Aero-Astro Magazine Highlight

The following article appears in the 2004–2005 issue of Aero-Astro, the annual report/magazine of the MIT Aeronautics and Astronautics Department. © 2005 Massachusetts Institute of Technology. Learn more about/download the Aero-Astro publication.

Distributed satellite systems offer vision for exploration and education

by David W. Miller

Astronomy has entered a golden age. We are starting to answer the age-old questions of how did it all begin, how will it end, and is there life beyond Earth. Discovering the answers to these questions raises daunting engineering challenges. Space telescopes — our premier investigatory tools — are becoming ever larger, exceedingly precise, and more exotic. Our challenge is to engineer a telescope effectively the size of a football field, that operates in an environment only slightly warmer than absolute zero, orbits 10 million miles from Earth to a precision less than the diameter of a hydrogen atom, and which we've never run in this operating environment prior to launch.

The MIT Aeronautics and Astronautics Department's Space Systems Laboratory is developing an innovative approach to satellite design that alleviates drawbacks which have plagued the industry: high design costs, complexity of system integration and validation, risk of large deployment, low reliability due to design customization, and limited design heritage and legacy. By modularizing typical satellite functions (e.g., propulsion, power, attitude control) and achieving subsystem interconnectivity through genderless docking ports and wireless command and data handling, we simplify assembly and test prior to launch as well as enabling self-assembly and reconfiguration once on orbit. The goal is to reduce the cost, risk, and time required for the deployment of new spacecraft by changing the fundamental methodology used in their development. Most spacecraft are developed as point-designs, optimized for their particular missions. Launch costs are so high, and opportunities so rare, that it is only natural for program managers to include as much functionality as possible into each spacecraft. For this reason, most spacecraft are one- or few-of-a-kind creations. While components may be reused from one spacecraft to the next, there is usually a high degree of customization, leading to inevitable increases in testing and verification costs, along with unavoidable decreases in reliability. This customization has several drawbacks. First, each spacecraft requires a substantial upfront design cost. Second, the resulting design lacks the risk reduction associated with flight heritage. Third, subsystem functionality cannot be truly tested until some level of system integration has been performed. Fourth, the designs lack the cost and learning curve savings of large production runs. Fifth, repair of subsystems prior to launch requires substantial disassembly in order to get access. Sixth, on-orbit repair, replenishment, and upgrade of subsystems are not possible.

This need to customize yet service and upgrade systems can be seen in the field of astronomy. Whether it is the Mount Wilson telescope in California or the Hubble Space Telescope in Earth orbit, dramatic vistas of the universe have been opened through periodic upgrade of the instruments located at the focus. This ability to service and upgrade has clear benefits. However, the cost of such upgrades is enormous, considering that the next generation of space telescopes will operate 10 million miles from Earth at the second Earth-Sun Lagrangian point [L2]. To improve angular resolution (the ability to distinguish between two closely-spaced objects), the Space Interferometry Mission and Terrestrial Planet Finder exploit multiple telescopes that are spread apart. In the case of the latter, these individual telescopes lie on separate spacecraft that are flown in formation.

Coordinating the use of multiple satellites to facilitate assembly, servicing, upgrade, and operation of future space-based telescopes is an emerging field. Distributing mission functionality across multiple satellites has the promise of revolutionizing space exploration in general, while also presenting unique challenges. The research program that I am privileged to lead integrates technology development, on-orbit research, mission design, and undergraduate education into a unique approach to make this vision a reality.

A Space Systems Lab student holds a Synchronized Position Hold Engage and Reorient Experimental satellite. SPHERES are designed to fly in formation with each other as compoenets of a distributed satellite system. (Photo ©Volker Steger / Science Photo Library)

Technology development

Satellites use propellant to maneuver from one orbit to another, much like an automobile uses gasoline. Formation flying satellites will also need to maneuver in order to keep formation and retarget. Unlike automobiles, these satellites do not have the advantage of maneuvering into a gas station as their propellant runs low. Instead, they must bring with them, at launch, all of the propellant they will need over their lifetime. This makes propellant a precious commodity. Electromagnetic Formation Flight (EMFF), using renewable solar energy, replaces the need for propellant in performing formation flight.

It has been shown both in theory and practice that by using a combination of electromagnetic dipoles and reaction wheels all relative degrees of freedom among a cluster of vehicles can be controlled without the use of propellant. Using current and future state-of-the-art in high temperature superconducting wire to generate the electromagnetic fields, low power and lightweight systems can be realized that are competitive with current high specific impulse propulsion, but are not life-limited by propellant consumption. Because of the low power requirements, and lack of consumables, much more aggressive maneuvers can be performed continuously over the lifetime of the mission. Any mission that can be satisfied by controlling only relative degrees of freedom is a potential application for EMFF. Potential applications include all cluster formation flying (reconfiguration) and formation keeping (fighting perturbations such as differential drag, gravitational variations, and solar pressure), as well as rendezvous and docking.

