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Unformatted text preview: s and their performance will be reviewed in the following material.
References  and  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  and is duplicated in this report
as Figure D-1.
(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
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.
- Spring '14