desired output patterns. This is then repeated until a representative set of test cases has been surveyed. Due to the additive nature of the weights, ‘learning’ one pattern does not wipe out ‘knowledge’ of those already surveyed. For the present case of the beam pointing system, the input layer consists of several units of the microwave detectors (i.e. selected rectenna elements). The number of units in the output layer are determined according to the desired accuracy of the main beam direction. Each unit in the output layer is expected, ideally, to give 1 when the beam hits it, otherwise to give 0. Figure 10 shows the actual output of the five output units of the test system. Weights were determined by learning at five angles of the main beam ( — 20, —10, 0, 10 and 20). Responses of each unit were measured by swinging the direction of the main beam from —20 to 20. To simulate distortion of the antenna structure, random phases were added into the beam. Results show that a neural system can recognize the direction of the input beam quite accurately. 2. Target Satellite (TGS) The target satellite (Fig. 11) is to be used as a receiving target for the energy transmission test. It would be stored in the housing payload unit (PLU) until necessary, then released from the SFU on a tether wire. The TGS is roughly a sphere 30 cm in diameter, and consists of a micro wave receiver (rectenna), transmitter to transmit the pilot signal, and battery. Printed cross dipole antennas with rectifiers are arranged on the surface of the TGS. Six flexible monopole antennas for pilot signal transmission are placed perpendicularly to the other (cross) antennas for omnidirectional transmission of the pilot signal. A block diagram is shown in Fig. 12. The rectenna can measure the antenna pattern of the PAA by scanning the main microwave beam. This tests the tracking ability of the TGS. Since the rectenna can rectify the received microwaves and provide power to the TGS battery, it is only the pilot signal transmitter which needs extra electric power.
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