Solar Power Satellites (SPS) have been studied extensively since the 1970s. One of the best-defined and most-heavily-studied mission concepts involves SPS's located in Geostationary Orbit (GSO), so as to be able to transmit uninterrupted power via microwaves to a receiving station on the ground below its longitude. To accomplish this, the SPS will need to include a propulsion system to counteract environmental forces --- primarily the Moon's and the Sun's gravity, and Solar Radiation Pressure (SRP) --- that tend to perturb its orbit, and move it from its desired Geostationary Orbit. Previous GSO SPS studies have assumed this station keeping would be done using the same technique used by all GSO communications satellites, i.e., using chemical rockets or electric (e.g., ion, or Hall-effect) thrusters to carry out this function. With that approach, the amount of delta-V needed (at least 50 m/s per year) would require a very large mass of propellant to be shipped to the SPS over its lifetime, significantly increasing the operations cost of the SPS, and thus negatively impacting its business-case; expelling such enormous amounts of thruster exhaust in GSO could also have negative consequences for the space environment, as well as potentially depleting non-renewable terrestrial resources (e.g., xenon, for some types of electric thrusters). Here we propose an alternative approach that has not been studied before now --- to use Solar Radiation Pressure (the propulsion technique used by solar sails) to create the thrust forces to accomplish station-keeping for an SPS. Initial design conceptualization and sizing calculations indicate that a SRP-propulsion subsystem for an SPS could counteract all environmental forces perturbing the orbit of the SPS, as well as providing attitude-control torque actuation for the SPS about all 3 axes (eliminating the need for expending propellant for attitude control). In the proposed study, we will define, size and document this technique.
