Autonomous Rendezvous, Capture and In-Space Assembly: Past, Present and Future

Douglas Zimpfer* Draper Laboratory, Houston, TX, 77062, USA

Peter Kachmar† and Seamus Tuohy‡

Draper Laboratory, Cambridge, MA, 02139, USA

All mission architectures for exploration involve rendezvous and capture and many include in-space assembly of critical space elements. In order to meet the exploration enterprise goals of affordability, safety and sustainability, the critical capabilities of rendezvous, capture and in-space assembly must become routine and autonomous. For these critical capabilities to become routine, a much more autonomous rendezvous, capture and in-space assembly capability must be employed. Several space programs have developed and routinely performed rendezvous and capture, albeit with significant human intervention from the crew or ground operators. Flight demonstration programs are nearing launch that will demonstrate subsets of the autonomous rendezvous and capture technologies necessary. The assembly of the International and the Russian Mir space stations is proving that in-space assembly is technically feasible. All of these programs are coalescing in the recently announced mission to robotically service the Hubble Space Telescope which will develop and utilize many of the autonomous rendezvous, capture and in-space assembly technologies necessary for the near-term exploration missions. This paper will provide a technical and historical perspective on autonomous rendezvous and capture, from the early days of Apollo through today’s exciting programs and on to the needs of tomorrow’s exploration missions. This historical perspective will be provided from the authors’ unique perspective of supporting every rendezvous, capture and assembly mission since Apollo. We will provide an overview of the requirements for autonomous rendezvous, capture and in-space assembly; define the technologies required to provide safe and routine flight operations for the exploration missions; and apply historical insights to describe how future exploration missions can apply autonomous rendezvous and capture capabilities.

Nomenclature ADEPT = All-Domain Execution and Planning Technology AR&C = Autonomous Rendezvous and Capture ATV = Automated Transfer Vehicle AVGS = Advanced Video Guidance Sensor CEV = Crew Exploration Vehicle DART = Demonstration of Autonomous Rendezvous Technology ETS-VII = Experimental Test Satellite VII EVA = Extra-Vehicular Activity GN&C = Guidance, Navigation and Control GPS = Global Positioning Satellite GSFC = Goddard Space Flight Center HRSDM = Hubble Robotic Servicing and Deorbit Mission HRV = Hubble Robotic Vehicle HST = Hubble Space Telescope * Program Manager Space Transportation. † Principal Member Technical Staff. ‡ Program Manager Space Programs.

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1st Space Exploration Conference: Continuing the Voyage of Discovery30 January - 1 February 2005, Orlando, Florida

AIAA 2005-2523

Copyright © 2005 by Draper Laboratory. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

HTV = H-2 Transfer Vehicle ISS = International Space Station JSC = Johnson Space Center LEO = Low Earth Orbit LIDAR = Light Detection and Ranging LIDS = Low Impact Docking System MSFC = Marshall Space Flight Center OASIS = Orbital Aggregation and Space Infrastructure System RVS = RendezVous Sensor SMART = Spacecraft Mission Assessment and Replanning Tool TCS = Trajectory Control Sensor

I. Introduction On January 14th, 2004 President Bush defined an exciting new vision for Space Exploration in the 21st Century.

This vision outlined an ambitious set of objectives that begins with returning the Shuttle to flight, completing assembly of the International Space Station and initiating a long term exploration program. The exploration program relies on a spiral approach to build on initial development of a Crew Exploration Vehicle (CEV) in this decade, followed in the next decade by returning humans to the moon as a stepping stone to further exploration of Mars in later decades. The exploration program also integrates human and robotic missions to advance these goals.

The spiral development approach1, shown in Figure 1, meets these objectives through a combination of the human and robotic mission programs and technology development programs. To build these programs, NASA must clearly define its key mission needs, the capabilities these missions require and finally the technology necessary to enable the capabilities. Some examples of the GN&C technologies necessary include:

• Safe Crew Aborts • Advanced autonomous flight

operations • Autonomous Rendezvous and

Docking • Lunar and Earth Landing

Systems • In-space Navigation • Highly reliable and

reconfigurable avionics Figure 1. Project Constellation Development Spiral

Given current launch vehicle technologies, all of the mission architectures proposed require the capability to perform rendezvous, capture and in-space assembly. Further, to make these operations routine, autonomous rendezvous and capture (AR&C) is required. Figure 2 shows an early Lunar exploration architecture2. The concept involved the assembly of an Earth-Moon Gateway at L1. This gateway was used as the intermediate transfer point for missions to and from the moon. The early missions involved assembly of the gateway and later missions all involved rendezvous and capture at the gateway.

