GNCDE as DD&VV environment for ADR missions GNC
Luigi Strippoli*, Nuno Gomes Paulino*, Julien Peyrard*, Pablo Colmenarejo*, Mariella Graziano*, Jürgen Telaar**
*GMV, Spain, , , , ,
**Airbus Defense and Space, Germany,
Abstract
GNCDE ([1]) is an integrated GNC development and verification environment, developed by GMV in the frame of an ESA-GMV co-funded activity. It contains templates of four different adaptable scenarios (Rendezvous and Docking, 3-axis stabilization, Formation Flying, Launchers), a complete set of libraries (based on ESA SPACELAB standard, [2]) for sensors, actuators, DKE and GNC blocks, and a set of tools to ease up the design and analysis of the mission, e.g. guidance trajectories design, control and estimation synthesis, covariance analysis, Monte Carlo campaign, Statistical analysis, Autocoding, 3D visualization through direct connection with tools like Celestia, etc. GNCDE has been already successfully used to design the GNC of different rendezvous missions such as Advanced Re-entry Vehicle ([3]) and Mars Sample Return Orbiter ([4]) and it is the current development environment for the formation flying software of PROBA-3 phase CDE ([5]).
This paper will focus on the utilization of GNCDE for assessing GNC concepts of two different ADR scenarios, both aimed at the post-life disposal of ENVISAT:
1) Design, development, verification and validation of the GNC for RDV and de-orbiting phases of E-Deorbit mission, currently in phase B1. E-Deorbit is so far the most advanced ESA activity with the objective of de-orbiting ENVISAT. It is unique in its operational complexity and requires a high reliable and strongly validated GNC design.
2) Quick preliminary feasibility evaluation from GNC point of view of PRIDE-ISV vehicle used as active debris removal spacecraft. PRIDE-ISV is the ESA program aimed at developing a reusable robotic spacecraft with different in-orbit servicing capabilities, among which the possibility to serve as an ADR vehicle.
The high flexibility of GNCDE has permitted to adapt very quickly the rendezvous and docking template (originally used for an ATV-ISS docking scenario) to the two different ADR scenarios, parametrizing it opportunely to include configuration, initial orbital and attitude data of both ENVISAT and the chaser spacecraft. Sensors and actuators parameters have been also modified to take into account the typical accuracies and errors in the two cases. The rendezvous trajectories have been tailored to these scenarios and the GNC laws adapted to their specific needs.
In the case of PRIDE-ISV scenario, the study preliminarily indicates that the vehicle could be potentially suitable for an ENVISAT ADR mission. Using the link between GNCDE and Celestia, a video showing the capture phase, including synchronization between PRIDE-ISV and ENVISAT, has been also set up. In the case of E-Deorbit, the work to be done has a longer schedule aiming at a fully validated GNC and the design is still on-going. The paper will present the process which is being followed for GNC DD&VV of this specific scenario, how this process is supported by the GNCDE environment and the available preliminary results.
Index Terms— GNCDE, PRIDE-ISV, E-DEORBIT, ENVISAT, ADR
1. GNCDE overview
The GNC design process usually comprises a set of different disciplines and requires involvement of a team composed of people with different background knowledge and formation.
The set of knowledge areas usually involved to a certain extent in the GNC design process includes:
- Mission design and planning
- Spacecraft systems knowledge
- Trajectory design
- Control design
- Sensor technology
- Navigation strategy and Navigation filters design
- Onboard SW coding
- SW verification
- System verification (including HW in the loop)
The GNC design loop is, in general, an iterative process, where the designer moves forward and backward through the several steps of the sequence, in order to refine the functions being designed. Indeed, initial requirements and assumptions can be usually reconsidered in view of the results obtained from either preliminary analyses or detailed performances evaluation. Due to this iterative nature of the work, the handling of the design data, including inputs and outputs to the process (requirements, synthesis models, parameterization of the models, mathematical representation of the navigation and control functions, etc...) needs to be managed in a coherent way and made available in a suitable shape to every support tool being used in the process.
