Exposure to the beam can be minimized in various ways. On the ground, assuming the beam is pointed correctly, physical access must be controllable (e.g., via fencing). Typical aircraft flying through the beam provide passengers with a protective metal shell (i.e., a Faraday Cage), which will intercept the microwaves. Other aircraft (balloons, ultralight, etc.) can avoid exposure by using controlled airspace, as is currently done for military and other controlled airspace. In addition, a design constraint is that the microwave beam must not be so intense as to injure wildlife, particularly birds. Suggestions have been made to locate rectennas offshore, but this presents serious problems, including corrosion, mechanical stresses, and biological contamination.