Implementation of Δv Commands

This chapter discusses the impact of ∆ v implementation error on the performance of spacecraft formation flying, rendezvous, and docking. It also explores how accelerometers can be used to improve performance by providing accurate measurements of the applied ∆ v . As sensing technology and the ability to design optimal trajectories for formations of spacecraft continue to improve, the need for control systems that accurately implement ∆ v is becoming more pronounced. The chapter develops a model of ∆ v implementation error and uses it to investigate the dangers of poor thruster performance. For a rendezvous and docking scenario similar to what might be attempted on the International Space Station, small implementation errors of a few percent are found to result in terminal errors on the order of meters. A feedback control system, using accelerometers to directly monitor thruster performance, is developed and analytical expressions describing its expected performance are obtained. It is found that for a thruster with 2% deviation from the expected thrust and realistic performance parameters, the closed-loop system improved the accuracy of the implemented ∆ v by two orders of magnitude.