If there is a total failure within the phase control system (for example, the uplink pilot beam transmitter is shut off), the subarrays will no longer be phased together and the total beam will be defocused. As shown in figure 1, the peak intensity Of the beam drops to .003 mw/cm? and the beam width greatly increases. Since this peak power density is less than even the stringent USSR guideline there will not be any health problem if the phase control system fails completely. This is the fail-safe mechanism mentioned previously. In addition there would be sensors near the rectenna to detect any large changes in incident power density; this information would immediately be transmitted to the antenna to cease operations. In addition to the sidelobe patterns near the rectenna as given in figure 1, the far-sidelobe patterns have been calculated. There had been some concern about the radio interference levels over eastern European countries because of frequency allocation problems. The SPS downlink power beam lies in the 2400-2500 MHz frequency band which has been reserved for Government and nonGovernment industrial, medical, and scientific (IMS) usage. By definition anyone operating in an IMS band must accept interference from any other user within this band. However, this 100 MHz band is not recognized by some of the European countries, including Albania, Bulgaria, Hungary, Poland, Rommania, Czechoslovakia, and the USSR. These countries reserve 2375 MHz + 50 MHz for the IMS band. The far sidelobe levels as shown in figure 2 indicate the peak levels for one 5 gigawatt SPS system are three to four orders of magnitude below .01 mw/cm?. For simultaneous operation of one hundred, ten gigawatt SPS systems, the average peak level is still one to two orders of magnitude lower than .01 mw/cm?. Grating lobes, which occur at 440 Km intervals from the rectenna, are functions of subarray size and mechanical misalinement of the subarrays within the 1 Km phased array. The grating lobes occur at spatial distances corresponding to angular directions off axis of the array where the signals from each of the sub- arrays add in-phase. When the boresights of the subarrays are not a- lined with the uplink pilot beam transmitter at the rectenna, the unwanted contributions of the array factor of the antenna do not lie in the null-points of the subarray pattern as shown in figure 3. Even though the phase control system will still point the composite beam at the rectenna, some energy will be transferred from the main beam into the grating lobes. The amount of energy in the grating lobes depends upon the misalinement (or how far the array factor is displaced from the null points of the subarray patter- These orating lobes are somewhat unique in that they do not spatially move with misalinement changes, rather they are stationary with an amplitude dependence upon the mechanical mi salinements. This behavior is due to the operating characteristics of the retrodirective phasing system. From environmental considerations the grating lobes are constraire: to be less than .01 mw/cnA The total mechanical alinement requirements for both the subarrays and the total array can be determined from this constraint. If the 10.4 meter X 10.4 meter subarray is considered to be the smallest entity for phase control, then the peaks of the grating lobe patterns at the ground are shown in figure 4. Since the distance between maxima for the grating lobes is inversely proportional to the spacings between subarrays, a 10.4 meter square subarray has peaks every 440 Km. If the phase control system for focusing the downlink beam is extended down to the power tube level, the maximum subarray size has its linear dimension reduced by a factor of two (there are a minimum of 4 tubes per subarray at the edge of the transmit array). The grating lobes would then have peaks spaced a minimum of 880 Km apart. Flgwe 3 GutlNg UH CkM»cten«tlc« Figure 4, CntlRg lobe ReafM There are two types of mechanical misalinements: 1) a systematic tilt of the entire antenna structure, and 2) a random tilt of the individual subarrays.
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