IV.A.2 MPTS/MCRS ANALYSIS G. D. Arndt, Avionics Systems Engineering Division INTRODUCTION The initial task in the Satellite Power Station study was to determine an appropriate sizing for the overall satellite system. The station consists of large solar arrays converting solar energy to DC electricity by the photovoltaic process. This electrical energy is transmitted back to the earth using a high power microwave transmission system. The microwave system, consisting of DC-RF amplifiers, a large planar phased array, and a ground antenna/rectifier scheme (rectenna), must be capable of operating at a high efficiency over a 30 year lifetime with a low failure rate. The microwave energy is rectified back to DC electricity in the rectenna and then collected and carried via buss bars to a power distribution interface with commercial landlines. The output DC power from the solar power station will be defined at a collection point near the ground rectenna, prior to any signal conditioning or DC/AC conversion. IV.A.2(a) SYSTEM SIZING The physical size and power capabilities for the Satellite Power Station (SPS) are dependent upon: (1) the amount of DC output power at the ground rectenna, (2) the transmit antenna size, and (3) the system efficiencies. The tradeoffs for defining these system requirements are as fol 1ows: (1) Amount of DC power out of rectenna - Three output power levels, 1GW, 5GW, and 10GW, were studied. Raytheon (ref. 1) and Lewis Research Center (ref. 2) indicate there may be some cost advantages in going to high power systems. The allowable output power level is also dependent upon the size of the microwave transmit array antenna in the SPS as discussed below. (2) Tradeoffs of SPS transmit antenna size - The size of planar phased array in the satellite is primarily dependent upon four factors: (a) Thermal constraints within the antenna - At the center of the phased array, temperature limitations due to heat radiated by the DC to RF converters determine a minimum size for the antenna. The maximum power density (on boresight) at the transmit antenna is given as a function of array diameter in Figure IV.A.2-1. A 10 dB taper aperature illumination for a 5GW system will just meet the thermal limitations for the antenna. (The 10 dB taper illumination rather than a 5 dB taper was.selected because of increased collection efficiency at the rectenna as will be discussed later.) For the model configuration, the maximum power density at boresight will be 20.88 KW/m2 when using a 1 km array 5GW system. This density level gives a temperature of 450° - 485°K at the antenna for a DC to RF amplifier conversion efficiency of 85-90%. This temperature is an upper limit
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