Harald J. Weigl

GeoCARB image navigation and registration performance

The geoCARB sensor uses a 4-channel slit-scan infrared imaging spectrometer to measure the absorption spectra of sunlight reflected from the ground in narrow wavelength regions. The instrument, which is to be hosted on a geostationary communication satellite, is designed to provide continual monitoring of greenhouse gas over continental scales, several times per day, with a spatial resolution of a few kilometers. The paper discusses the image navigation and registration (INR) of the geoCARB optical footprints on to the earth’s surface. The instrument acquires data in a step and stare mode with 4.08 s stare time and 0.34s step time on 1016 footprints spaced by 2.7 km at nadir in the NS direction along the slit, which is stepped in 3 km EW increments. Knowledge of the instrument line of sight is obtained through use of a dual-head star tracker system (STS), high-precision optical encoders for the scan mirrors, a GPS receiver, and a highly stable common optical bench to which the instrument components, the scan mirror assembly, and the heads of the STS are kinematically mounted. While attitude disturbances due to jitter and solar array flex affect spatial resolution, we show that the effect on INR is negligible. GeoCARB performs a star sighting every 30 minutes to compensate for its diurnal alignment variation relative to the STS, enabling a 1 sigma INR accuracy of 0.38 and 0.51 km at nadir in the NS and EW directions, respectively. Coastline identification may be used to improve accuracy by 6%, while an additional 20% improvement is achievable through identification of systematic errors via extensive post-processing. The paper quantifies all error sources and describes how each of them affects overall INR accuracy.

Predictive Momentum Control for High Fuel Efficiency GEO Spacecraft Stationkeeping

To reduce stationkeeping propellant and extend orbital maneuver life, a predictive momentum adjust system has been developed for the Lockheed Martin A2100 spacecraft. This system is designed for maneuvers where high-efficiency Arcjets (AJTs) fire continuously for Delta-V, Reaction Wheel Assemblies (RWAs) are used for attitude control, and hydrazine Rocket Engine Assemblies (REAs) are pulsed for momentum control. The predictive momentum control strategy improves fuel efficiency by optimizing the use of the REAs, which have a much lower specific impulse than the AJTs. To reduce the REA firing impulse, sequences of contiguous REA pulses, or pulse bursts, are executed to drive the RWA momentum error at the end of the maneuver to zero. The final maneuver momentum error is calculated based on an estimate of the AJT disturbance impulse as well as the commanded and measured RWA momentum. By using prediction, the system eliminates REA firing in response to that portion of the momentum error that would naturally be corrected by the AJT disturbance torque alone. Innovative control logic maximizes the REA duty cycles to reduce the total number of pulses and increase the specific impulse of the firings. The system executes pulse bursts interspersed with intervals of quiescent RWA control in a way that adjusts the RWA momentum and simultaneously maintains the RWA speeds within their allowable limits. The recursive implementation provides feedback to ensure the target momentum is achieved in the presence of REA torque uncertainties and the time-varying AJT disturbance torque. The paper provides an overview of the control system logic, and includes numerical simulation results and in-orbit A2100 spacecraft flight data that illustrate the benefits of the new approach.