A generic drag-free simulator has been developed to aid in the design, on-orbit and post-mission data analysis phases of increasingly complex future missions such as Gaia and STEP. Adaptation to the recent science mission Gravity Probe B (GP-B) has been carried out for a first simulator verification with actual flight data. Lessons learned from GP-B have shown that the controls simulator, developed concurrently with GP-B, has been invaluable to test flight control design and furthermore to resolve on-orbit anomalies in a time-saving manner. A complete mission software simulator including controls, full-body dynamics and comprehensive spacecraft environment disturbances has been established for Gravity Probe B. This simulator provides a reference and development platform for future mission design. The importance of this effort lies in the challenge to meet rising science requirements for future missions in the area of maximum disturbance rejection.
