0191 -9067/82/040255-26$03.00/0 Copyright 1983 SUNSAT Energy Council GRATING LOBE CHARACTERISTICS AND ASSOCIATED IMPACTS UPON THE SOLAR POWER SATELLITE MICROWAVE SYSTEM E. M. KERWIN and G. D. ARNDT NASA Johnson Space Center Houston, Texas 77058, USA Abstract — The object of this investigation is to determine the grating lobe characteristics for the SPS (solar power satellite) microwave power beam. Using techniques of Fourier array decomposition, equations, and numerical results are given for the complete SPS microwave antenna pattern (main beam plus grating lobes). In order to ensure the grating lobe peaks are below the environmental guideline of 0.01 mW/cm2, constraints are defined for the mechanical alignment (tilt) of the SPS antenna and the satellite attitude control system. Two conditions, phase control to the subarray level and to the power module (tube) level, are analyzed as to their effects upon grating lobes. 1. INTRODUCTION The high-power microwave beam from an SPS is electronically steered and phase controlled through the use of an active, retrodirective phasing system. The phase control system will precisely focus the main beam onto a 10 km diameter ground receiving/rectifying antenna (rectenna), maintaining approximately 88% power transfer efficiency from the satellite to the ground. Grating lobes, or replicas of the main beam, will also be formed on the ground at intervals determined by the physical separation of the phase control centers on the transmit antenna. The purpose of this paper is to determine the grating lobe characteristics (separation distances, amplitudes, alignment tolerances, etc.) for two antenna configurations: phase controlling to the antenna subarray level as originally proposed in the reference SPS system and phase controlling to the tube level. This latter configuration increases by an order of magnitude the number of elements to be individually phase controlled. Because of environmental considerations, the grating lobe characteristics can be translated into constraints upon mechanical alignment of the antenna and its subarrays and upon the satellite attitude control system. In particular, the grating lobe peaks must be below 0.01 mW/cm2 for the microwave radiation levels. 2, PHASE CONTROL SYSTEM The SPS antenna is an active retrodirective array using an uplink pilot beam signal to provide precise phase conjugation. The 1 km diameter antenna is mechanically divided into 10.4328 by 10.4328 m subarrays, numbering 7200. A 10 dB Gaussian
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