NASA - Johnson Space Center

Frame Decoder for Consultative Committee Space Data Systems

ABSTRACT GNU Radio has a default packet decoder that is not optimized for Consultative Committee Space Data Systems (CCSDS). CCSDS frames are 256 bytes with a header or frame synchronization pattern (0x1ACFFC1D). The problem with the generic packet decoder is that it adds more bytes to the CCSDS frames and does not permit bit error correction using Reed Solomon. In this project, the software-development environment used was GNU Radio. GNU Radio is a free and open source development toolkit that provides signal processing to implement software radios. It can be used with low-cost external RF hardware to create software-defined radios or without hardware in a simulation-like environment. GNU Radio applications are primarily written in Python and C++. The hardware used in this project was the Universal Software Radio Peripheral (USRP) Networked Series 210. The USRP is a computer-hosted software radio designed by Ettus Research. The USRP connects to a host computer via Gigabit Ethernet. Using the Universal Hardware Driver (UHD), we can run GNU Radio applications using the USRP. The algorithm developed for frame synchronization consists of a state machine. The state machine consists of three states: STATE FLAG SEARCH, STATE HAVE FLAG, and STATE HAVE HEADER. This state machine depends on a correlator that will search for the frame sync pattern and will add a fag indicating the beginning of the payload and end of the header. The first state of the machine will look for this fag. Once the flag is located, the machine will move to the second state, STATE HAVE HEADER. In this state, the machine will verify that the header flagged by the correlator is the frame sync pattern. If the header is the correct frame sync pattern, or within a configurable tolerance, then the machine moves to the final states, otherwise it keeps searching for the frame sync pattern. The last state is simply framing the CCSDS packets. In conclusion, this CCSDS Frame Decoder will optimize modulation and framing operations. This packet is not adding more bytes and is not creating more overhead like generic GNU Radio frame decoder. The developed CCSDS Frame Decoder will allow full benefit for bit error correction using Reed Solomon.