Drawing Number: 00014PP02 Rev: D

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Drawing Number 00014-PP02 Rev D

• SNAP Electrical Block Diagram Power Point
  Presentation
• Drawn by H. Heetderks
• Cognizant Engineer H. Heetderks
• Date 2003-11-03

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SNAP Electrical Block Diagram

• Henry Heetderks
• Space Sciences Laboratory, UCB

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Introduction

• The following slides consist of a simplified
  block diagram for the SNAP instrument and
  spacecraft followed by a detailed block diagram
  (controlled as SNAP drawing number 00010-AC14,
  currently at Rev H)
• The drawing shows interfaces between major
  instrument subassemblies and between SNAP and the
  JDEM spacecraft, and also shows how redundancy
  is implemented and single point failures are
  eliminated
• (To see details of this drawing zoom to 400)
• Following the discussion of architecture and
  redundancy is a block diagram showing the data
  flow for a single string from the visible imager
  CCD to the out put which modulates the spacecraft
  Ka band transmitter
• The final viewgraphs show the inputs and outputs
  and describe the operation of each block in the
  data chain

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Overall View of the SNAP Observatory
Secondary Mirror Hexapod Bonnet
Door Assembly
Main Baffle Assembly
Secondary Metering structure
Solar Array, Sun side
Primary Mirror
Optical Bench
Solar Array, Dark side
Instrument Metering Structure
Instrument Radiator
Tertiary Mirror
Instrument Bay
CCD detectorsNIR detectorsSpectrographFocal
Plane guiders Cryo/Particle shield
Fold-Flat Mirror
Spacecraft
ACS CD H Comm Power Data
Instrument Mass Memory (1 of 3 Units)
Shutter
Hi Gain Antenna
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SNAP Simplified Electrical Block Diagram
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Electrical System Features

• No single point failures
• Fully redundant and cross strapped spacecraft
• Redundant Observatory Control Units and Large
  Memories in instrument
• Instrument Detectors and Mechanisms modularized
  for failure tolerance
• Industry Standard 1553 interface between
  instrument and spacecraft
• Wide band downlink controlled directly by
  Instrument
• Memory Capability incorporated into the
  Instrument
• No SSR in spacecraft
• Spacecraft consists of 3 independent modules
  connected by a 1553 bus
• Power system
• ACS System
• CDH System

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Detailed Block Diagram shows full redundancy
plan
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CCD to Transmitter One String

• Data flow from one CCD to the transmitter with
  redundant elements not shown

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Imager CCD Module

• One back illuminated CCD is mounted on an AlN2
  circuit board which is mounted on an invar/
  molybdenum module mounted to the focal plane
• Four video outputs from the CCD are CDSed and
  ADCed by a custom ASIC (the CRIC chip) mounted
  on the rear of the circuit board
• The interface lines to this module include CCD
  clocks, a number of DC bias voltages, and the
  digital and control lineS which operate the ASIC
• Module operates at 135 degrees Kelvin

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CCD Clock Module

• One Clock Module is dedicated to each CCD Module
• Connection to the CCD is made via a 50-60 pin
  connector at one end
• Circuit provides CCD clocking and control of the
  CRIC chip
• System includes one or more ASICs and a DC-DC
  converter
• A connector at the other end has a pair of
  independent Power and Data interfaces for
  connection to the prime and back-up ICUs
• Controller multiplexes the 4 CRIC ADC outputs
  onto a single serial interface
• Circuit operates at 135 Kelvin

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Guider CCD Clock Circuit

• This circuit is similar to that used for the
  Imager CCDs except that its clock waveforms and
  timing are consistent with the video rate of the
  Guider CCDs

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ICU Interface Board

• The primary function of this circuit is to reduce
  the wire count coming from the focal plane to the
  ICU while isolating the redundant ICUs from each
  other
• Separate identical circuits are dedicated to the
  two redundant ICUs
• A relay matrix allows power to be applied to any
  combination of the 9 CCDs controlled by each of
  the four interface boards
• Clock and Strobe signals are buffered and fanned
  out to each CCD module
• Guider interface is separate from Imager CCDs

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Visible Detector Interface Board in ICU

• One such board or module in the ICU controls a
  block of 9 Imager CCDs
• Provides common Clock and Strobe lines for the
  serial data interface and takes data from 9 CCDs
• Board is set up by the ICU DPU via a CDI
  interface (a serial interface used on the last
  dozen or so SSL projects)
• Switched power is provided by the ICU DC-DC power
  system under DPU control
• Interleaved data from the four corners of each of
  the 9 CCDs are output from a single high speed
  serial interface to the Mass Memory (I have
  called this USB as a place holder). Data are
  written in a quasi-CCSDS Source Packet format.

