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Spaceflight Mechanics and Exploration Systems Laboratory

Attitude Dynamics.


Attitude model for spacecraft under solar radiation pressure with anisotropic reflection.

This research focused on a modeling technique of the solar radiation pressure (SRP) exerted on spacecraft which can incorporate general surface with anisotropic reflectance property. The SRP torque is a major disturbance for the attitude of spacecraft especially in deep space missions. It depends not only on the shape and attitude of spacecraft, but also on their surface optical reflectance property. The aim of this paper is to create a precise reflectance distribution model by measurement of a bidirectional reflection distribution functions(BRDF). A BRDF, a technique originally developed in the field of Computer Graphics, describes a reflectance map between arbitrary incident and emitted rays in a general mathematical form.

The SRP torque is a major disturbance for the attitude of spacecraft especially in deep space missions. It depends not only on the shape and attitude of spacecraft, but also on their surface optical reflectance property. Some missions even attempted to exploit the SRP to stabilize and reorient spacecraft attitude rather than compensating it. Two such typical missions are the JAXA’s solar sail technology demonstration spacecraft IKAROS and the asteroid explorer Hayabusa2. They modelled very precise SRP torque exerted on the spacecraft bodies (GSDM: Generalized Solar-sail Dynamics Model) and successfully extracted a useful behavior to control the spacecraft attitude. On the other hand, owing to precise SRP modeling used in the operation of IKAROS and Hayabusa2, at least 3% of directional inconsistency has found between the SRP torque estimated from the conventional SRP model used in the GSDM and the actual flight data.

Generalized Solar-sail Dynamics Model.

Generalized Solar-sail Dynamics Model.

Our detailed sensitivity analysis implies the error should be due to unmodelled anisotropic optical reflectance of a large area on the spacecraft, such as solar array panels (SAPs). Therefore, the aim of this research is to create a precise reflectance distribution model by measurement of a bidirectional reflection distribution functions (BRDF). A BRDF, a technique originally developed in the field of Computer Graphics, describes a reflectance map between arbitrary incident and emitted rays in a general mathematical form.

The BRDF measurement was performed for test cells of the Hayabusa2 SAPs. As a result, a distinctive anisotropic reflectance property was found which hadn’t been modelled in the conventional model. It was also suggested that this reflection should be due to the microscopic structure of the solar cell. Then a compact mathematical model of the reflection which reproduces the anisotropic property observed in the BRDF measurement has been developed. This model successfully describes the inconsistency between the conventional SRP torque and the flight data and relates material’s microscopic structure to the optical property.

experimental result of BRDF measurement of hayabusa2 materials.

Anisotropic Reflection in SAP in Hayabusa2 used.

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