Beta Angle

The beta angle, ft, is defined as the angle between the Earth-Sun line and the orbit plane when the spacecraft is closets to the sun (orbit noon). It varies seasonally between 0° and (i + io)°, where i = orbit inclination, and io = angle between equator and ecliptic plane, which is 23.45°.

The value of ft = 90° results in the greatest solar flux on the satellite body, and ft = 0° gives zero flux. In most satellites with the array always pointed to the sun by the solar array drive motor, the ft angle has an insignificant effect on the generation of electric power. However, the thermal control system is impacted by the ft angle. Low ft may require additional heaters, while high ft may require additional cooling. The ft angle impacts the solar array temperature, which in turn has a small secondary effect on the power generation. The most significant effect of the ft angle on the power system design comes from the eclipse duration as given by Equation 1.5. As ft increases, the eclipse duration decreases, which consequently requires a smaller battery and less charging power during sunlight.

The p angle is not to be confused with the sun angle, 0, often used to define the sunlight incidence angle on the solar array, which could be canted to catch the sun normally in case p is not 90°. The sun angle is defined as the angle between the solar array plane and the sun-pointing vector. The power generation is proportional to cos 0, so 0 of 90° results in the maximum power generation, and 0° gives zero power.

The power system engineer starts with the orbit parameters specified by the customer, which primarily set the orbit period, eclipse duration and the p angle. These parameters in turn have the greatest impact on the power system design.

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