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Guider System Interface Board in ICU

• The Guider Interface board has a dedicated 3 wire
  serial link to each of the 4 guider CCDs
• The DPU sets up modes and supplies power in the
  same way as for the Imager interface
• This board
• Locates bright spots on the Guider CCDs
• Calculates centroids
• Prepares a CCSDS packet with centroid data,
  brightness, quality, time, (velocities?), etc.
• Packet is dumped onto 1553 bus and also put into
  Mass Memory

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NIR Detector Interface Board in ICU

• Circuit operation is TBD but similar to that of
  visible imager

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DPU Board in ICU

• DPU sets up all ICU and Mass Memory functions
  using the CDI interface
• System has a port to read and write to the Mass
  Memory for diagnostic functions, but does not
  normally process or transfer high speed science
  data
• The DPU receives ground commands from the
  spacecraft via the 1553 and processes and
  distributes them to other subsystems
• It also collects SOH data via the CDI and
  generates CCSDS engineering packets and delivers
  them to the spacecraft via 1553
• The DPU controls a switch system routing science
  data to the various modules in the Mass Memory

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Controller Board in Mass Memory Unit

• All Mass Memory functions are controlled by the
  DPU via CDI command
• A cross point switch in the controller routes the
  various science data streams to the desired
  memory module
• The controller is provided with hardware
  capability to
• Perform block data transfers
• De-interleave science data and generate final
  CCSDS source packets
• Perform memory scrubs and EDAC
• Perform data compression on stored data

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128 Gbit Mass Memory Board

• Live chip map transparently maps out bad memory
  locations
• Boards are separately powered and have provision
  on interface lines to prevent power scavenging
• SEU latch-up circuit breaker included on board if
  needed

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Mass Memory Unit Power System

• DC-DC converter takes 28 volt power from the ICU
  and generates switched, limited, and monitored
  power for all memory subsystems
• System can be powered from either the prime or
  back-up ICU
• CDI command from Memory Controller board
  determines which services are powered

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DPU Power Converter Control Board

• The power converter in the ICU provides switched,
  limited, and monitored power for all ICU
  subsystems
• The system can be powered from either the prime
  or back-up spacecraft CDH
• CDI commands from the DPU determine which
  services are powered

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ICU Downlink Board

• This board combines CCSDS source packets into
  CCSDS transfer frames and inserts the final
  header information
• The data are output as complete transfer frames
  to the Ka band transmitter at a 300 Mhz rate
• The output may be sent via 1, 2 or 4 lines
  depending on the details of the transmitter
  design
• Input data from the Mass Memory may be
  transferred on as many as 4 pre-interleaved lines
  depending on the speed of the memorys serial
  data interface

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Calibration Source Controller Board

• This low data rate board has only power and CDI
  interfaces

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Motor Controller Board

• The Cassegrain shutter drive electronics requires
  a somewhat elaborate feed back control system to
  take position data from a magnetic angle resolver
  and control current to the limited angle torque
  motor so as to follow a prescribed opening
  waveform with high precision
• Control of the motorized bipods is expected to be
  quite a bit easier
• Plain stepper motor?

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Redundant Control of Mechanisms

• Separate motors on a common shaft actuate the
  same mechanism
• Separate position sensors for the prime and
  back-up interfaces
• Prime intf connects to prime ICU back-up intf
  connects to back-up ICU
• I.e. no cross strapping

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Structure Thermal Control System

• Thermal control board in OCU has 32 separately
  fused branches of two-wire interfaces consisting
  of a power line with superimposed data and status
  return
• A second level of fusing distributed in the
  harness drives 8 ASICs from each branch
• Connectors are inserted as needed to insure that
  structural elements are not captive by wiring
• Each ASIC contains four heater controllers
• Set point of each controller can be independently
  commanded by ICU
• IICU can read temperature of each sensor for
  inclusion in engineering telemetry
• Each heater consists of a prime and back-up
  element, one controlled by the prime thermal
  control system and the other controlled by the
  back-up system
• Overall, the system independently controls and
  monitors 1,024 heater elements

