The development of a testbed for the development and demonstration of technologies needed by tethered formation flying satellites is proposed. Such a testbed would support dynamic modeling and control algorithm development for missions such as NASA?s Submillimeter Probe of the Evolution of Cosmic Structure (SPECS) mission. The testbed would be based on the successful Synchronized Position Hold Engage Re-orient Experimental Satellites (SPHERES) system that has been developed and is being flown on-board the International Space Station. As part of its first flight, SPHERES will demonstrate metrology, formation flight, and autonomy algorithms. The effort proposed here would adapt the SPHERES system first for ground testing and subsequently for a follow-on mission to the ISS to serve as a testbed for Tethered Spacecraft Interferometer (TSI) autonomous software technologies. Tethers are being considered as the means for maneuvering sub-apertures spacecraft in order to eliminate reliance on propellant consumables. The SPHERES-Tether testbed would aid in streamlining the development of spacecraft formation initialization and deployment and retraction algorithm development, and significantly reduce the risk involved with testing this critical SPECS mission operation, including coarse spacecraft formation initialization, deployment and retraction, and multi-stage precision pointing control.