David Folta

SATELLITE FORMATION FLYING USING AN INNOVATIVE AUTONOMOUS CONTROL SYSTEM (AUTOCON) ENVIRONMENT

This paper describes the formation of a partnership between two competing technologies with very different approaches to the problem of enhanced formation flying (EFF) on the New Millennium Program (NMP) Earth Orbiter (EO)-l mission. This includes a brief description of the two approaches that were independently proposed by the Goddard Space Flight Center (GSFC)/Stanford University and JPL teams. The actual mission combines these two approaches within a single autonomous control architecture called AutoCon™. The software is designed so that a control mode switch can be set by the ground flight operations team to invoke either EFF algorithm. The advantage of this approach is that both EFF technologies can be incorporated onboard EO-1 within the AutoCon™ framework. In addition, the details of each proposed algorithm need not be divulged provided that the algorithms conform to the specifications of AutoCon™. Forming a partnership between two competing technologies represents a significant programmatic challenge. This paper discusses the programmatic issues and several of the technologies that have been developed to perform the EFF mission. In the process, several recommendations are provided that should streamline similar partnerships on future NMP missions.

A Formation Flying Technology Vision

Formation Flying is revolutionizing Earth and Space science and the way the space community conducts science missions. This technological transformation provides new, innovative techniques in spacecraft guidance, navigation, and control, and how the science community gathers and shares information between space vehicles and the ground. These technologies will also expedite the human exploration of space. Once fully matured, this technology will result in swarms of space vehicles flying as virtual platforms or distributed space systems and sensor webs which gather significantly more and better science data than is possible today. Formation flying will be enabled through the development and deployment of automated spaceborne guidance, navigation, and control (GN&C) systems integrated with intrasatellite communications infrastructure. Innovative spacecraft autonomy techniques, such as differential Global Positioning System (GPS) technology, optical, and vision-based navigation and celestial navigation allow autonomous navigation while other technologies provide for the autonomous control methods to maintain and optimize these formations. This paper provides a vision of the future of space exploration in the context of GN&C technology, and an overview of the current status of NASA technology development and its partnerships with the Department of Defense (DoD), Industry, and University to bring formation flying technology to the forefront as quickly as possible. We address the impediments that need to be overcome to achieve the distributive spacecraft vision and NASA GSFCs approach to transfer this technology to space. We will also describe some of the formation flying testbeds, such as the on-orbit Air Force Research Laboratory (AFRL) University Nanosats, and Orion test beds and ground based GSFC Formation Flying Test Bed, which are being developed to demonstrate and validate innovative position sensing and formation control technologies.

Multibody Orbit Architectures for Lunar South Pole Coverage

A potential ground station at the lunar south pole has prompted studies of orbit architectures that ensure adequate coverage. Constant communications can be achieved with two spacecraft in different combinations of Earth-Moon libration point orbits. Halo and vertical families, as well as other orbits near L1 and L2 are considered. The investigation includes detailed results using nine different orbits with periods ranging from 7 to 16 days. Natural solutions are generated in a full ephemeris model, including solar perturbations. A preliminary station-keeping analysis is also completed.

Formation Flying Satellite Control Around the L2 Sun-Earth Libration Point

A growing interest in formation flying satellites demands development and analysis of control and estimation algorithms for station-keeping and formation maneuvering. This paper discusses the development of a discrete linear-quadratic-regulator control algorithm for formations in the vicinity of the L2 sun-earth libration point. The development of an appropriate Kalman filter is included as well. Simulations are created for the analysis of the station-keeping and various formation maneuvers of the Stellar Imager mission. The simulations provide tracking error, estimation error, and control effort results. From the control effort, useful design parameters such as delta V and propellant mass are determined. For formation maneuvering, the formation spacecraft track to within 4 meters of their desired position and within 1.5 millimeters per second of their desired zero velocity. The filter, with few exceptions, keeps the estimation errors within their three-sigma values. Without noise, the controller performs extremely well, with the formation spacecraft tracking to within several micrometers. Each spacecraft uses around 1 to 2 grams of propellant per maneuver, depending on the circumstances.

A Survey Of Earth-Moon Libration Orbits: Stationkeeping Strategies And Intra-Orbit Transfers

Cislunar space is a readily accessible region that may well develop into a prime staging area in the effort to colonize space near Earth or to colonize the Moon. While there have been statements made by various NASA programs regarding placement of resources in orbit about the Earth-Moon Lagrangian locations, there is no survey of the total cost associated with attaining and maintaining these unique orbits in an operational fashion. Transfer trajectories between these orbits required for assembly, servicing, and positioning of these resources have not been extensively investigated. These orbits are dynamically similar to those used for the Sun-Earth missions, but differences in governing gravitational ratios and perturbation sources result in unique characteristics. We implement numerical computations using high fidelity models and linear and nonlinear targeting techniques to compute the various maneuver (Delta)V and temporal costs associated with orbits about each of the Earth-Moon Lagrangian locations (L1, L2, L3, L4, and L5). From a dynamical system standpoint, we speak to the nature of these orbits and their stability. We address the cost of transfers between each pair of Lagrangian locations.