Several efforts are underway to develop and demonstrate the necessary technologies to provide the autonomous rendezvous, capture and in-space assembly capability necessary to make the Exploration Vision a reality. The technologies under development for these missions include GN&C algorithms, autonomous mission management, sensor technology, mechanisms and robotic assembly techniques. These technologies are at varying stages of readiness, but many are quite mature. The key challenge in the development of any system to perform a rendezvous,

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capture and assembly is the integrated system design and analysis. Fortunately, tools and techniques have also been developed to meet this challenge.

This paper will provide the background necessary to understand the criticality of AR&C, the current state of the art and how these affect the future exploration missions. Section II will review the AR&C challenges posed by the exploration program and the capabilities necessary to meet these challenges. Section III will review the history and the on-going AR&C programs to provide a basis for the

current state of art. Section IV will then expand this history to summarize the state of the art in AR&C. Finally, Section V will provide some concluding remarks on how the on-going programs and current state of the art meet exploration mission needs.

Figure 2. OASIS Mission Architecture

II. Exploration AR&C Challenges As stated in the Introduction, NASA has defined an exciting and aggressive program for exploration that has

resulted in the development of several system of systems concepts.. These concepts vary greatly, but all have one thing in common. Each mission architecture has relied heavily on the ability to rendezvous and mate multiple elements in space. The longer term concepts often include the assembly of human outposts or supply depots. Figure 3 shows a sample of mission architectures3 for Exploration and each relies on rendezvous and capture.

An Apollo-like Lunar return mission would currently require rendezvous and capture in Earth orbit of a transfer stage, CEV and Lunar lander and re-assembly of the configuration in lunar orbit for a return to Earth. Even this simplest of missions, may require uncrewed flight and crewed flight, up to three rendezvous’ and operations in both Earth and Lunar orbits. Mission architectures that employ orbital outposts or depots in Earth orbit, Lunar orbit and/or at LaGrange points (e.g., L1) will require even more rendezvous operations as crewed and robotic vehicles dock with the outposts or obtain supplies from a depot. These operations may become more complex as servicing, assembly and/or repair of vehicles may be undertaken. Again, many variations of the operations may be required. A later scenario may involve the operations for advanced exploration of the solar system. These operations may have additional

Basic Lunar Mission Earth Depot

Low Thrust Transfer Mission Reusable

Figure 3. Possible Mission Architectures

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requirements such as rendezvous during cruise to a planet or rendezvous and capture around an asteroid for a sample and return mission.

Table 1 provides a summary of some rendezvous and capture capabilities required to meet the aggressive space exploration goals. Some of these capabilities have already been demonstrated on past rendezvous missions, some will be demonstrated during upcoming demonstration missions and some require future effort.

Table 1 Exploration AR&C Capabilities Earth and Lunar Orbit Operations Robotic Operations Multiple Vehicle Operations and Assembled Operations Operations with Infrequent or No Ground Communications Operations at L1, Mars or other planetary bodies (e.g., asteroids) Crew transfer operations

Several technologies are required to provide these mission capabilities. First, it should be noted that rendezvous

and capture must become much more automated than today’s Shuttle operations. For rendezvous and capture to become routine it must not require a large operational burden on ground operators or flight crews. For example, current Shuttle rendezvous and capture operations require significant pre-flight mission design and analysis, crew training, multiple crew activities during flight and a specific flight controller position to support the flight operations from the ground. Second, given the possible scenarios for exploration, autonomous operations are required to allow rendezvous and capture to be conducted with minimal ground intervention, either due to communications limitations or communication delays. For example, it is unreasonable to assume that any rendezvous and capture operations in Lunar orbit will occur on the front side of the moon or await the deployment of a fully operational Lunar communication system. Therefore, the remainder of this paper will address AR&C.

Table 2 outlines some of the technologies required to provide the capabilities in Table 1. Although, not explicitly shown in Table 2, the most important aspect in developing AR&C capabilities for future exploration missions will be recognizing that each mission scenario (e.g., Spiral 1 versus Spiral 2 operations) is unique and that unique integrated system design, analysis and test must be performed. The technologies and functional capabilities do not have to be re-developed for each scenario, but their integrated design must be adapted to meet each unique scenario or mission architecture. This does not mean that each repeated flight should require unique design and operations, as is currently done for Space Shuttle flights to the ISS.

