XI Workshop de Testes e TolerÃ¢ncia a Falhas (WTF) - SBRC 2010

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122 Anais The PUS, or the ECSS-E-70-41A standard, focuses on the ground and systems operations related to the utilization of telecommand and telemetry packets. It standardizes these packets and describes sixteen services which the OBDH (On-Board Data Handling) should provide. Figure 1 shows these sixteen services. The underlined service is the one used in this work. Each service has an identification called “Type”. Depending on the type of the service, there are specific activities, called “Subtypes”, which are responsible for performing the user´s request. Therefore, the telecommand and telemetry packets are variable: they may correspond to the chosen type and subtype. Figure 2 shows the fields of a telecommand packet, highlighting the field “Data Field Header”, where the type and subtype are defined in the request. The PUS addresses the shaded fields, although some of the white fields, such as “Packet ID Type” and “Packet ID Data Field Header Flag”, have also been defined in PUS with default values. Telecommand Verification Service Event Reporting Service On-Board Operations Scheduling Service On-Board Storage and Retrieval Service Device Command Distribution Service Memory Management Service On-Board Monitoring Service Test Service Housekeeping and Diagnostic Data Reporting Service Function Management Service Large Data Transfer Service On-Board Operations Procedure Service Parameter Statistics Reporting Service Time Management Service Packet Forwarding Control Service Event-Action Service Figure 1. PUS Services.
XI Workshop de Testes e Tolerância a Falhas 123 Version Number (=0) Type (=1) Packet ID Data Field Header Flag Packet Header (48 Bits) Application Process ID Packet Sequence Control Sequence Flags Sequence Count 3 Bits 1 Bit 1 Bit 11 Bits 2 Bits 14 Bits Packet Length 16 Bits 16 Bits 16 Bits Data Field Header Variable Packet Data Field (Variable) Application Data Variable Spare Variable Packet Error Control 16 Bits CCSDS Secondary Header Flag Boolean (1 Bit) TC Packet PUS Version Number Enumerated (3 Bits) Ack Enumerated (4 Bits) Service Type Enumerated (8 Bits) Service Subtype Enumerated (8 Bits) Source ID (Optional) Enumerated (n Bits) Spare (Optional) Fixed BitString (n Bits) Figure 2. PUS Telecommand Packet. 4.2. Telecommand Verification Service According to ECSS (2003), The Telecommand Verification Service provides the capability of checking the execution of the each telecommand packet, from its acceptance through to its completion of execution. There are four different stages for the telecommand verification. Although providing the verification of the telecommand, it is not necessary that every telecommand should be verifiable at each stage. The stages are: - Telecommand acceptance - Telecommand execution started - Telecommand execution progress - Telecommand execution completion Within the range between the telecommand acceptance and the telecommand completion of execution, the user can request an execution success report, which allows him to follow the exact point of the execution. The success report is requested through the “Ack” field of the telecommand packet. This field is shown in Fig. 2 above. When a failure occurs at any stage, this service must send a failure report to the user containing the error code and some additional information regarding the cause of this failure. It helps the user to understand the main reason of such failure. In short, each stage should have two reports: Success Report and Failure Report. It results in eight reports that the telecommand verification service shall provide. The type number of this service is 1. The subtype numbers of each report are listed in Table 1. For this work, only the “Telecommand Acceptance” and “Telecommand Execution Completion” stages were used.
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