All these facts point to the need for integrated GNC development environments that not only provide the tools able to support the analysis, synthesis and evaluation activities required but that also manage in an integrated way the data being used within the full process. GNCDE is such a kind of integrated GNC development environment, able to provide these tools and the data coherence mechanisms among them. The next figure summarizes the GNC DD&VV process and the support provided by GNCDE during the whole cycle.
Figure 1: GNCDE support along the GNC DD&VV Process
2. ENVISAT overview
ENVISAT ([6]) is a European satellite, launched in 2002 for Earth observation purposes. With 10 instruments aboard and at 8 tons, ENVISAT is the largest civilian Earth observation mission ever delivered on orbit. The ENVISAT mission ended on 08 April 2012, following the unexpected loss of contact with the satellite. Since then, ENVISAT has been coasting without any possibility to command it or even communicate with it. It is now considered the top priority target for an ADR mission. ENVISAT is a high priority ADR target because of its mass, its long decay time (at least 150 years) and its location on a Sun-synchronous orbit that is critical due to its proximity to other Earth observation satellites and debris.
ENVISAT’s position is constantly monitored from the ground, along with other debris, to prevent any collision with an active satellite through collision avoidance maneuvers. Currently, ENVISAT orbits at an altitude of 766km, on a nearly circular orbit, with an inclination of 98.346º. While the position and the physical characteristics of ENVISAT, like the mass, center of gravity or inertia tensor, are known with a relatively good precision, it is not the case with its attitude.
When the satellite was lost, ENVISAT started spinning, up to an angular velocity of several degrees per second, converting the satellite into a very fast spinning object (an outlier) with respect to other debris. The spinning period of the satellite has been measured at 135s, with the main rotation around the satellite y-axis. The angular velocity is slowly decreasing with the effect of the perturbations, primarily from the Earth gravity gradient. This paper considers a pessimist conservative value of 5 deg/s for the initial angular velocity, along the y-axis. Figure 2 shows the ENVISAT body reference axes. The high rotation speed and the mismatch between the alignment of the angular velocity and the main moment of inertia causes the target’s attitude to travel to any direction w.r.t. to the angular momentum (Figure 3).
Figure 2: ENVISAT axis
Figure 3: Initial directions of the angular velocity and momentum, where the approaching axis to target tumbles in the entire possible domain
ENVISAT’s body is made of carbon fibers, thus presenting a high risk of producing debris in case of shock or strong mechanical actions. Therefore, when using a robotic arm, a metallic part of the body must be selected for capture with the robotic arm. One of the most promising locations is the interface ring (origin of the axes in the ENVISAT picture above).
3. E-DEORBIT SCENARIO
The e.Deorbit mission ([7]) is to “Remove a single large ESA-owned Space Debris from the LEO protected zone”. To accomplish this, the mission is designed with a ‘chaser’ satellite launched by a small or medium launcher, which will autonomously perform a rendezvous and docking with the ‘target’ ENVISAT by mean of a robotic arm. The operations include capturing and removing the target from the LEO protected zone accordingly to mitigation rules and mission scenario requirements constrains issued by ESA, which include a robust design of the chaser, able to deal with uncertainties in the target status.
This mission represents a new type of mission with specific needs:
- Meet target satellite and keep a relative position to the target for capture operations
- The target is non-cooperative and non-passivated, tumbling, and the chaser shall stabilize it
- Perform a controlled de-orbiting of the combined target-chaser system
The capture technique employs a robotic arm combined with a clamping or fixation device, selected based on performance, safety, cost, and its verification prior to utilization in the mission.
One of the main critical aspects of the mission is the unknown attitude motion of the target, which puts a heavy demand on the GNC system during rendezvous and capture, where the Chaser spacecraft needs to keep a stationary relative position with respect to the target’s body frame, compensating the centrifugal acceleration while keeping a constant relative attitude. The attitude motion shown in Figure 3 demands a very generic guidance, and has a large impact on the delta-V necessary to achieve the capture point and keep it.
The current chaser wet mass is about 1470 kg, with more than 50% of propellant (~820 kg). 12+12 AOCS thruster of 22 N are foreseen, plus four 220 N and two 425 N thrusters acting as main engines. Camera at far range, LIDAR at medium range and LIDAR 3D at shortest ranges are assumed as relative sensors.