Table 2 AR&C Technologies for Exploration Autonomous Relative Navigation Sensors Autonomous GN&C Autonomous Mission Management Advanced Capture and Docking Mechanisms Autonomous Robotic Manipulation Inter-vehicle Communications

III. AR&C Historical Perspective Figure 4 provides a timeline of AR&C missions[4-17]. This section will summarize the highlights of each of these

programs. The Gemini and Apollo programs developed the initial concepts for rendezvous and capture. The Shuttle has demonstrated that these can be performed for a more general set of LEO missions. Near-term demonstration programs, such as DART, XSS-11 and Orbital Express, will demonstrate many of the technologies required for exploration. The Progress, ATV and HTV do or will provide robotic resupply of the Space Station. These missions coalesce in the Hubble Robotic Servicing and Deorbit Mission which will provide an aggressive operational demonstration of autonomous rendezvous, capture and in-space repair.

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Figure 4. AR&C Program Timeline

A. Apollo The Apollo4 program was the first program that required rendezvous and capture to meet its operational mission.

For Apollo, the Lunar Excursion Module (LEM) rendezvous and docked with the Command Service Module (CSM) following its ascent from the Lunar surface. The Apollo program even with its limited computer resources was able to demonstrate many of the automated guidance, navigation and control functions required today for AR&C. Figure 5 shows a sample Apollo co-elliptic trajectory. The Apollo mission relied on rendezvous radar for long range and crew observations for the final terminal stage. The rendezvous planning was performed on the ground, but the onboard system was able to target and automatically control burns. The final capture/docking phase was controlled manually by the crew. The docking mechanism for Apollo was a probe and drogue system, shown in Figure 6.

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Figure 7. Orbiter Docking System Figure 6. Apollo Docking Mechanism

B. Shuttle The Shuttle5,6 has operationally

demonstrated the capability to rendezvous and capture various types of spacecraft. The Shuttle has demonstrated the ability to perform robotic capture of payloads, such as the Hubble Space Telescope, and directly dock with both the Mir and International Space Station (ISS) using the Orbiter Docking System (see Figure 7). The Shuttle flies a stable orbit trajectory shown in Figure 8 with the maneuver sequence planned by ground operators. The onboard GN&C, via crew direction, is able to automatically perform many of the necessary rendezvous functions, including relative navigation, targeting and control. Still, the crew manually performs the final docking maneuvers using visual aids, the

Trajectory Control Sensor (LIDAR) and laptop situational awareness displays, shown in Figure 9.

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Figure 9. Shuttle Situational Awareness Display

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C. DART (Demonstration of Autonomous Rendezvous Technology)

The DART7 mission currently awaiting launch sometime in 2005 is a joint program between MSFC and Orbital Sciences to demonstrate several automated rendezvous and proximity operations technologies. Figure 10 shows the DART proximity operations trajectories. The DART mission will automatically perform flight operations from orbital insertion to close proximity operations (5 m) without any ground intervention. All maneuver sequences have been

preplanned, but will be sequenced and controlled onboard. DART will use GPS for long range measurements and the Advanced Video Guidance System (AVGS) for position and orientation at close ranges (see Figure 11).

D. Orbital Express Orbital Express8,9

is a mission sponsored by DARPA to develop and demonstrate satellite servicing technology. A Boeing team is developing a system that will demonstrate multiple dockings in space using a low-impact docking system and automatic robotic capture via a manipulator (see Figure 12). The Orbital Express vehicle will demonstrate several sensor technologies for long and short range navigation (see Figure 13). The AVGS is again used for close-range position and attitude for docking. An onboard mission manager, derived from the Draper Timeliner software developed for ISS, will autonomously execute ground-planned mission sequences to perform the mission.

Figure 10. DART Proximity Operations Trajectory

Figure 11. Advanced Video Guidance Sensor

Figure 12. Orbital Express Robotic

Capture

Figure 13. Orbital Express Sensor Suite

Distribution Statement A. Approved for Public Release. Distribution Unlimited

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E. XSS-11 The Air Force Research Laboratory and

Lockheed Martin are teaming to develop and fly the XSS-118,10 mission. This small, agile micro-satellite is designed to demonstrate technologies to quickly arrive at a space object and perform maneuvers around it without months of planning and with minimal ground support. To meet these objectives the XSS-11 will fly advanced planning algorithms for rendezvous onboard, along with an event planner, monitor and forward thinking resource manager for proximity operations. This system is based on the Draper All-Domain Execution and Planning Technology (see Figure 14). The vehicle will use an active LIDAR and a passive camera/star tracker for relative navigation (see Figure 15).

