The goal of the Orbital Express Space Operations Architecture program is to validate the technical feasibility of robotic, autonomous on-orbit refueling and reconfiguration of satellites to support a broad range of future U.S. national security and commercial space programs. Refueling satellites will enable frequent maneuvers to improve coverage, change arrival times to counter denial and deception, and improve survivability, as well as extend satellite lifetime. Electronics upgrades on-orbit can provide performance improvements and dramatically reduce the time to deploy new technology. The Orbital Express advanced technology demonstration will design, develop, and test on orbit a prototype servicing satellite (ASTRO), a surrogate next generation serviceable satellite (NEXTSat). The elements of the Orbital Express demonstration will be tied together by non-proprietary satellite servicing interfaces (mechanical, electrical, etc.) that will facilitate the development of an industry wide on-orbit servicing infrastructure. NASA will apply the sensors and software developed for autonomous rendezvous and proximity operations to enable future commercial resupply of the International Space Station.

Since the 1960s, NASA has performed numerous rendezvous and docking missions. The common element of all US rendezvous and docking is that the spacecraft has always been piloted by astronauts. Only the Russian Space Program has developed and demonstrated an autonomous capability. The Demonstration of Autonomous Rendezvous Technology (DART) project currently funded under NASA's SPace Launch Initiative (SLI) Cycle I , provides a key step in establishing an autonomous rendezvous capaibility for the United States. The Objective of the DART mission is to demonstrate, in space, the hardware and software necessary for autonomous rendezvous. Orbital Sciences Corporation intends to integrate an Advanced Video Guidance Sensor and Autonomous Rendezvous and Proximity Operations algorithms into a Pegasus upper stage in order to demonstrate the capability to autonomously rendezvous with a target currently in orbit. The DART mission will occur in April 2004. The launch site will be Vandenberg AFB and the launch vehicle will be a Pegasus XL equipped with a Hydrazine Auxiliary Propulsion System 4th Stage. All mission objectives will be completed within a 24 hour period. The paper provides a summary of mission objectives, mission overview and a discussion on the design features of the chase and target vehicles.

In general, autonomous rendezvous and docking requires that two spacecraft start at a remote distance (i.e., out of sight of each other), come together into a common orbit, rendezvous, dock, and control the new combined spacecraft in both orbit and attitude. Doing this requires developing and testing a variety of new technologies including absolute and relative autonomous navigation, autonomous rendezvous and docking hardware and software (both sensors and actuators), and autonomous control of a "new" spacecraft with different mass and inertia properties than either of the two original spacecraft. While these are very workable technologies, they do require a significant change in mindset -- turning over control of thrusters and other actuators to an on-board computer. While there is substantial potential for cost savings, risk reduction, and new mission modes by use of these technologies, there is a very strong reticence to allowing operational spacecraft to control their own destiny, particularly in firing thrusters. This paper summarizes work at Microcosm and elsewhere in each of the above technologies. Autonomous navigation and absolute orbit control have been demonstrated on orbit. In conjunction with Michigan Aerospace, autonomous rendezvous and docking hardware and algorithms have been demonstrated in parabolic flights and zero-g simulations. Approaches have been proposed for more precise and robust autonomous navigation and autonomous on-orbit estimation of combined mass and inertia properties, leading to efficient orbit and attitude control of the combined spacecraft. Many of these technologies can be tested at low cost in parabolic flights, suborbital flights, and evaluation of data from existing or planned missions. Thus, a "coordinated attack" on the complete problem of fully autonomous rendezvous and docking is both feasible and potentially very low cost.

Michigan Aerospace Corporation has developed a mechanism for microsatellite docking, which has been successfully demonstrated in a microgravity environment. This docking mechanism is specifically designed for soft-docking capability, tolerance to misalignment, and scalability. The current Autonomous Microsatellite Docking System (AMDS) design resulted from modifications to an earlier docking mechanism prototype that was tested at the Marshall Space Flight Center (MSFC) Flat Floor Facility. The AMDS was tested in a microgravity environment through the NASA JSC Reduced Gravity Program, where a KC-135 turbo jet flies a series of parabolic maneuvers. The test objectives of the KC-135 flight were to determine the docking mechanism cable assembly behavior in zero-g, test the full range of the docking envelope in a six degree of freedom test setup and determine the undocking capability and stability. The nature of the Michigan Aerospace docking mechanism enabled the entire docking cycle, including soft dock, auto-alignment and hard dock, to be completed within the 20 seconds of 'zero-g' time. Complete end-to-end docking and undocking was performed under a variety of initial conditions and docking parameters. The data collected during the KC-135 testing will be used to validate dynamic simulation models of the docking mechanism. The intent of these dynamic models is to examine a number of docking scenarios between a chaser and target satellite. This paper will discuss the results of the KC-135 docking tests and docking simulations.

