| The complexity of space communication has limited our access to space systems and kept mission operations costs high. Ultimately, this results in reduced mission capabilities and yields. In particular, ground stations, the access point between space and terrestrial networks, suffer from monolithic designs, narrow interfaces, and unreliability that raise significant financial barriers for low-cost, experimental satellite missions. This research reduces these barriers by developing technology for recovery-oriented, flexible access networks built from commercial-off-the-shelf (COTS) components. Based on our extensive small satellite experiences, we decomposed ground station services and captured them in an extensible framework that simplified reuse of ground station services and improved portability across heterogeneous installations. This capability, combined with selective customization through virtual machine technology, allowed us to deliver "just in time" ground stations for QuakeSat-1 at a fraction of the price of current commodity solutions. This decomposition is also informed by principles of robust system design. Thus, our ground station reference implementation called Mercury was a candidate for recursive recovery (RR), a high availability technique whose effectiveness in reducing recovery time has been demonstrated on research prototypes of Internet server systems. Augmenting Mercury to implement RR reduced recovery time of typical ground station software failures by a factor of four, dropping recovery time to within the "window of recovery" and effectively eliminating the adverse effects of these failures. Since the time of failures cannot be predicted, RR allowed us to mitigate the effects of the failures and greatly reduce their potential impact on ground station operations.{09}Our ground station architecture harnessed the benefits of COTS components, including rapid prototyping and deployment, while overcoming the challenges of COTS reliability and mission critical usage. Our ground station research has laid a foundation for an increase in reliable, flexible access networks. Our early ground station network prototype, the Mercury Ground Station Network (MGSN), is harnessing the idle resources of heterogeneous ground stations at the University of Wurzburg in Germany, the Norwegian University of Science and Technology (NTNU) in Norway, Aalborg University in Denmark, and the University of Iowa. |
