2.3 POWER TRANSMISSION The microwave power transmission system (MPTS) is one of the most' complex elements of the SPS and major technology advancements in electromagnetic systems design must be made before a high-confidence program decision can be made. The microwave (MW) antenna aperture size is orders of magnitude larger than any similar antenna constructed and the required techniques for amplitude/ phase control of this large aperture exists only in theory. The performance of the MPTS affects not only the design of the SPS ground and satellite elements but its impact on communications and the environment are unknowns. Because of the overall complexity of the MPTS, Rockwell has identified pertinent key technical issues which were of highest priority and has limited its study to these key issues: 1. DC-RF conversion devices 2. DC-DC efficiency 3. Waveguide structures 4. Phase control of MPTS To supplement the contract study, results obtained from JPL, Aerospace, Raytheon, NASA-MSFC and NASA-JSC studies were utilized to insure minimum duplication to key issue studies where Rockwell has agreed to the study results. These agreed-upon results were then established as technical guidelines for the study. One of these key technical guidelines for the MPTS is the operating frequency. The frequency selected for SPS study is 2.45 GHz, which is the U.S. industrial power frequency. Rockwell recommends that this frequency selection, however, be examined further for international implications, interference to various spectural lines, radio astronomers, etc. The 5-Gw, singleantenna system also was chosen as the baseline. The Rockwell study accepted previous study results as defined in the NASA studies relative to ionospheric losses and rectenna conversion efficiencies. The ionospheric losses and effects were not studied because of the limited study funding. It is, however, one of the key issues which must be resolved in terms of the effects of heating, RF communication, losses, and polarization rotation. Analytical models must be constructed and large-scale ground tests must be made to determine the full effect. The polarization of the rectenna elements can be determined only after detailed studies. Early test results from JPL indicate polarization effects over a very short range. Results of polarization rotation effects may require polarization diversity elements which further increase rectenna costs and possibly reduce conversion efficiency. Analyses of the rectenna were not conducted because of the in-depth effort currently in progress at JPL. Based on a cursory analysis of the rectenna design, Rockwell feels that key issues associated with the rectenna will be the diode type, diode efficiency as a function of power density, the polarization of the elements, and array efficiency if arraying is required.