The Thermosphere•Ionosphere•Mesosphere•Energetics and Dynamics (TIMED) program is developed by The Johns Hopkins University Applied Physics Laboratory (APL) for NASA as NASA's first Solar Terrestrial Probe mission in the Solar Connections program. It was successfully launched into its desired orbit on December, 7, 2001 and started its prime science mission in January 2002. TIMED employs four remote sensing instruments designed to provide measurements needed to characterize the temperature, density, and wind structures of the Earth's atmosphere extending from 60 to 180 kilometers and to understand the processes controlling the region's energy balance. TIMED is a low-cost mission and its spacecraft was designed with a high degree of autonomy to enable inexpensive Mission Operations using a relatively small Mission Operations Team. The keys to enabling this cost savings are the decoupled instrument operations approach based on real-time GPS navigation and event based instrument commanding on board the spacecraft. This paper provides an overview of the TIMED mission, mission architecture, spacecraft and ground system design. The focus of this discussion will be on cost reduction efforts, especially those related to advanced technology and the enabling concept of decoupled mission operations.

The Virtual System Integration and Testing (VSIT) paradigm is presented as a means for verification and cost and risk reduction of space system technologies. Components and benefits of the paradigm are explained, accompanied by examples of its application as carried out by the Boeing VSIT team. Particular attention is paid to the current application of VSIT to the Orbital Express (OE) program. Based on examples of the application of VSIT and its latest application to the OE program, the conclusion is drawn that VSIT is a valuable paradigm that can significantly reduce the costs and risks of space system technology development and provide for a cost-effective and thorough means of verification of system level performance.

The development, integration, test, and deployment of large space systems has grown increasingly complex. Simulation of specific critical events such as rendezvous/docking, deployments, separations, pointing and station keeping has been possible for some time and can be used for design decisions and operations planning. An emerging technology in space systems is the simulation of entire systems during the design process. Now a whole array of design and operational decisions can be made based on an integrated simulation model before the hardware has been built. The fidelity and complexity of this simulation model has increased to match the complexity of the systems that engineers are creating. This paper describes the latest developments in simulation technology to support simulation based design decisions for space systems. These developments are moving the simulation industry beyond parametric based analysis to template based design, complete integrated design process flow, and multidisciplinary design tradeoffs and optimization. The particular issues for simulation based design that impact the space system industry are addressed. These issues include proof of accuracy, parametric based exploration, interdisciplinary model integration, modeling of innovative systems. Examples are given from industry to illustrate the extent that simulation tools are now affecting the space system design process and how these models can be used to make operational decisions.

In recent years, Michigan Aerospace has approached the problem of gentle autonomous spacecraft rendezvous and docking using a flexible soft-dock cable that is extended from the docking spacecraft to the target spacecraft. Because of the nature of a soft-dock cable, testing and validation of the technology is difficult in normal gravity. To properly emulate the behavior of this soft-dock cable, we have performed dynamic computer simulations so that the effects of micro-gravity could be simulated. The Autonomous Satellite Docking System (ASDS) was initially prototyped and tested at Marshall Space Flight Center’s air-bearing floor facility. The test data was compared to the simulations and used to validate the model. Once a good correlation between the simulation’s predicted results and the actual data was shown, the model was used to predict future performance of the ASDS mechanism on several potential spacecraft for the Orbital Express program. A new dynamic simulation model was created and compared to test data from a recent KC-135 flight test to further validate the modeling approach used. This paper will describe the methodology used in modeling and simulating the ASDS mechanism. Correlation between the models and the test data will be discussed.

The advent of powerful Commercial Off The Shelf (COTS) real-time hardware and significant advances in dynamic modeling and simulation software, combined with strides in cost-efficient virtual instrumentation and animation tools, provides the means for development of innovative low cost spacecraft control solutions. In this paper we develop hardware and software system architectures for a flexible simulator which can function as a powerful platform for research, education and testing. The objective is to provide a basic system configuration that is modular and can be easily reconfigured to meet several different requirements, thus providing a single platform for different phases of the design and development process. The use of graphical modeling and automatic code generation tools that are tightly integrated with the hardware ensures rapid software reconfiguration. The paper also discusses two applications that have taken advatage of this architecture.

