Each of the data units and their performance will be

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Unformatted text preview: s and their performance will be reviewed in the following material. References [4] and [3] specify the details of constructing the above elements; this Annex describes how the on-board telecommand system handles each element and the performance that may be obtained. Throughout this note a binary symmetric telecommand channel with additive white gaussian noise is assumed. Issue 6 Page D-3 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE D-2.2 On-Board Telecommand Logic The Receiving End (i.e., spacecraft) telecommand decoder state diagram, upon which these discussions are based, is shown in Figure 3-4 of Reference [4] and is duplicated in this report as Figure D-1. E4 E1 S1 (INACTIVE) E3 S2 (SEARCH) S3 (DECODE) E2 E2 Figure D-1: TC Decoder State Diagram Inactive State. The initial state for the TC channel is State 1 (S1), the INACTIVE state, where no bit modulation is detected. When a TC bit stream is detected (telecommand modulation applied and bit synchronization achieved), Event 1 (E1) occurs, CHANNEL ACTIVE. If the TC signal is lost or there is a loss of bit lock, Event 2, CHANNEL INACTIVE, occurs, and the spacecraft decoder returns to the INACTIVE state (S1). Search State. When E1 occurs, the logic goes into the SEARCH state (S2). The unit that governs this event is the CLTU Start Sequence. The bit stream is searched for the CLTU Start Sequence; when the pattern has been detected, the decoder declares Event 3 (E3), START SEQUENCE FOUND, and assumes the next bit delimits the beginning of the first codeblock of a continuum of codeblocks of the CLTU. (Note: a codeblock is a codeword plus a filler bit.) The probability of E3 occurring depends upon: 1) the decoder actually being in the SEARCH state when the start sequence arrives; 2) the bit error rate (BER) of the channel; and 3) the decoding strategy used. If, at this point, the TC signal is lost or there is a loss of bit lock, E2 occurs and the telecommand decoder returns to S1. Decode State. Assuming E3 has occurred, the decoder enters the DECODE state, S3. The data unit that governs the DECODE state is the TC Codeblock. All TC Codeblocks are received and decoded in either the Triple Error Detection (TED) mode, or the Single Error Correction (SEC) mode. The use of mixed decoding modes may require additional coordination and more complex analysis. The data contents of decoded (valid) codeblocks are transferred to the layer above (i.e., the TC Frame Layer). The probability of the telecommand Issue 6 Page D-4 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE decoder remaining in this state is a function of the decoding strategy, the BER, the length of the codeword (n), in bits, and the number of codewords in the CLTU. Return from Decode State. If, during the decoding process, the decoder rejects a codeblock due to errors, no data from this codeblock are transferred to the layer above. This situation corresponds to Event 4 (E4), CODEBLOCK REJECTION, which returns the decoder to the SEARCH state (S2). To end a CLTU, a Tail Sequence is i...
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This document was uploaded on 03/06/2014.

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