Table 1. ESS on-board rendezvous sensors.
Résumé
Le projet ESS, financé par l'Agence spatiale allemande, est consacré à l'utilisation de la robotique pour les interventions à distance sur des satellites géostationnaires ou quasi géostationnaires. Il s'appuie sur les enseignements technologiques de l'expérience Rotex menée dans le cadre de la mission D2. La réussite de la mission dépend dans une grande mesure de la qualité du système de pilotage, guidage et navigation. Une étude de faisabilité de la mission et du principe d'ensemble a été effectuée en 1994, la mise au point et l'évaluation des éléments-clés du système devant prendre fin en décembre 1997.
Contractors:
Jena Optronik (D),
Dornier Satellitensysteme (D),
OHB (D), GTA (D),
or-Computers (D),
DLR (D), FGAN (D)
Funding:
German national programme
Even today, a geostationary satellite, whose useful life has ended, must be abandoned in space. The experimental servicing satellite ESS applies robotics to resolve the problem of servicing a non-cooperative target in or near to a geostationary orbit, a region of space still out of reach to manned spaceflight. A three-month demonstration flight of ESS has been planned and all phases of its mission have been defined. These include the acquisition, inspection and servicing of an orbiting satellite through to parking it in a "graveyard" orbit. The project builds on the experience gained with the robotics technology experiment Rotex, flown on the D2 Mission.
Studies have assumed an Ariane 44LP launch and mission control carried out by the German Space Operations Centre (GSOC). A spin-stabilised spacecraft TVSat1, has been selected as a typical target satellite. This spacecraft flies in the super-geostationary orbit and, due to its drift, is continuously visible for 30 days from GSOC. This target does not support rendezvous and docking.
The guidance, navigation and control system of ESS plays a fundamental role in the way it carries out its tasks. In general, all operational steps will be initiated and supervised by the ground station, and many will be executed autonomously by ESS.
ESS will be launched into a geostationary transfer orbit from which it will be raised into a full geostationary orbit and will then drift towards its target, maintaining a three-axis-stabilised attitude. Under surveillance of the ground station, it will then autonomously transfer to a super-geostationary orbit, homing-in to an initial aiming-point about 24 km behind its target. Once acquired, ESS autonomously tracks the target and moves to an inspection point about 100 m behind it. From here ESS performs an autonomous in-plane and out-of-plane fly-around inspection of the target.
Video cameras on-board ESS take images of the target spacecraft which are sent to the control station and used to determine the spin rate of the target. Under ground control, ESS is then manoeuvred to point about 100 m 'below' the target along its spin axis. At this point, a manipulator on-board ESS is deployed, during which attitude control counteracts disturbance forces and maintains ESS in its position relative to target.
With the manipulator still deployed, ESS enters a spin-stabilised mode and, locking onto the spin rate of the target satellite, moves to a point about 2 m away from it, along the spin axis. Having captured the target by means of the manipulator equipped with a suitable tool, the guidance navigation and control system re-orientates ESS in relation to the target, to assume a position from which servicing activities can be carried out. As a minimum, the composite spacecraft pair can then be reoriented and, under the automatic on-board control of ESS, the target may be retrieved or placed into a graveyard orbit.
The motion of ESS with six degrees-of-freedom is actuated by 16 ten-newton thrusters; attitude and rate measurements are provided by an inertial measurement unit, Earth- and Sun-sensors, and appropriate rendezvous sensors.
To carry out the entire rendezvous phase ESS requires special navigation sensors. The orbital position of the target can be determined from the ground by the tracking and imaging radar at FGAN near Bonn. Measurements have shown that the 3-sigma uncertainty of the positioning is below 24 km along- track, 17 km across-track and 2.6 km in range. Consequently, the on-board sensors have to make up for the difference and cover a range of 24 km down to 1 m. Since the target has no cooperating optical marks or active transponders, only known shape and contours can be used for recognition. The underlying concept of the sensor system is shown in
Table 1.
Table 1. ESS on-board rendezvous sensors.
The final stages of approach and the subsequent capture of the target are the most critical phases of the mission. The manipulator of ESS, equipped with a capturing tool, must follow the residual movements of a selected object on the target (e.g. the main thruster) by means of an image processing system whose data are passed through an extended Kalman filtering process. With the robot controller monitoring force, torque and travel, the capture tool is inserted into the cone of the thruster. After capturing the target satellite, the ensemble is stabilised and reoriented. To free the manipulator for servicing activities and to provide a stiff mechanical coupling, the target satellite is grasped by means of a docking mechanism (Figure 1).
Figure 1. An artist's impression of ESS capturing its target at the main thruster.
To perform its servicing task, the robot replaces the capture tool with an appropriate servicing tool such as a scissors or a gripper. This requires that a tool adaptor, fitted with an integrated force and torque sensor and a stereo camera be attached to the manipulatorÕs endmost section. The tool exchange process is executed automatically, but control of the repair task itself must be shared between the machine and a human operator at the ground station. To counter the transmission time delay, a predictive graphical simulation of the robot's behaviour in its environment is used at the ground station.
Although ESS is a highly complex automatic system, it is easy to maintain and its architecture is simple and extendable. This implies the use of modular hardware and software. To curb costs, standardised elements are used wherever possible to realise the basic satellite functions. The satellite is now sufficiently defined to allow component procurement (in the next stage of the project) to proceed.
In ESS, DARA is sponsoring a project to evaluate the technology for servicing satellites in space, something which appeals to a broad range of interests.
The technology needed, especially in the fields of automation and robotics, and guidance navigation and control are now available. All necessary control algorithms have been prototyped as real time embedded software. Simulation work, involving software and breadboard hardware, will be completed by the end of 1997.
Preparing for the Future Vol. 7 No. 2