Abstract
The Data Processing Unit (DPU) is the “heart” of the plasma suite SWA
and is the only interface with the S/C. The DPU is interfaced with EAS,
PAS and HIS sensors via SpW dedicated links and is in charge of
supporting EAS and PAS with power, functionality control, temporary
storage, communication and computational capability and, in addition,
supports HIS with communication to the S/C. Its architecture derives
from a trade-off analysis aiming to define a system able to perform the
needed computational tasks while keeping mass, volume and power within
the limits imposed by the constraints. Additionally, the DPU has been
designed to be “single fault” tolerant and the “cold-spare” concept
has been adopted as redundancy philosophy. It implements data and
command interfaces with the S/C via the redundant SpaceWire (SpW) data
links and the redundant power input HV-HPC command interface. Two
independent, executable SW images represent the overall SWA DPU SW: the
Boot SW (BSW) and the Flight SW (FSW). While the BSW manages the basic
hardware initialization, the FSW manages TC/TM, controls all the
processes related to the state of the sensors, validates and executes
TC, acquires, processes, compresses and formats science data prior to
downlink for EAS(1&2) and PAS sensors, while HIS autonomously processes
its scientific data. In particular, FSW is in charge of data
compression, moments calculation and telemetry generation restrictions
to keep each sensor within its respective telemetry allocation.
Remarkable level of data compression for EAS, which generates the
largest data volume, is reached via customized implementation of
lossless CCSDS 121.0 based on a “Complex Reordering” mechanism, which
avoids periodical jumps among acquisition directions in phase space.
Moments of proton and electron velocity distribution functions are
computed onboard. Different Look-Up Tables (LUT) for PAS and EAS allow
to perform moments calculation modulating the counts in each volume of
phase space by a combination of the factors contained in these tables.
Finally, since SWA data production greatly changes from normal to burst
mode, a book keeping algorithm (BKA) will monitor and control,
continuously along the orbit, the amount of burst mode, scheduled or
triggered, used against the pro-rata expectation.