For handling an emergency with a near earth

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Unformatted text preview: w data rate. For handling an emergency with a near-Earth spacecraft, the COP-1 expedited service can be sufficient, with telecommand frames managed one by one. For these deep-space scenarios, the proposed systematic retransmissions provide another means to attempt to deliver a sequence of telecommand frames, despite the very difficult conditions. 1.4 Scope of the systematic retransmissions The systematic retransmission is in addition to the existing automatic retransmissions of the sequencecontrolled service. Any changes to support the systematic retransmission mechanism will be limited to the sending end of the telecommand communications system. The desired effect of the retransmissions relies on the existing behaviour of the receiving end. The discussion in this document is therefore only concerned with the sending end of the frames for the sequence-controlled service. The expedited service of COP-1 has no retransmission mechanism: FOP-1 transmits each frame exactly once. The proposed systematic retransmission does not affect the expedited service. It is very important to limit the scope of the modifications and the impact on existing standards. The specifications associated with the new parameter should be designed to be compatible with existing implementations, so that existing compliant implementations remain compliant with the modified standards. The behaviour of a modified system with the default value of the new parameter should be identical to an unmodified system. TCretrans7 2008Sep09 I.B. + M.A. DE LANDE LONG SOFTWARE + CONSULTANCY Options for repeated retransmissions for telecommand Page 4 2. Background 2.1 COP-1 automatic retransmissions The sequence-controlled service includes a mechanism for the automatic retransmission of a sequence of TC Transfer Frames whenever one of them is not correctly received on board. The retransmission mechanism is “go-back-n”. COP-1 uses protocol status information which is passed from FARM-1 (the receiving end of COP-1) to FOP-1 (the sending en...
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