Most of the present day spacecraft use micro

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Unformatted text preview: various types and characteristics on space links. Most of the present-day spacecraft use micro-processors for processing data (e.g., for compressing data, checking the status of subsystems, executing timelines, etc.). As a result, they need to send and receive various types of data (e.g., compressed images, housekeeping telemetry, event reports, commands, timelines, etc.) that have different Quality of Service (QoS) requirements in terms of data volume, data rate, latency, reliability, and so on. However, since the processing capability available onboard spacecraft is limited due to the physical constraints imposed by the fact that the spacecraft are flying, the protocols must be simple enough to be implemented by small hardware and/or processors. The Space Data Link Protocols have the capability of transferring various kinds of data with different QoS requirements using relatively simple algorithms. Further, care was taken to ensure that these protocols are upper-compatible with the basic data formats used by earlier spacecraft. CCSDS has developed four Space Data Link Protocols: the TC Space Data Link Protocol (TCSDLP, reference [1]), the TM Space Data Link Protocol (TM-SDLP, reference [2]), the AOS Space Data Link Protocol (AOS-SDLP, reference [3]), and the Data Link Layer of the Proximity-1 Space Link Protocol (reference [7]). Since there is a separate CCSDS Report that explains the concept and rationale of the Proximity-1 Space Link Protocol (reference [8]), this Report deals only with TC-SDLP, TM-SDLP, and AOS-SDLP. 2.2 BASIC CONCEPTS A space link is defined to be a communications link between a spacecraft (which may be a lander or a rover on a distant planet) and its associated ground system or between two spacecraft. Therefore, at least one end of a space link is a spacecraft of some kind. A space link consists of one or more Physical Channels in one or both directions. A Physical Channel is defined to be a stream of bits transferred over a space link in a single direction. A space link usually consists of one forward Physical Link for sending commands from the ground system to the spacecraft (or from the controlling spacecraft to the target spacecraft), and one or multiple return Physical Links (possibly using multiple frequency bands) for sending telemetry from the spacecraft to the ground system (or from the target spacecraft to the controlling spacecraft) (see figure 2-1). CCSDS 130.2-G-1 Page 2-1 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS Spacecraft Forward Physical Link Ground System or Spacecraft Return Physical Links Figure 2-1: Space Link Each Space Data Link Protocol provides one-way transfer from the sending end to the receiving end of a Physical Channel, but the TC-SDLP usually uses a service provided by the TM-SDLP or AOS-SDLP on a different Physical Channel in the other direction of the same space link (see 4.4) to provide feedback from the receiving end to the sending end. However, it is possible to use the same Space Data Link Protocol for...
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