Experience with Delay-Tolerant Networking from Orbit

The disaster monitoring constellation (DMC), constructed by Surrey Satellite Technology Ltd (SSTL), is a multi-satellite Earth-imaging low-Earth-orbit sensor network where captured image swaths are stored onboard each satellite and later downloaded from each satellite payload to a ground station. The DMC is currently unique in its adoption of the Internet Protocol (IP) for its imaging payloads and for satellite command and control, based around reuse of commercial networking and link protocols. Earth images are downloaded from the satellites using a custom IP-based high-speed transfer protocol developed by SSTL, Saratoga, which works well in unusual link environments. Store-and-forward of images with capture and later download during passes over ground stations gives each satellite the characteristics of a node in a Delay/Disruption Tolerant Network (DTN). DTNs are under investigation in an Internet Research Task Force (IRTF) DTN research group (RG), which has developed a 'bundle' architecture and protocol. We experiment with use of this DTNRG bundle concepts onboard the UK-DMC satellite, by examining how Saratoga can be used as a convergence layer to carry the DTN Bundle Protocol, so that sensor images can be delivered to ground stations and beyond as bundles. This is the first use of the Bundle Protocol from orbit. We use our practical experience to examine the strengths and weaknesses of the Bundle architecture for DTN use, paying attention to fragmentation, custody transfer, and reliability issues. We similarly examine and discuss an alternative network stack, based around proposed use of the Hypertext Transfer Protocol (HTTP) that we have been architecting, which we believe has potential applications across a range of DTN networks. We use our practical experience to make suggestions about DTN use and adoption of IP in sensor networks.