Many potential future astrophysical science missions, such as extrasolar terrestrial planet interferometer missions, X-ray interferometer missions, and optical/ultraviolet deep/space imagers would call for instrument apertures or baselines beyond the scope of even deployable structures. The only practical approach for providing the measurement capability required by the science community’s goals would be precision formation flying (PFF) of distributed instruments. Future missions, such as those for Earth observation and extrasolar planet hunting would require effective telescope apertures far larger than are practical to build. Instead, a suite of spacecraft, flying in formation and connected by high-speed communications could create a very large “virtual” science instrument. The advantage is that the virtual structure could be made to any size. For baselines more than about a dozen meters, precision formation flying becomes the only feasible option. This type of technology has been identified as critical for 21st Century NASA astrophysical and Earth science missions. Specifically, formation flying refers to a set of distributed spacecraft with the ability to interact and cooperate with each other. In deep space, formation flying would enable variable-baseline, interferometers that could probe the origin and structure of stars and galaxies with high precision. In addition, such interferometers would serve as essential instruments for discovering and imaging Earth-like planets orbiting other stars. Future Earth science missions would require PFF. Potential future Earth-science missions, such as terrestrial probe and observation missions would also benefit from PFF technologies. These missions would use PFF to simultaneously sample a volume of near-Earth space or create single-pass interferometric synthetic-aperture radars.
JPL’s Distributed Spacecraft Technology Program for Precision Formation Flying has developed architectures, methodologies, hardware and software components for precision control of collaborative distributed spacecraft systems, in order to enable these new mission architectures and their unprecedented science performance. These technologies ensure that JPL is uniquely poised to lead and collaborate on future missions.
Non-NASA applications of PFF include synthesized communication satellites for high-gain service to specific geographical regions, e.g., a particular area of operations, high-resolution ground-moving target indicator (ground- MTI) synthetic-aperture radars, and arrays of apertures for high-resolution surveillance of and from geosynchronous Earth orbit (GEO). Recently, the concept of fractionated spacecraft (FSC) has been introduced. An FSC system calls for functions of a monolithic spacecraft to be distributed over a cluster of separate spacecraft or modules. Each cluster element would perform a subset of functions, such as computation or power. FSC offers flexibility, risk diversification, and physical distribution of spacecraft modules to minimize system interactions that lead to system fragility. Flexibility would bes increased by the ability to add, replace, or reconfigure modules and thereby continually update an FSC’s architecture throughout its development and operational life. Further, FSC systems could be incrementally deployed and degrade gracefully. PFF would achieve the benefits of FSC, cluster sensing, guidance and control architectures and algorithms, and actuation that must be distributed across modules and coordinated through communication. Each type of PFF mission scenario creates unique technology needs. For astrophysical interferometry, inter-spacecraft range and bearing knowledge requirements are on the nanometer and subarcsecond levels, respectively.
Improved wide field-of-view (FOV) sensors and high-fidelity simulation tools are essential to operate such missions and to validate system performance prior to launch. Precision, centimeter-level drag-free control, repeat-track control, and formation control all would require micropropulsion systems. This would require high-bandwidth, and robust inter-spacecraft communication systems and distributed command and sensing designs to coordinate these complex precise formations. Even smaller missions of only two or three spacecraft must develop distributed command systems to avoid large, expensive mission operation teams. Finally, advanced formation guidance, estimation and control architectures and algorithms would be necessary for robust, fuel-optimal formation operation of any formation; for example, to perform reconfigurations for science targeting and to ensure collision avoidance.
Selected Research Thrusts
Many future Earth and deep-space missions that would achieve a host of measurement capabilities, both in the NASA and non-NASA communities, would be enabled by precision formation flying (PFF). Essential precision collaborative flight of distributed spacecraft systems would require PFF-critical technology developments ranging from architectures to methodologies, to hardware and software.
Distributed-spacecraft architectures are fundamentally different from single-spacecraft architectures. They require the combination of distributed sensor measurements, path planning, and control capabilities, subject to communication capacity to guarantee formation performance. Distributed architectures could enhance collision avoidance, allow for allocation and balancing of fuel consumption, and allow for graceful degradation in the case of system failure. New, scalable, and robust classes of distributed multi-spacecraft system architectures must be developed that integrate formation sensing, communication and control. To function as a formation, the spacecraft must be coupled through automatic control. Such control requires two elements: inter-spacecraft range and bearing information to determine the present formation configuration, and optimal desired trajectories that achieve science goals. These two elements are, respectively, formation estimation and formation guidance. All three capabilities—guidance, estimation, and control—must function in a distributed manner since precision performance requirements coupled with computational, scalability, and robustness constraints typically prevent any one spacecraft in a formation from having full formation knowledge in a timely manner. Distributed architectures determine how a formation is coordinated and, hence, the possible stability and performance characteristics achievable for given communication and sensing systems. As such, distributed architectures must be able to support a wide range of communication and sensing topologies and capabilities and further, must be able to adapt to changing topologies. Future performance targets include the development of architectures of up to 30 spacecraft with sub-centimeter performance over a 10-year mission life, with consistent graceful degradation while meeting sensor/communication requirements.
