A Real-Time Computer Control Platform for an Experimental ... - CTR

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Figure 1: UPMSat2 external view.the hardware and software levels. Section 4 summarizes the methodological aspectsof the on-board software development. Finally, some conclusions and asummary of the remaining work to be done are recorded in section 6.2 On-Board Computer requirements2.1 On-board computer functionalityAll the computer-related functions on board of the UPMSat2 satellite are executedon a single computer platform, called the On-Board Computer (OBC).Its main functions are:Attitude determination and control (ADCS). The attitude of the satellite(i.e. its orientation with respect to the Earth surface) is computed fromthe signals provided by a set of three sensors that measure the intensity ofthe Earth magnetic field on the platform reference axes (magnetometers).The attitude is controlled most of the time by means of a passive mechanism,consisting on three sets of permanent magnets that tend to alignwith the Earth magnetic field. When this is not enough, a set of threemagnetic coils (magneto-torquers) are used as actuators in order to adjustthe attitude to the required angle values.Telemetry and telecommand management (TMTC). The satellite communicateswith the Earth station by means of two radio links. Telemetrydata (i.e. information sent by the satellite to ground) are coded and packagedby the OBC software according to standard protocols, and sent to thecommunications hardware (TMC) in order to enable the ground operatorsto monitor the operation of the satellite and its orbital parameters.Telecommands are produced by the ground equipment and sent by radioto the satellite. Upon reception by the communications hardware, theyare forwarded to the OBC for decoding and execution.Platform monitoring and control. This function includes periodic operationaland navigational data measurements (e.g. temperatures and voltagesat various points, orbit and attitude parameters), which are sent to2
TMC!sensors!- magnetometers!SW system!ADCS!TC!TM!actuators!magneto-torquers!platform!monitoring!payloadmgt!sensors!- temperature!- voltage!- intensity!OBC!payload!Figure 2: OBC context diagram.the ground station by telemetry. Monitoring functions also include detectingsome possible incorrect situations and signalling them to ground.Basic platform control functions (e.g. power supply and temperature) aremostly carried out directly in hardware. The computer functionality consistson setting references and changing operation modes when necessary(e.g. low battery voltage).Fault detection, isolation, and recovery (FDIR). One important functionis to detect and recover from faults of the computer system itself. The approachto FDIR in the project is mostly based on constructive mechanisms,e.g. decoupling of software modules, software cheeks and redundancy.Payload data management. The payload is the part of the satellite thatcarries out the mission objectives. Possible payloads for the UPMSat2mission include an Earth observation system and a marine emergencysurveillance system.Figure 2 shows a context diagram of the system including its main functionality.2.2 Software requirementsBased on the required functionality, a software system specification document(SSS) has been produced, according to the standards commonly used in theEuropean space industry (ECSS40). The document also includes some nonfunctionalrequirements, including temporal requirements for some functions,3
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Page 7: Juan A. de la Puente, Juan Zamorano

A Real-Time Computer Control Platform for an Experimental ... - CTR

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