... The primary observational constraints on the schedulability of most fixed targets in two-gyro mode are the position of the target in the sky, the required orientation or roll angle of the observatory (if any), and the required timing of the observation (if any). ... Figure 2.3 provides a graphical description of the general decision process involved in determining the schedulability and orbital visibility periods for fixed targets observed in two-gyro mode. ... Available Orientations . ... Two-Gyro ...