RELATIVE STATE VECTOR GENERATION ALGORITHM FOR ON-BOARD NAVIGATION FOR RENDEZVOUS DOCKING EXPERIMENT

摘要

Indian Space Research Organization is planning an on-orbit Rendezvous Docking experiment in the near future, todevelop and demonstrate the technology needed for rendezvous docking. In this experiment, two IMS(Indian MicroSatellite) Spacecrafts, one designated as target and the other designated as chaser, are launched by a PSLV launcherinto two slightly different orbits. No communication link between the target and chaser during the far rangerendezvous phase in which relative separation is around 50km to 5km range is envisaged and this phase is a groundguided phase. In the docking phase of the mission, docking sensors such as Laser Range Finder during the relativeseparation of 5 km to 0.25km, Docking Camera during the relative separation of 300m to 1m ,Visual Camera for realtime imaging during the relative separation of 1m to docking are used respectively. This paper presents the studycarried out to assess the accuracy achievable with CW(Clohessy Wiltshire) equation as it is one of the constituent forthe onboard navigation software based on Kalman fillter technique and foreseeing a need for relative statepropagation for longer duration in the absence of measurements. The study is carried out with different initialrelative separation between the target and chaser. The deviation of relative state obtained from CW equation withthat obtained from the individual precise state necessitated the incorporation of J2, the predominant Earth's gravityharmonic in the CW equation. Even with this improved CW model, considerable deviation in the relative state to anorder of few km over a day is noticed with respect to precise model. The periodicity of the deviation in the CWrelative state obtained from the individual precise state lead to representation of residual relative state by Fourierpower series representation with an accuracy of around 10mt. The combination of CW equation with Fourier powerseries representation for relative state vector results in better accuracy for relative state limited mainly bydetermination accuracy and the ground propagation accuracy of the individual state vector. This non abstruseapproach for relative state generation makes it suitable for onboard implementation.