Libration Point Navigation Concepts Supporting the Vision for Space Exploration

This work examines the autonomous navigation accuracy achievable for a lunar exploration trajectory from a translunar libration point lunar navigation relay satellite, augmented by signals from the Global Positioning System (GPS). We also provide a brief analysis comparing the libration point relay to lunar orbit relay architectures, and discuss some issues of GPS usage for cis-lunar trajectories.

Formation Control of the MAXIM L2 Libration Orbit Mission

The Micro-Arcsecond X-ray Imaging Mission (MAXIM), a proposed concept for the Structure and Evolution of the Universe (SEU) Black Hole Imager mission, is designed to make a ten million-fold improvement in X-ray image clarity of celestial objects by providing better than 0.1 micro-arcsecond imaging. Currently the mission architecture comprises 25 spacecraft, 24 as optics modules and one as the detector, which will form sparse sub-apertures of a grazing incidence X-ray interferometer covering the 0.3-10 keV bandpass. This formation must allow for long duration continuous science observations and also for reconfiguration that permits re-pointing of the formation. To achieve these mission goals, the formation is required to cooperatively point at desired targets. Once pointed, the individual elements of the MAXIM formation must remain stable, maintaining their relative positions and attitudes below a critical threshold. These pointing and formation stability requirements impact the control and design of the formation. In this paper, we provide analysis of control efforts that are dependent upon the stability and the configuration and dimensions of the MAXIM formation. We emphasize the utilization of natural motions in the Lagrangian regions to minimize the control efforts and we address continuous control via input feedback linearization (IFL). Results provide control cost, configuration options, and capabilities as guidelines for the development of this complex mission.

Representations of Invariant Manifolds for Applications in Three-Body Systems

The lunar L1 and L2 libration points have been proposed as gateways granting inexpensive access to interplanetary space. To date, individual transfers between three-body systems have been determined. The methodology to solve the problem for arbitrary three-body systems and entire families of orbits is currently being studied. This paper presents an initial approach to solve the general problem for single and multiple impulse transfers. Two different methods of representing and storing the invariant manifold data are developed. Some particular solutions are presented for two types of transfer problems, though the emphasis is on developing the methodology for solving the general problem.

Libration Orbit Mission Design: Applications of Numerical & Dynamical Methods

Sun-Earth libration point orbits serve as excellent locations for scientific investigations. These orbits are often selected to minimize environmental disturbances and maximize observing efficiency. Trajectory design in support of libration orbits is ever more challenging as more complex missions are envisioned in the next decade. Trajectory design software must be further enabled to incorporate better understanding of the libration orbit solution space and thus improve the efficiency and expand the capabilities of current approaches. The Goddard Space Flight Center (GSFC) is currently supporting multiple libration missions. This end-to-end support consists of mission operations, trajectory design, and control. It also includes algorithm and software development. The recently launched Microwave Anisotropy Probe (MAP) and upcoming James Webb Space Telescope (JWST) and Constellation-X missions are examples of the use of improved numerical methods for attaining constrained orbital parameters and controlling their dynamical evolution at the collinear libration points. This paper presents a history of libration point missions, a brief description of the numerical and dynamical design techniques including software used, and a sample of future GSFC mission designs.

Navigating THEMIS to the ARTEMIS Low-Energy Lunar Transfer Trajectory

THEMIS – a NASA Medium Explorer (MIDEX) mission – is a five-spacecraft constellation launched in February 2007 to study magnetospheric phenomena leading to the aurora borealis. During the primary mission phase, completed in the summer of 2009, all five spacecraft collected science data in synchronized, highly elliptical Earth orbits. Both mission design and efficient navigation and flight operations during the primary mission resulted in appreciable fuel reserves. Therefore, an ambitious mission extension, ARTEMIS, became feasible. ARTEMIS involves transferring the outer two spacecraft from Earth to lunar orbits where they will conduct measurements of the Moon’s interaction with the solar wind and its crustal magnetic fields. Earth departure of these two spacecraft is accomplished by successively raising the apogees of their orbits until lunar perturbations become the dominant forces significantly altering their trajectories. This orbit raise sequence requires over forty maneuvering events, with multiple lunar approaches and fly-bys, before setting the two spacecraft on low-energy transfer trajectories to lunar orbit in February and March 2010. This paper addresses overcoming the navigation and operational challenges presented by the ARTEMIS mission, consisting of two spacecraft that were not designed to leave Earth orbits.