One potential solution is to offset the frequencies of different satellites by small amounts, to introduce lobe rotation and thus reduce the averaging time needed to reduce the interference. However, a careful study is needed, as inappropriate choices for the offset frequencies can lead to potential rf intermodulation problems, making the situation worse rather than better. THE "RUSTY BOLT EFFECT A major contribution to spurious emissions in the radio frequency spectrum will be generation of harmonics and intermodulation products in nonlinear elements in the SPS and its environment. Nonlinear effects in the transmitting array and in the rectenna are mentioned elsewhere in this report. Here we call attention to the possibility, indeed likelihood, that nonlinear devices accidentally present in the field of the transmitter will produce serious modulation products. The following well-known effect occurs in situations in which many antennas transmitting on different frequencies are assembled in a small area. In the geographical vicinity of such "antenna farms" the spectrum is frequently cluttered with out-of-band intermodulation products. These are the result of the interaction of currents of different frequencies in rectifying junctions in metal structures--thus the appellation "rusty bolt effect." Normally, the intermodulation products would be attenuated rapidly with distance from an antenna farm. The extremely high-field strength in the main beam of the SPS, of the order of 200 volts/meter at the earth's surface, makes it virtually certain that nonlinear conduction will occur in any structure containing corroded joints or fasteners or other nonlinear elements. Even if it were possible to eliminate all electrically nonlinear joints in the vicinity of the rectenna, the diodes in the array will serve as modulators, producing the same effect unless elaborate filtering provisions are made. Other signals of much lower strength, such as those from television or radar transmitters some distance away, can be expected to interact with the SPS fundamental frequency and many of its harmonics. It is conceivable that these modulation products may be of
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