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to enable the system engineer to select values of relevant coding parameters and operational
strategies so that system performance requirements may be satisfied. Telecommand operating
strategies are described for different environments, as well as the procedures that are elements
of the strategies. D-2 PERFORMANCE CRITERIA AND NATURE OF THE PROBLEM The objective of the Telecommand Service is to provide a highly reliable uplink service.
"Reliable" service is considered to be achieved when the following two performance criteria are
simultaneously met by the overall telecommand system:
(1) TC Frame Deletion Rate
A maximum of one telecommand frame is deleted (rejected) for every 1000 frames
transmitted. This operating point is defined to be "Command Threshold". (2) TC Frame Undetected Error Rate
A maximum of one telecommand frame for every 109 telecommand frames
transmitted is erroneously accepted (that is, contains one or more undetected bit
errors). It should be noted that since performance is measured in terms of frames, considerations
discussed herein extend through the physical and coding layers of the Channel Service and the
transfer layer of the Data Routing Service. D-2.1 Performance Components The basic accounting unit of Telecommand is therefore the TC frame. To meet these
performance objectives for the frame, the Channel Service and Data Routing Service specify
standard techniques for constructing and handling the TC frame and its components:
(1) To achieve bit acquisition (synchronization). (2) To delimit the beginning of a continuum of bits (CLTU) comprising the
telecommand bit stream. Issue 6 Page D-2 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE (3) To encode the bit stream into a series of standard codeblocks which can provide
error detection and/or correction. (4) To delimit the beginning and end of each telecommand frame. (5) To provide an optional error check over each telecommand frame to improve
protection against undetected errors. (6) To delimit the end of a CLTU to prepare the spacecraft decoder to recognize the
start of the next continuum (CLTU). In addition, Reference  describes certain optional strategies (e.g., no errors vs. one error
allowed in the CLTU start sequence). The performance of each of these is also discussed. STRATEGIES
Reference , Section B-4, recommends two alternative strategies for combining telecommand
options when a CLTU is sent. These are:
- CLTU Start Sequence with no errors allowed
CODEBLOCKS - Decoded in Triple Error Detection mode
- Tail Sequence Codeblock Rejection or Stop modulation Strategy 2:
- CLTU Start Sequence with 0 or 1 error allowed
CODEBLOCKS - Decoded in Single Error Correction, Double Error Detection mode
- Tail Sequence Codeblock Rejection or Stop modulation In general, Strategy 1 may be used when it is important to reduce the undetected error rate,
while Strategy 2 may be used to reduce the operational complexities of retransmission.
Each of the data unit...
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This document was uploaded on 03/06/2014.
- Spring '14