product comprising providing a first gas stream including a major component and at least one impurity component, determining the concentration of the at least one impurity component, and comparing the concentration so determined with a reference concentration for that component. Navigation instructions are obtained concerning a particular data record in response to the data navigation instructions. As a result, the VTOL UAV can takeoff from and land on inclined surfaces without the use of landing gear mechanisms designed to level the UAV on the inclined surfaces.