Performance data are presented to enable the system

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Unformatted text preview: ata are presented 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 [4] 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 [4], Section B-4, recommends two alternative strategies for combining telecommand options when a CLTU is sent. These are: Strategy 1: START - CLTU Start Sequence with no errors allowed CODEBLOCKS - Decoded in Triple Error Detection mode FINISH - Tail Sequence Codeblock Rejection or Stop modulation Strategy 2: START - CLTU Start Sequence with 0 or 1 error allowed CODEBLOCKS - Decoded in Single Error Correction, Double Error Detection mode FINISH - 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.

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