On-orbit research

The SPHERES formation flight laboratory on the International Space Station (ISS) is the culmination of a succession of dynamics and controls research laboratories developed by the Space Systems Laboratory and flown on the Shuttle and ISS. By exploiting platforming concepts, where a common chassis with standardized interfaces allows modular components to be added, these laboratories have been extensible in both hardware and software to accommodate a myriad of diverse research objectives. Furthermore, these laboratories are operated in the risk-tolerant shirtsleeve environment where software is not needed as a safety control. This allows the research to push the limits of engineering capability as well as rapidly iterate on design in much the same way as is done in terrestrial research laboratories.

Consider that we formation fly every day on the interstate with surprisingly few collisions. However, we do not toss the keys to our expensive car to our 16-year-olds. Instead, we have them practice in a less expensive car in a risk-tolerant environment (e.g., parking lots) until handling nominal and off-nominal conditions becomes second nature. The goal for satellite formation flight, rendezvous and docking is to show that it is not only feasible but also robust. SPHERES provides exactly that environment for formation flight and on-orbit assembly.

Funded primarily by the Defense Advanced Research Projects Agency's Orbital Express, SPHERES is a multi-satellite docking laboratory designed to mature metrology, autonomy, and path-planning algorithms for autonomous rendezvous and docking in the risk-tolerant yet long duration micro-gravity environment inside ISS. Five flight-qualified SPHERES have been built; three of which are, at this writing, awaiting launch after Shuttle return-to-flight. Uplinking software, downlinking data and attaching payloads to the SPHERES expansion ports facilitates spiral algorithm development and hardware extensibility.

Professor Dave Miller in his lab with an electromagnetic formation flight testbed. EMFF — spacecraft positioning themselves through magnetic attraction/repulsion of other spacecraft — would use solar energy, obviating a dependence on finite fuel supplies brought with them from earth. (William Litant photograph)

Mission design

For monolithic telescopes, the cost of the primary mirror grows faster than its area. This has caused designers of future systems to consider sparse apertures to achieve the fine angular resolution associated with a large telescope. Sparse apertures combine the light from multiple smaller telescopes to achieve this effect. The modularity inherent in a sparse aperture can then be exploited throughout the spacecraft to embody the functions of assembly, servicing, upgrade, and operation. To quantify the attributes of modular telescope design, the Space Systems Laboratory developed the Adaptive Reconnaissance Golay-3 Optical Satellite (ARGOS) to quantify the savings associated with building a sparse aperture primary, the additional cost of providing the beam train that combines the light from the multiple apertures to the requisite precision, and the system scale at which such an architecture becomes favorable over monolithic systems such as the Hubble Space Telescope. The data clearly shows that modular optical systems are more cost-effective than monolithic systems for larger telescopes, based purely on fabrication costs. When one also considers the opportunities for assembly and servicing that modularity provides, such architectures hold the promise of revolutionizing the next generation of space telescopes.

Undergraduate Education

To extend the design-build experience beyond graduate students, staff and subcontractors, the SPHERES, ARGOS, and EMFF prototypes were developed through a three-semester undergraduate Aero-Astro capstone class developed as part of the Department's CDIO Initiative; an innovative educational framework that stresses engineering fundamentals set in the context of Conceiving — Designing — Implementing — Operating real-world systems and products. As an alternative to conventional design and laboratory classes, the integrated design-build sequence allows the students to take a concept through design, fabrication and testing and thereby gain a working knowledge of the engineering lifecycle. Furthermore, while working on their specific subsystems in small teams, the students also contribute to team wide activities such as requirements formulation, design reviews, system integration, and field testing. Indeed the very first spheres were designed, developed, fabricated and flown by undergraduates on NASA's KC-135 Zero-Gravity Simulator aircraft. Not only does this innovative educational environment enrich the experience of the undergraduates, it also provides an advance rapid prototyping team supporting my graduate research program. The numerous follow-on research programs funded by government and industry testify to the merit of integrating undergraduate education with cutting-edge research.

The research activities within the Space Systems Laboratory address all aspects of the engineering lifecycle of space telescopes from systems architecture, to development of enabling technology, to on-orbit technology maturation. The goal is to develop new engineering practices that help the next generation of space telescopes keep pace with the new scientific questions being asked.

David W. Miller, director of the Space Systems Laboratory, is an associate professor in the MIT Department of Aeronautics and Astronautics. He may be reached at millerd@mit.edu.