Implementing the New CCSDS Housekeeping Data Compression Standard 124.0-B-1 (Based on POCKET+) on OPS-SAT-1

Implementing the New CCSDS Housekeeping Data Compression Standard 124.0-B-1 (Based on POCKET+) on OPS-SAT-1

Utah State University, Logan, UT

The number of telemetry parameters available in a typical spacecraft is constantly increasing. At the same time, the bandwidth available to download all that information is rather static. Operators must therefore make hard choices between which parameters to downlink or not, in which different situations, and at which sampling rates. This tradeoff is more problematic for missions with higher communication latency beyond LEO. Since 2009, The European Space Agency’s European Space Operations Center (ESA/ESOC) has been promoting the compression of housekeeping telemetry as a solution to this problem. Most spacecraft housekeeping telemetry parameters compress extremely well if they are pre-processed correctly. Unfortunately, most spacecraft record telemetry packets in flat packet stores so accessing different packets within them is too CPU and memory intensive for flight computers. Using traditional compression schemes such as zip or tar are not compatible with the traditional “fire and forget” mode of operation i.e., occasional packet losses are expected. This would render entire compressed files unusable. ESOC invented an algorithm called POCKET+ to solve this problem. It is implemented using very low-level processor instructions such as OR, XOR, AND, etc. This means that it can run with low CPU usage and, more importantly, with a short execution time. It is designed to run fast enough to compress a stream of incoming packets as they are generated by the on-board packetiser. The output is a smaller stream of packets. The compressed packets can be handled by the on-board system in an identical fashion to the original larger uncompressed packets. Robustness with respect to the occasional packet loss is built into the protocol and does not require a back channel. In 2018, POCKET+ was proposed to the CCSDS data compression working group and after extensive research by other agencies the core idea has been Evans 2 36th Annual Small Satellite Conference incorporated into a proposed new standard for “Robust Compression of Fixed Length Housekeeping Data.” The second supporter for the mission is CNES, supported technically by the University of Barcelona (UAB). Both CNES and UAB have suggested changes that make POCKET+ even more powerful. POCKET+ is already flying on OPSSAT, a 3U CubeSat launched by the European Space Agency on December 18th, 2019. The mission has updated the Onboard Software (OBSW) and ground control software to be compliant with the latest POCKET+ standard. The standard is set to be available for an ESA review. This paper describes the latest algorithm and how it is implemented on OPS-SAT, including how the same core software has been successfully deployed in two completely different scenarios/environments. One compresses files offline and then uses a transport protocol with a completeness guarantee; the other compresses a packet stream in real-time and uses the classic transport protocol where completeness is not guaranteed. The results show that compression ratios between eight and ten are usual for the OPSSAT mission. Improvements made during the development of the planned CCSDS standard for “Robust Compression of Fixed Length Housekeeping Data” are also presented.