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Thermal Control ASIC

• One ASIC controls four independent heater
  circuits
• A single 2 wire interface controls all four
  circuits
• Power with superimposed command data and status
  return
• ASICs are used in a pair of redundant circuits
  which control four heaters with redundant
  elements
• Redundant thermistors provide temperature sense
  for the prime and back-up ASICs

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Thermal Control Board in ICU

• All heaters on either the prime or the back-up
  system are run from a single 28 volt service
• A system layers of fuses protects lower
  branches of the tree from faults in the higher
  branches
• Control and status information are multiplexed
  onto the 28 volt power line

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Pyro Controller Board in ICU

• There is currently one mechanical deployment
  baselined on the SNAP observatory the opening
  of the telescope front cover
• Options for effecting this include an EUVE type
  bolt cutter or a Frangibolt of the type used on
  THEMIS
• In either case a DPU controlled power switch is
  required to actuate the mechanism

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Spacecraft Power System
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Spacecraft Power System Notes

• System operates solar array, performs battery
  charge control, and supplies switched, limited,
  and monitored 28 volt /-2 power to OCUs
  and spacecraft systems
• Includes a pair redundant independent
  controllers, either of which can control all
  solar array elements and both batteries
• Each controller has a pair of redundant 1553
  interfaces to provide full cross strapped
  redundancy to the spacecraft 1553 bus
• Every powered subsystem (ICUs and S/C
  components) has a redundant set of protected
  power input interfaces
• I.e. failure on one side will not propagate to
  the other side
• The Power System is stand alone with a pair of
  dedicated control units which are independent
  mechanically and electrically from the spacecraft
  CDH and ACS systems. The only connection
  between it and the other S/C systems is the 1553
  interface and the power service lines. There
  shall be no aspect or feature of the system which
  limits the ability of the SNAP Project to procure
  the power system from a different vendor than
  those supplying the other two major spacecraft
  subsystems.

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Spacecraft CDH System
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Spacecraft CDH System Notes

• The CDH system includes the CDH controllers,
  the S-band transponders, S-band antennas, the
  Ka band transmitters, and fixed 120 cm Ka band
  gain antenna.
• Either of the redundant independent controllers
  can control either set of telemetry components
• Each controller has a pair of redundant 1553
  interfaces to provide full cross strapped
  redundancy to the spacecraft 1553 bus. The CDH
  controller is the bus master.
• The CDH controller tasks include
• Command decoding
• S-band antenna switching
• S/C Engineering T/M collection and transmission
• Control of high gain antenna gimbal
• Modulation of the Ka band transmitters is done
  directly by the OCU
• The CDH System is stand alone with a pair of
  dedicated control units which are independent
  mechanically and electrically from the spacecraft
  Power and ACS systems. The only connection
  between it and the other S/C systems is the 1553
  interface and the power service lines.

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Spacecraft ACS / Propulsion System
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Spacecraft ACS / Propulsion System Notes

• The ACS / Prop system includes the ACS
  controllers, Reaction Wheels, Coarse Star
  Trackers, IRUs, Sun Sensors, Propellant
  Tanks, Thrusters, and propellant valves and
  lines
• The current baseline has 3 star trackers instead
  of the 2 shown
• Each controller has a pair of redundant 1553
  interfaces to provide full cross strapped
  redundancy to the spacecraft 1553 bus
• Interface between the ACS subsystem components
  is done by a separate redundant 1553 bus
• All ACS components including Instrument Fine
  Guider interface via the 1553 bus
• Interface to propulsion components is done by a
  dedicated custom board in the controller
• The propellant system includes 6 redundant fuel
  tanks connected to a redundant pair of manifolds
  with a redundant set of thrusters
• The ACS / Prop system controller tasks include
• Implementation of Safe-Hold mode on initial power
  up
• Operation of the ACS control algorithms
  including
• Coarse (2 arcsec) pointing of the spacecraft
• Fine pointing using Fine Guider centroid data
  received from the ICU via the S/C 1553 bus
• Control of prop system for R/W dump
• Control of prop system for orbit injection and
  station keeping
• Control of prop system for S/C disposal
• The ACS / Prop System is stand alone with a pair
  of dedicated control units which are independent
  mechanically and electrically from the spacecraft
  Power and CDH systems. The only connection
  between it and the other S/C systems is the 1553
  interface and the power service lines.