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When a solar sailing craft is successfully getting force from solar photon pressure, that force applies a small torque to the craft because the center of pressure is never perfectly aligned with the center of mass. Therefore, solar sailing craft need an attitude control system to maintain the correct angle to the sun. This can be done using reflective control devices on the solar sail itself (like IKAROS), by translating masses around to change the center of mass, or any other means of attitude control.

However, these control devices need control inputs to function. How have previous solar sailing missions provided these control inputs? Did it come from measurements of attitude over time? Measurement of the forces on the sails? Simulated or estimated forces on sails?

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    $\begingroup$ @uhoh The links were from a section I wrote, but deleted before publishing. $\endgroup$ – BobTheAverage Nov 21 at 19:11
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In short, its measurement of attitude compared to external references.

I couldn't say for "pure" solar sailing missions - where the aim of the mission is to demonstrate a large sail, however:

Context

Some geostationary communications satellite missions employ solar sailing as an attitude control technique exactly as the OP describes with a reflective surface whose orientation can be steered relative to the rest of the satellite. Actually in this case these are tabs on the end of the solar arrays and steering the solar arrays differentially produces more or less torque. Its a good solution creating gentle attitude control torques rather than bang-bang thruster control for attitude control and desaturating momentum wheels.

Attitude measurement

Finally the answer: such satellites use an array of conventional sensors for attitude control. Generally this could be a star sensor, Earth sensor, or coarse sun sensor. The latter are interesting in their simplicity, often being a pyramid arrangement of solar cells facing slightly different directions so that the difference between their solar input produces a voltage dependent on the angle to the sun.

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