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The box holding the Instrument Data Processing Unit (IDPU) attached to the Electrostatic Analyzer (ESA)
Mass: approximately 4.75kg
Average Power: 8W for the core system, 15W for the full instrument
Development Institutions: University of California, Berkeley (UCB)
IDPU Lead: Michael Ludlam and Peter Harvey, UCB
Purpose: To control the instruments, sample, store, compress and telemeter their data
The Instrument Data Processing Unit (IDPU), houses most of the electronics for the instruments on the THEMIS spacecraft: the Flux Gate Magnetometer (FGM); Electrostatic Analyzer (ESA); Electric Fields Instruments (EFI); Search Coil Magnetometer (SCM); and the Solid State Telescope (SST). As the processing and power supply for the instrument package, it provides the interface to the spacecraft, receiving power and commands and sending back the data collected from the instrument sensors. These instrument electronics control how the fields and particles in space will be measured. Individual instrument boards within the IDPU process the data and send it to the main IDPU processor.
Many spacecraft have combined instrumentation but few have taken the single electronics box quite as far as the THEMIS IDPU. Similar in concept to the Fast Auroral SnapshoT (FAST) IDPU, the THEMIS IDPU was compressed by a factor of five. More than fitting all the instrument electronics into one box, the IDPU's role is making all five instruments work together in concert so that the science is maximized for the power and mass used by the IDPU. Building the six IDPUs for THEMIS took less than 10 months from initial engineering units to finished flight units.
The core of the IDPU consists of three parts. The Low Voltage Power Supply (LVPS) takes power from the spacecraft battery and converts it into the different voltage supplies that each of the instruments needs to run. This can be thought of as taking a battery of a certain voltage, like a car battery, and making many batteries of different voltages, similar to AA or AAA batteries. The Power Control Board (PCB) switches these voltages to each of the instruments. This is important because the switches isolate the instrument electronics from each other. If there is a failure on one instrument then it will not bring down the others. The Data Controller Board (DCB) contains the IDPU processor, a computer that receives and acts upon commands from the ground, commands the individual instruments, formats, stores and compresses the data from those instruments and then sends them to the ground via the spacecraft computer.
The IDPU also controls and processes: the EFI boom signals using the Boom Electronics Board; the EFI and SCM signals using the Digital Fields Board; the Fluxgate sensor signals using the Flux Gate Electronics board; and, the SST telescopes using the Digital and Analog Processor Board. Finally, the IDPU's ESA and SST Circuit keeps these particle telescopes synchronized and puts their counts into bins as the spacecraft rotates.
The IDPU computer analyzes the data and collects high time resolution data when the ESA or SST see particles going at high velocity or when strong electric or magnetic fields are detected by the EFI, SCM or FGM sensors. The computer uses the sun sensor to keep track of which direction the flux is coming from so that only the magnetotail direction events are selected. This will be the first time high-rate data is taken on five satellites aligned during a substorm. Once the data has been collected, it is compressed in order to minimize the data return time. There are 256M Bytes of memory in the IDPU, an amount similar to the RAM in your personal computer. This memory is used to store the data taken and then compress it by factor of two or more.