Planck Pre-Launch Status Papers - APC - Université Paris Diderot ...

M. Bersanelli et al.: Planck pre-launch status: Design and description of the Low Frequency InstrumentThe main functions of the data processing unit (DPU) includemonitoring and control of the RAA, instrument initialisation,error management, on-board time synchronisation, managementof instrument operating modes, and control of theoverall LFI data rate and data volume. Switching the FEMs andBEMs on and off, aswellasvoltageadjustments,areaddressedby the DPU with a configuration that allows flexible setup commands.The DPU interface provides all commands for the DAE,while the SPU interface is in charge of retrieving the fixed formatraw data from the RCAs. Both the DPU and the SPU arebased on an 18 MHz CPU. The link between the REBA andthe DAE is implemented through IEEE 1355 interfaces and bymeans of data flag signals that ensure hardware and softwaresynchronisation.Finally, the data acquisition unit (DAU) is in charge of functionsthat are internal to the REBA, and it has no interfaces withthe RAA. It converts the primary power received from the spacecraftto the secondary regulated voltages required by the REBAand performs analogue-to-digital conversion of REBA housekeepingdata.5. Thermal interfaces5.1. LFI 20 K stageThe LFI front-end is cooled to 20 K by a closed-cycle hydrogensorption cryo-cooler (Wade et al. 2000; Bhandari et al. 2004;Morgante et al. 2009), which also provides 18 K pre-cooling tothe HFI (Fig. 20). The cooler provides ∼1 Wofcoolingpowerfor the LFI FEU. The system operates by thermally cycling a setof compressors filled with La 1.0 Ni 4.78 Sn 0.22 powder alternatelyabsorbing and desorbing H 2 gas as their temperature is cycledbetween ∼270 K and ∼450 K, thus providing the working fluidin a Joule-Thomson (JT) refrigerator.Heating of the sorbent beds is obtained by electrical resistanceheaters, while cooling is achieved by thermally connectingthe compressor element to a radiator at ∼270 K in the warmspacecraft. The hydrogen flow lines are connected to the threeV-groove radiators and passively pre-cooled to