We propose a Microwave Energy Transmission in Space (METS) experiment using the future Space Flyer Unit (SFU) to establish basic technologies for microwave energy transmission and to examine nonlinear interactions between high power microwaves and the ionospheric plasma. Among all the technologies necessary for microwave energy transmission, control of the microwave beam is the most essential and the most complicated. An SPS system requires very accurate beam-pointing but a relatively limited angle range in scanning. Conversely, space-to-space energy transmission between an orbiting transmitter and orbiting receiver requires a wide scanning angle range with high speed control. Our METS system covers both possibilities with its versatile active-phased transmitting array. The system also uses a retrodirective antenna and computer controls. The retrodirective antenna uses two-tone pilot signals to unambiguously determine the phase of the transmitting wave. Computer control uses a newly developed neural network to identify input beam direction. To assess the effect of the high-power microwaves on the space plasma environment and vice versa, we will carry out an active experiment injecting microwave energy into the plasma. The beam will be focused on a nearby spot in space so that a very intense microwave field is produced which in turn causes a ‘hot spot’ in the plasma. Plasma response in the hot spot will be observed by diagnostic probes extended from the SFU. METS Experimental Objectives Research areas of the METS experiment are as follows: (1) Research and development of microwave power transmission technology: a microwave beam control, a semiconductor transmitter able to handle the high power involved, transmitting array antenna and a high-efficiency rectenna. (2) Study of possible interactions of microwave beam with the ionosphere and the neutral atmosphere. Beam Pointing Control of the energy beam is highly important as the power station should aim accurately at its moving target regardless of any distortion of the transmitting antenna structure. A retrodirective antenna appears to be the best bet to realize the very stringent beam pointing requirement. The retrodirective type of antenna—called a self-phasing system—uses a standard technique to control the main microwave beam using a pilot signal transmitted from the receiving site. Each element of the emitting array transmits an amplified microwave with a phase conjugate to the received pilot signal. Using this phase-conjugate transmission the beam is automatically directed towards the receiving site. It is, however, impossible to use the same frequency for the pilot signal and the retransmitted energy beam because then the pilot signal cannot be discriminated from it. Choosing a slightly different frequency for the pilot signal can solve this isolation problem, yet this induces a pointing error when the microwave beam is scanned at directions different from the normal to the antenna. The pointing error △ 0 is given by
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