F. ATV/HTV/Progress The Russian Progress11, European

Automated Transfer Vehicle (ATV)12 and the Japanese H-2 Transfer Vehicle (HTV)13 all have the mission of providing robotic resupply of the ISS. These missions all have

or will demonstrate the ability of robotic vehicles to automatically perform rendezvous and capture operations. These operations will be planned by the ground control teams. The Progress and ATV directly dock to the Russian Soyuz probe and drogue docking system, while the HTV is planned to be grappled by the ISS robotic arm. For long range operations the HTV will use relative GPS which the Japanese demonstrated on ETS-VII14,15. In close, the Progress utilizes the Russian KURS radar system, while the ATV and HTV will utilize European Rendezvous Sensors which uses

lasers and target reflectors mounted on the ISS16.

Figure 14. Autonomous Mission Manager Architecture

Figure 15. XSS-11 Sensors

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G. HRSDM The Hubble Robotic Servicing

and Deorbit Mission (HRSDM)17 is led by the Goddard Space Flight Center and Lockheed Martin is the prime contractor for development of the Deorbit Module rendezvous and docking system. The Hubble Robotic Vehicle (HRV) will employ many advanced AR&C capabilities for the robotic rendezvous and capture of the Hubble Space Telescope (HST), Figure 16. HRSDM Uncooperative Capture Approach

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including the capability to autonomously identify and capture a tumbling HST (see Figure 16). The HRV will employ both active and passive vision-based systems to identify the relative position and orientation of the uncooperative HST (no previous targets have been located on the HST for these operations), which will be the first time pose of an uncooperative target will be demonstrated for space operations (see Figure 17). An onboard autonomous GN&C system will execute the operations with ground supervision and be able to identify contingency situations and command aborts to safely terminate the capture operations.

IV. AR&C State of the Art As the previous section indicates pment in the area of AR&C. Several

pro

A. AR&C Unique Integrated Analysis,

rated analysis, design and test

NFIR sensor package requires internal projection of images upon 3-D geometric models to provide a texture template to correlate with incoming camera 2-D optical data

Figure 17. Pose Estimation to an Uncooperative Target

there has been significant develograms are going to fly in the near future, culminating in the Hubble Repair Mission. As such, many of the

technologies outlined in Section II are quite mature. The following sections will briefly summarize the state-of-the-art in several key technology areas.

Design and Testing Tools and Techniques

The integing for each unique AR&C scenario is

critical to the overall success of the system development. Many lessons have been learned from the previous programs, in particular the operational lessons of the Space Shuttle. Tools and processes have been developed to perform many of the system design and analyses required. Figure 18 shows an example of a linear covariance analysis which can be performed to assess the navigation and maneuver performance during the rendezvous. Tools of this type, combined

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Figure 18. Linear Covariance Analysis Tool

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with modern simulation techniques and expertise in the system issues related to AR&C are critical to the development of the AR&C system. A final key ingredient is the capability to test and evaluate the system designs. Excellent facilities exist at the MSFC and the Naval Research Laboratory for dynamic simulation and hardware-in-the-loop testing.

B. Autonomous Guidance, Navigation and Control The GN&C required for nearly all of the exploration objectives has been developed and simulated. For example,

Draper had performed high-fidelity simulations of autonomous Shuttle Rendezvous and Capture with cooperative targets nearly fifteen years ago. More recent programs have built on these experiences to develop and refine these algorithms for other LEO missions and will soon flight demonstrate these capabilities. The HRSDM mission will extend these efforts to demonstrate their capabilities for AR&C with a non-cooperative target.

A significant effort in each of the mission designs is to develop the baseline trajectory for rendezvous and capture. Various techniques have been described in Section III as they applied to the specific programs. A key ingredient in the development of the AR&C capabilities for exploration will be the application of these techniques to the various mission scenarios, including integration of the vehicle constraints, e.g., thermal and power, with the sensor field of view and lighting constraints. The lessons learned in these areas can be directly applied for many of the exploration missions in Lunar or Martian orbits, but new trajectory designs and techniques may be required for L1 or low-gravitational bodies such as asteroids.

The GN&C systems must also be extended beyond LEO. Many of the targeting and guidance algorithms are geared towards orbiting large body assumptions and high thrust systems. Guidance techniques to accommodate low thrust propulsion and its application to rendezvous has not been fully explored.

C. Autonomy The autonomy that has been developed for terrestrial applications is slowly making its way to space operations.

