Acknowledgments Chapter 1: Introduction to Two Gyro-Mode 1.1 Introduction 1.2 Two-Gyro Pointing Jitter and Image Quality 1.3 Guide Star Acquisitions 1.4 Scheduling and Target Visibility 1.5 Science Instrument Performance Chapter 2: Planning Observations in Two-Gyro Mode 2.1 All-Sky Availability of Fixed Targets 2.1.1 Overview 2.1.2 All-sky Availability Movie 2.1.3 Number of Available Days During the Course of a Year 2.2 Scheduling Considerations and Visibility Periods for Fixed Targets 2.2.1 Overview 2.2.2 Unconstrained Fixed-Target Observations 2.2.3 Constrained Fixed-Target Observations 2.2.4 Examples 2.3 Verifying Scheduling Constraints for Phase I 2.4 Two-Gyro Orbit Calculations for Phase I 2.5 Continuous Viewing Zones 2.6 Moving Targets