ACADEMIA

NASA’s MESSENGER team is using the CCSDS File Delivery Protocol (CFDP), a highly specialized protocol designed to overcome space operations communications challenges, to download data captured during a successful flyby of Earth last week. A team of international space data communications experts, collaborating through the Consultative Committee for Space Data Systems (CCSDS), developed CFDP to reliably and efficiently downlink files from a spacecraft even in the strenuous environment of deep space. Since the MESSENGER spacecraft’s launch a year ago, it has successfully used CFDP to enable mission communications and will use it throughout its 7.9-billion kilometer journey to Mercury. In using CFDP, MESSENGER communications represents a change in the standard method of storing science and housekeeping data on spacecraft built by the Johns Hopkins University Applied Physics Laboratory (JHU/APL). MESSENGER is also the first U.S. space flight mission to use CFDP in mission operations. Prior to MESSENGER, JHU/APL missions used a raw storage model of storing data, but new mission and operational requirements meant that MESSENGER would have to incorporate a file system of data storage into its spacecraft software architecture. A reliable method of downlinking files to the ground had to be found and CFDP was chosen by mission planners to do the job. CFDP is included in the MESSENGER software architecture through a reuse of a NASA Jet Propulsion Lab (NASA JPL) implementation on the ground and a JHU/APL “CFDP-lite” implementation on the flight side. The NASA JPL implementation is also used on NASA’s highly successful Deep Impact mission. “JHU/APL engineers integrated CFDP software developed by NASA JPL into the MESSENGER mission's ground system, which communicates with a CFDP flight software implementation developed by JHU/APL on the spacecraft,” said Christopher Krupiarz, senior professional staff member, JHU/APL Space Department Embedded Systems Group in Laurel, Maryland (USA). “Being able to use an international standard like CFDP was a key factor in getting two systems developed by two different organizations to work for one Mercury bound spacecraft.” CFDP is designed to function reliably despite the long data propagation delays and frequent, lengthy interruptions in connectivity found in deep space. It uses powerful forward error correction coding that minimizes data loss in communication across deep space, and also supports optional “acknowledged” modes of operation in which data loss is automatically detected and a retransmission of the lost data is automatically requested. Some of the world’s leading space communications experts working within CCSDS collaborated at bi-annual working group sessions (similar to those scheduled to take place next month in Atlanta, Georgia) to first standardize CFDP. They defined the protocol according to space file transfer requirements articulated by CCSDS participating space agencies, including NASA, the European Space Agency (ESA), the British National Space Centre (BNSC), the Centre National d'Etudes Spatiales (CNES) and the Japan Aerospace Exploration Agency (JAXA). The protocol’s ability to maintain a high level of data transfer reliability even across interplanetary distances makes it critical to successful communications on deep space missions like the MESSENGER mission to Mercury and is expected to have a high level of applicability to future Lunar exploration missions. CFDP also benefits space flight missions in another important way: cost savings. CFDP allows an instrument to record an observation in a file and transmit the file to Earth without having to consider whether or not physical transmission is possible at that time. Sequestering outbound data management and transmission planning functions within CFDP can simplify flight and ground software, which reduces mission costs - an important benefit to lower cost missions like MESSENGER. CCSDS will continue to foster global scale technical cooperation to develop recommendations for space communication like CFDP that increase interoperability, as well as reduce risk and mission operation costs. Currently, the organization is investigating extending the use of CFDP in emerging delay-tolerant networking technology to Interplanetary Internet operations, and specifically to the use of CFDP in complex mission configurations, which should further enhance the usefulness and value of CFDP to space exploration missions in the future. Scott Burleigh, CCSDS working group chair and lead CFDP system engineer at NASA JPL in Pasadena, Calif. commented, “The successes of CFDP on MESSENGER and the Deep Impact mission bring us closer to having an automatic interplanetary communication fabric that can support deep space science and exploration the way the Internet supports science on Earth."