At most orientations the power generated by the arrays is more than sufficient to run the spacecraft. When the spacecraft is tipped at a large angle from the sun, however, it may be necessary to draw additional power from the spacecraft batteries. The primary function of the batteries is to provide power to IUE during Earth shadow periods. Shadow seasons occur when a portion of the IUE's orbit passes through the Earth's shadow once each day for 3 weeks in the late summer and winter. Note time change between EST and EDT on the first Sunday in April and the last Sunday in October.
Pitch is the angle between the sun and the telescope pointing.
At zero degrees pitch, the telescope would be pointed at the sun while a pitch angle of 180 degrees corresponds to the anti-solar position. The Beta angle is then defined as 180 degrees minus the pitch angle. The OBC also performs other important functions, such as controlling camera exposures. Prior to August 1985, the OBC used data from three or more of IUE's six gyros to control attitude. The FSS measures the orientation of IUE with respect to the sun.