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The launch date of Compass-1 is not yetdetermined as a previous launch opportunityhad been canceled. It is planned to concludethe development for launch acceptancetesting by May 2005.2. PAYLOAD AND EXPERIMENTALSYSTEMSAbove all, the satellite will be a platform forexperimental technologies composed ofcomponents that have not been originallydesigned for use in space (i.e. commercialof-the-shelf(COTS) products), as well ascutting-edge space technologies.2.1 Camera ModuleThe OV7648 CMOS camera module hasbeen selected as the imager payload. Itintegrates a fourth generation sensor chipfrom Omnivision and is offered fullyassembled with lens system and electricalinterface. The sensor can capture VGAresolution pictures which will result in earthimages of about 380 km x 450 km at nadirfor the designated orbit.The camera’s exposure time is programmedvia the two-wire I²C bus interface and data isstreamed out over an 8-bit parallel bus. Theimages are in raw data format. Pictures canbe triggered and received by ham usersworldwide that comply with the communicationsarchitecture of the spacecraft.2.2 Attitude Determination and ControlActive attitude control on Compass-1 isachieved by three mutually perpendicularmagnetorquers using a linear quadraticregulator. The targeted nadir-error envelopeis 8° or better. The attitude information isextracted by means of an extended Kalmanfilter from vector observations from a 3-axismagnetometer and five 2-axis MOEMS 1 sunsensors [3]. The sun sensors have beendeveloped by the Denmark Technical1 Micro-Opto-Electro-Mechanical SystemUniversity, Copenhagen, for application on aspacecraft similar to Compass-1. Thesesensors are currently undergoing a testcampaign in order to validate their properfunction under vacuum and thermal cyclingconditions. Also, the electronic sensorinterface is in a re-design stage with the aimto reduce mass, size and power consumption.2.3 Miniature GPS ReceiverThe GPS receiver is originally COTS, butuses advanced software, developedspecifically for LEO satellites by the GermanAerospace Center. The 22g receiver, calledPhoenix [4], has never been flight tested andas such is considered part of the payload ofCompass-1. As long as activated, it providesthe ADCS with (autonomous) orbitinformation, which will be computationallypropagated in between the receiver’soperational phases. The target orbit forCompass-1 is a sun-synchronous orbit at600km altitude with an inclination of about98°.2.4 Command and Data HandlingCore of the Command and Data HandlingSystem (CDHS) is an 8051-based microcontrollerwith powerful features that ease itshandling and respond to the systemrequirements. It is a low power IC with tinyTGFP-64 footprint. A JTAG interface allowsin-system programming and debugging ofthe internal Flash memory which stores theflight software. A FIFO is used to buffer thedata from the payload which is streamed outat a very high frequency. Later on the buffercontent is transferred to the external Flashdevice for non-volatile data storage. None ofthose ICs has yet been tested in space. Inparticular the characteristic of the Flashmemory, when exposed to LEO environment,is of interest for future missions.3. APPROACHESFacing the design and development of anentire satellite is always a challenging
venture, regardless of the satellitesproportions. In fact, the creation of picosatellitesseems to be even more demandingsince it is not achievable by simply downsizingthe subsystems and components inorder to derive a small satellite from itslarger example model.Picosatellite engineers have to go other ways.They have to nurture new solutions for theexisting requirements. The use ofminiaturized electrical and mechanical partsis essential and its integration into the wholesystem becomes a crucial aspect. Thefollowing chapters will elaborate thoseaspects in regards to the Compass-1 CubeSat.3.1 COTS ComponentsTo date, with a few exceptions, cost andtechnical budget requirements make originalspacecraft components prohibitive forCubeSat developers. It is well understoodthat implementing COTS components comeswith a certain risk. However, the comparablylow cost of picosatellites sets the wholeconcept of reliability into a new perspective.As part of this awareness, many developersconduct environmental testing. These teststypically include vibration, vacuum, thermalcycling and rudimentary radiation testing,possibly a combination of the above. Onlythose parts with acceptable stability will beimplemented in the final flight model.Ultimately, only their exposure to the harshspace condition in LEO orbit will show ifthey can be reliably used for subsequentmissions.3.2 Sun Sensor Re-DesignThe sun sensor developed by the MIC atDTU is an excellent example of howpicosatellite applications can benefit fromMEMS technology. The sensor area itselfhas an extremely small low profile outline of(7 x 8) mm² and does not require any power.A drawback is the size and powerrequirement of the necessary interfaceelectronics. In fact, the interface size is oneof the most common arguments against theuse of MEMS in space. But other thandiscarding the indisputable potentials ofMEMS, solutions have to be found for theinterface problem. Important inputs can begained from other miniaturization advances,such as those made in mobile telecommunication,which faces very similarconstraints. Solutions adopted from this fieldare power efficient microcontrollers, smalloutline interconnects and light-weight flexcableconnections.3.3 Modular System ArchitectureOne of the design philosophies the teamagreed on in the initial design stage was amodular architecture of the satellite system.Generally speaking, this can be achieved byminimizing the interfaces (electrical, data,mechanical) between individual subsystems.In Compass-1 this is done by agreeing on asimple, yet efficient, common bus system.Since all subsystems are arranged in a slotconfiguration, with the CDHS board actingas the ‘motherboard’, as opposed to thepopular stack configuration for instance, theelectrical connections also serve as amechanical interface. The connectors havebeen selected specifically to withstand thehigh loads during launch.The modular approach also means that allsubsystems contain their own processingunits, such that only a limited volume ofcommands and high-level data will becommunicated between the distributedprocessors over the common system bus.This enables an easy implementation of thedeveloped subsystems on other CubeSats aslong as the few interface requirements aremet.Another example of how modularityenhances the flexibility of the systems as it isnecessary when implementing them ondifferent platforms is highlighted by anattempt to separate the ADCS hardware fromthe (application) flight software. This isparticularly interesting for a system which isas software-driven as an attitude controlsystem. This way, spacecraft specificparameters can be easily adjusted. Evencompletely different flight software (e.g.
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