text: The potential exposure of humans and animals on the ground to the high power microwave beams is a significant concern with these systems. At the Earth's surface, a suggested SPSP microwave beam would have a maximum intensity at its center, of 23 mW/cm. While this is less than 1/4 the solar irradiation constant, microwaves penetrate much deeper into tissue than sunlight, and at this level would exceed the current United States Occupational Safety and Health Act (OSHA) workplace exposure limits for microwaves at 10 mW/cm At 23 mW/cm, studies show humans experience significant deficits in spatial learning and memory. If the diameter of the proposed SPSP array is increased by 2.5x, the energy density increases to 1 W/cm. At this level, the median lethal dose for mice is 30-60 seconds of microwave exposure. While designing an array with 2.5x larger diameter should be avoided, the dual-use military potential of such a system is readily apparent. 
  With good array sidelobe design, outside the receiver may be less than the OSHA long-term levels  as over 95% of the beam energy will fall on the rectenna. However, any accidental or intentional mis-pointing of the satellite could be deadly to life on Earth within the beam.
  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.
  A commonly proposed approach to ensuring fail-safe beam targeting is to use a retrodirective phased array antenna/rectenna. A "pilot" microwave beam emitted from the center of the rectenna on the ground establishes a phase front at the transmitting antenna. There, circuits in each of the antennas subarrays compare the pilot beams phase front with an internal clock phase to control the phase of the outgoing signal. This forces the transmitted beam to be centered precisely on the rectenna and to have a high degree of phase uniformity; if the pilot beam is lost for any reason (if the transmitting antenna is turned away from the rectenna, for example) the phase control value fails and the microwave power beam is automatically defocused. Such a system would not focus its power beam very effectively anywhere that did not have a pilot beam transmitter. The long-term effects of beaming power through the ionosphere in the form of microwaves has yet to be studied.