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Simulation of the Apollo Guidance Computer. Julian Webb University of the West of England, Bristol, UK julia[email protected] Introduction. Main aim of project:

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simulation of the apollo guidance computer
Simulation of the Apollo Guidance Computer

Julian Webb

University of the West of England, Bristol, UK

[email protected]

G146/MAPLD 2004

introduction
Introduction
  • Main aim of project:
    • To produce a fully functional Block II LGC (Lunar Module version of the AGC) simulator/emulator running the original MIT-produced code and interacting with the user via reasonably accurate simulations of spacecraft controls
  • Though currently based on the LGC, the simulator would, with some alteration, also be able to represent the CMC
  • The AGC simulator is very much a “work in progress”!

G146/MAPLD 2004

motivations
Motivations
  • Why?
    • To provide a working AGC based on original materials rather than a functional simulation
    • To provide an interesting and accessible history of computing educational resource
    • To honour the efforts of the programmers, engineers and managers involved at MIT/Draper Labs
    • To preserve some AGC materials in a machine-readable form
    • To facilitate statistical analysis of AGC performance
    • To allow better comparison with other 1960’s computers (has the AGC had a bad press?)

G146/MAPLD 2004

simulator basics
Simulator basics
  • The simulator provides a real-time emulation of the LGC, DSKY and other associated LM systems in a package for use under Windows XP
  • Written in C++ (the AGC itself) and Visual Basic (DSKY and other systems)
    • these two major components communicate via DLLs
    • also possible to interface other programs to AGC
  • The AGC simulator adjusts its speed automatically to give the correct 11.7s clock rate

G146/MAPLD 2004

sources used
Sources used
  • Available sources
    • Documentation
      • History of Recent Science and Technology project (mostly ex-MIT/Draper Labs documents)
      • Apollo Lunar Surface Journal (operational documentation, checklists etc.)
    • Source code listings (pdf files)
      • Luminary 1C (for LGC - two versions prior to Apollo 13 flight release)
      • Colossus 249 (for CMC - as flown on Apollo 9)
    • See References slide for more details

G146/MAPLD 2004

construction
Construction
  • Method
    • Luminary source code entry (c1500 pages)
      • available as MS Excel and text files
    • ‘YUL assembler’ written (in Perl)
    • Assembled machine code checked against original core listing (and later against another AGC enthusiast’s version)
    • AGC written and tested
    • Interface DLL written
    • DSKY and other components written
    • Testing against original LM checklists

G146/MAPLD 2004

difficulties encountered
Difficulties encountered
  • Difficulties - documentation
    • Lack of coherent documentation - many documents available but spread over a period of 10 years
    • Some documentation misleading (to the historian) due to numerous version changes - care required to interpret the documents in relation to their context
    • Documentation mainly aimed at informing MIT staff (for obvious reasons) - hence much background research required
    • Little documentation for YUL assembler
    • Lack of machine-readable source code

G146/MAPLD 2004

technical difficulties
Technical difficulties
  • Difficulties - technical
    • Precise understanding of operation of several instructions required investigation of the microcode and...
    • some microcode control pulses are not well documented
    • A few questions remain concerning the YUL assembler output
    • Uplink loads not documented - necessary to simulate these to provide (e.g.) appropriate mission time, vehicle mass

G146/MAPLD 2004

system boundaries
System boundaries
  • Difficulties - system boundaries
    • LGC linked to other LM systems and to the mission control environment
    • Necessary to provide at least a basic simulation of many of these external systems
    • Full emulation limited (at present) to LGC, DSKY and AOT
    • Other systems provided in simulation as required by LGC functionality
    • A major omission is the AGS which seems to have little internal technical documentation remaining (especially regarding the AEA)
    • Microcode - simulation of this is a proposed later stage of this project

G146/MAPLD 2004

educational features
Educational features
  • One aim of this project is to provide an educational facility
  • Demonstration mode:
    • allows pre-prepared scripts to interact with the simulation whilst providing an explanatory commentary
    • features of the AGC can be shown either in isolation or in the context of a mission
    • this facility can be seen in the poster presentation
  • Also available is a fault-creation facility to demonstrate the effect of (e.g.) parity errors, DSKY display malfunctions etc.

G146/MAPLD 2004

progress so far
Progress so far
  • LGC
    • Full instruction set operational
    • Timers and external interrupts operational
    • Fully functional DSKY
  • Associated spacecraft systems
    • Major associated LM switches and circuit-breakers implemented
    • Mechanically functional AOT (no simulated starfield)
    • Limited ACA/TTCA hand-controller simulation and RCS graphic
    • Limited IMU functionality
  • External systems
    • Basic telemetry downlink display
    • Executive and Waitlist display
    • Basic statistical display

G146/MAPLD 2004

work to do
Work to do
  • The following features are planned for the future (not in any particular order):
    • Fully implement IMU
    • Simulate vehicle dynamics
    • Finish implementation of AOT to permit star-based alignments
    • Emulation of microcode instructions
    • Improve statistical performance analysis

G146/MAPLD 2004

conclusion
Conclusion
  • The current simulator provides enough functionality to gain useful insights into the operational use of the LGC and associated environment
  • Future work will extend the simulation to provide for spacecraft manoeuvring
  • The intention is to allow a full LM descent, landing, ascent and rendezvous to be demonstrated

G146/MAPLD 2004

acknowledgments
Acknowledgments
  • Thanks are due to Ron Burkey (who has been working independently on an AGC simulator) for providing a series of stimulating questions during my work and for permitting the checking of the assembled Luminary code against his own machine code created in an alternative way

G146/MAPLD 2004

acronyms
Acronyms
  • ACA – Attitude Controller Assembly
  • AEA – Abort Electronics Assembly (the AGS digital computer system)
  • AGC – Apollo Guidance Computer (cf CMC and LGC)
  • AGS – Abort Guidance System – backup to PGNS to allow rendezvous
  • AOT – Alignment Optical Telescope
  • CMC – Command Module Computer
  • DSKY – DiSplay and KeYboard for AGC
  • IMU – Inertial Measurement Unit
  • ISS – Inertial SubSection
  • LGC – Lunar module Guidance Computer
  • PGNCS – Primary Guidance, Navigation and Control Section
  • PGNS – Primary Guidance and Navigation Section
  • RCS – Reaction Control System (on LM 16 jets arranged in two systems)
  • TTCA – Throttle/Translation Controller Assembly
  • YUL – AGC assembler (original written at Christmas)

G146/MAPLD 2004

references
References
  • Some resources of interest to AGC historians...
    • History of Recent Science and Technology - 77 documents more-or-less relating to the development of the AGC

http://www.hrst.mit.edu/hrs/apollo (N.B. site is not always available)

(A set of convenient indices to these documents is available at my website:

http://www.cems.uwe.ac.uk/~jtwebb/agc)

    • Apollo Lunar Surface Journal - contains several interesting LM operational documents plus complete surface transcripts

http://www.hq.nasa.gov/alsj

    • Eldon C. Hall, Journey to the Moon: The History of the Apollo Guidance Computer, AIAA, Reston, VA, 1996

Should be read in conjunction with: Hugh Blair-Smith, Annotations to Journey to the Moon, http://www.hrst.mit.edu/hrs/apollo/ public/blairsmith.htm

G146/MAPLD 2004

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