Analysis and Flight Validation of Momentum-Biased Attitude Stability During Open-Loop Deployment on NISAR

The deployment of large unfurlable reflectors presents significant challenges for spacecraft attitude stability, particularly when performed in open-loop (no control) mode due to uncertainties in deployment dynamics and limitations on active control authority. For the NASA–ISRO Synthetic Aperture Radar (NISAR) mission, a 12 m reflector was deployed in orbit using an open-loop sequence. To minimize drift from the Sun and ensure uninterrupted power generation, a momentumbiased configuration was implemented using reaction wheels. Unlike conventional spin-stabilization, which is unsuitable for reflector missions due to telemetry and telecommand pointing constraints, the reaction-wheel momentum bias provided controlled passive stability while maintaining spacecraft orientation. The bias value was derived through analytical modeling of spacecraft dynamics with reaction wheels under a smallangle approximation about the Earth-pointing reference frame, explicitly accounting for gravity-gradient torques and inertia properties. This analysis established stability margins and guided the selection of the commanded momentum bias. The approach also maximized the allowable open-loop duration, providing sufficient time for deployment even under contingency scenarios. Pre-flight predictions showed stable behavior with minimized solar drift, which was subsequently confirmed through on-orbit telemetry. Flight data demonstrated close agreement with analytical results, providing a flight-proven validation of the momentum-biased attitude stability approach for large reflector deployment.