The Engineering Directorate of NASA Johnson Space Center has developed a nanosatellite-class free-flyer intended for future external inspection and remote viewing of human spaceflight activities, including International Space Station (ISS) operations. The Miniature Autonomous Extravehicular Robotic Camera (Mini AERCam) technology demonstration unit has been integrated into the approximate form and function of a flight system. The spherical Mini AERCam free flyer is 7.5 inches in diameter and weighs approximately 10 pounds, yet it incorporates significant additional capabilities compared to the 35 pound, 14 inch AERCam Sprint that flew as a Shuttle flight experiment in 1997. Mini AERCam hosts a full suite of miniaturized avionics, instrumentation, communications, navigation, imaging, power, and propulsion subsystems, including two digital video cameras and a high resolution still image camera. The vehicle is designed for either remotely piloted operations or supervised autonomous operations including automatic stationkeeping and point-to-point maneuvering. Free-flyer testing has been conducted on an air-bearing table and in a six degree-of-freedom closed-loop orbital simulation. The orbital simulation models the three-dimensional dynamics of the free-flyer in proximity to the ISS, and produces corresponding God's eye views and simulated free-flyer camera views. A high-fidelity simulation is achieved by directly interfacing to free-flyer thruster driver signals, emulating the MEMS gyro responses in hardware, and using the "truth" state to drive a GPS signal generator connected to the free-flyer GPS receiver.

A long-range scanning laser range imaging system designed for 3D imaging applications is presented. The system will be compact, lightweight and low power: ideally suited for remote and robotic applications. It will feature a fully-programmable scanner with a wide field of regard, and a precise time-of-flight laser range measurement system that will provide high-speed, accurate point-cloud data from very short to very long ranges. The potential applications of this technology to be briefly discussed here, both terrestrial and in space, are numerous. They include: robotic vision; autonomous navigation and guidance; mapping and surveying; on-orbit rendezvous and docking; planetary landing; visual geology; and rover navigation. This paper will discuss the physical characteristics of the system as well as the performance of the lidar itself. Test results and some sample imagery will be presented. The paper will also discuss some of the applications for which the system may be suited.

Our approach to onboard processing will enable a quicker return and improved quality of processed data from small, remote-sensing satellites. We describe an intelligent payload concept which processes RF lightning signal data onboard the spacecraft in a power-aware manner. Presently, onboard processing is severely curtailed due to the conventional management of limited resources and "power-unaware" payload designs. Delays of days to weeks are commonly experienced before raw data is received, processed into a human-usable format, and finally transmitted to the end-user. We enable this resource-critical technology of onboard processing through the concept of Algorithm Power Modulation (APM). APM is a decision process used to execute a specific software algorithm, from a suite of possible algorithms, to make the best use of the available power. The suite of software algorithms chosen for our application is intended to reduce the probability of false alarms through post-processing. Each algorithm however also has a cost in energy usage. A heuristic decision tree procedure is used which selects an algorithm based on the available power, time allocated, algorithm priority, and algorithm performance. We demonstrate our approach to power-aware onboard processing through a preliminary software simulation.

This article discusses the principle and performance of a controlled sensitive element. A concept of the cryogenicoptical sensor based on competitive the adaptive sensitive element applicable to a gravity meter sensor is considered. The sensor element is based on a magnetic levitation phenomenon, high-precision optical registration of levitating body mechanical coordinates, and robust signal processing tools. A controlled self-bearing probe dynamics is also analyzed. An dynamical approach to highly sensitive measurement of weak signal is presented. The robust signal estimation problem is considered, when signal are estimated via application of neural networks and when nonlinear measurements are used. The construction of the sensor is described. Simulation results support the mathematical, and the system characteristics are thus optimized.

An ideal Rendezvous and Capture (R&C) sensor on a seeker Space Vehicle (SV) would provide accurate relative 6 degree of freedom data for the Guidance Navigation and Control System (GNCS) from far and near, operate autonomously, and provide multifunctional capability. Flash LADAR has the potential to fulfill these requirements. Sandia has developed Scannerless Range Imaging (SRI) LADAR sensors for a multitude of applications. One of the sensors, LDRI, flew onboard the STS97 mission to install the P6 truss and solar panels on the International Space Station. When compared to scanning LADAR, Scannerless LADAR is smaller, lighter, not mechanically complex, and has a much faster image acquisition time. Recently Sandia has demonstrated Flash Scannerless Range Imaging. Flash LADAR enables the capture of a full scene 3-D range image in one acquisition, thus, enabling freeze motion. The technology’s proven ability to accurately image an object as well as capture the image on the move has the potential to provide very accurate static and dynamic position data for the target vehicle relative to the seeker SV. Since no specific requirements are imposed on the target vehicle, the sensor will work equally well on cooperative and uncooperative target vehicles. This sensor technology can also provide docking feature inspection data and perform a detailed inspection of the target vehicle. This paper will describe the applicability of a Flash LADAR sensor for on-orbit cooperative and uncooperative rendezvous and capture.