GDS Overview

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GDS Overview
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GDS Description

• GDS provides a way to fool the flight computer
  into thinking it is in space.
• Provides simulations of flight components
• Provides simulations of space environment
• GDS is a product line
• Currently used on SDO, LRO, ELC
• Planned use on GPM, MMS
• High levels of reuse between missions
• GDS is a partnership between codes 596 and 582
• A. Imasuen/596 has overall responsibility for the
  GDS on MMS
• George Heisey/596 has overall responsibility for
  the GDS on LRO and GPM
• Stephe Leake/582 has overall responsibility for
  GDS multi-mission software development
• GDS website http//fsw.gsfc.nasa.gov/gds/

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GDS software reuse
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GDS Description

• Typical uses of GDS
• Testing hardware interfaces to flight components
  during flight computer development
• CDH lab
• propulsion lab
• Testing/debugging Flight Software interaction
  with flight components, during FSW development
• FSW lab
• Final acceptance testing of FSW
• FLATSAT
• System-level testing of mission scenarios
• FLATSAT
• Support spacecraft IT
• Spacecraft operations and operator training
• After launch
• Typically FLATSAT becomes the MOC spacecraft
  simulator

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GDS Description

• GDS Users Guide
• Basic GDS commands
• Describes operation of each model, giving input,
  parameter, output symbols
• http//fsw.gsfc.nasa.gov/gds/user_guide.pdf
• Algorithms
• Some models need more detail than Users Guide
  provides
• Separate algorithms documents all on GDS website
• Schematics, assembly drawings
• In project CM

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GDS Description

• GDS user interface is ASIST or ITOS
• Completely operated from the TC workstation
• All GDS commands issued from STOL procs
• All GDS telemetry displayed and archived by TC
  workstation
• GDS is configurable
• If a component that GDS normally simulates is now
  hardware, run a different GDS configuration.
• All labs run the same GDS software difference is
  in run-time configuration/startup files.
• GDS software
• Mostly Ada
• Provides twice the productivity of C
• Finds bugs at compile time
• Enables truly reusable code
• C code for device drivers, some heritage models
• Runs on LynxOS - a commercial Real-Time Operating
  System
• Measured 125 microsecond interrupt response time

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GDS Description

• Models allow simulation of all reasonable
  anomalous conditions
• Alignment, scaling, noise parameters
• Specific failure modes modeled
• Broken wires at hardware level
• Stuck thruster valve
• Arbitrary initial conditions
• All parameters can be set at any time before or
  during a simulation
• Hardware mode
• No models running
• User can set all hardware outputs, view all
  hardware inputs
• Used for low-level hardware interface testing
• Models mode
• All models running
• Models use hardware inputs, drive hardware outputs

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GDS Description

• Existing reusable models
• Spacecraft
• Orbit affected by gravity, thrusters, arbitrary
  input
• Attitude affected by thrusters, reactions wheels
• Mass affected by thruster firing (propellant
  usage)
• Inertia affected by thruster firing, gimbal
  motions
• Ephemeris
• Solar position in ECI from Astronomical
  Algorithms
• Lunar position in ECI from JPL DE405
• Gravity
• Spherical model for Earth, Moon, Sun
• Gravity gradient for all three
• Coarse Sun Sensor
• Generic model cosine or 8th order polynomial
• Generic analog hardware interface

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GDS Description

• Existing reusable models (cont)
• Articulated mechanisms
• Anything on gimbals
• Computes positions, inertia
• Heaters
• Generic temperature ramp
• IRU
• Generic model
• Kearfot hardware (pulses)
• Honeywell hardware (1553)
• RWA
• Generic Model
• Feeds torque to spacecraft model
• Integrates torque to get momentum
• GSFC Model
• Adds command/telemetry detail
• Generic analog hardware interface
• GSFC hardware interface (1553)

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GDS Description

• Existing reusable models (cont)
• Star Tracker
• Generic model
• Computes ST quaternion from spacecraft quaternion
• Galileo model
• Adds command/telemetry details
• Galileo hardware interface (1553)
• Digital Sun Sensor
• Generic model
• Computes angle of Sun in DSS sensor frame
• Adcole model
• Adds digitizing details
• Adcole hardware interface (serial)
• Guide Telescope
• Generic model
• Thermistors
• Generic model
• Generic hardware interface

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GSD Description

• Existing models that must be customized
• Thrusters
• Number of tanks
• Number of thrusters
• Valve number and location in fluid schematic

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GDS hardware

• VME chassis
• One CPU card
• Multiple commercial Industrial Pack (IP) plug-in
  cards
• Analog I/O
• Serial I/O
• FPGAs for timing, complex IO
• Custom card chassis
• Protects flight/ETU hardware
• Analog voltage scaling
• Low-level pulse detection (for thrusters, valves,
  relays)
• Connects to IP cards in VME chassis
• Load shelf
• High-power resistors, inductors, relays, etc
• Emulate high-power flight loads
• Propulsion valves
• Heaters
• Other relays

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GDS load shelf LRO
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GDS VME rack
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GDS hardware

• Power Monitoring/Relay Drive (PMRD)
• Reusable card design
• Handles typical propulsion drive signals
• Also other power loads, discrete outputs
• Provides some ground isolation
• Flight signal processing
• Load shelf provides flight-like loads
• Load shelf provides relays
• Valve status signals
• Pyro load opening
• Custom card does analog threshold, digital
  debounce
• FPGA times edges to microsecond accuracy
  (ignoring analog rise times)
• Each propulsion timing FPGA can handle about 5
  thruster, latch or pyro valves
• FPGA drives relays to get response time right
• Customizable for each mission.
• Gains via resistor packs
• Redundant thruster coils, pyro via jumpers

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GDS Hardware

• FPGA module
• Altera EP1K100 FPGA (5000 gates)
• 48 programmable IO pins
• All TTL, all RS485, half half
• Standard IP bus inteface
• Programmed in VHDL
• Standard GDS IP architecture simplifies
  programming new applications

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GDS Hardware

• Thermistor resistor simulation
• Used for thermistors, hinge pots, PRT, etc
• Reusable card design
• Optically isolated digitally controlled
  potentiometers
• Local transformer-coupled power supply
• 48 Channels
• Resolution as low as 40 Ohm
• Range as high as 1 MOhm
• Analog I/O
• Scales A/D, D/A signals to flight levels
• Provides current output
• Provides some ground isolation
• 16 output channels
• 4 input channels

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GDS Validation

• Validation of GDS
• Heritage from previous missions
• Reused models are reused verbatim
• Internal testing
• All internal tests are rerun for each release
• Compare to actual hardware
• Run same tests on GDS models, actual flight
  components
• Demonstrate an adequate match between HiFi and
  GDS/FSW runs
• Compare two independent implementations of
  simulator models both must be right.
• FSW tests pass
• Demonstrates that FSW team and GDS team interpret
  component ICDs in the same way
• Shared parameters
• Many parameters must match between HiFi, FSW, and
  GDS
• For example, reaction wheel inertia, thruster
  placement
• GDS negotiates a sharing mechanism for these
  parameters