Input/Output (I/O) Requirement for Telemetry and Command Since hardware mass and power are subject to the I/O points for telemetry and command, I/O requirement would be set up early in the design phase. A standard communication satellite, which is intermediate between simple satellite and complex satellite, has 400 to 700 I/O points for telemetry and 600 to 800 I/O points for command. Considering the space solar power mission to be neither simple nor complex, the solar power satellite would have the same scale of I/O points as the standard communication satellite. The total number of I/O points for both telemetry and command would amount to about 2000 points. In this case, the hardware mass including telemetry equipment and command equipment would range from 50 kg to 60 kg and the power required for those two equipment's would be about 135 W. RF Communication Equipment and Antenna For RF communication equipment, an antenna (omni antenna for S-Band), telemetry transmitter, command receiver, wave guide, coax, and so on would be needed. If the same band frequency would be used for both uplink and down link, only one antenna could be used commonly for transmitting and receiving. However, the diplexer should be put behind the antenna to divide between the transmitted signals and the received signals. Data rate, which is a very important factor to hardware, would be selected to be 250 bits per second for the command receiver, and 1000 to 5000 bits per second for the telemetry transmitter. If the sun-synchronous orbit is selected, the ground station for TT&C on the high latitude should be better for RF communication, because the higher latitude the ground station is located on, the more time it can see the spacecraft. Budgets Spacecraft Power estimation Power level is the most important parameter in designing the space solar power satellite. The key driver is the ground power level requirement, and we must take into account of other requirements such as the constraints of cost, launcher capability, time limitation and the current technology availability. Given the power level on ground and the microwave transmitting frequency, we can decide the power level in space and the size of transmitting antennae. For most large satellites, the power consumed by payload is about 40 to 80% of the total energy generated by the solar array. For the case of solar power satellite, the percentage will be in the range of 80%, since the only purpose of solar power satellite is to beam power to die Earth. With the basic assumption that the spacecraft is made of two solar arrays of 50 m by 10 m each, a service module and a phase array transmitting antennae using 35 GHz frequency, we can make our power budget with the current technology available to us. We assume that the current Si solar array's conversion efficiency is 9% for space use, the specific performance is 120 W per square meter and the mass specific is 30 W/kg. The life span is five years and the degradation rate is 20% at the end of life. At the beginning of life ,the total power generated by the Power Subsystem is 200 kW among which 150 kW could be converted to micro wave to the Earth and 50 kW is used by the spacecraft bus for thermal control, TTC, GNC and other subsystems. As for the power used by the spacecraft bus, GNC subsystem takes about 50% of it. Since the solar power satellite is much larger than the existing satellites, its increased solar panel and transmitting antennae are the major sources of disturbance for both attitude and orbit control because of the atmospheric drag and solar radiation. So larger magnetic torquers with more power input are needed to compensate the attitude drifts. As for the power subsystem, no batteries or only limited amounts are needed, because the solar power satellite demo is in a sun-synchronous Orbit of 1000 km high which means that there will be almost no eclipse during the whole operational period. So we do not need to store energy for using in the shadow area. Spacecraft Mass Estimation First we try to use the maximum capability of the current launch vehicles available to put satellites into sun-synchronous orbit with height of 1000 km. Again the satellite's mass is divided into four different parts: solar panels, phase array transmitting antennae, service module and propellant.
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