The XSS-11 vehicle is planned to extend these capabilities to onboard planning, monitoring and execution for AR&C. These capabilities should demonstrate a level of trust to lead to their use in crewed systems. Care must still be taken to develop the appropriate level of autonomy for a given mission, which is being assessed by the Spacecraft Mission Assessment and Replanning Tool (SMART)18 team at JSC. Also, autonomous ground systems can be employed to work interactively (and hopefully seamlessly) with onboard systems. The Draper ADEPT system framework, combined with the trusted execution capabilities of Timeliner deployed on Orbital Express can provide the necessary flexibility to tailor the autonomy for human and robotic systems.

Another key feature in the autonomous flight operations is the integration of vehicle health management and autonomous fault management. The integration of these systems with an autonomous mission manager provides the onboard capability to sense problems, identify the appropriate sub-system response and re-configure the mission plan to accommodate the updated vehicle system performance.

D. Sensors Sensor technology is critical to the development of AR&C for exploration. Each of the on-going development

programs is extending the sensor capabilities. DART, Orbital Express, XSS-11, HTV and ATV will demonstrate sensor technologies and systems for navigating to cooperative targets that can be employed for exploration. The HRSDM will extend these sensor capabilities to navigating to uncooperative targets. Additionally, imaging technologies are being developed for Shuttle Tile Inspection that may be applicable to relative navigation. These programs will significantly extend the relative navigation sensor technology.

Each of the on-going programs is being performed in LEO and many rely on GPS for absolute orbit determination. Technology must be developed to perform autonomous navigation outside of LEO, for example, systems to navigate in cislunar space, during transfer to Mars, etc.

E. Mechanisms The ability to mate or capture is key to the success of AR&C. The Shuttle, Progress and ATV all rely on heavy

high impact docking systems which are not applicable to the exploration missions. HTV, Orbital Express and HRSDM will demonstrate the capability for robotic manipulator capture and berthing of the vehicles. Both Orbital Express and HRSDM include low impact direct docking systems. Yet, these systems don’t involve the exchange of crew members through a large pressurized tunnel. Engineers at JSC have been developing an advanced docking system19 for application to human exploration vehicles that incorporates active elements to assist the docking (see Figure 19).

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F. Assembly and Servicing Techniques The current state of the art for assembly and

servicing of spacecraft is a manually intensive operation. The Space Shuttle has performed four servicing missions of the Hubble Space Telescope that require a significant amount of Astronaut Extra Vehicular Activity (EVA). The Shuttle has also demonstrated human assembly of the ISS, again, with significant human interaction and EVA. These

operations are both costly and involve human risk. Future space assembly and servicing must become less

costly and less human intensive to reduce safety risks. To meet these objectives the operations must become robotic and simplified. The Orbital Express and HRSDM programs will significantly increase the capabilities to perform these operations robotically. Orbital Express will demonstrate that systems can be designed for satellite servicing, while HRSDM will robotically perform many of the tasks completed by human EVA today (see Figure 20). Both of these developments will be critical to the evaluation and development of assembly and servicing for future exploration missions.

Figure 19. Advanced Docking System

Figure 20. Hubble Robotic Servicing

V. Conclusion Section II outlined the AR&C capabilities and technologies needed to meet the exciting challenges posed by the

exploration initiative. These capabilities and technologies have or are currently being matured through several on-going programs. These programs were reviewed in Section III. The current maturity was discussed in Section IV.

Hopefully, this discussion has shown that the current state-of-the-art is mature for many of the technologies required for AR&C, assuming that the upcoming planned missions are executed successfully. Also, it should be clear that each of the planned missions is developing a piece of the puzzle and all are providing valuable experience and systems that will be needed to provide AR&C for NASA exploration missions.

A key element of the exploration AR&C development will be the ability to perform the AR&C unique design, analysis and integrated testing for each mission spiral. The lessons learned from past programs must be employed and built upon. Additionally, further autonomy must be employed in the future operational systems, including human systems, to provide routine and cost affordable AR&C operations.

Finally, some additional development activities should be considered to assure adequate AR&C for all exploration missions. These include technologies needed to meet longer term mission scenarios, such as low-thrust trajectories and planetary navigation. Also, NASA and its partners must perform unique AR&C design and analysis for the CEV and other exploration vehicles in the context of the planned exploration mission scenarios to ensure the CEV can evolve to meet the challenge.

Acknowledgments As this is a summary paper and highlights the efforts of many organizations the authors would like to

acknowledge those individuals as a whole, specifically those at the DARPA, NASA, Boeing, Lockheed Marin, Mitsubishi Electric and Orbital Sciences. The data provided is from the public domain and is not an acknowledgment by any other parties of the opinions expressed herein.

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