The two sidebands are demodulated in the RF receivers in the subarrays (or power modules) and the carrier is reconstructed. This prevents beam squint problems arising from different uplink and downlink frequencies, and it allows the proper phase conjugation to be made. The ionosphere constrains the frequency separation Af between the sidebands and the downlink carrier to be greater than 10 MHz, the maximum plasma resonance frequency. This limitation is to prevent intermodulation products between the uplink pilot beam and the downlink power beam from creating parametric instabilities associated with overdense ionospheric heating, (3) coding of the pilot beam for security and pilot discrimination. Since multiple SPS satellites will be illuminated by a single pilot beam transmitter, each satellite has to recognize which pilot beam signal it should respond to. In addition, coding will prevent power drain from any intentional interfering signals. (4) ground safety control system (ground sensors for interpreting beam shape) with a command link capability to the satellite. RFI Characteristics -■ The radio frequency interference comes primarily from the DC-RF power converter tubes. This interference can be divided into three main categories: (1) interference from the high power downlink beam due to sidelobe and grating lobe radiations, (2) spurious noise generated near the carrier frequency by the tubes, and (3) harmonic generation within the tubes. The sidelobe and grating lobe levels were previously examined. Within the phase control system, the phase lock loop around each power tube will reduce the spurious noise close to the carrier frequency. A representative loop might have a 5 MHz bandwidth with a second or third order filter. This loop will not affect the tube noise characteristics outside the 2450 + 50 MHz band. However, the klystron tubes will have a multiple cavity design which provides additional filtering (24 dB/octave) to reduce the out-of-band noise. The SPS noise density characteristics are summarized in figure 26 (reference 31), The CCIR (International Radio Consultative Committee) requirement for power flux density at the earth's surface is -180 dBW/m /Hz for S-band frequencies with an angle of arrival above 25°. As shown in figure 26, the RFI
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