5. THE IMPACT OF MODIFIED IONOSPHERE ON THE SPS The uplink pilot beam of the SPS is intended to provide a source of phase information used in forming the SPS microwave beam and directing the beam toward the proper rectenna. The pilot beam transmitter is located at the center of the rectenna and operates on a pair of sidebands equally spaced from the microwave beam frequency. The sidebands are sampled at a number of receiving elements distributed over the SPS transmitting antenna and combined to derive signals at the beam frequency. The phase of the received signal at a given point is compared with the phase of the local SPS transmitters. Thus, a phase distribution for the SPS transmitting antenna is produced and this distribution forms the SPS beam and aims it at the proper rectenna. A comparison of the geometries of the uplink pilot beam and the downlink microwave beam is instructive. For a rectenna in the southern United States the distance to the ionosphere from the rectenna is about 400 km and to the SPS about 40,000 km. The two rays from the pilot transmitter to the edges of the SPS antenna are only separated by 10 m in the ionosphere, whereas the rays connecting the edges of the SPS antenna with the edges of the rectenna are separated by about 10 km in the ionosphere. On the basis of ray theory, then, the uplink pilot beam “sees” or propagates through only a small cross section of the ionosphere that is traversed by the (downlink) SPS microwave beam. In no sense are these reciprocal propagation paths. Ray theory will apply if the perturbations introduced by the medium are small. If the perturbations are large, then a full wave solution to the propagation problem (or a better approximation than ray theory) is in order. In this case the uplink transmission will be influenced by more than the 10-m circle estimated for ray theory and the increased cross section will depend on the precision of the theory required. (The first Fresnel zone has a diameter of 400 m; the important scale in thin screen diffraction is 50 m.) In any practical case the cross section will be limited in size to less than 400 m and more nearly 10 m or a few tens of meters. A physical picture of the perturbation of the uplink pilot beam is readily available. One can think of the shimmering of the image of an object seen through the turbulent eddies of a hot pavement. The object may be discerned, but its shape is altered in a random way that varies with time. Through a heated ionosphere the SPS will see a pilot transmitter whose position dances with time. If the apparent departure of the transmitter from its true location is small, the effect is tolerable and it is possible to define small and tolerable in terms of energy lost from the system by the beam spilling over the edge of the rectenna. The problem, then, is to determine what is the effect on the uplink pilot beam of an ionosphere heated by the SPS microwave beam. Will the phase fluctuations observed at the receiving sites on the SPS be large enough to cause the beam not to form, or to misalign the beam? Our best estimate at the present is that the beam will form, it will be directed at the center of the rectenna, and the phase fluctuations observed at any pick-up point on the SPS will have an RMS of a few phase degrees. Tests are needed. In pursuing the study we have uncovered a related problem. Simply stated, one must be certain that the uplink pilot frequencies are far enough removed from the downlink SPS frequency that the frequency differences do not pump the ionospheric plasma exciting parametric instabilities, for these are known to produce field-aligned striations that have significant effects on communications. This problem can be avoided if the frequency separation between the uplink and downlink
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