Figure 4: e.Deorbit capture concept explored in Phase B1
3.1. Mission phases and contributions
The rendezvous phase is split into far, medium range and close range (synchronisation) phases. The Stack phase concerns the operations of the composite system, including stabilization, pointing and deorbiting.
GMV provides the guidance and control during the rendezvous sub-phases (excluding the relative navigation):
- Homing (far rendezvous phase), from 8km to 800 m from target
- Safe hold relative orbit at 800 m (an ellipse trajectory never crossing V-bar)
- Closing (medium range rendezvous), from 800 m to 100 m
- Parking hold point, with station keeping over V-bar at 100 m
- Inspection trajectory: from 100 m perform fly-by around the target and back to parking hold point.
- Approach along V-bar to 30m
- Fly-by to the direction of target’s angular momentum vector
- Attitude synchronization, and approach to 7 m
- Final approach and fly by to capture point in target’s body frame
Figure 5: Example of trajectory for final approach
3.2. GNCDE utilization
The design and simulations are carried out in the GNCDE, using extensively its pre-validated SpaceLab libraries (both for real world dynamics and OBSW), the ACED Tool for control synthesis/verification and the MonteCarlo engine. Further, through simulations, analyses are carried out, such as consumption evaluation and sensor placement.
Using the GNCDE template framework, two MIL simulators are developed: the RDV template and the STACK template. The former runs the rendezvous phase from far range up to synchronization, while the latter is in charge of simulating the composite body (chaser+target) phases (detumbling and deorbiting).
Figure 6: GNCDE template structure
Each GNCDE template contains an OBSW module which includes the GNC and management modules.
Figure 7: Structure for the On Board SW in the GNCDE template
For an overview, Figure 7 displays the structure adopted in the simulator at this analysis phase:
- Mission and Vehicle Management: it manages the GNC mode transitions and selection/deselection of equipment (sensors and actuators) and GNC functions for each of the GNC modes. A programmable model is already available in GNCDE libraries according to SPACELAB standard, which will be tailored for each simulator.
- Control algorithms: they include the robust control laws for each GNC mode, implemented with ACED Tool according to synthesis/analysis techniques, expressed by means of state-space formulation.
- Navigation algorithms: for STACK it includes the filtering of the absolute sensors while in the RDV template it relies on a performance model of the visual/LIDAR based navigation filter (far range visual camera navigation during the homing, LIDAR performance model at safe hold point).
- Guidance algorithms: they include the generation of the chaser reference relative position and attitude profiles, as well as the computation of the required feed-forward actions from known spacecraft dynamics. Although some guidance libraries were available in GNCDE, given the novelty of the scenario, new libraries were added and integrated in GNCDE for specific parts of the trajectory such as the spiral approaches, fly-by along the angular momentum, and station keeping in target’s body frame.
- The actuation management computes the allocation of each thruster magnitude with a simplex optimization, based on the commanded torques and forces from control. Also this model was already available in GNCDE/SPACELAB framework.
The control design is carried within the H∞ framework, where the system’s description is combined with the weighting transfer functions to create the augmented plant used in the synthesis. Any set of requirements expressed in frequency through W(s) becomes embedded in the system.
As mentioned, the design and robustness analysis is supported by the Automatic Control and Estimator Design Tool (ACEDTool) which supports the user in the GNC design process by allowing control and estimator design and analysis for linear time invariant (SISO or MIMO and continuous or discrete) systems. The stability and performance objectives are achieved by tuning the weighting functions, and the ACEDTool contains a very generic augmented plant, allowing the expression of requirements on error, output, actuation, and also characterization of generalized inputs, Figure 8. Furthermore, it takes in uncertain systems, and supplies the tools for robustness analysis of the closed loop system, for a first validation of the performance and stability of the controller in the presence of uncertain parameters and unmodelled dynamics.
Figure 8: MIMO robust synthesis and analysis framework, built in the ACEDTool
While for the far and medium rendezvous the control relies on attitude pointing control, for closer operations there is the need for a more agile 6DoF controller to deal with coupled attitude and position relative dynamics, while keeping good tracking in the vicinity of the target. During the STACK phase, a much heavier body implicates a slower dynamics, affected also by the flexible modes of ENVISAT’s solar panel.