This paper will describe the results of a research experiment aiming to implement a simplified communication system for a small satellite on-board computer (OBC) on a single reconfigurable programmable logic chip (RSC-OBC) [1] using the Consultative Committee of Space Data Systems (CCSDS) protocol. The development of the reconfigurable single chip OBC is part of a long-term research programme at the Surrey Space Centre (SSC) named "ChipSat". Other related hardware implementation will also be presented.

Small Satellites aim to achieve low-cost, fast access to space and this is normally supported by the use of off-the shelf components and development tools. System-on-a-Programmable-chip is a major enabling technology. Such a single-chip solution to the core data handling, communications and control systems will not only dramatically reduce size, complexity and cost of small satellites electronics, but will also allow on-board hardware modification.

A simplified version of a Surrey Satellite Technology Limited (SSTL) on-board computer (OBC) is in a process of implementation as a system-on-a-chip (SoC) device. A XILINX Virtex FPGA is used for prototyping of the SoC. This single-chip OBC is composed of Microprocessor IP Core, Memory Error Detection and Correction Unit, Bootstrap Loader, HDLC Controller, CAN Interface, Network Interface, True IDE Interface, Cordic Co-Processor and Peripheral Bus Interface. Synthesis results of the microprocessor and peripheral cores have been reported in [2].

Being standard space industry communication protocol the Consultative Committee of Space Data Systems (CCSDS) protocol has been employed on numerous missions ranging from relatively simple low earth missions to deep space probes. Using the CCSDS standard has proven important and advantageous since it could lead to spacecraft interoperability, re-usable systems and mission cross support – not just for in-house missions but across the CCSDS space agencies members – thereby reducing costs for on-board, group and test equipment, as well as for spacecraft testing and in-orbit operations.

At present, the complete CCSDS TLM and TC implementation is still very complex for low-cost small satellites and hardware implementations are expensive. A CCSDS software package has been developed in SSC which represents a simplified yet reliable standalone alternative software implementation of the CCSDS TLM and TC Command Operation Protocol (COP-1) [3]. The COP-1 service is based on a streamlined version of the CCSDS Frame Acceptance and Report Mechanism (FARM) and Frame Operation Protocol (FOP). Both the FARM and FOP systems satisfy the essential requirements for a reliable CCSDS communication system. The software imposes minimal memory footprint and performance requirements on the On-Board Computer. The CCSDS communication system will implement the Packetisation Layer, the Transfer Layer and the Coding Layer of both the TC and TLM Systems. The CCSDS Reed-Solomon (R-S)/Turbo encoder and decoder are used for the TLM Channel Coding. The Bose, Chaudhuri and Hocquenghem (BCH) cyclic detecting error code is used for the TC coding.

The functions of the ground segment are to format CCSDS TC packets and CCSDS TC frames; calculate the BCH check; insert TC packets into the Data Field of TC frames; insert TC frames into Codeblocks; format Control Link Transfer Units (CLTU) to ensure synchronisation; implement the R-S or Turbo decoding; receive CCSDS TLM frames; subtract CCSDS TLM packets from the Data Field of the TLM frames; decode the CRC and if no error is detected, the raw TLM data is passed to be analysed and displayed. The functions of the spacecraft segment are to format CCSDS TLM packets; format CCSDS TLM frames; insert TLM packets into the Data Field of the TLM frames; calculate the CCSDS recommended Cyclic Redundancy Check (CRC) cyclic code of the frame inserted at the end of TLM frames; format the Attached Synchronized Marker (ASM); implement R-S or Turbo encoding; receive and decode CCSDS TC frames; detect whether a transmission bit error has been identified by the BCH code. If there is no error, the telecommand will be passed to the on-board Controller Area Network (CAN) bus and accepted by the corresponding CAN node. The TC retransmission system utilises the COP-1 "go-back-n" automatic retransmission protocol, which consists of the pair of synchronized procedures, FOP (in the ground segment) and FARM (in the spacecraft segment). The FOP transmits TC transfer frames to the FARM. The FARM returns Command Link Control Words (CLCW) within the TLM transfer frames.

The experimental set-up for the simulation of the CCSDS communication system consists of a XILINX Virtex 800 FPGA prototyping board supporting the on-board operation and a personal computer (PC), supporting the ground operation. The on-board segment will be run by the RSC-OBC. An on-board CAN bus system will be used to verify the validity of the whole system. The CAN interface is implemented as a VHDL IP core and is integrated with the LEON processor IP core. An external CAN card will take responsibilities of an on-board payload which will generate TLM data and receive TC data. The CCSDS TLM encoder and decoder are provided by the European Space Agency (ESA). The encoder is implemented in software which will run on the RSC-OBC. The decoder will be implemented as a VHDL core in a XILINX FPGA. The RSC-OBC will communicate with the PC via an asynchronous RS-232 serial port.

The paper will be structured in the following way. The CCSDS end-to-end link model will be overviewed and its TLM and TC simulation models will be presented. The CCSDS software structure and interfaces will be illustrated. An approach to the simulation of the CCSDS communication will be outlined. Finally, simulation results will be presented and illustrated.

References:

1. D.Zheng, T.Vladimirova, H.Tiggeler, Prof. Martin Sweeting. "Reconfigurable Single-Chip On-Board Computer for a Small Satellite", 52nd International Astronautical Congress, Toulouse, France, October 1-5, 2001, IAF-01-U3.09.
2. H.Tiggeler, T.Vladimirova, D.Zheng. "A System-on-a-Chip for Small Satellite Data Processing and Control", Proceedings of Military and Aerospace Applications of Programmable Devices and Technologies International Conference (MAPLD’2000), P20, September 2000, Laurel, Maryland US, NASA.
3. I. Rutter, T.Vladimirova, H.Tiggeler."A CCSDS Software System for a Single-Chip On-Board Computer of a Small Satellite", AIAA Small Satellites Conference, 2001, SSC01-VI-4.