Wireless Data Transfer
High-throughput, low-latency, multipoint (cross-linking) communications with adaptable routing and robustness to fading is necessary to support formation-flying missions. Throughput and latency directly impact inter-spacecraft control and knowledge performance as well as payload operational efficiency. Real-time control quality of service must be maintained over large dynamic ranges, some latency, and varying number of spacecraft and formation geometries. Payloads would require tens to thousands of megabit-per-second data rates for target recognition/science-in-the-loop applications. Coordinating multiple spacecraft would require distributing locally available information (e.g., a local inter-spacecraft sensor measurement) throughout a formation. Health and high-level coordination information must also be disseminated, such as a spacecraft’s readiness to perform a certain maneuver. For these reasons, and unlike any single-spacecraft application, formations would require closing control loops over a distributed wireless data bus. For example, a sensor on one spacecraft might be used to control an actuator on another.
The overall precision performance of the formation can be limited by the ability of inter-spacecraft communications. While technologies such as cellular towers are fine for terrestrial voice applications, formations would require highly reliable systems free of single-point-failures and which would have high bandwidth and guaranteed low latency. Dropped packets could cause a synthesized instrument to stop functioning, severely reducing observational efficiency. Finally, the range over which formations operate means that the communication system must be capable of simultaneously talking to a spacecraft hundreds of kilometers away without deafening a spacecraft tens of meters away, a problem area referred to as cross-linking. Short-term performance targets for wireless data transfer for PFF would include operating 30 spacecraft at 100 Mbps data rates, with seamless network integration.
Formation Sensing and Control
Formations would require inter-spacecraft knowledge to synthesize virtual structures for large instruments. Direct relative optical and radio frequency sensing of inter-spacecraft range and bearing would be essential, especially for deep space and GEO missions that cannot fully utilize global positioning system (GPS) capabilities. For astrophysical and exoplanet interferometry, the range and bearing knowledge between spacecraft must be sensed to the nanometer level for science and to the micrometer-to-millimeter level for precision formation control. Space-qualified, high-precision metrology systems with a large dynamic range and the ability to simultaneously track multiple neighboring spacecraft would be required. Further, variable lighting conditions and several orders-of-magnitude dynamic ranges must be accommodated, while maintaining reasonable mass/power/volume and ease of integration. Finally, beyond GPS, knowledge based on Deep Space Network (DSN) information would not be sufficient for formation spacecraft to find one another. So, the first step after deployment would be to initialize the formation: spacecraft must establish communication and search for each other with onboard formation sensors. The capability of sensors, particularly their field of view (FOV), would drive situational awareness within a formation and could enable attendant collision-avoidance capability. Sensors must provide relative knowledge from submeter/degree-to-micrometer/arcsecond level of range/ bearing performance to support robust science observations over operating distances of meters to tens of kilometers. For large formations, sensors must function with multiple spacecraft in FOV and minimal coupling to flight systems. For control, advanced formation guidance and estimation and control algorithms are necessary for robust, fuel-optimal formation operation, including reconfiguration and collision avoidance. The algorithms and methodologies are the low-level counterpart to the high-level distributed architectures.
Selected Research Projects
Balloons/Aerobots for Planetary Exploration
Balloons would offer unparalleled promise as vehicles of planetary exploration because they can fly low and cover large parts of the planetary surface. This research explores ways to predict and control the motion of balloons.
Tethers would provide a unique capability to deploy, maintain, reconfigure, and retrieve any number of collaborative vehicles in orbit around any planet. Control techniques for tethered formation reconfiguration must allow the tethered spacecraft to act as a single unit, while the tether length could change depending on the mission profile. Tethers would also offer a high survivability low fuel alternative to scenarios in which multiple vehicles and light collectors must remain in close proximity for long periods of time. In this way, distributed tethered observatories with kilometer class apertures could be built that enable the resolution needed in the optical and microwave bands.
Precision metrology system for state-determination and control of instruments on board distributed spacecraft missions.
The MSTAR task has developed a Modulation Sideband Technology for Absolute Ranging (MSTAR) sensor concept that enables absolute interferometric metrology. The concept is now being used to develop a two-dimensional precision metrology sensor. This technology would be applicable to any mission of scientific exploration in which there is a need for a precision sensor to be used for formation flying control of separated elements. The developed sensor may also find use in the lithography for semiconductor manufacturing and precision machining applications.