On-ground tele-operation and on-board autonomous control for the ETS-VII’s rendezvous docking and space robot experiments

In the near future space activities, the in-orbit servicing capability which means transportation of logistics, maintenance of spacecraft and assist of scientific activities such as space experiments are highly required. These work will be partly conducted by astronauts and manned spacecraft. However, most of these works should be done by unmanned spacecraft and robots from the safety and cost reasons. NASDA developed and launched ETS-VII (Engineering Test Satellite No.7) to test and establish the rendezvous docking and space robot technologies. ETS-VII’s rendezvous docking (RVD) is characterized by the autonomous rendezvous docking between unmanned spacecraft. ETS-VII consists of two satellites, the chaser satellite and the target satellite. ETS-VII robot (RBT) mission is conducted using a 2 m long robot arm on the chaser satellite. Since mission operations are conducted using TDRSS, communication time delay should be considered in a operation systems. In addition to this, RVD and RBT operations are series of time critical and dynamic operations. Therefore, role assignment among the on-board segment, ground segment and human operator is critical. This paper introduces (1) ETS-VII Satellite System, (2) Mission of ETS-VII, (3) Operation systems of RVD and RBT , and (4) First results of the mission. 1. ETS-VII Satellite System The ETS-VII is an experimental satellite to develop and demonstrate the rendezvous and space robot technologies. ETS-VII consists of two satellite named a chaser and a target which weighs 2.5 ton and 0.4 ton respectively. RVD experiments are conducted using the two satellites, while the RBT experiments use the robot arm on the chaser satellite. Both satellites were launched together by NASDA’s H-II launch vehicle on November 28, 1997 and injected into the mission orbit which is 550km height and 35degrees inclination. Designed life is 1.5 years. Figure.1 shows ETS-VII satellite. ETS-VII rendezvous docking system is characterized as unmanned and fully autonomous system. The chaser satellite measures the relative position/velocity to the target satellite using three kinds of sensors depending on the relative distance between the two satellites. A relative GPS navigation is used in the long range (500má). A Rendezvous laser Radar (RVR) and a CCD camera Proximity Sensor (PXS) are used in middle (2má500m) and in short (0má2m) range respectively. Three sets of latching mechanism are used to connect two satellites rigidly for docking. Figure.2 shows RVD components layout of the two satellites. The robot arm on the chaser is 6 degree-of-freedom and 2m in length. Besides the basic endeffector, two endeffectors specialized for handling target satellite and task-board respectively are prepared. Several robot experimental payloads are on-board the satellite. A task board is used to evaluate the robot performance while an ORU (Orbital Replacement Unit) is used to demonstrate the in-orbit equipment exchange.